McMurrich1914 Chapter 1
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McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.
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Chapter I. The Spermatozoon and Spermatogenesis; The Ovum and its Maturation and Fertilization
The Spermatozoon
The human spermatozoon (Figs. 4 and 5) is a minute and greatly elongated cell, measuring about 0.05 mm. in length. It consists of an anterior broader portion or head (Fig. 5, H) , which measures about 0.005 mm - i n length and, when viewed from one surface (Fig. 4, 1), has an oval outline, though since it is somewhat flattened or concave toward the tip, it has a pyriform shape when seen in profile (Fig. 4, 2). Covering the flattened portion of the head and fitting closely to it is a delicate cap-like membrane, the head-cap (Fig. 5, He), whose apex is a sharp edge, this structure corresponding to a pointed prolongation of the cap found in the spermatozoon of many of the lower vertebrates and known as the perforatorium. Immediately behind the head is a short portion known as the neck (Fig. 5, N), which consists of an upper more refractive body, the anterior nodule, and a lower clearer portion. To this succeeds the connecting or middle-piece (Figs. 4 and 5, m) which begins with a posterior nodule, from the center of which there passes back through the axis of the piece an axial filament, enclosed within a sheath, this latter having wrapped around it a spiral filament. At the lower end of the middle-piece this spiral filament terminates in the annulus, through which the axial filament and its sheath passes into the jiagellum or tail (Fig. 4,/). This portion, which constitutes about four-fifths of the total length of the spermatozoon is composed simply of the axial filament and its sheath, this latter gradually thinning out as it passes backward and ceasing altogether a short distance above the end of the axial filament.
Fig. 4. - Human Spermatozoon. 1, Front view; 2, side view of the head; e, terminal filament; k, head; /, tail; m, middle-piece. - (After Retzius.)
Fig. 5. - Diagram Showing the Structure of a Human Spermatozoon.
Af, Axial filament; Ann, annulus; H, head; He, lower border of head -cap; m, middle- piece; N, neck; Na and Np, anterior and posterior nodule; S, sheath of axial filament; Spf, spiral filament. - (Bonnet, after Meves.)
The filament thus projects somewhat beyond the actual end of the tail, forming what is known as the terminal filament or end-piece (Fig. 4, e).
To understand the significance of the Various parts entering into the composition of the spermatozoon a study of their development is necessary, and since the various processes of spermatogenesis have been much more accurately observed in such mammalia as the rat and guinea-pig than in man, the description which follows will be based on what has been described as occurring in these forms. From what is known of the spermatogenesis in man it seems certain that it closely resembles that of these mammals so far as its essential features are concerned.
Spermatogenesis
The spermatozoa are developed from the cells which line the interior of the seminiferous tubules of the testis. The various stages of development cannot all be seen at any one part of a tubule, but the formation of the spermatozoa seems to pass along each tubule in a wave-like manner and the appearances presented at different points of the wave may be represented diagrammatically as in Fig. 6.
Fig. 6. - Diagram showing Stages of Spermatogenesis as seen in Different Sectors of a Seminiferous Tubule of a Rat. s, Sertoli cell; sc l , spermatocyte of the first order; sc 2 , spermatocyte of the second order; sg, spermatogone; sp, spermatid; sz, spermatozoon. - (Modified from von Lenhossek.)
In the first section of this figure four different generations of cells are represented; above are mature spermatozoa lying in the lumen of the tubule, while next the basement membrane is a series of cells from which a new generation of spermatozoa is about to develop. The cells of this series are of two kinds; the larger one (s) will develop into a structure known as a Sertoli cell, while the others are parent cells of spermatozoa and are termed spermatogonia (sg). In the next section the Sertoli cell is seen to have become considerably enlarged, its cytoplasm projecting toward the lumen of the tubule, and in the third section the enlargement has increased to such an extent that the spermatogonia are forced away from the basement membrane, with which the Sertoli cell alone is in contact. In the fourth section ("he spermatogonia are seen in process of division; one of the cells so formed will persist as a spermatogone, while the other forms what is termed a primary spermatocyte (sc 1 ). The results of the division are seen in the last section, where four spermatogonia are seen again in contact with the basement membrane and above them are four primary spermatocytes. Returning now to the first and second sections, the layer of primary spermatocytes may still be seen, indications of an approaching division being furnished by the arrangement of the chromatin in those of the second section, and in the third section the division is seen in progress, the two cells which result from it being termed secondary spermatocytes (sc 2 ). These cells almost immediately undergo division, as shown in the fourth section, each giving rise to two spermatids (sp), each of which becomes later on directly transformed into a spermatozoon (sz). From each primary spermatocyte there have been formed, therefore, as the result of two mitoses, four cells, each of which represents a spermatozoon.
