Book - A Laboratory Text-Book of Embryology 2 (1903)

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Minot CS. A Laboratory Text-Book Of Embryology. (1903) Philadelphia:P. Blakiston's Son & Co.

A Laboratory Text-Book of Embryology: 1. General Conceptions | 2. Early Development of Mammals | 3. Human Embryo | 4. Pig Embryos | 5. Chick Embryos | 6. Blastodermic Vesicle and Ovum Segmentation | 7. Uterus and the Foetal Appendages in Man | 8. Methods | Figures | Second edition | Category:Charles Minot
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This historic 1903 embryology textbook by Minot describes human development.


This textbook was republished in a second edition 1917: Minot CS. A Laboratory Text-Book Of Embryology. (1917) Philadelphia:P. Blakiston's Son & Co.


See also his earlier 1897 textbook; Minot CS. Human Embryology. (1897) London: The Macmillan Company.

Minot Links: Harvard Collection | 1889 Uterus And Embryo - Rabbit | 1905 Harvard Embryological Collection |1897 Human Embryology | 1903 A Laboratory Text-Book of Embryology | 1905 Normal Plates of Rabbit Embryo Development | Category:Charles Minot


See also: Historic Embryology Textbooks

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Pages where the terms "Historic Textbook" and "Historic Embryology" appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms and interpretations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Chapter II. The Early Development Of Mammals

The Spermatozoon

Fig. 1. Human Spermatozoa. A, Complete spermatozoon. B, Head seen from the side. C, Extremity of the tail. ft. Head, mi, Middle piece, m, Main piece, e, End piece. All highly magnified. — (After Retzius.)

The spermatozoa of mammals are filaments consisting of a short thick end called the head, and a very long, delicate thread called the tail. They are of minute size as compared with the ovum. The head varies greatly in shape according to the species. It contains chromatin, hence it stains darkly with those histological dyes which color nuclei. The tail consists of three parts: First, the middle piece, which is next the head, is short, and the thickest of the three parts, contains an axial thread, and probably always has a very fine spiral thread running round it; second, the main piece, which is the longest part of the tail; and, third, the end piece, which is not more than a line, even as seen with very high microscopic powers. The Human Spermatozoon. — The human spermatozoon is 0.055 mm - long — the head being 0.005 mm., the tail 0.050, and the middle piece 0.009. It is shown in two views in Fig. 1 . The head is flattened and pointed. Seen from the fiat side, it appears oval (Fig. 1, A) with the front end generally tapering a little, but never pointed. The anterior half or two-thirds has a brighter and more transparent part. Seen on edge (Fig. 1, B), the head has a pointed form with a posterior, thicker, round dark part. By adjustment of the focus it can be ascertained that the sides near the point are depressed, somewhat like those of human red blood-corpuscles. Some writers maintain that there is a special tip projecting from the head as a cylinder thread, with a hook at its end. The middle piece, mi, is directly united with the head by a transverse joint. It is cylindrical and about as long as, or a little longer than, the head. Its surface is often granular or rough, and there cling to it a few shreds of protoplasm. It has a spiral thread, which is easily overlooked on account of its extreme fineness. The main piece, m, of the tail is about half as thick as the middle piece. It gradually tapers and ends abruptly at the beginning of the still finer and very short end piece, e.


The spermatozoa, when free in the fluids in which they normally occur, are capable of active locomotion. This is achieved by means of the tail, which acts as the swimming organ by vibratory undulations which drive the spermatozoon along, head foremost. The tail has often been compared to the flagellum which serves as. the locomotive organ for many of the unicellular organisms.

The Fully Grown Ovum before Maturation

Fig. 2. Full-grown Human Ovum before Maturation. cor.r, Part of corona radiata. Z, Zona pellucida. PI, Protoplasm. Y, Yolk. nu, Nucleus. — (After W. Nagel.)

The structure to be here described is not the true sexual element, but is only the modified germ-cell which has accomplished its period of growth and is ready to be transformed into the genuine female sexual element. This transformation is called the maturation, and is accomplished essentially by the expulsion of the so-called polar granules. The full-grown mammalian ovum is found in the ovary in the center of the discus proligerus of the Graafian follicle. It measures from o. 10 to 0.15 mm. in diameter. It is approximately spherical. In some cases observers have found a very delicate vitelline membrane covering the protoplasm. Others have failed to observe this. Outside there is a thick envelope measuring from 0.02 to 0.03 mm. in diameter and known as the zona pellucida or radiata. Against the outside of the zona rest the cells of the discus proligerus which constitute the so-called " corona radiata." The nucleus is large, spherical, contains a distinct nucleolus, and always occupies an eccentric position.* The protoplasm of the cell is large in amount, granular in appearance, forms a distinct reticulum, and contains a greater or less number of yolk granules which vary considerably in character, size, and distribution in different mammals. They are usually more or less concentrated in the central portion of the ovum, leaving the outer portion, known as the protoplasmic zone, more or less free.


The Human Ovum. — The full-grown human ovum is distinguished among mammalian ova for the clear development and ready visibility of all its parts, a peculiarity due chiefly to the small amount of the yolk and the fewness of the fat granules it contains. Fig. 2 represents an ovum from a woman of thirty years. The specimen was obtained by ovariotomy, examined and drawn in the fresh state, being in the liquor folliculi. The specimen gave the following measures: The diameter of the whole ovum, including the zona radiata, 165-170 fi; thickness of zona, 20-34 !'■ ! perivitelline fissure, 1.3 // ; the clear outer zone of the yolk, 4-6 it; the protoplasmic zone, 10-21, n; the zone of yolk granules, 82-87 /*; nucleus, 25-27 /i. The corona radiata, cor. r, consists of elongated radiating cells with rounded ends and oval nuclei. The zona pellucida shows a distinct radial striation. This is probably due to the presence of minute canals running through the zona. The ovum proper is separated by a narrow fissure, the perivitelline space, from the zona, within which it lies free and loose. Hence when a fresh specimen is examined, the same side of the ovum, that containing the nucleus and which is the lightest part, is always found uppermost. In this ovum no vitelline membrane was observed. The body of the ovum may be divided into an inner kernel containing the yolk granules, and an outer protoplasmal zone, of which the very thin outermost layer is clear and therefore more or less differentiated from the broader, deeper layer, which is granular and constitutes most of the zone, PI. The yolk grains are 1 ft or less in diameter. They are highly refringent and of various kinds. Their characteristics have not yet been accurately investigated. The nucleus is nearly spherical and has a conspicuous nucleolus. In fresh specimens the nucleolus shows amoeboid movements, even at ordinary summer temperatures, for several hours after removal from the ovary. It is only in hardened specimens that the reticulum of the nucleus can be clearly observed.


  • The nucleus was formerly termed "germinal vesicle" ; the nucleolus, " germinal spot."

Ovulation

The discharge of the ovum from the ovary is called ovulation. It results from structural changes in the Graafian follicle, and these changes continue after the departure of the ovum, transforming the Graafian follicle into a so-called corpus lutcum. The exact history of these changes does not fall within the scope of this work. The essential steps in the process are the growth of the Graafian follicle and the thinning of its wall at a point at the surface of the ovary. The thin part is called the stigma. This breaks through and establishes an opening by which the ovum surrounded by the corona radiata, together with the liquor of the follicle, can escape into the periovarial chamber, whence it makes its way into the Fallopian tube. The growth of tissue in the walls of the collapsed Graafian follicle fills up the space of the same, constituting a mass which is known as the corpus luteum on account of its yellow color. The most characteristic elements of this structure are the large cells which contain the pigment. Each cell has a rounded nucleus and a large protoplasmic body, which is also more or less rounded in form. The lutein granules are in these cells. The function of the corpus luteum was long entirely unknown. Recently the theory has been suggested by Born that these cells exert an influence upon the uterus by which it is prepared to receive the ovum. This influence may be suggested to act by means of a chemical substance produced by the lutein cells and added to the blood, which then affects the uterus. There are some experimental observations tending to prove the correctness of this theory.


The brilliant color of the corpus luteum is especially characteristic of man, and has determined the name of the structure. In sheep the pigment is pale brown, in the cow dark orange, in the mouse brick red, in the rabbit and pig flesh-color.


The Maturation of the Ovum

Maturation is the term applied to the series of changes by which the fully grown egg-cell is transformed into a true female sexual element. Viewed externally in the living ovum, the process manifests itself chiefly by the separating off of one, or usually two, small bodies of protoplasm, each of which contains some nuclear material. These small bodies are generally known by the name of polar globules. They take no further part in the development, ultimately disintegrate, and are lost. The remaining ovum is capable of impregnation. It is now known that the production of the polar globules is the result of a special form of cell division, which we term the "reduction division." When the first polar globule is formed, the egg-cell divides into one very large cell and a second very small one. When the second polar globule is formed, the larger of the cells again divides, producing a second small cell and a new large one. This large one is the true female element. When an ovum is about to mature, its nucleus moves nearer that point of the surface which may be regarded as the center of the socalled animal pole, or region of the ovum, which contains most of the protoplasm and less of the yolk material. During the migration of the nucleus, the cell as a whole usually contracts so that a space appears between it and the zona radiata. Concerning the force that moves the nucleus we have no definite knowledge. When near the surface, the nucleus as such disappears. Older writers supposed that it was lost altogether, but we now know that the disappearance of the nucleus is only apparent, not actual, being in reality a metamorphosis. It is probable that the first step is the discharge of the nuclear fluid into the surrounding protoplasm, causing the nucleus to become more or less shriveled. The second step is the dissolution of the membrane of the nucleus so that the nuclear contents are brought into direct contact with and partly mixed with the protoplasm of the cells. The third step, which in time more or less accompanies the second, is the gathering of the chromatin of the nucleus into a definite number of separate granules or chromosomes. These chromosomes are always conspicuous -and are larger than those formed during ordinary ceH division. Their number is also highly characteristic. As is now well known, there appear during the process of indirect cell division a fairly definite number of chromosomes, a number which is characteristic for each species. In numerous cases it has been observed that the number of chromosomes in the maturing egg-cell is exactly one-half of that found during the ordinary cell divisions of the same species. The chromatin granules lie at first irregularly. Fourth, there arises a characteristic spindle figure such as accompanies mitosis. The chromatin forms an equatorial plate, each granule being associated with one of the spindle threads. The shape of the spindle varies, as does also the distribution of the granules of the equatorial plate. In guinea-pigs the ends of the spindle are pointed and the threads are straight, the outline of the spindle being like a diamond ; in the bat the spindles are barrelshaped and the threads are curved. In many cases it is possible, and it may be found to be true of all cases, that the axis of the spindle is at right angles to the radius of the ovum. The nuclear spindle now changes its position, becomes first oblique, and then radial. One end of the spindle lies close to the surface of the ovum. The first step is the division proper. The spindle, driven by an undiscovered power, moves centrifugally until it is partly extruded from the egg. The projecting end is enclosed in a distinct mass of protoplasm which gradually increases and soon becomes constricted around its base. The fragments of chromatin have each divided into two parts, and one-half of each fragment moves toward one end, and the other half toward the other end of the spindle. The half fragments of each set move together, hence there seem to be two plates within the spindle. The translation of the groups of chromatin continues until they reach the end of the spindle. The achromatic threads then break through in the middle, and thus the original nucleus, or at least a part of it, has been divided. There are now two masses of nuclear substance, one in the ovum, the other in the polar globule. The result of the whole process is the formation of two cells extremely unequal in size, and each containing in its nuclear elements half the number of chromosomes characteristic of the body-cells. The number of chromosomes has, therefore, been reduced, hence the term reduction division. It will be noted that the actual reduction in the number of chromosomes took place when they were first formed in the maturing ovum, while the spindle or mitotic figure was developing. In most eases a second polar globule is produced a short time after the first.


When this occurs, the nuclear remnants in the ovum do not reconstitute themselves into a membranate nucleus, as occurs in ordinary cell division, but they change directly into a second spindle, which lies, as did the first, within the protoplasm of the animal pole. This second spindle occupies an oblique position, or may even be parallel with the surface. But it also soon takes up a radial position and produces a second polar globule in similar manner to the first. The second globule is usually smaller than the first.


It may also happen that the first polar globule may itself divide, so that three polar globules appear.


The Formation of the Female Pro-nucleus. — The nuclear material, which remains in the main ovum after the separation of the polar globules, is known as the female pro-nucleus. The nuclear remnant lies close to the animal pole and in clear protoplasm. The details of its further history vary according to the species of animal. Three tendencies are known to affect the pro-nucleus: viz., to move toward the central position in the ovum, to unite with the male pronucleus as soon as that is formed out of the spermatozoa which enters the ovum, and to assume the character of a membranate nucleus. As the time of the formation of the male pro-nucleus is variable, the other tendencies being more constant, the exact history of the female pro-nucleus may be said to depend principally on the appearance of the male pro-nucleus. The earlier that event, the less does the female pro-nucleus move centrifugally and the less does it assume the membranate form. Even among mammals there is variation.


Time of Maturation. — The time when the polar globules are formed varies according to the animal, and may be before or after the egg-cell leaves the ovary. In placental mammals, maturation always begins, so far as known, in the ovary, and is said in some cases to be completed there. But in other cases it is certainly completed only after ovulation or when the ovum has passed into the Fallopian tubes.


Impregnation of the Ovum

Impregnation is the union of the male and female elements to form a single new cell, capable of initiating by its own division a rapid succession of generations of descendent cells. The process of union is commonly called the entrance of the spermatozoon into the ovum. The new cell is called the impregnated or fertilized ovum.


