CHAPTER I

THE REPRODUCTION OF LIVING BEINGS _History of the Germ:--Cell-division--Parthenogenesis-- Conjugation--Mneme--Embryological Development--Difference of the Sexes--Castration--Hermaphrodism--Heredity--Blastophthoria._ A general law of organic life decrees that every living individual is gradually transformed in the course of a cycle which is called individual life, and which terminates with death, that is by the destruction of the greater part of the organism. It then becomes inert matter, and the germinative cells alone of all its parts continue its life under certain conditions. =The Cells: Protoplasm. The Nucleus.=--Since the time of _Schwann_ (1830) it is agreed that the cell is the most simple morphological element which is capable of living. Among the lower organisms this element constitutes the entire individual. There is no doubt that the cell is already a thing of high organization. It is formed of infinitely small elements of very different value and chemical constitution, which form what is called _protoplasm_ or the cell-substance. But these infinitely small elements are so far absolutely unknown. It is in them that must be sought the change from inanimate matter, that is the chemical molecule, to living matter, a change which was formerly believed to lie in the protoplasm itself, before its complicated structure was known. We need not concern ourselves here with this question which remains an open one. Life being established, the cell remains its only known constant element. The cell is composed of protoplasm which contains a rounded nucleus formed of _nucleo-plasma_. The nucleus is the most important part of the cell, and governs its life. =Cell-division.=--The lowest unicellular organisms, as each cell of a multicellular organism, reproduce themselves by division or _fission_. Each cell originates from another cell in the following manner: the cell divides in the center as well as its nucleus, and in this way forms two cells which grow by absorbing by _endosmosis_ (filtration) the nutritive juices which surround them. Death or destruction of the cell is therefore death of the entire organism when this is unicellular. But it has been previously reproduced. We find here already the special and fundamental act of conjugation, that is the fusion of two cells into one, which serves to strengthen reproduction. This act, common to all living things including man, shows us that continuation of life is only possible when from time to time different elements, that is elements which have been exposed to different influences, combine together. If this conjugation is prevented and life is allowed to continue indefinitely by means of fission or by budding (_vide infra_), there results a progressive weakening and degeneration which leads to the disappearance of the whole group thus reproduced. It is necessary to explain here the results of recent scientific work on the intimate phenomena of cell-division, for they are closely allied to those of fecundation. The nucleus of an ordinary cell presents itself in the form of a nearly spherical vesicle. Delicate methods of staining have shown that the nucleus encloses several round nucleolar corpuscles, and also a reticulum which is attached to its membrane and spreads through its whole substance. The liquid part of the nucleus fills the meshes of this reticular tissue, which stains easily and for this reason is named _chromatin_. The phenomena of cell division in well-developed cells with nuclei is termed _mitosis_. Certain lower forms of cells exist in which the nucleus is not well differentiated. Mitosis begins in the nucleus (Plate I). Figure 1 represents the cell before division has commenced. In the protoplasm, by the side of the nucleus, is formed a small corpuscle (_c_) which is called the _centrosome_. The nucleus itself is marked _b_. When the cell commences to divide, the meshes of the network of chromatin contract and the centrosome divides into two parts (Fig. 2). Shortly afterward the particles of chromatin concentrate in the form of convoluted rods called _chromosomes_ (Figs. 3 and 4). The number of these varies according to the species of organism, but remains constant for each animal or vegetable species. At the same time the two centrosomes separate from each other on each side of the nucleus. The chromosomes then become shorter and thicker while the nucleus is completely dissolved in the protoplasm of the cell, and its membrane disappears (Fig. 4). Directly afterwards the chromosomes arrange themselves regularly in line, like soldiers at drill, following one of the larger diameters of the cell, and forming a barrier between the two centrosomes (Fig. 5). Each of the chromosomes then divides into two parallel halves of equal thickness (Fig. 6). Figures 3 and 4 show that, while these changes are being produced, each of the two centrosomes is surrounded by stellate rays. Some of these rays extending in the direction of the chromosomes, become attached to one of their extremities and draw it toward the corresponding centrosome (Fig. 7). Thus around each centrosome are grouped as many chromosomes as the mother