During these divisions important departures from the typical method of mitosis occur, these departures leading to a reduction of the chromosomes in each spermatid to one-half the number occurring in the somatic cells. The general plan by which this is accomplished may be described as follows: In the division of the spermatogonia the number of chromosomes that appears is identical with that found in the somatic cells, so that in a form whose somatic number is eight, eight chromosomes appear in each spermatogonium, and divide so that eight pass to each of the resulting primary spermatocytes. When these cells divide, however, the number of chromosomes that appears is only one-half the somatic number, namely, four in the supposed case that is being described (Fig. 7, sc 1 ). The further history of these chromosomes indicates that each is composed of four elements more or less closely united to form a tetrad, and during mitosis each tetrad divides into two dyads, four of which will therefore pass into each secondary spermatocyte. These cells (Fig. 7, sc 2 ) undergo division without the usual reconstruction of the nucleus and each of the dyads which they contain is halved, so that each spermatid receives a number of single chromosomes equal to half the number characteristic for the species (Fig. 7, sp).
Fig. 7. - Diagram Illustrating the Reduction of the Chromosomes During Spermatogenesis. sc 1 , Spermatocyte of the first order; sc 2 , spermatocyte of the second order; sp, spermatid.
This account of the behavior of the chromosomes during spermatogenesis assumes that all the chromosomes of the primary spermatocytes are of equal value and behave similarly during mitosis. It has been found, however, that in a number of forms (insects, spiders, birds, etc.,) this is not the case and recent observations by Guyer indicate that in man certain of the spermatocytic chromosomes differ decidedly from their fellows. At the division of the primary spermatocytes twelve chromosomes make their appearance, but two of these differ from the rest in that they do not divide, but pass directly to one of the poles of the mitotic spindle (Fig. 8). When the division is completed, accordingly, one of the two daughter secondary spermatocytes will have received two undivided or accessory chromosomes plus ten ordinary chromosomes, resulting from the division of ten of the primary spermatocytic chromosomes; the other daughter cell, on the other hand, will have received only ten ordinary chromosomes in all, so that two classes of secondary spermatocytes are formed, in one of which the cells possess twelve chromosomes and in the other only ten.
In this respect, then, the spermatogenesis in man differs from the general plan described above and the division of the secondary spermatocytes reveals a second difference. For in these mitoses instead of twelve and ten chromosomes, seven and five, respectively, make their appearance. This may be explained on the supposition that the ten ordinary chromosomes, present in each class of secondary spermatocytes, have united to form five bivalent chromosomes, while the two accessory chromosomes, present in one of the classes have remained distinct. During the mitosis the accessory chromosomes divide just as do the ordinary ones, so that from each spermatocyte of one class two spermatids are formed, each containing seven chromosomes, while from each spermatocyte of the other class two spermatids, each containing five chromosomes, result (Fig. 8). Since the spermatids are directly transformed into spermatozoa, half of these latter will have received seven chromosomes, and the remaining half will have received five, or, since the five ordinary chromosomes are bivalent and the two accessories are univalent, the spermatozoa of one class will each have received the equivalent of ten plus two, i. e., twelve univalent chromosomes, while those of the other class will have received the equivalent of only ten.*
The transformation of the spermatids into spermatozoa takes place while they are in intimate association with the Sertoli cells, a number of them fusing with the cytoplasm of an enlarged Sertoli cell, as shown in Fig. 6, s, and probably receiving nutrition from it. In each spermatid there is present in addition to the nucleus, an archoplasm sphere and two centrosom.es that have migrated from the archoplasm and lie free in the cytoplasm. The centrosomes and the archoplasm sphere take up their position at opposite poles of the nucleus, the archoplasm eventually forming the head-cap of the spermatozoon, and from one of the centrosomes a slender axial filament grows out and soon projects beyond the limits of the cytoplasm (Fig. g, A). The other centrosome becomes a rod-shaped structure which applies itself closely to the posterior pole of the nucleus, becoming the anterior nodule, while the lower one, from which the filament arises, becomes at first pyramidal in shape (Fig. 9, B) and later separates into a rod-like portion to which the filament is attached and a ring, through which the filament passes (Fig. 9, C). The rod-like portion becomes the posterior nodule, and the ring separates from it to form the annulus (Fig. g,D). The nucleus becomes the head of the spermatozoon, the cytoplasm surrounding it becoming reduced to an exceedingly delicate layer, so that the head is composed almost entirely of nuclear substance, if the head-cap be left out of consideration. The spiral filament of the middle-piece is, however, a derivative of the cytoplasm and according to some authors this portion of the spermatid also furnishes the material for the sheath of the axial filament, though this has been denied (Meves), the sheath being regarded as a differentiation of the axial filament. Each spermatozoon is, then, one of four equivalent cells, produced by two successive divisions of a primary spermatocyte and containing one-half the number of chromosomes characteristic for the species.