In all multicellular animals impregnation is effected by three successive steps:

  1. The bringing together of the male and female elements
  2. the entrance of the spermatozoon into the ovum and the formation of the male pronucleus
  3. fusion of the pro-nuclei to form the segmentation nucleus.


Meeting of the Sexual Elements. — In all amniota the seminal fluid is transferred from the male to the female passages during coitus, and spermatozoa are thereafter, in mammals, found in the uterus. In default of actual knowledge it is commonly believed that the spermatozoa make their way by their own motions into the Fallopian tubes. The ovum, meanwhile (probably, in mammals, while completing its maturation), travels down the tube. The meeting-point, or site of impregnation, in placental mammals is about one-third way down from the fimbria to the uterus. The exact spot, is remarkably constant for each species. Nothing is known by direct observation as to the site of impregnation in man, but there is no reason to suppose, as has unfortunately been often done, that the site is either variable or essentially different from that in other mammals.


The Entrance of the Spermatozoon into the Ovum. — It is probable, in mammals at least, that only one spermatozoon enters the yolk of the ovum and accomplishes its fertilization. It has been observed in those animals in which, as in the rabbit, there is formed a more or less considerable space between the yolk and the zona radiata, that a number of spermatozoa appear in this space, but apparently only one actually fuses with the substance of the ovum. The manner in which additional spermatozoa are excluded, after the first has entered, is still under discussion. The hypothesis has been suggested that the attractive power of the ovum is annulled or weakened by the formation of the male pro-nucleus from the spermatozoon which first enters. With our present knowledge the assumption appears unavoidable that the ovum exerts a specific attraction upon spermatozoa of the same animal species. Recent authorities incline to the view that this attraction is of a chemical nature, for it has been observed that certain chemical substances may attract very strongly unicellular organisms capable of free locomotion. The phenomenon is called ehemotropism. According to this interpretation, the attraction of the ovum for the spermatozoon would be termed ehemotropic.


At the time of fertilization the ovum in the Fallopian tube is surrounded by a number of spermatozoa. In the case of the rabbit, perhaps by a hundred, more or less. They are all, or nearly all, in active motion, for the most part pressing their heads against the zona radiata. Several of them may make their way through the zona into the interior. According to Hensen, only those spermatozoa which enter the zona along radial lines can make their way through. Those which take oblique courses remain caught in the zona. The female pronucleus is already present, either formed or at least forming as a membranate nucleus. A single spermatozoa makes its way into the yolk proper, passing a short distance into the interior. It is uncertain whether the whole tail of the spermatozoon enters the ovum or not. In some of the lower vertebrates and in other animals, it appears to do so. It is probably always the case that at least v ■the'vmain piece of the tail enters the yolk. The tail as such very soon disappears, while the head of the spermatozoon enlarges, probably by the imbibition of fluid from the surrounding yolk. The head of the spermatozoon is rich in chromatin, which forms a series of irregular masses as the head enlarges, producing a network appearance, and is thus converted into a nucleus-like body, the male pro-nucleus. At the same time the growing head surrounds itself in some animals by a membrane.


We now have a cell which contains two nucleus-like bodies, one derived from the head of the spermatozoon and the other from the nucleus of the eggcell. They are termed respectively the male and female pro-nucleus. Each pro-nucleus, when it first appears, is small and gradually enlarges, probablv in both cases by the imbibition of fluid. The relative size of the two pro-nuclei varies considerably in different species, and is probably a secondary and relatively unimportant relation. The proportion between the two probably depends upon the time when the male pro-nucleus is formed. If the spermatozoon enters earl}- while the female pro-nucleus is forming, it may make a pro-nucleus as large as that from the egg-cell. If, on the other hand, the spermatozoon enters late, the female pro-nucleus enlarges, acquires a start, and the growing male pronucleus is, therefore, smaller.


Concerning the fate of the middle piece of the spermatozoon and its share in the fertilization in the ovum of mammals, we possess no satisfactory information. It has been shown, however, in other animals that this middle piece produces a centrosome, and the only centrosome which appears in the fertilized ovum. The theory has been advanced that the ovum, after its maturation, has no centrosome, that a centrosome is always brought into the ovum by the spermatozoon in the manner just indicated. If we regard the centrosome as a permanent cell element, then we must further interpret the addition of the male centrosome as one of the most important phenomena of fertilization. Whether this hypothesis is correct or not, we are unable at present to decide.


Astral figures play a conspicuous part in the phenomenon of fertilization in many animals. Astral figures are produced in the protoplasm of the ovum by its assuming a special radiating structure. Astral figures may appear around both the male and female pro-nuclei (Fig. 3). In other cases the astral figure arises only in association with the head of the spermatozoon or male pro-nucleus. In mammals, so far as known, no astral figures are developed about either of the pro-nuclei. There is a clear space in the protoplasm around each nucleus, and such a clear space has often been noted also when the astral figure is present. It may possibly be interpreted as a rudimentary aster or center of astral formation. The two pro-nuclei usually lie at some distance from one another. As soon as they are formed, or perhaps when they are fully differentiated, they tend to move toward one another and toward the center of the ovum. Concerning the path of the male pro-nucleus we possess interesting information from the study of the ova of the frog and axolotl. In these ova the spermatozoon leaves a trail of pigment, which consists of two limbs, one passing through the cortical layer of the ovum nearly perpendicular to the surface, and the other forming an angle with the first and leading directly to the female pro-nucleus. The female pronucleus tends always to move toward a central position. The force which draws the pro-nuclei together is unknown. The hypothesis that this force is chemotropic has met with favor.

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Fig. 3. Ovum of a Worm (Sagitta) with Two Pro-nuclei. Around Each Pro-nucleus is shown the Aster. — (After O. Hertwig) Fig. 4. Ovum of a Rabbit, Seventeen Hours after Coitus, with the Pro-nuclei about to Conjugate. p.g, Polar globules. — (After Rein.)


The fusion of the pro-nuclei. — In the rabbit, as probably in all mammals, both pro-nuclei lie at first eccentrically, but both move toward each other and toward the center, meeting when the central position is attained. As they near one another, both pro-nuclei perform active amceboid movements. After they meet they still continue their amceboid movements, and travel together to the center of the ovum (Fig. 4). One of the pro-nuclei assumes a crescent shape and embraces the other. In the mouse the history is similar. After the two pronuclei in this animal have met, they remain side by side, they are without membranes, the chromatin threads become distinct and draw closer together. Between them appears first a small spot or centrosome with a few radiating lines between it (Fig. 5). From the centrosome arises a spindle of achromatic threads (Fig. 6 I.


The chromatin of each pro-nucleus now forms a group of well-defined, elongated, somewhat crooked chromosomes. The two groups of chromosomes are quite distinct, and are separated from one another by the intervening spindle. The spindle continues to grow, and the chromosomes of the male pro-nucleus on the one side, and the female pro-nucleus on the other, attach themselves to the equatorial region of the spindle (Fig. 7). The spindle continues to grow; the chromosomes become V-shaped and arrange themselves as the so-called equatorial plate, in which the chromosomes of the two pro-nuclei can no longer be distinguished from one another. At each end of the spindle is a distinct centrosomewith a very faint, small astral radiation in the neighboring protoplasm. This spindle is the beginning of the division of what we may call the segmentation nucleus. In the mouse the two pro-nuclei do not actually fuse into a single nucleus before the formation of the spindle, which initiates the first division of the fertilized ovum, so that, strictly speaking, there is no fusion of the pro-nuclei to make a segmentation nucleus. There is, nevertheless, a true fusion of the pro-nuclei accomplished, although it is somewhat masked by the early commencement of the first segmentation spindle, which develops at the same time that the fusion of the pro-nuclei is being completed. Whether the processes as described in the mouse are typical for mammals we do not know, the white mouse being the only species in which the process has been followed in detail. The student will find some additional information in the practical part.

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Fig. 5. Ovum of White Mouse. Beginning of the Conjugation of the Pro-nuclei X 1500 diams. (after Sobotta) Fig. 6. Ovum of White Mouse. Conjugation of the Pro-nuclei, and Formation of the Segmentation Spindle X 1500 diams. (after Sobotta) Fig. 7. Ovum of White Mouse. First Segmentation Spindle with the Chromosomes of the Pro-nuclei still forming Two Distinct Groups X 1500 diams. (after Sobotta)


It is now believed that the pro-nuclei never unite to form a distinct membranate nucleus, the so-called segmentation nucleus of earlier writers, but that the fusion always takes place during the absence of the membranes of the pronuclei by the mingling of their contents. The time of mingling, however, varies as regards the formation of the chromosomes. It may take place before or after the chromosomes are developed. When, as in the mouse, the chromosomes appear as two distinct groups, it is possible sometimes to determine their number. In the mouse counting is difficult, but there seems little doubt that each pronucleus forms twelve chromosomes. Hence it results that there are twenty-four chromosomes in the segmentation spindle. This number, twenty-four, so far as has been determined, is the number which appears during later stages of segmentation and in all subsequent cell divisions of this animal. It is believed to be a general law that the male and female pro-nuclei each contribute the same number of chromosomes to the segmentation spindle. This number is identical with the number which appears during the reduction divisions which lead to the maturation of the ovum, on the one hand, and the development of the spermatozoon on the other; and, further, the number is one-half the number of chromosomes which appear during ordinary cell divisions of the species. The most thorough study of the phenomenon which has yet been made is that by a succession of able investigators upon the large nematode Ascaris megalocephala. An admirable summary of the process of fertilization in Ascaris has been given by Oscar Hertwig.* Segmentation of the Ovum.


Shortlv after the entrance of the spermatozoon into the ovum the segmentation spindle is developed by the union of the pro-nuclei, as described in the previous section. This spindle leads to a division of the ovum into two cells. These cells further rapidly divide. As stated on page 26, these early cell divisions are called the segmentation of the ovum.


The position of the segmentation spindle is always eccentric, and appears to be approximately, if not exactly, the same as that of the egg-cell nucleus before maturation. The axis of the spindle varies greatly in its direction. It is sometimes at right angles to the radius of the ovum, which passes through the polar globules, but it is more usually oblique to this radius. It was at one time thought that the plane of division was always at right angles to the radius of the extrusion of the polar globules, but this view cannot be upheld. After the ovum has divided into two cells, the polar globules lie in the angle between these two cells (Fig. 8), because there the globules find room. It is to be noted that the globules accommodate themselves to the segmentation spheres, and that the forma-' tion of the spheres is not accommodated to the original position of the globules.



  • " Lehrbuch der Entwicklungsgeschichte," sixth edition, 1898. The large Ascaris is a particularly favorable object. The student who wishes to pursue the practical study of impregnation further should select this form. Material suitably preserved may be obtained from the Preparalor of the Department of Zoology, University of Pennsylvania.


The degree of the eccentricity of the segmentation spindle varies in different ova, chiefly according to the amount of yolk ; the greater the quantity of yolk in the ovum, the more marked is the eccentricity.


The actual first cell division (first cleavage or first segmentation) of a mammalian ovum has never been followed by direct observation, the practical difficulties not having hitherto been successfully overcome. Various phases of the division have, however, been seen and the internal changes have been studied by means of sections. It accordingly seems expedient to interpolate the following account of the external appearances of the first segmentation in the living ovum of the snail, Limax campestris. The eggs of this animal, by their size and in their mode of segmentation, have a certain resemblance with mammalian ova. The following description is taken from the account by E. L. Mark, published in 1 88 1 ; it is nearly in his own words : In Limax, after impregnation, the region of the segmentation nucleus remains more clear, but all that can be distinguished is a more or less circular, ill-defined area, which is less opaque than the surrounding portions of the vitellus. After a few moments this area grows less distinct. It finally appears elongated. Very soon this lengthening results in two light spots, which are inconspicuous at first, but which increase in size and distinctness, and presentlv become oval. If the outline of the egg be carefully watched, it is now seen to lengthen gradually in a direction corresponding to the line which joins the spots. As the latter enlarges the lengthening of the ovum increases, though not very conspicuously. Soon a slight flattening of the surface appears just under the polar globules; the flattening changes to a depression (Fig. 9), which grows deeper and becomes angular. A little later the furrow is seen to have extended around on the sides of the yolk as a shallow depression, reaching something more than half-way toward the vegetable or inferior pole, and in four or five minutes after its appearance the depression extends completely around the yolk. This annular constriction now deepens on all sides, but most rapidly at the animal pole; as it deepens it becomes narrower, almost a fissure. By the further deepening of the constriction on all sides there are formed two equal masses connected by only a slender thread of protoplasm, situated nearer the vegetative than the animal pole, and which soon becomes more attenuated and finally parts. The first cleavage is now accomplished. Botli segments undergo changes of form; they approach and flatten out against each other, and after a certain time themselves divide.


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Fig. 8. Ovum of a Rabbit of Twenty-four Hours. The first cleavage has been completed ; the two cells (segmentation spheres) are appressed ; above the cells lie the polar globules ; numerous spermatozoa lie in and within the zona pellucida. (After Coste. ) Fig. 9. Ovum of a. Snail (Limax campestris) during the First Cleavage. The Envelopes of the Ovum are not Drawn in X 200 diams. (After E. L. Mark. )


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Fig. 10. Ovum ok White Mouse. First SEGMENTATION Spindle with Equatorial Plate of Chromosomes. X 'S 00 diams. — (After Sobotta. )

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Fig. 11. Ovum of White Mouse. First Segmentation Spindle.


The chromosomes have divided and have migrated toward the poles of the spindle, forming two groups. -,' 1500 diams. — (After Sobotta.)