cell possessed itself (Fig. 8). Simultaneously, the cell enlarges and its protoplasm commences to become indented at each end of the diameter previously formed by the chromosomes. From this moment the nuclear liquid concentrates itself around each of the groups of chromosomes, the rays disappear and the cell divides into two halves, each containing a group of chromosomes (Fig. 9); the indentation increases so as to form a partition across the protoplasm. The chromosomes then form a new meshwork of nuclear chromatin, and we have then two cells each with a nucleus and a centrosome like the mother cell (Fig. 10). This is what takes place in the reproduction of all cells of the animal and vegetable kingdoms. In the simplest unicellular organisms which are known fission constitutes the only means of reproduction. In the complicated organisms of the higher plants and animals each cell divides in the manner indicated above, both in the embryonic period and later on during the development of each of the organs which forms the organism. This fact shows more than any other the intimate relationship which connects all living organisms. The most remarkable thing, perhaps, is the almost mathematical division of the chromosomes into two halves, a division which results in the equal distribution of their substance through the whole organism. We shall return to this point later on. =Reproduction by Budding. Parthenogenesis.= In the animal and vegetable kingdoms the higher organisms become more and more complicated. They are no longer composed of a single cell, but of an increasing number of these cells combined in a whole, of which each part, adapted for a special purpose, is itself formed of cells, differentiated as much by their organic form as by their chemical and physical constitution. In this way, in plants, are formed the leaves, flowers, buds, branches, trunk, bark, etc.; and in animals the skin, intestine, glands, blood, muscles, nerves, brain, sense organs, etc. In spite of the great complication of the divers living multicellular organisms, one often finds among them the power of reproduction by fission or by budding. In certain animals and plants, groups of cells vegetate in buds which separate from the body later on and form a new individual; this occurs among the polypi and plants with bulbs, etc. One can even form a tree by means of a cutting. Ants and bees, which have not been fecundated, are capable of laying eggs which develop by _parthenogenesis_ (virgin parturition) and become complete individuals. But these degenerate and disappear if reproduction by parthenogenesis or budding is continued during several generations. Among the higher animals, the vertebrates and man, there is no reproduction without conjugation; no parthenogenesis or budding. So far as we have studied the question we see in the animal and vegetable kingdoms sexual reproduction, or conjugation, as a _sine qua non_ for the indefinite continuation of life. =The Sexual Glands. The Embryo.= However complicated the organism, it always possesses a special organ, the cells of which, all of the same form, are reserved for the reproduction of the species and especially for conjugation. The cells of these organs, called _sexual glands_, have the power of reproducing themselves so that they reconstruct the whole individual (the type of the species) from which they arose, in an almost identical form, by conjugation (sometimes also, for a certain time, by parthenogenesis) under certain fixed conditions as soon as they leave its body. We can thus say with _Weismann_, speaking philosophically, that these germinal cells continue the life of their parents, so that in reality death only destroys part of the individual, namely, that which has been specially adapted for certain exclusively individual ends. Each individual, therefore, continues to live in his descendants. The germinal cell divides into a number of cells called embryonic, which become differentiated into layers or groups which later on form the different organs of the body. The embryonic period is the name given to the period between the exit of the germinal cell from the maternal body and the final complete development which it acquires in becoming the adult individual. During this period the organism undergoes the most singular metamorphoses. In certain cases it forms a free embryo which appears to be complete, having a special form and mode of life, but which finally becomes transformed into an entirely different sexual individual. Thus from the egg of a butterfly there first emerges a caterpillar, which lives and grows for some time, then changes to a chrysalis and finally to a butterfly. The caterpillar and the chrysalis belong to the embryonic period. During this period every animal reproduces in an abbreviated manner certain forms which resemble more or less those through which its ancestors have passed. The caterpillar, for example, resembles the worm which is the ancestor of the insects. _Haeckel_ calls this the _fundamental biogenetic law_. We are not concerned here with embryology, and will content ourselves with some of the main points. =Germinal Cells. Hermaphrodites.