Fig. 8. - Diagram Illustrating the Behavior of the Chromosomes in Human Spermatogenesis. The upper figure shows the mitotic spindle of a primary spermatocyte with the two accessory chromosomes passing to one pole. The two figures in the second row represent the chromosomes of such a spindle in an anaphase ; seen from either pole, and the figures of the last row represent spermatids derived from the two classes of secondary spermatocytes. - (Based on Guyer.)
- Doubt has been thrown upon the accuracy of these observations by Gutherz, who, while he finds a structure in the human spermatocyte which he identifies as an accessor)' chromosome, claims that it divides similarly to the other chromosomes. He does not find, therefore, any numerical difference in the chromosomes of the spermatids dividing them into two classes, although there may be qualitative differences indistinguishable by our present technique.
Fig. 9. - Stages in the Transformation of a Spermatid into a Spermatozoon. - (After Meves.)
The number of spermatozoa produced during the lifetime of a single individual is very large. It has been found that 1 cu. mm. of human ejaculate contains 60,876 spermatozoa, a single ejaculate, therefore, containing over 200,000,000. This would indicate that during his lifetime a man may produce 340 billion spermatozoa (Lode).
The Ovum
The human ovum is a spherical cell measuring about 0.2 mm. in diameter and is contained within a cavity situated near or at the surface of the ovary and termed a Graafian follicle. This follicle is surrounded by a capsule composed of two layers, an outer one, the theca externa, consisting of fibrous tissue resembling that found in the ovarian stroma, and an inner one, the theca interna, composed of numerous spherical and fusiform cells. Both the these are richly supplied with blood-vessels, the theca interna especially being the seat of a very rich capillary network. Internal to the theca interna there is a transparent, thin, and structureless hyaline membrane, within which is the follicle proper, whose wall is formed by a layer of cells termed the stratum granulosum (Fig. 10, mg) and inclosing a cavity filled with an albuminous fluid, the liquor folliculi. At one point, usually on the surface nearest the center of the ovary, the stratum granulosum is greatly thickened to form a mass of cells, the discus proligerus (dp), which projects into the cavity of the follicle and encloses the ovum (0) . Usually but a single ovum is contained in any discus, though occasionally two or even three may occur.
Fig. 10. - Section through Portion of an Ovary of an Opossum (Didephys virginiana) showing Ova and Follicles in Various Stages of Development. b, Blood-vessel; dp, discus proligerus; mg, stratum granulosum; o, ovum; s, stroma; th, theca folliculi.
Fig. 11. - Ovum from Ovary of a Woman Thirty Years of Age. cr, Corona radiata; n, nucleus; p, protoplasmic zone of ovum; ps, perivitelline space; y, yolk; zp, zona pellucida. - (Nagel.)
The cells of the discus proligerus are for the most part more or less spherical or ovoid in shape and are arranged irregularly. In the immediate vicinity of the ovum, however, they are more columnar in form and are arranged in about two concentric rows, thus giving a somewhat radiated appearance to this portion of the discus, which is termed the corona radiata (Fig. u, cr). Immediately within the corona is a transparent membrane, the zona pellucida (Fig. n, zp), about as thick as one of the cell rows of the corona (0.02 to 0.024 mm.) , and presenting a very fine radial striation which has been held to be due to minute pores traversing the membrane and containing delicate prolongations of the cells of the corona radiata. Within the zona pellucida is the ovum proper, whose cytoplasm is more or less clearly differentiated into an outer more purely protoplasmic portion (Fig. n, p) and an inner mass (y) which contains numerous fine granules of fatty and albuminous natures. These granules represent the food yolk or deutoplasm, which is usually much more abundant in the ova of other mammals and forms a mass of relatively enormous size in the ova of birds and reptiles. The nucleus (n) is situated somewhat excentrically in the deutoplasmic portion of the ovum and contains a single, well-defined nucleolus.