The Internal Changes in the Mammalian Ovum. — At the close of impregnation the segmentation spindle is completely formed (Fig. 10). The chromosomes of the equatorial plate now divide probably by splitting longitudinally so that the number of chromosomes is doubled. During the splitting the chromosomes tend to draw apart from one another. At the same time the spindle, without changing its length, becomes somewhat narrower. The chromosomes now move apart from the equator toward the two poles, forming two groups, each group I containing half of the total number of chromosomes (Fig. n), and at the same time the whole ovum becomes somewhat elongated in the direction corresponding with the axis of the spindle. The chromatin granules accumulate at the two poles of the spindle. The achromatic threads between the poles break through. Then the actual cleavage of the elongated ovum into two cells becomes marked in the protoplasm, and the line of separation of the two cells passes through the equator of the spindle. The accumulated granules of chromatin then reconstitute the resting membranate nucleus (Fig. 12). In brief, the segmentation of the ovum is a typical indirect or mitotic cell division. In the mouse the first cleavage is completed about twenty-six hours after the coitus. The second cleavage is not completed until twenty-four hours later. When first formed, two segmentation spheres are oval and entirely separated from one another, but subsequently they flatten against one another and become appressed, a phenomenon of which we have no explanation. In most mammals which have been studied there is more or less space between the segmenting ovum and the zona (see Fig. 8), but in the mouse this space is reduced to a minimum and the zona is often stretched into irregular forms during the changes of the ovum.

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Fig. 12. Ova of White Mouse with Two Segmentation Spheres or Cells. A, Telophase of the division ; the chiomosomes are reconstituting the nucleus. j9, Membranate nucleus reconstituted. /, First cell of segmentation. nu. Nucleus, p.g, Polar globules. Z, Zona pellucida. X S°° diams. — {Af/tr Sobolta.)


The succeeding cleavages of segmentation need to be followed out in greater detail than yet recorded. In many cases there appear to be three cells in the next stage, because one of the two primitive segmentation spheres divides sooner than the other. The more commonly received view is that four cells are produced next, but it may very well be that there is really a three-cell stage preceding the four-cell stage of which two figures are presented. The first of these (Fig. 13) represents the four-cell stage of the ovum of a bat, and the second (Fig. 14) represents the four-cell stage of the ovum of the Virginian opossum. That of the bat resembles the picture which we obtained from a number of animals, such as the rabbit, the guinea-pig, the dog, and others. That of the opossum differs so much from anything known in other mammals that it may be questioned whether it is entirely normal. In the mouse the zona is much thinner and assumes an irregular form, adapting itself to the pressure of the single spheres.


After the four-cell stage, the segmentation proceeds apparently with considerable irregularity, but we are soon able to see that the cells are grouping themselves into an uninterrupted external layer and an internal accumulation of cells. The outer layer is in contact, or nearly in contact, with the zona radiata, and may, therefore, be termed the subzonal layer (Fig. 16, s.z). The inner accumulation of cells is designated as the inner mass, i.m. Fig. 15 represents a rabbit ovum of about seventy hours, according to the observations of van Beneden. He represents the subzonal layer, Ec, as interrupted at one point, where one of the cells of the inner mass, i.m, is exposed. It is probable, however, that van Beneden is in error in regard to this, and that the subzonal layer is really continuous. In the next stage (Fig. 16) we find that the ovum has become larger by the appearance of a cavity in its interior. This cavity appears between the inner mass, i.m, and the subzonal layer, but at one side the inner mass remains adherent to and closely connected with the subzonal layer. We now have reached the stage in which the developing ovum may be designated as the blastodermic vesicle.

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Fig. 13. Ovum of a Bat (Vespertilio Murina) with Four Segmentation Spheres. — {Afttr van Bentden and Jitlin.)

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Fig. 14. Ovum of a Virginian Opossum, with Four Segments. — {After Emil Selenka.)


As to the interpretation of the parts, it is probable that the subzonal layer is the ectoderm, and that the inner mass is the entoderm. At the stage we have now reached the blastodermic vesicle has a large part of its walls formed by the subzonal laver only, so that we call this the stage of the one-layered blastodermic vesicle.


Arrival in the Uterus. — During the stages described the ovum travels along the Fallopian tube and reaches the uterus in an early phase of the stage which we designate as the blastodermic vesicle. The transit requires about eighty hours in the mouse, about five days in the opossum, four days in the rabbit, and eight to ten days in the dog. The time necessary in man is unknown. It may be supposed to be about one week.


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Fig. 15. Rabbit's Ovum of about Seventy

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Fig. 16. Young Blastodermic Vesicle of a Hours. Mole.

Ec, Outer layer, i.m, Inner mass of cells. Z, Zona i.m. Inner mass of cells, s.z, Outer or subzonal pellucida. — (Afltr E. van Beneden. ) layer, z, Zona pellucida. — (After W. Heap?.)


Pro-chorion. — The ovum in many mammals becomes surrounded by a gelatinous covering, which is secreted by the glands of the uterus. It may be compared with the white of the bird's egg. In the rabbit this envelope becomes enormously thick about the blastodermic vesicle and in other rodents is voluminous. In the dog it is less developed, but presents the further peculiarity that the secretion in the tubular glands may be hardened in connection with the envelope itself, which, therefore, appears, when the ovum is removed from the uterus, to be studded over with fine threads resembling villi. The gelatinous envelope has been termed by Hensen the pro-chorion. The thread-like projections seen in the dog were taken by Bischoff for true villi, and they have sometimes been referred to as the pro-chorionic villi. The term pro-chorion has been applied to other structures, as, for instance, to the subzonal layer of the blastodermic vesicle. The student needs to be warned against confusing the term pro-chorion in its various applications.

The Blastodermic Vesicle

The blastodermic vesicle always consists at first of the subzonal layer and an inner cell mass attached at one point to the subzonal layer, and has a cavity between the inner mass and the subzonal layer; the vesicle itself is always enclosed in the zona radiata. The variations offered in different mammals are so great that a description less general than that given would hardly be applicable, even to the placental mammals.


The next step in development is the production of a complete second layer out of the cells of the inner mass. This layer extends completely around the vesicle and lies close against the subzonal layer, and encloses the main cavity of the vesicle. The way in which this inner vesicular layer is developed varies greatly. In the hedgehog it appears very precociously, while the blastodermic vesicle is very small, and afterward it expands rapidly, while the vesicle as a whole is growing. In the rabbit and in the mole it is formed much later, and the one-layered vesicle expands to a considerable diameter before the inner mass begins to spread out. The striking changes through which the inner mass passes in the mole are illustrated in Fig. 17. It forms at first a small globe, A. The inner mass subsequently flattens out, becoming lens-shaped, thinner, and larger in area, B. It continues spreading laterally and separates into three layers. The two outer layers enter into the formation of the true ectoderm, C. In the rabbit, and perhaps in the mole, the outermost layer is temporary only in existence. In some rodents it acquires a very great development and leads to the curious phenomenon known as the inversion of the germ-layers. The innermost of the layers grows at its edges, and its cells spread out gradually further and further under the subzonal layer until they extend completely around the vesicle and form, by meeting at the opposite pole of the ovum, a closed vesicle. Very similar is the process in the rabbit. The cells at the expanding edge of the inner layer are found to spread rapidly, so that during the expansion they are more or less widely separated from one another. But they continue their expansion and multiplication until they form a complete inner epithelial layer.


The point where the inner mass and the subzonal layers are connected with one another marks the site of the future embryonic area.


The blastodermic vesicle grows rapidly in size, partly by the multiplication of its cells, partly by their becoming flattened out so as to cover a larger surface. The interior of the vesicle is filled with fluid. As the vesicle grows the fluid increases in amount, and is presumably derived by the ovum from the walls of the uterus. It is under pressure within the vesicle, as is shown by the manner in which it spurts out if the vesicle is broken. Nothing exact as to the composition of this fluid is known, though we may suppose it to resemble more or less the serous fluids of the adult body. The size and form of the vesicle offer characteristic variations in mammals. It starts as a more or less nearly spherical body. In the rabbit it assumes an oval shape, and by the seventh day measures about 4.0 mm., and soon thereafter becomes attached to the wall of the uterus. In the hedgehog, the guinea-pig, and the mouse the ovum, while very small and more or less rounded in form, becomes imbedded in uterine tissue and develops into a special shape in adaptation to its new situation. In the ungulates the vesicle grows enormously, becoming a very long and slender sack. Thus, for example, in the sheep it may measure on the fourteenth day not less than 50 cm. in length.


Another respect in which the blastodermic vesicles differ greatly from one another in various mammals is in regard to the early development of the subzonal layer, or, as we may call it, the ectoderm. In many cases the entire layer undergoes a precocious development, its cells multiply very rapidly, so that the layer becomes several cells thick. This thickened layer is known as the trophoblast. In other placental mammals this thickening is confined to a limited area of the ectoderm. For further description see Trophoblast.

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Fig. 17. Sections through the Inner Mass of Blastodermic Vesici.es of the Mole at Three Successive Stages. Ec, Outer or subzonal layer, c, ,r, Zona pellucida.


i.m, Inner mass of cells. hy, Entoderm. — [After IV. Heaps.)


The Embryonic Shield

Sooner or later in the early history of every blastodermic vesicle, and always as the first indication of the development of the embryo proper, there appears a thickening of a small oval area of the outer layer of the blastodermic vesicle. This thickening is known as the embryonic shield. In the fresh specimen it marks itself hy the greater opacity which it causes in the walls of the ovum where it lies. It is produced always at the point where the inner cell mass was originally attached to the subzonal layer. In those cases where the thickening of the ectoderm to form the trophoblast extends over the entire blastodermic vesicle, it is very difficult to follow the early history of the embryonic shield. In other cases, however, where the trophoblast occupies a special restricted area, the history of the embryonic shield may be more readily followed. The animals in which it has hitherto been chiefly studied are the rabbit, dog, cat, and sheep. In all of these the embryonic shield is simply a thickening of the outer layer (Fig. 18). The embryonic shield is at first small, but it rapidly expands and assumes a rounded or oval form. There next appears, in a more or less central position in the shield, a small, darker spot, which marks what is known as the primitive knot, a peculiarity of which is that it corresponds to an intimate union of the cells of the inner with those of the outer layer of the blastodermic vesicle. Soon a linear shadow becomes visible extending from the primitive knot toward a point at the periphery of the embryonic shield — Fig. 19, p. s, which represents the embryonic shield of a dog at about two weeks. The shadow from the primitive knot is termed the primitive streak, and it very soon becomes further characterized by the formation of a fine groove caused by a depression in the outer layer of cells. This is known as the primitive groove, and has been observed in all amniote embryos. Its exact significance has never been satisfactorily ascertained, and its interpretation is still a matter of scientific discussion. A transverse section through the primitive streak of a vesicle of a common European mole is shown in Fig. 23. At about the time the primitive streak appears the embryonic shield becomes ova! in form. In those animals, such as the carnivora and ungulates, which have a large elongated blastodermic vesicle, we find that the long axis of the embryonic shield is nearly at right angles to the long axis of the vesicle. The size of the shield is about the same in all mammals which have been heretofore studied.

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Fig. 18. Transverse Section through the Embryonic Shield of the Blastodermic Vesicle of a Dog of Eleven or Fifteen Days (Precise Age Unknown). O.L, Outer layer. Ent, Entoderm. \ 200 diams. — (After Bonnet.)

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Fig. 18. Surface View of the Embryonic Shield of the Blastodermic Vesicle of a Dog of Thirteen to Fifteen Days (Precise Age Unknown).


The specimen had been preserved with sublimate and stained with borax-carmin. S/i, Embryonic shield. A'u, Hensen's knot. f>.s, Primitive streak. >( IO ° diams. — {After Bonnet.)

Origin of the Mesoderm

The development of the primitive streak and groove is accompanied by the appearance of the third or middle germ-layer, the mesoderm (Fig. 20, mes). As shown in the section there figured, the three germ-layers are fused together underneath the primitive groove, and are there thicker than elsewhere. As we pass laterally from the groove, the ectoderm and mesoderm both become thinner and are distinctly separated from one another. The entoderm consists of a single thin layer of cells very closely connected with the mesoderm. The mesoderm occupies at first only a small area in the immediate neighborhood of the primitive streak. It grows rapidly, so that its edge extends further and further over the blastodermic vesicle. The mesoderm is to be regarded as the product of the entoderm. Its exact origin in mammals has not yet been adequately traced. We know, however, that in birds, reptiles, and elasmobranchs the cells of the inner layer multiply rapidly, so that the inner layer becomes more than one cell thick, The upper cells soon split off from the lower and thus form themselves into the middle- germ-layer. The mesoderm therefore is said to be formed by delamination. It seems probable that in mammals the process is the same.

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Fig. 20. Central Portion of a Sheep's Blastodermic Vesicle of Twelve to Thirteen Days.

S/i, Embryonic shield. Ah, Hensen's knot, 'lies. Shadow caused by mesoderm developing around the shield. ■ 34 diams. — (After Bonnet. )


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Fig. 21. Blastodermic Vesicle of a Rabbit of Seven Days. portion of the mesoderm of the Area Oi'ACA. — {After K'olliker. )


It may be mentioned that, according to Bonnet, the development of the mesoderm is not quite as above described. It can be first distinguished at the stage when the primitive knot has appeared, and before the primitive streak is developed. In the fresh specimen it is seen as a slight turbidity of the vesicular walls just outside the edge of the shield (Fig. 20), while in the region of the shield there is no middle layer whatever. By the time the primitive streak has appeared in the sheep, the formation of the mesoderm has extended under the embryonic shield, and the relations between the germ-layers then become essentially as above described.