= We now come to _conjugation_. In order to avoid complications we will leave aside plants and speak only of animals. Among multicellular animals, sometimes in the same individual, sometimes in different individuals, occur two kinds of sexual glands, each containing one kind of cells--the male cells and the female cells. When both kinds of sexual glands occur in the same individual, the animal is said to be _hermaphrodite_. When they develop in two different individuals the animals are of distinct sexes. Snails, for example, are hermaphrodite. There also exist lower multicellular animals which reproduce by budding, but among which conjugation takes place from time to time. We shall not consider these animals any further, as they are too remote to interest us here. =Spermatozoa and Ova.=--In all the higher animals, including the hermaphrodites, the male germinal cells, or _spermatozoa_ are characterized by their mobility. Their protoplasm is contractile and their form varies according to the species. In man and vertebrate animals they resemble infinitely small tadpoles, and their tails are equally mobile. The female germinative cell, on the contrary, is immobile and much larger than the male cell. Conjugation consists in the movement of the male cell, by means of variable mechanism, toward the female cell, or egg, into the protoplasm of which it enters. At this moment it produces on the surface of the egg a coagulation, which prevents the entrance of a second spermatozoid. The egg and the spermatozoid both consist of protoplasm containing a nucleus. But, while the spermatozoid has only a small nucleus and very little protoplasm, the egg has a large nucleus and a large quantity of protoplasm. In certain species the protoplasm of the egg grows in the maternal organism in a regular manner to form the _vitellus_ (yolk of egg) which serves as nourishment for the embryo for a long period of its existence. This occurs in birds and reptiles. =Conjugation.=--The phenomena of conjugation were made clear by _van Beneden_ and _Hertwig_. These phenomena, as we have seen, commence among unicellular organisms. In these they do not constitute reproduction, but the vital reënforcement of certain individuals. Conjugation takes place in a different manner in different cases. For example, a unicellular animal applies itself against one of its fellows. The nucleus of each cell divides into two. Then the protoplasm of the two cells fuses over the whole surface of contact, and half the nucleus of the first cell penetrates the second cell, while half the nucleus of the latter enters the first cell. After this exchange the cells separate from each other and each exchanged half of the nucleus fuses with the primitive half of the nucleus remaining in the cell. From this moment each cell continues to reproduce itself by fission, as we have seen above. In another form, two cells meet and fuse completely. Their nuclei become applied against each other and each exchanges half its substance with the other as in the preceding case, so that the final result is the same. In both cases the two conjugated cells are identical, and one cannot call them male and female. =Penetration of the Spermatozoid into the Egg.=--In all the higher animals in which the germinal cells are of two kinds, male and female, conjugation takes place in rather a different manner. Here, the female cell or egg only reproduces itself exceptionally by parthenogenesis. It usually contains no chromosomes and often too little chromatin, so that it perishes when conjugation does not occur. The spermatozoid swims by means of its tail to meet the egg. As soon as it touches it it penetrates it and the coagulation which we have mentioned is produced. This coagulation forms the _vitelline membrane_, which prevents the entry of other spermatozoids. If, from pathological causes the entry of several spermatozoids takes place, there results, according to _Fol_, a double or triple monster. In Fig. 11 on Plate II, we see the egg with its vitelline membrane and nucleus, the chromatin network of which is marked in blue: _b_ shows the protoplasm of the egg or _vitellus_; _a_ the vitelline membrane; _d_ the spermatozoid which has just entered, and the nucleus of which, composed chiefly of chromatin, is colored red, while its tail has performed its task and is about to disappear. The letters _e_, _f_, and _g_, show a spermatozoid which has arrived too late. Before the head of the spermatozoid which has entered, appears a centrosome (Fig. 12) which it brings to the egg with its small amount of protoplasm, and around this centrosome rays form, as in the case of cellular fission. At the same time a nuclear liquid arising from the protoplasm of the egg becomes concentrated around the chromatin of the spermatozoid, while the nucleus of the egg remains in place and does not change. The nucleus of the spermatozoid, on the contrary, begins to grow rapidly. It forms half the number of chromosomes corresponding to the cell of the species to which it belongs, and grows at the expense of the vitellus of the egg. During this time the centrosome