A follicle with the structure described above and containing a fully grown ovum may measure anywhere from five to twelve millimeters in diameter, and is said to be "mature," having reached its full development and being ready to burst and set free the ovum. This, however, is not yet mature; it is not ready for fertilization, but must first undergo certain changes similar to those through which the spermatocyte passes, the so-called ovum at this stage being more properly a primary oocyte. But before describing the phenomena of maturation of the ovum it will be well to consider the extrusion of the ovum and the changes which the follicle subsequently undergoes.
Ovulation and the Corpus Luteum. - As a rule, but a single follicle near maturity is found in either the one or the other ovary at any given time. In the early stages of its development a follicle is situated somewhat deeply in the stroma of the ovary, but during its growth it approaches the surface and eventually forms a marked prominence, only an exceedingly thin membrane separating the cavity of the follicle from the abdominal cavity. This thin membrane finally ruptures, and the liquor folliculi, which is apparently under some pressure while contained within the follicle, rushes out through the rupture, carrying with it the ovum surrounded by some of the cells of the discus proligerus.
The immediate cause of the bursting of the follicle is not yet clearly understood. It has been suggested that a gradual increase of the liquor folliculi under pressure must in itself finally lead to a rupture, and it has also been pointed out that just before the maturation of the follicle the theca interna undergoes an exceedingly rapid development and vascularization which may play an important part in the phenomenon.
Normally the ovum when expelled from its follicle is received at once into the Fallopian tube, and so makes its way to the uterus, in whose cavity it undergoes its development. Occasionally, however, this normal course may be interfered with, the ovum coming to rest in the tube and there undergoing its development and producing a tubal pregnancy; or, again, the ovum may not find its way into the Fallopian tube, but may fall from the follicle into the abdominal cavity, where, if it has been fertilized, it will undergo development, producing an abdominal pregnancy; and, finally, and still more rarely, the ovum may not be expelled when the Graafian follicle ruptures and yet may be fertilized and undergo its development within the follicle, bringing about what is termed an ovarian pregnancy. All these varieties of extra-uterine pregnancy are, of course, exceedingly serious, since in none of them is the fetus viable.
Fig. 12. - Ovary of a Woman Nineteen Years of Age, Eight Days after Menstruation.
d, Blood-clot; /, Graaffian follicle; th, theca. - (Kollmann.)
With the setting free of the ovum the usefulness of the Graafian follicle is at an end, and it begins at once to undergo retrogressive changes which result primarily in the formation of a structure known as the corpus luteiim (Fig. 12). On the rupture of the follicle a considerable portion of the stratum granulosum remains in place, and usually there is an effusion of a greater or less amount of blood from the vessels of the theca interna into the follicular cavity. The split in the wall of the follicle through which the ovum escaped soon closes over and the cavity becomes filled with cells separated into groups by trabecular of connective tissue containing blood-vessels (Fig. 13). These cells contain a considerable amount of a peculiar yellow pigment known as lutein, the color imparted to the follicle by this substance having suggested the name corpus luteum which is now applied to it.
Fig. 13. - Section through the Corpus Luteum of a Rabbit, Seventy Hours post coitum. The cavity of the follicle is almost completely filled with lutein cells among which is a certain amount of connective tissue, g, Blood-vessels; ke, ovarial epithelium. - (Sobotta.)
In later stages there is a gradual increase in the amount of connective tissue present and a corresponding diminution of the lutein cells, the corpus luteum gradually losing its yellow color and becoming converted into a whitish, fibrous, scar-like body, the corpus albicans, which may eventually almost completely disappear. These various changes occur in every ruptured follicle, whether or not the ovum which was contained in it be fertilized. But the rapidity with which the various stages of retrogression ensue differs greatly according to whether pregnancy occurs or not, and it is customary to distinguish the corpora lutea which are associated with pregnancy as corpora lutea vera from those whose ova fail to be fertilized and which form corpora lutea spuria. In the latter the retrogression of the follicle is completed usually in about five or six weeks, while the corpora vera persist throughout the entire duration of the pregnancy and complete their retrogression after the birth of the child.