The cells of the mesoderm are at first quite closely packed, but as the layer grows they begin to move apart, though remaining connected with one another by protoplasmic processes. The moving apart of the cells is least near the primitive streak and becomes more marked as we go toward the periphery of the layer (Fig. 21 ), which represents a part of the peripheral region of the mesoderm of a blastodermic vesicle of a rabbit of seven days.


In the details of its expansion the mesoderm varies greatly in different mammals. In some forms it develops very early and rapidly expands over the entire blastodermic vesicle, which then becomes three-layered. This seems to be the method of its growth in man and other primates. In other cases, as in the dog and cat, it grows more slowly, but ultimately encloses the entire entoderm. In the rabbit, on the contrary, it never expands more than about three-fifths of the way over the blastodermic vesicle, one part of which, therefore, — viz., that opposite the embryo, — never has any mesoderm whatever. This, however, is to be regarded as a special modification, since we must consider that primitively the mesoderm extended over the entire vesicle.


The Primitive Axis

The next stage of development is characterized by the appearance of an accumulation of cells which extends forward from the primitive knot in the axial line. This thickening is termed the primitive axis. German writers commonly designate it as the "head process" (Kopffortsatz). The primitive axis may be easily distinguished in transverse sections from the primitive streak by the fact that in the former the thickening occurs in the mesoderm and entoderm, which are closely united, and it is separated from the outer layer; whereas in the latter the cells of the thickening are fused with both the entoderm and the ectoderm.


The primitive axis corresponds to the region in which the body proper of the embryo develops, and represents the beginning of embryonic development in this restricted sense. It grows quite rapidly in length and width, and as it grows encroaches more and more upon the territory of the primitive streak, which is gradually obliterated by merging into the caudal end of the developing embryo, so that it can no longer be distinguished. The obliteration of the primitive streak is gradual, and there is a series of stages easily observed in amniota in which we find the embryonic development in the region of the primitive axis more or less advanced, while part of the primitive streak still presents to us, more or less clearly, its original condition.

The Notochordal Canal

In regard to this canal our knowledge is imperfect. Any account of it which we can give may need correction. It is a very small canal which runs through the center of the primitive axis. It ends blindly in front, but opens through the ectoderm at its posterior end, at a point corresponding perhaps exactly to the position of the primitive knot. The first indication of the formation of the canal is an alteration in the form of the cells in the center of the primitive axis. These cells elongate in directions at right angles to the axis. Their nuclei become oval and are radially placed. The change begins posteriorly and progresses forward. The radial cells move apart, so that there arises a longitudinal canal. It may happen that in mammals, as in birds, the canal is not actually open at its posterior end. If that should be found to be the case in any instance, it would not alter our interpretation, for we should then consider that the walls had simply closed together. There are many instances of tubular structures being temporarily solid in embryonic stages. Such a condition, for example, lias been observed in the oesophagus of elasmobranehs, in the large intestine of birds, and in other cases.


The opening of the notochordal canal is termed the blastopore, and is supposed to be identical with the blastopore of the anamniota.


After the notochordal canal is formed the blastodermic vesicle has, of course, two cavities: first, the small cavity of the canal; second, the large main cavity of the vesicle which is surrounded by entoderm. This larger space is designated as the yolk-cavity. After the canal has acquired a not inconsiderable length its lower wall develops a series of irregular openings (Fig. 22, nch) on its ventral side, by which it comes into communication with the large underlying yolk-cavity. These openings grow until the ventral wall of the notochordal canal is entirely lost. We then have the two cavities completely fused making a single cavity, bounded by a continuous layer of cells, the majority of which represents the lining of the yolk-cavity, but the small minority represents the cells of the notochordal canal. The continuous layer of cells is known as the permanent entoderm. At about this time, probably sometimes earlier, sometimes later, according to the species, the blastopore becomes permanently closed and the entodermal cavity no longer has an opening to the exterior.


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Fig. 21. Germinal Area of Guinka-pigat Thirteen Days and Twenty Hours, skkn FROM the Under (EntoderMu.i Side.

n. o, Areaopaca. a.p, Area pellucida, ncA, Notochordal canal with several irregular openings through the entoderm.


The cells on the dorsal side of the notochordal canal have a different destination, for they become thickened to make the anlage of the future notochord. It is to this fact that the canal owes its name.


The Notochord

The notochord {chorda dorsalis) is a rod of peculiar tissue constituting the primitive axial skeleton of vertebrates. It begins in the embryo immediately behind the pituitary body and extends to the caudal extremity. It occurs as a permanent structure in some of the lower vertebrates and as a temporary one in the embryos of amniota. It appears very early in the course of development, being differentiated from the median dorsal wall of the notochordal canal, beginning at a time when the medullary groove (compare page 70) is not fully marked out posteriorly, and is nowhere closed. The notochordal anlage can be first detected as an axial band of cells, which at first is not well marked off from the mesoderm of the primitive axis. The cells of the anlage are larger than those of the adjacent entoderm (Fig. 23, nch ). The differentiation of the notochordal cells begins usually at the anterior end of the canal and progresses backward. It appears merely as a specialized part of the entoderm of the blastodermic vesicle, but has a very sharp demarcation.


The notochordal anlage separates off and the entoderm proper closes across under it, so that the notochordal band lies between the entoderm and the overlying ectoderm (floor of the medullary groove or canal). The two primitive germ-layers come into actual contact in the median line, along which, therefore, when the notochord first separates from the entoderm, there is no middle germlayer present. The separation of the anlage does not take place at the anterior extremity of the notoehord until somewhat later, so for a considerable period the cephalic end of the notoehord remains fused withjihe entoderm. The separation from the entoderm is effected in mammals by the entoderm proper shoving itself under the notoehord toward the median line. When the cells from one side meet, those of the other are united with them and form a continuous sheet of entoderm below the notochordal cells.

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Fig. 23. Transverse Section of a Mole Embryo I Heape's Stage H). am. Amnion. MJ, Medullary groove. My, Primitive segment. Ca, Coelom. En, Entoderm, nch, Notochord. no, Aorta. z'/.a. Vitelline artery. -Vow, Somatic mesoderm. Spm, Splanchnic mesoderm. — [After IV. Heap!.)


After its separation the notoehord is a narrow band of cells, which starts anteriorly from the entoderm (the future lining of the alimentary tract), running backward to the blastopore. So long as the blastopore is open the notoehord terminates in the epithelium lining it. For a certain period the notoehord continues to grow tailward by accretion of cells from the walls of the blastoporic passage; and after the canal is permanently obliterated, the notoehord may still continue to lengthen by acquisitions at its caudal end of additional cells from the primitive streak.


After it is once formed as a band of cells, the notoehord passes through various changes of form, but ultimately becomes a cylindrical rod with tapering extremities. It attains considerable size in the embryos of most vertebrates, but in those of placental mammals it is always small. It is probable that in mammals the notoehord, when first separated from the entoderm, is a broad, flat band, and that this band subsequently draws together, diminishing its transverse and increasing its vertical diameter until it has acquired a rounded form. Finally its outline becomes circular in cross-section. This series of changes begins near the anterior end of the notoehord and progresses both forward and backward.


In later stages the mesoderm again grows across the median line of the embryo, completely surrounds the notoehord, and forms a special sheath about it. Still later the mesoderm forms around the notoehord groups of cells which we can identify as the anlages of the vertebrae and of certain parts of the skull.


The Ultimate Fate of the Notochord

As the vertebral column develops the notoehord degenerates. It first ceases to be continuous by breaking apart at points corresponding approximately to the center of each vertebra. The fragments of the notoehord contract and form little masses situated in cavities in the intervertebral spaces. These cavities have a distinct boundary and present characteristic forms in different mammals. The notochordal cells do not fill the cavity. The sheath of the notoehord is lost ; the walls of the cells disappear; the tissue becomes granular and breaks up into multinucleate, irregularly reticulate masses which are gradually resorbed (Fig. 24). Tissue of this character may be easily observed in human embryos of the third and fourth month. The cavity in which these notochordal remnants are lodged is supposed not to be identical with the intervertebral cavity of the adult.

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Fig. 24. Degenerating Tissue of the notochord from the central porTION of the Intervertebral Disc of a Cow's Embryo. — (After Leboucq.')

The Origin of the Nervous System

The first step in the differentiation of the central nervous system is the formation of a thickening of the ectoderm, which is known as the medullary plate, and which begins to appear shortly after the formation of the primitive streak. It extends over the primitive axis, the primitive knot, and the anterior end of the primitive streak (Fig. 25, Md), and also extends some distance to the right and left of the axial line. It is rounded in front and behind, where, however, it gradually fades out. It will be remembered that the ectoderm of the embryonic shield has at first a considerable thickness, for it consists of cuboidal or low cylindrical epithelial cells. The stage which follows next after the appearance of the primitive axis is characterized by the gradual thinning out of the ectoderm over the peripheral portions of the shield, while in the neighborhood of the axial line the full thickness of the outer germ-layer is not only retained, but is actually increased. For a time there is a gradual passage between thicker and thinner parts, but as development progresses the demarcation rapidly becomes sharper. At the same time that the medullary plate is being thus differentiated, the central portion becomes depressed, making the deep, narrow furrow which begins just in front of the primitive knot and extends nearly to the anterior edge of the medullary plate. This axial depression is known as the dorsal furrow. Its appearance is shown in cross-section as illustrated by Fig. 26. The furrow is narrow and deep. Its upper edge is rounded or curving. By the formation of the furrow the ectoderm of the medullary plate is brought into actual contact with the anlage of the notochord (Fig. 26, ch), so that the mesoderm can be no longer in the median line, and is consequently divided into right and left parts, as above mentioned in describing the formation of the notochord. As the blastopore lies at or near the side of the primitive knot, it becomes included in the medullary plate. It may remain open while the medullary plate is being transformed into the nervous system, and in that case may establish a connection between the cavity of the central nervous system and that of the entoderm. Such a communication is termed a neurenteric canal. Fig. 64 represents a wax model reconstructed from the sections of a human embryo in the stage of the medullary plate. It shows clearly the form of the plate, the deep dorsal groove, the- opening of the neurenteric canal, and the remnants of the primitive groove behind the canal. As the development progresses the medullary plate extends further backward and encroaches upon the territory of the primitive streak until this latter is obliterated.

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Fig. 25. Surface View of the Embryonic Shield of a Dog Embryo, with Medullary Plate.


A.o, Area opaca. A.p, Area pellucida. A'n, Hensen's knot. Md, Medullary plate. md.F, Medullary furrow, pr.s, Primitive streak. X 15 diams.


The Medullary Groove. — Almost or quite as soon as the medullary plate is formed its lateral portions begin to arise on each side, so that the two halves of the plate together form a broad open trough known as the medullary groove, into which, of course, the dorsal groove is merged, so that it no longer can be recognized. While the groove is being formed the medullary plate increases considerably in thickness. These nuclei multiply rapidly and lie irregularly scattered at various heights. The ectoderm alongside the medullary plate or groove thins out still further. The development is most rapid at a point corresponding to the posterior region of the future head. The further from this point we go, the less advanced do we find the formation of the groove, so that at a certain stage there is a well-marked medullary groove in the cephalic region, the medullary plate behind that, and the primitive streak at the hind end of the embryo. But when the streak has disappeared, the medullary groove is found to extend the entire length of the embryo. Owing to this peculiarity, it is possible in a single embryo to follow all the principal stages of the formation of the medullary groove by the examination of a series of transverse sections. Such a stage is found in the rabbit at nine days, or in the chick at from thirty to forty hours of normal incubation (Figs. 167, 168, and 173).

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Fig. 26. Cross-section of a Human Embryo of 1.54 MM.

f. Dorsal furrow, ek, Ectoderm, ct, Somatic mesoderm, p, Beginning of the embryonic coelom. g, Junction of the extra-embryonic somatic and splanchnic mesoderm. df, Splanchnic mesoderm, en, Entoderm. me, Mesoderm. ch. Notochord. — {After Conn/ Spec. I


The Medullary Canal. — The medullary groove gradually deepens, its sides rising higher and higher and arching more and more toward one another until the edges meet and coalesce, thus changing the groove into a tube — the medullary canal. The closure of the groove occurs in the cervical region first, and spreads from there in both directions. As the closure progresses forward it completes the canal in the region of the head. It occurs in such a manner that there is a very small opening, which is the last point to close. This opening seems to be a fixed point, occupying always the same relative position in all vertebrates. It is called the anterior neuropore. At this time the caudal end of the medullary groove may be still open, and it is the last portion to close. Of the entire length of the primitive canal, about one-half is the anlage of the brain, while the other half is the anlage of the spinal cord. In the subsequent development of the brain the transverse expansion of the canal is most conspicuous, while in the development of the spinal cord the elongation of the canal predominates. The dilatation of the brain begins very early. - , The medullary canal produces the entire central nervous system. Some of/' the cells from its walls migrate out of the wall itself on either side. These cells produce the ganglia.


The Structure of the Medullary Canal

When the medullary canal is first formed, it tends to present a rounded outline in transverse section. But its lateral walls being thicker than the wall on the dorsal and ventral sides of the canal, the internal cavity appears somewhat flattened (Fig. 27). On its ventral side it lies against the notochord. On its dorsal surface it is in contact with the overlying ectoderm, from which it has, however, completely separated, and it causes the overlying ectoderm to rise up somewhat. Its sides are in contact with the mesoderm, which is there developing into the primitive segments, page 79. The nuclei in the wall of the canal are very numerous, oval in form, and usually with a single nucleolus. The nuclei are placed in the radial lines. For some time after the canal has become closed the nuclei multiply very rapidly by indirect division, but all of the mitotic figures are found close to the inner surface of the canal, which surface, it will be remembered, corresponds to the original outer surface of the ectoderm.