divides into two halves, which progress slowly on each side toward the periphery of the egg, as in the case of fission (see Plate I), while the chromatin of the chromosomes of the spermatozoid is dissolved in the network. The nucleus thus formed by the spermatozoid enlarges more and more (Figs. 13 and 14) till it attains the size and shape of that of the egg (Fig. 15). The male and female chromatin are colored red and blue respectively. Then only commences activity of the nucleus of the egg, at the same time as fresh activity on the part of the nucleus of the spermatozoid. Before this, however, the nucleus of the egg has thrown off a part of its chromatin called a _polar_ body, and it now possesses only half as much chromatin as the other cells of the body of the individual. The nucleus of the egg and that of the spermatozoid then begin at the same time to concentrate their chromatin in the form of chromosomes (Fig. 16) which arrange themselves regularly in the middle line exactly as shown in Plate I, and divide longitudinally into two halves which are then attracted in opposite directions by the rays of each of the centrosomes (Fig. 17). Figure 17, of Plate II, thus corresponds exactly to Fig. 6, of Plate I. In fact, the growth of the nucleus of the spermatozoid has given to its substance the same power of development as to that of the nucleus of the egg. Both enter into conjugation in equal parts, which symbolizes the social equality and the rights of the two sexes! The signification of these facts is as follows: as soon as, in the course of development, the conjugated nuclei divide again into two cells, as in Figs. 7 to 10, of Plate I, each of these two cells contains almost the same quantity of paternal as maternal chromatin. We do not say exactly as much, for the paternal and maternal influences are not divided equally in the descendants. This phenomenon may be explained by what _Semon_ calls alternating ecphoria in mnemic dichotomy. (_Vide infra._) As cell division continues in the same way during embryonic life, it follows that each cell, or at least each nucleus of the future organism, will contain on the average half its substance and energy from the paternal and half from the maternal side. =Heredity. The Mneme.=--The secret of heredity lies in the phenomena which have been just described. Hereditary influence preserves all its primary power and original qualities in the chromosomes, which enlarge and divide, while the vitelline substance, absorbed by the chromosomes and transformed by the vital chemical processes into the specific substance of the chromosomes, loses its specific and plastic vital energy, as completely as the food which we swallow loses its energy in forming the structure of our living organs. We do not acquire any of the characters of the ox by eating beefsteaks; and the spermatozoid, after eating much vitelline protoplasm, preserves its own hereditary energies, increased and fortified, but without change in their qualities. In this way the nuclear chromatin of our germinal cells becomes the carrier of all the hereditary qualities of the species (hereditary mneme), and more especially those of our direct ancestors. The uniformity of the intracellular phenomena in cell division and conjugation proves, however, that, without being capable of reproducing the individual, the other non-germinal cells of the body may also possess these hereditary energies, and that there exists, hidden behind all these facts, an unknown law of life, the explanation of which is reserved for the future. However, a recent work based on an idea of the physiologist, _E. Hering_, which looks upon instinct as a kind of memory of the species, opens up a new horizon. I refer to the book of _Richard Semon_: "The _mneme_ considered as the conservative principle in the transmutations of organic life." (_Die Mneme als erhaltendes Prinzip im Wechsel des organischen Geschehens_, Leipzig, 1904.) _Conception of Irritation._[1]--By the aid of the fundamental facts of morphological science, biological and psychological, _Semon_ proves that _Hering's_ idea is more than an analogy, and that there is a fundamental identity in the mechanism of organic life. In order to avoid the terminology of psychology which tends to be equivocal, _Semon_ employs some new terms to designate his new ideas, based on the fundamental conception of _irritation_ in its physiological sense. _Semon_ defines _irritation_ as an energetic action on the organism which determines a series of complicated changes in the irritable substance of the living organism. The condition of the organism thus modified, which lasts as long as the irritation, is called by _Semon_ the _state of irritation_. Before the action of irritation, the organism is in a condition which _Semon_ calls the _primary state of indifference_, and after its action, in the _secondary state of indifference_. _Engram. Ecphoria._--If, when an irritation has entirely ceased, the irritable substance of the living organism becomes modified permanently during its secondary state of indifference, _Semon_ calls the action _engraphic_. To the modification itself