Two very different views are held as to the origin of the lutein cells. According to one, which may be termed von Baer's view, the cells of the stratum granulosum remaining in the follicle rapidly undergo degeneration and completely disappear, and the lutein cells and connective-tissue trabecular are formed entirely from the cells of the theca interna, which increase rapidly both in size and number. The other view was first advanced by Bischoff and may be known by his name. It is to the effect that the granulosa cells do not disintegrate, but, on the contrary, increase rapidly in number and become converted into the lutein cells, only the connective tissue and the blood-vessels being derived from the theca interna.
Which of these two views is correct is at present uncertain. The majority of those who have within recent years studied the formation of the human corpus luteum have expressed themselves in favor of von Baer's theory. Sobotta has, however, made a thorough study of the phenomena in a perfect series of mice ovaries and has demonstrated that in that form the lutein cells are derived from the granulosa cells. It would be strange if the lutein cells had a different origin in two different mammals, and the observations on mice are so thorough that one is tempted to regard different results as being due to imperfections in the series of ovaries studied, important steps in the development of the corpora lutea being thus overlooked. This temptation is, moreover, greatly increased by the fact that Sobotta's observations have been confirmed in the cases of several other animals, such, for instance, as the rabbit (Sobotta, Honore, Cohn), certain bats (van der Stricht), the sheep (Marshall), the marsupial dasyurus (Sandes), the spermophile (Volker), and the guinea-pig (Sobotta). The weight of evidence is at the present time strongly in favor of Bischoff's view, but until the adverse results obtained by Clarke and others from the study of the human corpus luteum and those obtained by Jankowski fiom the pig have been shown to be incorrect, the question as to the invariable derivation of the lutein cells from the stratum granulosum must be left open. Since it is held that both the granulosa and theca cells are derivatives of the embryonic ovarial epithelium the essential differences between the two origins that have been ascribed to the lutein cells may not be so great as has been supposed. Indeed, it is possible that both the follicular and thecal cells may in some cases contribute to the formation of the corpus luteum.
The persistence of the corpus luteum throughout the entire period of pregnancy and its disappearance within a few weeks if pregnancy does not supervene, have suggested the probability of its being related to the changes that take place in the uterus in connection with the implantation of the ovum in its wall. Experimental removal of the corpus luteum in rabbits either before or shortly after the implantation of the ovum produces a failure of pregnancy (Fraenkel), and similar results have been obtained in mice and bitches (Marshall and Jolly). It has accordingly been held that the corpus luteum is an organ of internal secretion directly concerned in the production and maintenance of the modifications of the uterus necessary for the implantation and further development of the ovum.
The Relation of Ovulation to Menstruation
It was long believed that ovulation was coincident with certain periodic changes of the uterus which constitute what is termed menstruation. This phenomenon makes its appearance at the time of puberty, the exact age at which it appears being determined by individual and racial peculiarities and by climate and other factors, and after it has once appeared it normally recurs at definite intervals more or less closely corresponding with lunar months ii. e., at intervals of about twentyeight days) until somewhere in the neighborhood of the fortieth or forty-fifth year, when it ceases.
In each menstrual cycle four stages may be recognized, one of which, the intermenstrual, greatly exceeds the others in its duration, occupying about one-half the entire period. During this stage the mucous membrane of the uterus is practically at rest, but toward its close the membrane gradually begins to thicken and the second stage, the premenstrual stage, then supervenes. This lasts for six or seven days and is characterized by a . marked proliferation and swelling of the uterine mucosa, the subjacent tissue becoming at the same time highly vascular and eventually congested. The walls of the blood-vessels situated beneath the mucosa then degenerate and permit the escape of blood here and there beneath the mucous membrane, this leading to the third, or menstrual, stage in which the mucous membrane diminishes in thickness, those portions of it that overlie the effused blood undergoing fatty degeneration and desquamation, so that the stage is characterized by more or less extensive hsemorrhage. The duration of this stage is from three to five days and then ensues the postmenstrual stage, lasting from four to six days, during which the mucous membrane is regenerated and again returns to the intermenstrual condition.
It seems but natural to regard these changes as the expression of a periodic attempt to prepare the uterus for the reception of the fertilized ovum, this preparation being completed during the premenstrual stage, the succeeding menstrual and postmenstrual being merely the return of the uterine mucosa to the resting intermenstrual stage, pregnancy not having occurred. If this be the real significance of the menstrual cycle, one would expect to find ovulation occurring at a more or less definite portion of the cycle, at such a time that the ovum, if fertilized would be able to make use of the premenstrual preparation for its reception.