The differentiation of the brain and spinal cord is indicated even during the stage of the medullary groove. The extreme anterior end of the groove is found to widen out so as to produce a pair of lateral expansions. As development progresses and the canal closes, these expansions become more marked and are themselves, of course, also closed over, so that when the canal is completed they appear as lateral diverticula or evaginations of the tube, which are known as the primary optic vesicles. While the vesicles are developing the medullary tube expands in diameter throughout its cranial or anterior half without any noticeable change in the general histological structure of its walls. Very soon the expansion becomes unequal, and the inequalities are such that they produce three dilatations, which are known as the three primary cerebral vesicles. The first vesicle is in the region of the optic outgrowth ; the second is just behind this, and the third is as long as the first and second combined and merges into the spinal cord. At the time these vesicles become recognizable they occupy about half the entire length of the medullary tube. Between the first and second vesicles there is a constriction, and one also between the second and the third. The three vesicles are the anlages respectively of the fore-brain, mid-brain, and hind-brain.

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Fig. 28. Transverse Section of a Rabbit Embryo of Eight Days and Two Hours.


Md, Medullary canal. Seg, Primitive segments. Clio, Chorion. Am, Amnion. Som, Somatopleure. Ca, Coelom, Spl, Splanchnopleure. Ent, Entoderm. CA, Notochord. Ao, Aorta.



In the region of the spinal cord the medullary tube soon becomes somewhat flattened from side to side, and therefore acquires a characteristic oval configuration as seen in cross-section (Fig. 28). We can now recognize in the cross-sections four regions : first, the two thick sides; second, in the median dorsal line the thin portion which we call the deck-plate, and in the median ventral line the thin portion which we call the floor-plate. Later on, each lateral portion becomes subdivided into two longitudinal bands, known as the zones of His, and distinguished from one another as the dorsal and ventral zones. After this stage there are six longitudinal zones in the embryonic cord. These are, first, the deckplate; second and third, the dorsal zones of His; fourth and fifth, the ventral zones of His ; and sixth, the floor-plate. These six zones also appear in the region of the brain, where, however, they undergo characteristic modifications. The zones of His dominate the entire morphology of the central nervous system.


The Early History of the Mesoderm

Concerning the precise origin and early development of the mesoderm authorities are by no means agreed, and in the interpretations offered there has been more of hypothesis than of observation. The most accurate observations have so far been made on the elasmobranchs, lizards and chick. In these forms the entoderm (or segmenting yolk) in the neighborhood of the primitive streak produces cells which take their place so as to form a layer next to the entoderm. This layer gradually becomes more and more distinct until it can be definitely recognized as a separate layer, the mesoderm. It is probable that a similar process goes on in amphibia and in mammals, so that it is safe to say that the mesoderm probably arises by this process, which we call delamination, in all vertebrates. In its first stage the mesoderm has no distinct boundary against the underlying entoderm. It is thickest in the neighborhood of the primitive streak and thins out from that in all directions. It very early comprises two easily recognizable classes of cells. One of these forms a more or less distinct layer next to the yolk, and so distributes itself as to form a network of cavities of which these cells become the boundaries, thus developing the first bloodvessels. The cells which form them constitute the angioblast. A portion of the angioblastic cells comes to lie in the cavities of these primitive blood-vessels and is transformed into the first red blood-corpuscles of the embryo. The second class of cells constitutes the mesoderm proper, and forms a more continuous sheet of undifferentiated, somewhat closely compacted cells, extending out from the primitive streak and lying between the angioblast and the ectoderm.


The Expansion of the Mesoderm. — After the mesoderm is once formed as a distinct layer, it seems to have no longer any connection with the entoderm or ectoderm, except in the axial line. Its further expansion is due to the proliferation of its own cells. During this early expansion the mesoderm assumes in all amniota a definite and characteristic series of outlines. It is at first pearshaped (Fig. 29, A), the anterior end being pointed. It extends a short distance only in front of the primitive streak and is widest a little distance behind the area pellucida, Ap. (For a description of the area pellucida see Chapter V.) The condition in the chick at about the twentieth hour of incubation is indicated by Fig. 29, B, drawn on the same scale as A, and at the close of the first day by Fig. 29, C. In the last stage figured it will be noticed that the mesoderm is expanding unequally in front, having sent two lateral wings which leave a median space between them without mesoderm. These wings continue their growth, and finally meet in front, so that in the anterior part of the area pellucida there is a small tract without any mesoderm, although it is completely enclosed bv mesoderm. This tract is the pro-amnion. The actual expanding edge of the mesoderm is quite irregular. The regularity shown in Fig. 29 is entirely diagrammatic.

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Fig. 29. Three Diagrams of Embryonic Areas of Chicks to show the Growth of the Mesoderm.

The mesoderm is indicated by vertical shading, the area opaca by horizontal shading. A.o, Area opaca. A.p, Area pellucida. mes, Mesoderm. pr, Primitive streak. — {After Duval.) .


The extent of the growth of the mesoderm over the extra-embryonic region of the mammalian blastodermic vesicle is very variable. Usually it extends completely around the vesicle, but in some cases, as in the rabbit, only part way (compare page 65, ante).


The Origin of the Coslom. — The next step in the differentiation of the middle germ-layer is the appearance of two slit-like cavities in it, one on each side. These cavities do not extend across the median line, for when they appear there is no mesoderm in the median line of the embryo. The coelom is the anlage of the body-cavity, and in part persists in the adult as the pericardial, pleural, and abdominal cavities. Certain parts of its walls share in the production of muscles and of the excretory organs. The complete history of the coelom is very complex. As the coelomatic cavities appear, the cells bounding them take on a distinctly epithelial character. This limiting layer is termed the mesothclium.


The earliest phases in the development of the coelom have been exactly followed only in a very few instances. In these it has been found that numerous fissures appear in the mesoderm and unite themselves so as to form a network of channels which grow, and produce by their fusion the coelom. The fusion occurs so that two cavities are developed, one on either side, and parallel with the axis of the embryo. As the head of the embryo grows the two cavities grow into its cervical end, following the penetration of the mesoderm, and unite so as to form below the developing pharynx a single median cavity, the anlage of the future pericardial cavity. In the Sauropsida and in many mammals the pericardial coelom merges into two large expansions of the body-cavity which lie just alongside of the head of the embryo and are known as the amnio-cardiac vesicles (Fig. 167). (Compare also the account of the splanchnocoele, page 82.) There are very great variations in the development of the coelom in mammals. In some cases the coelom grows so as to appear at an early stage in the body of the embryo (Fig. 27). In other cases it is developed in the entire extra- ' embryonic region of the blastodermic vesicle before it is developed in the embryo proper. This condition has been observed in primates, including man. It results in the formation of a layer of mesoderm surrounding the yolk-sac, and another layer underlying the extra-embryonic ectoderm, with a wide coelomatic space between the two mesodermic layers. This space we call the extra-embryonic coelom. These relations are illustrated in Fig. 3] .


As soon as the coelom has appeared the mesoderm is divided into two layers, an outer and an inner. The outer layer is in close contact with the ectoderm. It is called the somatic mesoderm. The inner layer is in close contact with the entoderm; it includes the entire angioblast, there being in early stages no blood-vessels or blood in the somatic mesoderm. The inner layer is called the splanchnic mesoderm.


Somatopleure and Splanchnopleure

The somatic mesoderm, together with the overlying ectoderm, constitute the somatopleure or primitive body-wall. The splanchnic mesoderm, together with the underlying entoderm, constitute the splanchnopleure. The somatopleure and splanchnopleure are, to a large degree, the elementary anatomical parts out of which the adult structure is produced. Although they each comprise cells belonging to two germ-layers, they nevertheless develop each almost as a unit, the cells of the two germ-layers entering into intimate co-operation with one another in the differentiation of organs. In both somatopleure and splanehnopleure it is convenient to distinguish two main regions; namely, the i mbryonic, which enters into the constitution of the embryo proper, and the extra-embryonic, which enters into the formation of the so-called appendages of the embryo, that is to say, of parts which exist during embryonic life, but are lost at the time of birth, and take no share in the permanent body.


In the primitive type of vertebrate development there are no embryonic appendages. This condition is illustrated by Fig. 30, which is a transverse section of a young stage of an axolotl. This may be readily compared with a blastodermic vesicle of a mammal, if we imagine the mass of yolk or entoderm reduced to a single layer of cells. We can then easily distinguish the ectoderm and the underlying somatic mesoderm, which together completely enclose the section. The splanchnic mesoderm lies close against the yolk and is separated from the somatic by the intervening coelom.


The general homologies of this primitive type of vertebrate embryos with the type which we find in the amniota may be readily grasped by the aid of the accompanying diagrams (Fig. 31), which are based somewhat on the processes as actually found in the chick. The embryonic structures properly so called are distinguished by shading. The yolk-sac is large and more or less a separate structure from the embryo. It is surrounded by a layer of mesoderm represented by a dotted line. In the direction of the embryo the mesoderm has continued to form part of the wall of the intestinal canal, In; hence we may say that the splanehnopleure forms the wall of the primitive intestinal canal and of the yolk-sac. The yolk-sac represents a lower portion of the splanehnopleure. It can be readily seen that we may compare it with the condition noted in the newt, and have to deal fundamentally with a question of relative proportions. The somatopleure, Som, enters into the formation of the embryo itself, but it also extends beyond. Its disposition becomes complicated in the amniota by the formation of the amnion itself. We shall consider here only what is looked upon as the primitive method of the production of the amnion, and note only that the exact steps of the process are considerably modified in many mammals, in connection with the early modifications which the ovum undergoes in order to secure its attachment to the walls of the uterus (see the section on the trophoblast). The somatopleure forms two folds, one on each side of the embryo. These folds arch up over the back of the embryo. The inner leaf or part of each fold is the anlage of the amnion, Am. It consists of a layer of ectoderm next to the embryo, and a layer of mesoderm, represented by the dotted line, turned away from the embryo. The remaining portion of the extraembryonic somatopleure, Cho, extends around both the amnion and yolk-sac, forming a membrane called the chorion, which likewise consists, of course, of ectoderm, which, however, faces away from the embryo, and of mesoderm (dotted line), which is turned toward the embryo. As regards the embryo, therefore, the position of the two germ-layers in the amnion is reversed in the chorion. The two folds continue to grow until they meet above the back of the embryo and unite. The amnion (Fig. 31, Am) has thus become the closed membrane surrounding the embryo, and the chorion, Clio, has become a closed membrane surrounding the amnion, the embryo, and the yolk-sac.

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Fig. 30.

Early Stage of an Axolotl. Ec, Ectoderm. met, Mesoderm. Md, Medullary groove. Ch, Notochord. /:';;/, Entoderm. Yk, Yolk. Ach, Archenteron or primitive entodermal cavity. — (After Bellonci.")

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Fig. 31. Generalized Diagram (if an Amniote Vertebrate Embryo.


The first figure shows the condition before, the second after, the separation of the amnion from the chorion. Am, Amnion. C/i,\ Chorion. Ca-, Coelom. /;;, Intestinal canal. Som, Somatopleure. Sp!, Splanclinopleure. Yolk, Yolk-sac.


By the processes indicated we have produced an embryo with its three primary appendages — the chorion, amnion, and yolk-sac. To these there is to be added a fourth appendage, the allantois, which also begins its development very early, and arises as a hollow outgrowth from the under side of the caudal end of the embryo and expands into the extra-embryonic ecelom or space between the yolk-sac and the chorion.


The Embryonic Coelom

In the body of the embryo proper the ecelom acquires a very complicated disposition. It forms, first, a series of small cavities alongside of the medullary tube. The walls of these cavities are termed the primitive segments. It forms two large main cavities, which partially unite in later stages on the ventral side of the embryo, the primitive segments lying more on the dorsal side. These two large coelom spaces constitute the splanchnocele, a term which has reference to the fact that this space surrounds the splanchnic viscera. Finally, it forms a series of so-called head-cavities, of which there are probably always three on each side of the head. The walls of these head-cavities in part produce the muscles of the eye. We must now consider the development of these divisions of the ecelom in the order indicated.


The Primitive Segments. — A segment consists of a pair of cavities symmetrically placed and bounded by mesothelium. The cavities are portions of the embryonic coelom. For convenience of description the term segment is usually applied to one of the pair of structures which constitute a whole segment. The primitive segments appear very early; the first pair can be recognized in the chick after twenty to twenty-two hours' incubation; in the rabbit, at the beginning of the eighth day. In both cases the medullary groove is still nowhere closed. In amniote embryos, just before the first segment appears, the mesoderm on either side of the axial line is considerably thicker than further awav from it. We can, therefore, distinguish two zones — namely, the thicker segmental zone near the axis, and the thinner, but much wider lateral or parietal zone. The first step in the formation of the first segment is a loosening of the cells in the segmental zone, along a narrow transverse line. In the chick this occurs about 0.14 mm. in front of the primitive streak, at a time when only a portion of the medullary groove is formed. Very soon there appears, close by, a second similar transverse loosening of the cells. The mesoderm of the segmental zone is thus cleft twice, the mesodermic cells between the two clefts constituting the first segment, which is somewhat cuboidal in form. The first segment appears in what later becomes the occipital region. Two or, according to some authorities, three segments are formed on the cephalic side of the first segment; and, meanwhile, the number of segments is also increasing on the caudal side of the first, but much more rapidly. The primitive segments, owing to their form and their proximity to the anlage of the central nervous system, were taken by early embryologists to be the beginnings of the vertebrae, and were, therefore, called the proto-vertebrcc. This name is still used, although the idea, upon which it was based, is known to be erroneous because the primitive segments form much more than the vertebrae.