he gives the word _engram_. The sum of the hereditary and individual engrams thus produced in a living organism is designated by the term _mneme_. _Semon_ gives the name _ecphoria_ to the revival of the engram by the repetition of part only of the original irritation, or by the entire but weakened reproduction of the whole state of irritation of the organism, which was originally produced in a synchronous manner with the primary irritation. Thus, an engram may be ecphoriated (that is to say, reproduced or revived) by the return of one part of the complex of primary irritations which produced it. A young dog, for example, is attacked by urchins who throw stones at it. It experiences two kinds of irritation: (1) the urchins stooping down and throwing stones (optic irritation); (2) the pain caused by the stones (tactile irritation). In its brain are produced two associated series of corresponding engrams. Previously, this dog did not react when it saw people stoop down. From this moment it will run away and howl at the sight, without any stone being thrown at it. Thus the tactile engram will be ecphoriated by the repetition of the original associated irritation. In the same way, the image of a tree in a known landscape will ecphoriate the entire landscape. Moreover, an engram may be revived by the enfeebled return of the primary irritating agent which produced it, or by an analogous enfeebled irritation. Thus, the sight of a photograph will revive the image of a known person. A certain kind of maize imported for a long time into Norway and influenced in that country during many generations by the sun of the long summer days, finally accelerated its time of maturation. When imported again to the south of Europe it first preserved its faculty of accelerated maturation in spite of the shortness of the days (_Schübeler_). _Semon_ gives a series of analogous examples which show how engrams repeated during several generations accumulate and end by becoming ecphoriated when they have acquired enough power. Engrams may be associated simultaneously in space, such as those of sight. But they may also be associated in succession, such as those of hearing and of ontogeny. Simultaneous engrams are associated in every direction with the same intensity. Successive engrams, on the contrary, are associated more strongly forwards than backwards, and have only two poles. In the succession _a b_, _a_ acts more strongly on _b_ than _b_ on _a_. In the successions of engrams it often happens that two or more analogous engrams are associated in a manner more or less equivalent to a preceding engram. _Semon_ calls this phenomenon dichotomy, trichotomy, etc. But in the successions, two engrams cannot be ecphoriated simultaneously. Hence the phenomenon which _Semon_ names _alternating ecphoria_; that is sometimes one, sometimes the other of the constituent engrams, for example, of a dichotomy, which arrives at ecphoria. Similarly, the engram of the ecphoriated dichotomy is most often that which has been previously most often repeated. In the laws of ontogeny and heredity alternating ecphoria plays an important part. The branch less often repeated remains latent and the other only is ecphoriated. But certain combinations which reënforce the latent branch or paralyze the other may induce ecphoria of the first to the second generation. _Semon_ also shows that the phenomena of regeneration in the embryo, as well as those of the adult, obey the law of the mneme. _Homophony._--The terms engram and ecphoria correspond to the well-known introspective phenomena in psychology of memory and the association of ideas. Engrams are thus ecphoriated. At the time of such phenomena every mnemic irritation of the engrams vibrates simultaneously with the state of synchronous irritation produced by a new irritation. This simultaneous irritation is named by _Semon_ _homophony_. When a partial discord is produced between the new irritation and the mnemic irritation, the organism always tends to reëstablish homophony (harmony). This is seen in psychological introspection by activity of attention; in embryology by the phenomenon of regeneration; and in phylogeny by that of adaptation. Relying on these convincing facts, _Semon_ shows that irritative actions are only localized at first in their zone of entry (primary zone); but that afterward they irradiate or vibrate, gradually becoming weaker in the whole organism (not only in the nervous system, for they also act on plants). By this means, engraphia, although infinitely enfeebled, may finally reach the germinal cells. _Semon_ then shows how the most feeble engraphias may gradually arrive at ecphoria, as the result of numerous repetitions (in phylogeny after innumerable generations). This is how the mnemic principle allows us to conceive the possibility of an infinitely slow heredity of characters acquired by individuals, a heredity resulting from prolonged repetition. The facts invoked by _Weismann_ against the heredity of acquired characters lose nothing of their weight by this, for the influence of crossing (conjugation) and selection transforms the material organic forms in