Attempts to determine the relation of ovulation to menstruation have been made by estimating the age of the corpora lutea occurring in ovaries removed in the course of operation from patients, the date of whose last menstruation was known. The results obtained by this method have, however, proved somewhat discordant. Thus, Fraenkel records out of eighty-five cases ten in which the operation was performed immediately before or after menstruation, and in none of these was any corpus luteum present; further, in twenty cases a newly formed corpus luteum was found and in these cases the last menstruation had occurred on the average nineteen (13-27) days previously. Villemin, too, reached a similar result, concluding that ovulation took place about fifteen days after menstruation. On the other hand, Leopold and Ravano found that in ninety-five cases ovulation coincided with menstruation in fifty-nine, while in the remaining thirty-six it occurred during other stages of the cycle.
If any conclusion may be drawn from these contradictory results it would seem to be that in the human species ovulation may take place at any stage of the menstrual cycle. Indeed, it may also be said that ovulation may take place independently of the menstrual cycle, since cases are on record of pregnancy having occurred in girls who had not yet menstruated. In other words, it seems probable that ovulation does not depend upon the condition of the uterine mucous membrane, but upon some other factor as yet undetermined.
- The conditions in lower animals seem also to point in this direction. In these ovulation is, as a rule, associated with a certain condition known as oestrus or "heat," this being preceded by certain phenomena constituting what is termed the procestrum and corresponding essentially to menstruation. In several forms, such as the dog and the pig, ovulation appears to occur regularly in association with "heat," but in others, such as the cat, the mouse and probably the rabbit, it occurs at this time only if copulation also occurs. Furthermore, it has been observed that although female monkeys menstruate regularly throughout the year, nevertheless there is but one annual cestral period when ovulation takes place (Heape).
The Maturation of the Ovum
Returning now to the ovum, it has been shown that at the time of its extrusion from the Graafian follicle it is not equivalent to a spermatozoon but to a primary spermatocyte, and it may be remembered that such a spermatocyte becomes converted into a spermatozoon only after it has undergone two divisions, during which there is a reduction of the number of the chromosomes to practically one-half the number characteristic for the species.
Fig. 14. - Ovum of a Mouse Showing the Maturation Spindle.The ovum is enclosed by the zona pellucida (z.p), to which the cells of the corona radiata are still attached. - (Sobotta.)
Similar divisions and a similar reduction of the chromosomes occur in the case of the ovum, constituting what is termed its maturation. The phenomena have not as yet been observed in human ova, and, indeed, among mammals only with any approach to completeness in comparatively few forms (rat, mouse, guinea pig, bat and cat); but they have been observed in so many other forms, both vertebrate and invertebrate, and present in all cases so much uniformity in their general features, that there can be little question as to their occurrence in the human ovum.
Fig. 15. - Diagram Illustrating the Reduction of~the Chromosomes during the Maturation of the Ovum. 0, Ovum; oc l , oocyte of the first generation; oc 2 , oocyte of the second generation; p, polar globule.
In typical cases the ovum (the primary oocyte) undergoes a division in the prophases of which the chromatin aggregates to form half as many tetrads as there are chromosomes in the somatic cells (Fig. 15, oc 1 ) and at the metaphase a dyad from each tetrad passes into each of the two cells that are formed. These two cells (secondary oocytes) are not, however, of the same size; one of them is almost as large as the original primary oocyte and continues to be called an ovum (oc 2 ), while the other is very small and is termed a polar globule (ft). A second division of the ovum quickly succeeds the first (Fig. 15, oc 2 ), and each dyad gives a single chromosome to each of the two cells which result, so that each of these cells possesses half the number of chromosomes characteristic for the species. The second division, like the first, is unequal, one of the cells being relatively very large and constituting the mature ovum, while the other is small and is the second polar globule. Frequently the first polar globule divides during the formation of the second one, a reduction of its dyads to single chromosomes taking place, so that as the final result of the maturation four cells are formed (Fig. 15), the mature ovum (o),and three polar globules (ft), each of which contains half the number of chromosomes characteristic for the species.