The association in time of the development of the medullary groove and primitive segments is important. By the formation of the groove the space between the ectoderm and entoderm alongside the groove is increased, and it is this space which gives the mesoderm the opportunity to grow in thickness so as to form the segmental zone next to the medullary groove.


In the amniota when the primitive segments are first formed they contain no actual cavity, but we must consider that one is morphologically present, since we can easily observe the. line of contact between the opposite walls of the segments. As observed in transverse sections the segments when first developed are triangular in outline. The base of the triangle extends along the side of the medullary canal; the apex of the triangle lies next to the splanehnoce'le, and at the point of the triangle the somatic and splanchnic mesoderm separate widely from one another. Very soon the apex of the triangle forms a narrower piece (Fig. 32, N), which is known commonly as the nephrotome or intermediate cell mass. While the nephrotome is being marked off the proximal portion of the segment enlarges, the cells assume a more distinctly epithelial character (Fig. 32, My), enclosing a considerable space, which, however, is completely filled by a mass of cells, C, which arise by a proliferation of the cells from the lower side of the segment. The line around this mass of cells marking it off from the other wall of the segment indicates the morphological cavity. In the sheep and the chick it has been observed that the cavities of the first four segments can be traced through the nephrotome to the splanchnocele. This represents a primitive condition, one which we find in all the segments of some of the lower vertebrates. Did we know the development of the amniota only, we should not have been able to identify the cavity of the segment as morphologically a portion of the coelom. The development in fishes shows conclusively that it must be so regarded.

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Fig. 32. Transverse Section from a Chick Embryo with about Eighteen Segments. Only the mesoderm of one side has been drawn. The section passes through a recently formed segment. At) , Secondary segment. C, Core of the segment. IV. J, Wolffian duct. N, Nephrotome. Car, coelom. Som, Somatic mesoderm. Sp/, Splanchnic mesoderm. \ 227 diams.


The Separation of the Nephrotome. — The nephrotome early loses its connection on the one side with the enlarged central portion of the segment, and on the other with the mesodermic walls of the splanchnocele, so that each nephrotome forms a little mass of cells isolated from, but in definite topographical relation to, the other parts of the mesoderm. It may be noted that during these early stages one can always find the anlage of the Wolffian duct on the ectodermal side, and on the entodermal side the anlage of a blood-vessel. Very soon the nephrotome assumes a rounded form, and a cavity appears in its interior; it is then often called a segmental vesicle (Fig. 33, Nephr). The exact details of the process by which the nephrotome is separated from the other parts of the middle germlayer have not yet been carefully studied. Each nephrotome is the anlage of one of the excretory tubules of the Wolffian body.


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Fig. 33. Section of a Very Young Cat Embryo. (Transverse Series 413, section 181.) Am, Amnion. Ao, Aorta. A/,/, Medullary tube (spinal cord). My, Outer, My', inner wall of primitive segment. Nek, Notochord. Nephr, Nephrotome (segmental vesicle). Som, Somatopleure. Spl, Splanchnopleure. Ve, Blood-vessel. W.D, Wolffian duct. )< 5° diams.


The portion of the primitive segment which is isolated by the formation of the nephrotome lies, of course, next to the medullary canal. The term primitive segment (as also proto- vertebra) is often applied to this structure as well as to the original primitive segment before the separation of the nephrotome, but it would be better to refer to it as the secondary segment

  • The secondary seg
  • This is a new term, here proposed for the first time.



merit, when first formed, appears more or less nearly square in surface views, and triangular in cross-sections. As the medullary canal grows, so does the secondary segment, and it becomes, therefore, somewhat elongated in its dorso-ventral diameter. After this change in its shape we can distinguish in transverse sections of an embryo (Fig. 28) the outer wall, which lies under the ectoderm, and an inner wall, which lies toward the medullary canal and notochord. In the further history of the secondary segment we can distinguish the following steps : First, the production of the myotome, with the accompanying transformation of a portion of the cells of the inner wall of the segment into the mesenchyma; next, the production of the true muscle plate ; third, the breaking-up of the outer wall of the myotome. These portions are sufficiently described in the practical part, Chapter V.


The Splanchnocele. — The splanchnocele makes its first appearance in the parietal zone of the mesoderm in the manner above described. It rapidly increases in size, so that a considerable space separates the somatic from the splanchnic mesoderm, as shown in Fig. 165 and Fig. 162. When it first appears, it is a narrow fissure. It rapidly widens, extends toward the axis until it almost reaches the primitive segments, and also spreads out laterally into the so-called extra-embryonic region. As above stated, the rate and extent of its extraembrvonic development vary greatly in different mammals. It develops earlier and acquires a greater distention at first in the future cervical region, where it produces the amnio-cardiac vesicles, in the median portion of whose united cavities the heart is lodged. The splanchnocele of the body proper appears after the primitive segments, and its expansion takes place at first only in the part of the mesoderm next to the primitive segments. Everywhere as the splanchnocele develops the mesodermal cells about it assume gradually more and more distinctly an epithelial character, so that it soon becomes proper to speak of the mesothelium or boundary epithelial wall of the coelom.


The splanchnocele is also designated by several other names, and is sometimes called simply the body-cavity or somatic cavity. Others term it the ventral coelom. By English embryologists it is usually called the phuro- peritoneal space. Its future subdivisions become early indicated by a transverse ridge of tissue which is known as the septum trunsversum. This septum is situated at the posterior end of the heart, and is developed to allow the great veins to have access to the heart itself. It is the anlage of the future diaphragm. It separates the coelom around the heart from that of the abdomen. It is a product of the splanchnopleure, so that it rises up on the ventral side of the coelom. We have, as soon as this septum is present, the pericardial cavity on its cephalic side, the abdominal cavity on its caudal, and a small pleural cavity on its dorsal side.


The Coelom of the Head. — No adequate investigation of the early stages of the mesoderm in the head of amniota has yet been made. We know, however, that in the lower vertebrates there appear at least three distinct cavities resembling portions of the true coelom and bounded by epithelial cells, similar to the mesothelium in character. These cavities are generally regarded as portions of the true coelom, and by many writers have been interpreted as true primitive segments. But this interpretation is not yet beyond doubt. The largest of these cavities is called the mandibular, because it has a prolongation which extends into the mandible of the young embryo. In front of it is the first or premandibular cavity, which is much smaller, and behind it is the third or hyoid cavity, which is intermediate in size between the first and second. The headcavities are best known in the elasmobranchs. They have also been found clearly developed in reptiles and certain birds. In mammals no actual cavities have been recorded. There are found the anlages of the muscles of the eye, and these are, by hypothesis, homologous with the cells of the walls of the headcavities in lower vertebrates, which cells produce the muscles of the eye.


The Mesenchyma

By the term mesenchyma we designate the whole of the mesoderm of the embryo, except the mesothelial lining of the coelom. When fully differentiated histologically, it consists of more or less widely separated cells, connected with one another by intervening threads of protoplasm, which form a network between the cells. The remaining space is filled by a homogeneous structureless matrix or basal substance. It gives rise to a large number of adult tissues, as shown in the table on page 35.


In the early development, or histogenesis, of the mesoderm we can distinguish four stages: First, that of distinct cells; second, the formation of the cellular network; third, the formation of the mesothelium; and, fourth, the differentiation of the mesenchyma. The first stage is known chiefly through observations on the early stages of elasmobranchs, reptiles, and birds. In these types the first cells, which are delaminated from the entoderm to form the anlage of the mesoderm, are of quite large size and lie between the entoderm, or yolk, and ectoderm, and are without connection with one another. The number of mesodermic cells increases both by the multiplication of the cells already delaminated, and by the addition of others from the entoderm. Whether this stage occurs in mammals, or not, we do not know at present. In the second stage the primitive cells are found to have acquired connection with one another, the protoplasm of one cell uniting by a process, or prolongation, with the protoplasm of another cell, and so on until the whole tissue becomes a network. When the primitive streak has been formed in the mammalian blastodermic vesicle, we find the mesoderm in this condition. The third stage is brought about by the development of the coelom as above described, and it seems probable that all the cells of the mesoderm are transformed into mesothelium. But this probability is not at present wholly beyond question. It is certain that nearly, if not quite, all the mesodermic cells become mesothelium. To produce the fourth stage, single cells leave the mesothelium or migrate out of it on the side away from the coelom. These cells are found to be connected both with one another and with the mesothelial cells by protoplasmatic processes, but they do not lie close together, as in the epithelium, so that there is a considerable, though variable, amount of intercellular space. By the migration of the cells and their multiplication, the mesenchyma is produced. It fills up all the room between the mesothelium and the two primary germ-layers so far as it is not occupied by the developing blood-vessels.


  • The anlages may be seen in a pig embryo of 10 mm. between the jugular vein and the internal carotid artery as a group of embryonic cells quite distinct from the surrounding mesenchyma.



Apparently the entire mesothelium may participate in the production of the mesenchymal cells. Its different regions, however, do not so participate all to an equal degree, or at the same time. The throwing off of mesenchymal cells may be observed in certain parts of the embryo in somewhat advanced stages of development, and it seems not impossible that the process may be found to occur even in adult life.


The mesoderm, by the formation of mesenchyma, becomes very early unlike the other germ-layers. Both ectoderm and entoderm are epithelial membranes. The mesoderm is partly epithelial, partly mesenchymal, and from the mesenchyma arise special kinds of tissue which are characteristic of the middle germlayer, and never are produced from either the outer or inner germ-layers.


The Germ cells

Concerning the primitive origin of the germ-cells in vertebrates our knowledge is scanty. The most accurate information we have refers to their development in the dog-fish. In this species the germ-cells are delaminated from the entoderm togel her with other cells of the mesoderm, and cannot, with our present knowledge, be distinguished from other mesodermic cells. They soon, however, became recognizable, because while the majority of the mesodermic cells are passing into the second stage (compare the section on Mesenchyma, page 83) these germ-cells change but little, if at all, so that they can be recognized as something distinct from the neighboring cells. For a short time they are found gathered into two compact groups, symmetrically placed in the extra-embryonic region, but not far from the embryo. The cells then break apart from one another and gradually become separated, and migrate by unknown means, first over the wall of the intestine, which has meanwhile been differentiated, then over the surface of the mesentery into the anlage of the genital gland. During their entire migration they are lodged in the mesothelium, and when they have reached their final destination they are still in the mesothelium of the genital anlage, where they remain until finally differentiated in the adult. The epithelium, with the germ-cells in their definite position in it, is called the germinal epithelium (compare page 39). The germinal epithelium has been observed in all vertebrates, but the origin of the germ-cells in amniota is entirely unknown. The hypothesis may be accepted, that they arise in a manner essentially similar to that known in the dog-fish. For some of the theories based on the known development of the germ-cells, see page 40.


The Yolk=sac

General Morphology. — The yolk-sac is the container of the nutritive yolk destined to be assimilated by the embryo. The principal factor in its morphological constitution is the entoderm, which, after the differentiation of the definitive germ-layers, contains nearly all of the yolk material. In the primitive vertebrates, as exemplified by the marsipobranchs, ganoids, dipnoi, and amphibia, we find this yolk material lodged in the walls of the primitive digestive tract. It is situated chiefly on the ventral side of this tract, and extends from the point where the heart is formed toward the tail of the embryo to the point where the- allantois is formed. In other words, it is situated in a region corresponding to the territory of the future abdominal cavity. In the primitive types just referred to, the yolk-bearing entoderm becomes divided into distinct cells which form a large mass. The condition may be understood from Fig. 30, which represents a transverse section of the early stage of an axolotl embryo. The cavity of the entodermal canal (digestive tract) is small. It is bounded on its dorsal side by a single layer of cells distinctly epithelial in their development, and on the ventral side by a great mass of rounded cells heavily laden with yolk granules, and containing conspicuously large nuclei. These large nuclei differ by their size and minute structure very much from the other nuclei in the embryo. The corresponding nuclei in higher animals are sometimes called parablast nuclei. Outside of the entoderm comes the second portion of the yolk-sac, the splanchnic leaf of the mesoderm. If we imagine the amount of yolk to be gradually increased, so that it would appear more distinct from the embryo proper, we should then apply to it the term extra-embryonic. The yolk-sac of the higher forms differs from that of the lower forms only by its size, as is illustrated by Fig. 34, which represents a diagrammatic transverse section of an early stage of the chick, before the formation of the amnion has begun. The essential relations may be seen by comparing Figs. 30, 34, and 31 . As shown in the section, Fig. 34, the yolk-sac, if we may so call it, is completely enclosed by the somatopleure of the embryo, and in the amniote embryo the condition is the same. The yolk-sac is surrounded by the somatopleure, which, however, in the amniota we call extra-embryonic. The extra-embryonic somatopleure around the yolk-sac is called in birds the membrana serosa, and in mammals the chorion.

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Fig. 34. — Diagrammatic Section of the Yolk of a Hen's Egg at an Early Stage to show the Relation of the Primitive Entodermal Cavity, Aeh.


Civ, coelom. ///, Intestinal cavity. Som, Somatopleure. Spl, Splanchnopleure.


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Fig. 35. — Wall of the Yolk-sac in the Region of the Area opaca of a Chick of the Second Day. Ales, Mesoderm. V, V, Bloodvessels, containing a few young blood-cells. ^///.Entoderm. c, Four distinctly shown entodermal cells.