an infinitely more rapid and intense manner than individual mnemic engraphias. The latter, on the other hand, furnish the explanation of the mutations of _de Vries_, which appear to be only sudden ecphoria of accumulated long engraphic actions. The way in which _Semon_ studies and discusses the laws of the mneme in morphology, physiology and psychology, is truly magisterial, and the perspective which opens out from these new ideas is extensive. The mneme, with the aid of the energetic action of the external world, acts on organisms by preserving them and combining them by engraphia, while selection eliminates all that is ill-adapted, and homophony reëstablishes the equilibrium. The irritations of the external world, therefore, furnish the material for the construction of organisms. I confess to having been converted by _Semon_ to this way of conceiving the heredity of acquired characters. Instead of several nebulous hypotheses, we have only one--the nature of mnemic engraphia. It is for the future to discover its origin in physical and chemical laws. I must refer my readers to _Semon's_ book, for this volume of 343 pages, filled with facts and proofs, cannot be condensed into a few paragraphs. =Each Cell bears in itself Ancestral Energy.= As we have already seen, the germinal, cells are not the only ones which possess the energies of all the characters of the species. On the contrary it becomes more and more certain, from further investigation, that each cell of the body bears in itself, so to speak, all the energies of the species, as is distinctly seen in plants. But in all the cells which are not capable of germinating, these energies remain incapable of development. It results that such energies, remaining virtual, have no practical importance. In an analogous sense we may say that all the cells of the body are hermaphrodite, as all germinal cells, for each possesses in itself the undifferentiated energies of each sex. Each spermatozoid contains all the energies of the paternal and maternal ancestry of man, and each egg those of the paternal and maternal ancestry of woman. The male and the female are only the bearers of each kind of germinal cells necessary for conjugation, and each of these bearers only differs from the others by its sexual cells and by what is called correlative sexual differences. But we must not forget that the germinal cells themselves are only differentiated at a certain period in the development of the embryo; they are thus hermaphrodite originally and only become male and female later. New experiments made on the eggs of sea urchins and other organisms have shown that conjugation may be replaced by an external irritating agent; for example, the action of certain chemical substances is sufficient to make eggs develop by parthenogenesis which would have died without this action. An entire being has been successfully produced from an egg divided into two by means of a hair. And even from the protoplasm of the egg without its nucleus, with the aid of a spermatozoid. We must not, however, base premature hypotheses on these facts. When a female cell, or egg, develops without fecundation (parthenogenesis) its nucleus enlarges and divides in the same manner as conjugated nuclei (mitosis). A point of general interest is what is called the _specific polyembryony_ of certain parasitic insects (hymenoptera of the genus _Encyrtus_). According to _Marchal_, their eggs grow and divide into a considerable number of secondary eggs, each of which gives rise to an embryo and later on a perfect insect. By shaking the eggs of certain marine animals they have been caused to divide into several eggs and thus to produce several embryos. All the individuals arising from the division of the same egg of _Encyrtus_ are of the same sex. [Illustration: PLATE I CELL DIVISION FIG. 1. Cell before division. FIG. 2. Division of centrosome. FIG. 3. Formation of chromosomes. FIG. 4. Dissolution of nucleus. FIG. 5. Lining up of chromosomes. FIG. 6. Division of chromosomes. FIG. 7. Division of chromosomes. FIG. 8. Attraction of chromosomes by centrosomes. FIG. 9. Concentration of nuclei. Division of cell. FIG. 10. Formation of new chromatin.] [Illustration: PLATE II FERTILIZATION OF THE OVUM BY THE SPERMATOZOID DIAGRAM OF OVUM AND SPERMATOZOID FIG. 11. _a_, Vitelline membrane; _b_, protoplasm, or vitellus; _c_, nucleus with chromatin; _d_, spermatozoid penetrating egg; _e_, another spermatozoid arrested by the vitelline membrane. FIG. 12. Formation of centrosome. FIG. 13. Formation of male nucleus by spermatozoid. Division of centrosome. FIG. 14. Development of nucleus of spermatozoid. FIG. 15. Nucleus of spermatozoid attains same size as that of ovum. FIG. 16. Formation of male and female chromosomes. FIG. 17. Lining up of male and female chromosomes.] =Embryology.=--It is not necessary to describe here in detail the different changes which the two conjugated cells pass through to become an adult man. This is the object of the science of embryology. We shall return to this in Chapter III. A few words are necessary, however, to explain the general principles. =Ovulation. The corpus luteum.