The similarity of the maturation phenomena to those of spermatogenesis may be perceived trom the following diagram:
Spermato ( J cyte I
Spermatocyte II
Ovum
Oocyte II O O OO
OO OO Spermatids
Polar globules
In both processes the number of cells produced is the same and in both there is a similar reduction of the chromosomes. But while each of the four spermatids is functional, the three polar globules are non-functional, and are to be regarded as abortive ova, formed during the process of reduction of the chromosomes only to undergo degeneration. In other words, three out of every four potential ova sacrifice themselves in order that the fourth may have the bulk, that is to say, the amount of nutritive material and cytoplasm necessary for efficient development.
The Fertilization of the Ovum
It is perfectly clear that the reduction of the chromosomes in the germ cells cannot very long be repeated in successive generations unless a restoration of the original number takes place occasionally, and, as a matter of fact, such a restoration occurs at the very beginning of the development of each individual, being brought about by the union of a spermatozoon with an ovum. This union constitutes what is known as the fertilization of the ovum.
The fertilization of the human ovum has not yet been observed, but the phenomenon has been repeatedly studied in lower forms, and a thorough study of the process has been made on the mouse by Sobotta, whose observations are taken as a basis for the following account.
The maturation of the ovum is quite independent of fertilization, but in many forms the penetration of the spermatozoon into the ovum takes place before the maturation phenomena are completed. This is the case with the mouse. A spermatozoon makes its way through the zona pellucida and becomes embedded in the cytoplasm of the ovum and its tail is quickly absorbed by the cytoplasm while its nucleus and probably the middle-piece persist as distinct structures. As soon as the maturation divisions are completed the nucleus of the ovum, now termed the female pronucleus (Fig. 16, ek), migrates toward the center of the ovum, and is now destitute of an archoplasm sphere and centrosome, these structures having disappeared after the completion of the maturation divisions. The spermatozoon nucleus, which, after it has penetrated the ovum, is termed the male pronucleus (spk), may lie at first at almost any point in the peripheral part of the cytoplasm, and it now begins to approach the female pronucleus, preceded by the middle-piece, which becomes an archoplasm sphere with its contained centrosome and is surrounded by astral rays. The two pronuclei finally come into contact near the center of the ovum, forming what is termed the segmentation nucleus (Fig. 16), and the archoplasm sphere and centrosome which have been introduced with the spermatozoon undergo division and the two archoplasm spheres so formed migrate to opposite poles of the segmentation nucleus, an amphiaster forms and the compound nucleus passes through the various prophases of mitosis. Since, in the mouse, the male and female pronuclei have each contributed twelve chromosomes, the equatorial plate of the mitosis is composed of twenty-four chromosomes, the number characteristic for the species being thus restored.
In describing the spermatogenesis it was shown (p. 16) that two classes of spermatozoa were formed, those of one class containing the equivalent of twelve chromosomes, while those of the other class contained only ten. A similar separation of the ovum into two classes probably does not occur, the accessory chromosomes in the oocytes dividing just as do the ordinary ones, so that each ovum possesses twelve chromosomes. When, therefore, the union of the male and female pronuclei takes place in fertilization, those ova that are fertilized by a spermatozoon with twelve chromosomes will possess twenty-four of these bodies, while in those in which the fertilization is accomplished by a spermatozoon with ten chromosomes, only twenty-two will occur. The number of chromosomes in the fertilized ovum determines the number in the somatic cells of the embryo that develops from it and hence there will be two classes of embryos, one in which the somatic cells possess twentyfour chromosomes and another in which there are twenty-two.
That this condition occurs in the human species is at present merely a conjecture based partly on what occurs during spermatogenesis and partly on what has been shown to occur in a number of invertebrates (insects). In these, two classes of spermatozoa have been found to occur as in man, and two classes of individuals, differing in the number of chromosomes in their somatic cells, develop from the fertilized ova; and it has been further found that in these forms those with the greater number of chromosomes become females and those with the smaller number males. If, as seems probable, this condition also obtains in the human species, it is evident that the sex of the future individual is determined at the fertilization of the ovum and is correlated with the number of chromosomes present in the ovum at that stage.
Fig. 16. - Six Stages in the Process of Fertilization of the Ovum of a Mouse. After the first stage figured it is impossible to determine which of the two nuclei represents the male or female pronucleus, ek, Female pronucleus; rk l and rk 2 , polar globules; spk, male pronucleus. - (Sobotia.)