In amniota we can distinguish in the entoderm of the embryo, or yolk sac, three distinct regions. The first of these includes the whole of the entoderm of the embryo and a certain territory around it. In this region, after the earliest stages are passed, the entoderm is found to be a very thin layer and to contain very few yolk-granules, and such few as it contains are small. This portion of the entoderm, therefore, seems translucent, an appearance which can easily be noted with the naked eye, and which has led to the name area pcllucida, which has long been applied to this region. The region all around the area pellucida appears in the fresh specimen darker, and this is called the area opaca, the second region. The entoderm in this part consists of columnar cells (Fig. 35, c, and Fig. 36). In the chick the cells are high cylinder cells of somewhat irregular shape, containing a loose network of granular protoplasm. The lower ends of the cells are rounded and projecting, and have a well-marked border of dense protoplasm. The nuclei are variable in size, but for the most part large, often three or four times greater in diameter than the neighboring mesodermic nuclei. They usually have one, sometimes two, conspicuous nucleoli. The nuclei always lie at the upper or basal ends of the cells, chiefly near one side of the cell. The cells contain yolk-grains which appear to be undergoing resorption. Toward the area pellucida the cells are smaller, the network of protoplasm closer, and the yolkgrains are either absent altogether or, if present, small in size and few in number. The transition to the thin entoderm of the area pellucida is quite abrupt. In the opposite direction the area opaca passes gradually, by changing its structure, into the general mass of the yolk, or area vitellina, the third of the regions of the yolk-sac, so called because it contains the bulk of the yolk material. The transition of the area opaca into the area vitellina is marked by a considerable accumulation of cells which are arising from the yolk. This accumulation of cells is called the germinal wall. It is the connecting-link between the epithelium on the dorsal side of the entodermal cavity and the yolk or area vitellina, which forms the ventral boundary of the cavity. If we follow successively the stages, we find that the area pellucida grows at the expense of the area opaca, and the area opaca at the expense of the area vitellina. These facts are to be interpreted as phases in the process of the assimilation of the nutritive yolk. The thin cells of the area pellucida are those in which the absorption of the yolk has been completed. The larger cells of the opaca are those in which the assimilation is going on, and it can be easily seen that it is most advanced in those cells which are nearest the embryo and least advanced in those cells which are nearest to the germinal wall. In mammals the area pellucida is well marked and resembles that of birds. The area opaca has well-defined cylinder cells (Fig. 36) which have rounded ends, but are much smaller than in birds and contain very little yolk material. Cells of this character extend over also what we should call the area vitellina, which does not present the special features which it has in birds, for the reason that the yolk in mammals is so small in amount. Later on the cells pass through degenerative changes, which need to be more exactly studied. In man the degenerative change in the cells of the yolk-sac takes place very earlv. The mesoderm of the yolk-sac is at first a thin layer. Very early there appears an angioblast, or the anlage of the first blood-vessels and blood. In all cases in which the process has been accurately followed the angioblast makes its first appearance in the region of the area opaca, where it forms a network of primitive blood-vessels close against the surface of the yolk. The region occupied by these blood-vessels is called the area vasculosa. Its boundary in the direction away from the embryo is everywhere well defined. Gradually the development of blood-vessels progresses from the region of the area opaca into the region of the area pellucida and extends into the body of the embryo. We even have the embryo almost completely surrounded by a region of extra-embryonic blood-vessels — the definitive area vasculosa. Now, it will be remembered that the area opaca is the territory in which the etitodermal cells are actively assimilating the yolk, and we must believe that the blood-vessels which are thus early developed in close contact with the cells of this area are destined to take up food material digested by the entodermal cells and carry it to the embryo. Hence we interpret the early development of the extra-embryonic vessels as due to physiological necessities.


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Fig. 36. — Wall of the Yolk-sac in the Region of the Area opaca of a Rabbit Embryo of Thirteen Days. V, Blood-vessels containing young red blood-cells, bl. mes, Mesoderm.


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Fig. 37.— Section of the Yoik-sac of a Young Human Embryo, 2.15 mm. Long.

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Fig. 38.—Human Embryo. lA/ler IV. His.) En/, Entoderm, met, Mesoderm. ;■, Bloodvessels. — (After Keibel. )


The mesoderm at first forms a very thin layer over the angioblast. It next thickens by the multiplication of its cells, and we can then distinguish in it both the outer mesothelium and the inner mesenchyma. The mesothelium is the permanent external cover of the yolk-sac. The mesenchyma grows in between the primitive blood-vessels, and finally penetrates, at least in part, between the blood-vessels, and the entoderm of the yolk-sac, a condition which is reached very early in the human embryo (Fig. 37).


The human yolk=sac is characterized by its small size and by the precocious expansion of the area vasculosa, so that in the very earliest stage known to us by observation blood-vessels are found over the entire sac. At the beginning of the third week the diameter of the yolk-sac is about equal to the length of the embryo (Fig. 65). By the end of the third week the sac has become distinctly pear-shaped, its narrower pointed end being that by which it is connected with the intestinal canal of the embryo (Figs. 38, 39). The sac continues growing, up to the end of the fourth week, after which it enlarges very slightly, if at all. Its diameter is only from 7 to 1 1 mm. It is then a pear-shaped vesicle attached by a long stalk to the intestine, the stalk having been formed by the lengthening of the neck of the yolk-sac (Fig. 40). The cavity of the stalk early becomes obliterated and the entoderm in the stalk disappears altogether.

Minot1897 fig039.jpg

Fig. 39. — Human Embryo of 2.6 mm. — [After W. His.)


The Origin of the BIood-vessels and Blood

As stated above (pages 88 and 89), the first blood-vessels appear in the circumscribed region in the mesoderm of the yolk-sac and lie close against the entodermal cells of the area opaca. The region which they occupy is termed the area vasculosa. From the area vasculosa the development of blood-vessels extends, as stated, across the area pellucida into the embryo.* During these early stages the only blood-vessels are in the splanchnopleure. After their formation has extended into the body of the embryo, it spreads into the somatopleure also, which, therefore, acquires its blood-vessels at a later stage. It should be noted, however, that the development of the blood-vessels begins before the coelom has been developed over the area vasculosa. While they are forming, the coelom expands; and after it has appeared, the primitive blood-vessels are found always exclusively in the splanchnic mesoderm.


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Fig. 40. — Human Embryo of 9.8 mm. Probable Age Thirty Days. X 5 diams.


Definition. —The essential part of a blood-vessel is its endothelial wall. In early stages all the blood-vessels consist only of endothelium. Arteries and veins differ but little, if at all, in histological structure during early embryonic stages, and are distinguished chiefly by the direction of blood-currents passing through them. Capillary blood-vessels and sinusoids have, as a rule, throughout life merely an endothelial wall. Arteries and veins become strengthened by the development of special coats around the endothelium which arise by transformations of the mesenchymal cells in the immediate neighborhood of the vessels.


  • It has been recorded that in lizards the vascular anlages appear first in the area pellucida.


The Development in the Chick

The first indication of the blood-vessels is a reticulate appearance, which can be recognized in the mesoderm in surface views of the fresh or hardened embryo at the end of the first day. The reticulate structure increases rapidly in extent and distinctness during the second day of incubation. It is confined to the region of the mesoderm surrounding the embryo proper, and which is, therefore, known as the area vasculosa, as above stated. As soon as there are several primitive segments in the embryo, the network in the mesoderm shows traces of coloration in irregularly shaped reddishyellow spots, which are largest and most numerous around the caudal end of the embryo. These spots are called blood-islands because the cells in them are transformed into the first blood-corpuscles. The network appearance is due to the development of the angioblast, which is a set of cells delaminated from the N^J entoderm or the yolk, and intervening between the mesoderm proper and the entoderm. The angioblast at first assumes the form of more or less solid cords. The meshes of the angioblast are partly or wholly filled by mesodermic cells. The coelom now appears in the extra-embryonic area, and thereafter the anlages of the blood-vessels are connected with the splanchnic mesoderm only. The anlages of the blood-vessel at this stage form a thick network without distinction of stem or branch, except that the edge of the area, bounded by a broad band of angioblast, gives rise to a single large vessel, which is known as the sinus terminalis. The anlages are all in one layer, none overlying the others, and up to this stage they are all solid. The terminal sinus becomes connected with the venous system.


The blood-islands are spots where there is a cluster of cells, which remain attached to one another and to the walls of the vessels. The cells develop hemoglobin in their interior, hence the clusters have a reddish color which renders the islands very conspicuous in surface views of fresh specimens. Blood-islands appear first in the area opaca, but almost immediately after in the pellucida also. They have at first a rounded or branching form. In the inner part of the latter they are small and stand alone. Toward the periphery they are larger, closer set, and more united with one another. Their development is greater around the caudal end of the embryo.


In the next stage the vascular anlages become hollow, and then may be called true blood-vessels. When they acquire a lumen, the blood-islands are found to remain attached usually to the upper side of the vessel like a thickening of its wall (Fig. 41, bl. is). Very soon after the vessels have become hollow the cells of the blood-islands break apart and lie free in the cavity of the vessel, thus forming the first blood-corpuscles. They are characterized by having a rounded nucleus with a very distinct nucleolus, and a minimal covering of protoplasm only. After the cells have become free the amount of protoplasm in each cell increases. The cells multiply rapidly by mitotic division. According to the prevalent hypothesis, all of the colored blood-corpuscles are descendants of these cells derived from the blood-islands.


The angioblast continues growing by the development of buds from the vessels already formed. These buds are rounded or pointed, forming, as it were, spurs. They often end by meeting one another and uniting. They are usually hollow from the first, and after they meet one another or an adjacent vessel, the cavities become continuous, and thus the vascular network is extended.


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Fig. 41. — Section of the Area vasculosa of a Chick Embryo of the Second Day. Som, Somatopleure. Spl, Splanchnopleure. Ec, Ectoderm. En, Entoderm. bl.is, Blood-islands. V, V Vlood-vessels. x 227 diams.

The Development in Mammals

The origin of the blood-vessels in mammals is not adequately known. The solid primary anlages appear in the extraembryonic area vasculosa and extend later into the embryo. They present wellmarked blood-islands, which make their first appearance in rabbit embryos of the eighth day, just before the appearance of the first primitive segments. It is characteristic of mammals that the entire yolk-sac, probably owing to its small size, becomes, very early indeed, vascularized throughout.


The Growth of the Vessels into the Embryo

The entrance of the vessels into the embryo chick begins toward the end of the second day. The buds which form the extra-embryonic angioblast grow first toward, then into, the embryo. The penetrating vessels follow certain prescribed paths. Part of the vessels run along the posterior edge of the amniocardiac vesicles, and enter into connection with the posterior end of the heart, which has meanwhile been developing, and which — owing to the early separation of the head end of the embryo from the yolk — is the onlv part of the heart which the vessels can reach directly. While the vessels are approaching the heart their differentiation into various sizes is going on, the smallest ones to remain as capillaries, the larger ones to become arteries or veins. The only two veins in the first stage are those above mentioned, which are called the omphalo-mesaraic. Another set of vessels penetrates along the sphlanchnopleure of the body on each side until they attain the small space between the notochord and myotome and the entoderm, where they fuse so as to form a longitudinal vessel, the anlage of the descending aorta. It should be noted that this anlage is primitively double. The aorta appears first in the region toward the head. It grows forward above the pharynx, bends ventrally just behind the mouth, dividing as it bends, one branch going around each side of the future pharynx and uniting again on the ventral side of the pharvnx in the median ventral line, in order to join the anterior end of the tubular heart. The heart begins to beat before the vessels unite with it. The first blood-cells have already been formed; hence as soon as union is accomplished the blood circulation starts up, the blood passing through the aorta to the body, thence by numerous lateral branches to the area vasculosa^ and returning by the two omphalo-mesaraic veins to the heart. It will thus be seen that almost the entire circulation is extra-embryonic.


The other embryonic blood-vessels are developed by buds from the walls of the vessels already present in the embryo, in the same general manner as new vessels are formed in the area vasculosa. These buds give rise to the endothelium only of the embryonic vessels. When a vessel becomes an artery or a vein, the media and adventitia are added, as above stated, by differentiation of the surrounding mesenchyma.


The "vasoformative cells" of Ranvier are probably degenerating bloodvessels and not the anlages of vessels, as Ranvier assumed.

The Blood - corpuscles

The red blood-cells are the only elements contained in the blood during the earliest stages of the vertebrate embryo. When the circulation begins, the number of corpuscles is small, but rapidly increases by division of the corpuscles. The cells in amniota are at first round; in the chick they are from 8.3 to 12.5 fi.


The cells are at first granular and slightly colored. As the protoplasm increases about each nucleus the cell-body becomes more distinct, more colored with hemoglobin, and more homogeneous.