=--The ovaries of woman (Fig. 18) contain a considerable number of cells or ovules, although infinitely less than the number of spermatozoids contained in the testicles. From time to time some of these ovules enlarge and are surrounded by a vesicle with liquid contents, which is called the Graafian follicle. At the time of the monthly periods an egg (sometimes two) is discharged from its Graafian follicle, from one or other ovary. This phenomenon is called _ovulation_. The empty follicle becomes cicatrized in the ovary and is called the _corpus luteum_ (yellow body). The egg after its discharge arrives at the abdominal orifice of the Fallopian tube, which communicates directly with the abdominal cavity. Some authors state that the end of the tube becomes applied against the ovary by the aid of muscular movement and, so to speak, sucks in the discharged ovule, while others hold that the movements of the vibratile cilia, with which the epithelium of the tubes is furnished, suffice to draw the ovule into its cavity. Figure 18 explains this phenomenon. Having arrived in the tube, the ovule moves very slowly in the almost capillary tube by means of the vibratile cilia and arrives in the cavity of the womb. Fecundation probably takes place most often at the entrance to the tube or in its canal; sometimes possibly in the womb. On some occasions a squad of spermatozoids advances to meet the descending egg, and numerous spermatozoids are often found in the tubes, even as far as the abdominal cavity. =Fixation of the egg. Formation of the Decidua.=--After fecundation, the egg becomes attached to the mucous membrane of the cavity of the womb. This mucous membrane proliferates and becomes gradually detached from the womb to form the _membrana decidua_ which envelops the egg or ovule. An egg fecundated and fixed in this way may keep its position and grow during the first weeks of pregnancy, by the aid of villosities covering its envelope which penetrate the wall of the womb. [Illustration: FIG. 18. Diagrammatic section in median plane of the female genital organs. It shows the position of an ovule which has just been discharged lying in the opening of the right tube, and that of another ovary fecundated and surrounded by the decidual membrane. In reality this could hardly coexist with the other ovule freely discharged. In the right ovary are seen ovules in various degrees of maturity in their Graafian follicles: also a corpus luteum--an empty Graafian follicle after expulsion of the ovule. The figure also shows the end of the penis in the vagina at the moment of ejaculation of semen, and the position of a preventive to avoid fecundation.] [Illustration: FIG. 19. The mouth of the tube applied to the ovary at the moment of expulsion of the ovule.] =The womb. The placenta.= The womb or uterus is the size of a small egg flattened in one direction. It terminates below in the neck or _cervix_, which is prolonged into the vagina as a projection, called the vaginal portion of the uterus. The cavity of the womb is continued into the neck and opens below in the vagina by an aperture which is round in virgins and is called the external _os uteri_. The walls of the womb consist of a thick layer of unstriped muscle. When childbirth takes place it causes tearing which makes the external os uteri irregular and fissured. During copulation the aperture of the penis or male organ is placed nearly opposite the os uteri, which facilitates the entrance of spermatozoa into the uterus. (For the illustration of these points see Fig. 18.) The vitellus and the membrane of the egg enlarge with the embryo and absorb by endosmosis the nutritive matter necessary for the latter, contained in the maternal blood. The womb itself enlarges at the same time as the embryo. [Illustration: FIG. 20. Human egg of the second week: magnified eight times. (After _Kölliker_.) _Chor._ Chorion or envelope of the egg. _Vill._ Villi of the chorion. _Emb._ Embryo (near the head are seen the branchial arches). _Umb._ Umbilical vesicle. _Am._ Amnion.] The fasciculus attached to the embryo is the allantois which becomes the umbilical cord. The vertebræ are already easy to recognize in this embryo. The embryo is formed from a portion of blastoderm, that is to say, from the cellular layer applied to the membranes of the egg and arising from the successive divisions of the two primary conjugated cells and their daughter cells. The embryo has the form of a spatula with the head at one end and the tail at the other. From its walls is detached a surrounding vesicle (Fig. 20) called the _amnion_, while another vesicle, the _umbilical vesicle_, grows from its ventral surface and serves, in birds, for the vitelline circulation of the egg which is detached from the mother's body. In man, the umbilical vesicle is unimportant. In its place the circulation of the blood takes place by the aid of another vesicle, called the _allantois_, which arises from the intestine of the embryo, and which becomes attached to the walls of the womb in the form of a thick disk called the placenta. [Illustration: FIG. 21. Embryo of four weeks (After _Kölliker_).