It seems to be a rule that but one spermatozoon penetrates the ovum. Many, of course, come into contact with it and endeavor to penetrate it, but so soon as one has been successful in its endeavor no further penetration of others occurs. The reasons for this are in most cases obscure; experiments on the ova of invertebrates have shown that the subjection of the ova to abnormal conditions which impair their vitality favors the penetration of more than a single spermatozoon (polsypermy), and, indeed, it appears that in some forms, such as the common newt (Diemyctylus) , polyspermy is the rule, only one of the spermatozoa, however, which have penetrated uniting with the female pronucleus, the rest being absorbed by the cytoplasm of the ovum.
Fertilization marks the beginning of development, and it is therefore important that something should be known as to where and when it occurs. It seems probable that in the human species the spermatozoa usually come into contact with the ovum and fertilize it in the upper part of the Fallopian tubes, and the occurrence of extra-uterine pregnancy (see p. 22) seems to indicate that occasionally the ovum may be fertilized even before it has been received into the tube.
It is evident, then, that when fertilization is accomplished the spermatozoon must have traveled a distance of about twenty-four centimeters, the length of the upper part of the vagina being taken to be about 5 cm., that of the uterus as 7 cm., and that of the tube as 12 cm. A considerable interval of time is required for the completion of this journey, even though the movement of the spermatozoon be tolerably rapid. The observations of Henle and Hensen indicate that a spermatozoon may progress in a straight line at about the rate of from 1.2 to 2.7 mm. per minule, while Lott finds the rate to be as high as 3.6 mm. Assuming the rate of progress to be about 2.5 mm. per minute, the time required by the spermatozoon to travel from the upper part of the vagina to the upper part of a Fallopian tube will be about one and a half hours (Strassmann). This, however, assumes that there are no obstacles in the way of the rapid progress of the spermatozoon, which is not the case, since, in the first place, the irregularities and folds of the lining membrane of the tube render the path of the spermatozoon a labyrinthine one, and, secondly, the action of the cilia of the epithelium of the tube and uterus being from the ostium of the tube toward the os uteri, it will greatly retard the progress; furthermore, it is presumable that the rapidity of movement of the spermatozoon diminishes after a certain interval of time. It seems probable, therefore, that fertilization does not occur for some hours after coition, even providing an ovum is in the tube awaiting the approach of the spermatozoon.
But this condition is not necessarily present, and consequently the question of the duration of the vitality of the sperm cell becomes of importance. Ahlfeld has found that, when kept at a proper temperature, a spermatozoon will retain its vitality outside the body for eight days, and Diihrssen reports a case in which living spermatozoa were found in a Fallopian tube removed from a patient who had last been in coitu about three and a half weeks previously. As regards the duration of the vitality of the ovum less accurate data are available. Hyrtl found an apparently normal ovum in the uterine portion of the left tube of a female who died three days after the occurrence of her second menstruation, and Issmer estimates the duration of the capacity for fertilization of an ovum to be about sixteen days.
It is evident, then, that even when the exact date of the coitus which led to the fertilization is known, the actual moment of the latter process can only be approximated, and in the immense majority of cases it is necessary to rely upon the date of the last menstruation for an estimation of the probable date of parturition. And by this method the possibilities for error are much greater, since, as been pointed out, ovulation is not necessarily associated with menstruation. The duration of pregnancy is normally ten lunar or about nine calendar months and it is customary to estimate the probable date of parturition as nine months and seven days from the last menstruation. From what has been said, it is clear that any such estimation can be depended upon only as an approximation, the possible variation from it being considerable.
Superfetation
The occasional occurrence of twin fetuses in different stages of development has suggested the possibility of the fertilization of a second ovum as the result of a coition at an appreciable interval of time after the first ovum has started upon its development. There seems to be good reason for believing that many of the cases of supposed superfetation, as this phenomenon is termed, are instances of the simultaneous fertilization of two ova, one of which, for some cause concerned with the supply of nutrition, has later failed to develop as rapidly as the other. At the same time, however, even although the phenomenon may be of rare occurrence, it is by no means impossible, for occasionally a second Graafian follicle, either in the same or the other ovary, may be so near maturity that its ovum is extruded soon after the first one, and if the development of the latter and the incidental changes in the uterine mucous membrane have not proceeded so far as to prevent the access of the spermatozoon to the ovum, its fertilization and development may ensue. The changes, however, which prevent the passage of the spermatozoon are completed early in development and the difference between the normally developed embryo and that due to superfetation will be comparatively small, and will become less and less evident as development proceeds, provided that the supply of nutrition to both embryos is equal.
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McMurrich JP. The Development Of The Human Body. (1914) P. Blakiston's Son & Co., Philadelphia, Pennsylvania.
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