By examining the blood of chick embryos of successive ages we can trace the differentiation of the red cells. We find that the protoplasm enlarges for several days, and that during the same time there is a progressive diminution in the size of the nucleus, which, however, is completed before the area of protoplasm reaches its ultimate size. The nucleus is at first granular, and its nucleolus or nucleoli stand out clearly. As the nucleus shrinks, it becomes round and is colored darkly, and almost uniformly, by the usual nuclear stains. The bloodcells of mammals pass through the same metamorphosis as those of birds. For example, in rabbit embryos of eight days (Fig. 42, A) the cells have reached the stage with a granular nucleus and well-developed cell-body. Corpuscles of this kind are characteristic of fishes and amphibia, and this may, therefore, be designated as the ichthyopsidan stage. Two days later the nucleus is already smaller, and by the thirteenth day has shrunk to its final dimensions. This condition of the corpuscles is characteristic of the reptiles and birds, and may be designated, therefore, as the sauropsidan stage. The nucleated stage of the cells is tvpical of embryonic life only in mammals. During the fetal period the nuclei of the red cells gradually disappear and the cells are transformed into the nonnucleated corpuscles, which occur only in mammals, so that this last may be designated as the mammalian stage. The successive stages of the blood-corpuscles in mammals illustrate the law of recapitulation (page 41). There has been much discussion as to the manner in which the nucleus disappears in order to convert the nucleated cell into the non-nucleated mammalian blood-corpuscles, but authorities are not yet agreed. When the nucleus disappears, the corpuscle becomes smaller. In the human embryo at one month, the red cells are the only blood-corpuscles. At two months they are still the most numerous, although the non-nucleated corpuscles have begun to appear. At three months the nonnucleated corpuscles constitute by far the majority of all corpuscles in the blood.


Leucocytes

The origin of the first leucocytes in the embryo is still uncertain. Blood is found to contain for some time only the red cells, the leucocytes not appearing in the chick until about the eighth day, in the rabbit about the ninth, and in the elasmobranchs not until the embryo is well advanced in its development. It is generally believed that the leucocytes do not arise in the bloodvessels, and that they have no genetic relationship to the red blood-corpuscles. It is probable that leucocytes are of several kinds, and of several distinct origins.

The Origin of the Heart

At about the time blood-vessels are developing in the area vasculosa, the head of the embryo is found to have grown so much that it projects forward somewhat. This process is illustrated by the diagrams in figure 43, which are intended to illustrate the method by which the so-called separation of the embryo and yolk takes place. It is often said that the embryo is constricted off from the yolk. This, however, is incorrect. The size of the connection between the embryo and the yolk remains absolutely about the same, or even increases in dimensions, but the embryo grows rapidly, so that its head end projects forward and later its caudal end also, hence, though the connection with the yolk may remain unchanged, the growth of the embryo causes that connection to appear relatively smaller. In early stages the head of the embryo grows more rapidly than the caudal end (Fig. 43, C). At the cervical end of the anlage of the head, where the tissues of the embryo bend over to join the yolk, a portion of the coelom is early developed. It extends across the median line. This coelom is the beginning of the pericardial cavity. In connection with it the development of the heart occurs. The formation of this organ is probably initiated by an ingrowth of the future cells of the angioblast, which give rise to the endothelium of the heart. The mesothelium of the dorsal side of the primitive pericardial coelom produces the muscular walls of the heart. The early development and primitive relations of this organ can be understood by the account given in the practical part of the structure of a chicken embryo with seven segments.



Fig. 43. — Diagrams to Illustrate the Separation of the Embryo from the Yolk.


bl, Blastopore. //, Head of embryo. AtA, Arclienteron or entodermal cavity, ec, Ectoderm.



The Germinal Area

The germinal area is that portion of the amniote ovum (mammalian blastodermic vesicle) in the center of which the embryo is differentiated. It comprises, therefore, both the embryo proper and the region immediately surrounding it. In its center we find the anlages of the embryonic structures proper. In its extra-embryonic part we find the three primitive germ-layers. Underneath the entoderm is the cavity of the yolk-sac. In the mesoderm we have occurring the development of the coelom,and in the somatic mesoderm the differentiation of the primitive blood-vessels. These primitive vessels occupy the sharply defined territory, the edge of which is marked by the sinus terminalis. The first differentiation in the germinal area which can be clearly recognized by the naked eye is the appearance of the area pellucida, which is due to the thinning of the entoderm over the central area. Next ensues the differentiation of the primitive streak. Further progress results in the gradual differentiation of the embryo, in the sharp demarcation of the area pellucida, which becomes pear-shaped, and in the appearance of the blood-vessels and the resulting differentiation of the area vasculosa. Figure 167, on page 296, represents the embryonic area of a hen's ovum after about twenty-seven hours' incubation. The embryo is well advanced in development, for, although the primitive streak, pr, still remains in part and the medullary groove is still open behind, the brain is already marked out and the head has become partly free. Alongside the medullary canal lie eight pairs of segments. Around the embryo one easily recognizes the somewhat pear-shaped area pellucida, A. p, and the darker area opaca, A. 0, by- which it is enclosed. The area vasculosa stands out conspicuously and is bounded by the already distinguishable sinus terminalis, st. Around and underneath is the translucent proamnion, pro. am, from which the mesoderm is altogether absent, and which therefore, cannot contain any blood-vessels. Nor are there at this state any vessels in front of the pro-amnion.


The Main Vessels of the Area Vasculosa

Soon after the capillary network of the areas opaca and pellucida has penetrated the embryo, certain lines of the network begin to widen, and soon distinctly assume the size and functions of main trunks; some of these unite with the posterior venous end of the heart, which has meanwhile been formed in the embryo, and others become connected with the anterior or aortic end; even before this the heart has begun to beat, so that, as soon as all connections are made, the primitive circulation starts up. The arrangement of the vessels is not the same in birds and mammals, although commonly so stated. The disposition in birds is indicated by the diagram shown in figure 44, in which, it should be remembered, the embryo and the capillary network are drawn many times too large in proportion to the area vasculosa. The area is bounded by a broad circular vessel, the sinus terminalis, 5. T., which constitutes a portion of the venous system in birds, for in front of the head of the embryo the sinus leaves a gap, and is reflected back along the sides of the body of the embryo to make two large veins, which, after uniting with the other venous channels coming from various parts of the area vasculosa on each side, enter the embryo as two large trunks, Om. V., known as the omphalo-mesaraic veins; these two veins unite in a median vessel, the sinus venosus, S. V., which runs straight forward and enters the posterior end of the heart. The sinus venosus also receives the veins from the body of the embryo, namely, the jugulars, /w<7.,and cardinals, card.; the former from in front unite each with the cardinal of the same side, making a short transverse trunk ktiown as the ductus Cuvicri, D. C; the two ducts empty into the sinus venosus. The entire venous current is thus brought to the heart in a united stream; it passes out through the aorta, the greater part ascends the aortic arches and passes back as shown in the figure, Ao., and divides at the posterior fork of the aorta, the bulk of the two currents passing out through omphalic arteries, Om. A., and thence to the capillaries of the area vasculosa and so on to the venous trunks again. As shown in the figure, which presents the under side of the area, the left omphalo-mesaraic vein preponderates, and in the latter stages this difference becomes more marked, until finally the right stem is very inconsiderable in comparison with the great left vein. The time at which the disparity commences is extremely variable, as is also the degree of inequality between the two veins.



Fig. 44. — Diagram of the Circulation in a Chick at the End of the Third Day, as seen from the Under (Entodermal) Side. The embryo, with the exception of the heart, is dotted; the veins are black. Ao, Aorta. Arc, Aortic arches. 1 Cardinal vein. D. C, Duct of Cuvier. Hi, Heart. Jug, Jugular vein. Om.A, Oiuphalo-mesaraic or vitelline artery. Om, V, ( Imphalo mesaraic or vitelline vein. .V. 7\ Sinus terminalis. S. V, Sinus venosus.


The following description probably represents what was the primitive condition of vessels in the mammalian area vasculosa. It applies to an early stage in the rabbit, which has been figured by Bischoff , whose figure is copied in Kolliker's " Grundriss." An essentially similar arrangement of the vessels exists also at a corresponding stage in the dog. The veins are much more symmetrical than in the chick, and have the same general plan; the sinus terminalis belongs to the venous system, so that the connection with the arterial circulation, found later, is secondary; the aorta of the embryo is double, and gives off on each side fsegmentally arranged?) transverse branches, one of which develops into the large trunk shown in figure 45 ; the network of small vessels forms two layers, of which the upper is connected with the arteries, the lower with the veins. The change from the earlier condition to the later has still to be followed.



Fig. 45. — Area Vasculosa of a Rabbit, Presumably of about Twelve Days.— (.4/?fr Van Bent-Jen and Julin.)


According to Van Beneden's recent researches on the rabbit, the arrangement of the main vessels in the area vasculosa at a later stage is quite different. The sinus terminalis forms a complete ring (Fig. 45), and is connected with the arterial system by a single trunk, which corresponds to the left omphalic artery of the bird. For some time the connection between the embryonic arteries and the area vasculosa is entirely through capillaries, and the arterial trunk on the vascular area does not appear in the rabbit for several days. There are two veins, one arising from each side of the body and passing out on to the area vasculosa over the back of the embryo; they are the two large upper vessels in the figure.


The Liver

When the omphalo-mesaraic veins, the first large veins to appear, are developed, they are situated in the splanchnopleure and join the heart. They are of such large size as to cause a projection into the coelom. This projection is the septum transversum (p. 82). As shown in the diagram (Fig. 43), the entoderm of the digestive canal of the head of the embryo passes over behind the pericardial cavity and behind the septum transversum into the yolk-sac. Out of the entoderm covering the septum transversum on its caudal side, the anlage of the liver is developed. This anlage is produced by a rapid proliferation of the entodermal cells, and they grow toward the space occupied by the omphalo-mesaraic veins. An intergrowth of the liver cells and of the endothelium of the veins takes place. The cavity of the veins becomes subdivided into smaller blood channels which we call sinusoids to distinguish them from capillary vessels. The liver cells arrange themselves in the form of cords which are termed the hepatic cylinders. Each hepatic cylinder is closely invested by the venous endothelium. The liver consists at first only of hepatic and endothelial cells and is situated in the septum transversum.


When the liver becomes larger, it protrudes from the septum transversum, but does not separate from it, so that in the adult the liver is always found attached to the diaphragm, which is merely the modified septum transversum.


The Oral and Anal Plates

These two structures resemble one another. Each occupies a small area and is formed by the intimate union of the entoderm with the ectoderm. When the union is first formed the two layers are distinct, but they soon fuse, so that no boundary can be recognized between them. I 'ltimately both plates break down.


their cells disappearing, and they are replaced by openings, that of the oral plate forming the opening between the mouth-cavity and the pharynx, that of the anal plate forming the primitive anal opening. The anal plate, before it breaks down, makes a considerable growth, forming an epithelial mass which plays an important part in the anatomical modeling of the region. The oral plate disappears very early; the anal plate much later.


As soon as the head of the embryo has grown so much as to project as an independent part, we find that the oral plate lies on the under surface of the head, a little in front 'of the heart. The pro-amnion, pro. am., arises from the somatopleure enclosing the heart, hi., so that when the oral plate becomes perforate, the cavity of the entoderm, Ent., will communicate directly with the cavity enclosed by the pro-amnion, or, in other words, with the permanent amniotic cavity.


A similar anal plate at the posterior end of the embryo also lies within the amnion (Fig. 46). This figure is taken from a sheep embryo in a very early stage, so that the anal plate appears to lie on the dorsal side. By the curling ventralwards or the bending over of the tail end of the young embryo the anal plate is gradually transferred or rolled over on to the ventral side, where it permanently remains.



46. — Longitudinal Section of the Posterior End of a Sheep Embryo of Sixteen Days.


, Amnion. a. in, Anal membrane (or plate), pr.s, Primitive streak. En, Entoderm. Acli, Archenteron, or entodermal cavity of the embryo. All, Anlage of allantois. met, Mesoderm. — (After Jf. Bonnet. )


The Excretory Organs

No less than three distinct excretory organs are known to occur in vertebrates.


Of these, the first is termed the pronephros, or head kidney, on account of its position toward the head and in the neighborhood of the heart. It is well developed and the only excretory organ in many fishes and in the larval stages of amphibia. In elasmobranchs, which occupy in this respect an exceptional position, and in amniota it exists in a rudimentary form only, except as to its duct, which plays an important role in the further development. The pronephros consists of a few epithelial tubes which take a somewhat twisting course, but may be said to run, in general terms, transversely. Each tube begins with a ciliated funnel-shaped opening (Fig. 47, /) not far from the median line of the embryo, and ends, after a more or less contorted course, in a longitudinal duct, which, after receiving all of the tubules, runs toward the posterior end of the embryo and opens into the extremity of the entodermal or digestive canal. Opposite the funnels, and separate from the pronephros proper, there is a so-called glomus (Fig. 47, gl), which is a projection of not inconsiderable size from the mesentery. When fully developed the glomus contains a rich network of blood-capillaries, so that it somewhat resembles the glomerulus of the kidney. The circulation of the pronephros is sinusoidal.


The second of the excretory organs is termed the mesonephros, Wolffian body, or foetal kidney. It is absent in many fishes, but it is well developed in elasmobranchs. In adult amphibians it replaces the pronephros, which is purely a larval structure. It is present in the embryos of all amniota, but undergoes a partial degeneration before adult life, being itself replaced in adult amniota by the true kidney. The mesonephros resembles somewhat the pronephros, especially as found in the ichthyopsida. It occupies a much larger region of the body than the pronephros. It has no glomus associated with it, but eacli tubule contains a glomerulus very similar in its general structure to the glomerulus of a true kidney. In the ichthyopsida each tubule begins with a ciliated funnel, and, after making several coils, opens into the pronephric duct. The circulation of the^ organ is sinusoidal. In the amniota the mesonephros, or, as it is more eommonly called in these animals, the Wolffian body, is essentially an embryonic structure. Its tubules, however, do not have at any stage the ciliated funnels to be found in amphibia and fishes, but they have glomeruli and they open into the pronephric duct, which, on account of its relations to the organs, is in this type more commonly spoken of as the Wolffian duct. The circulation of the organ is sinusoidal.


Fig. 47.— Frog (Rana temporary) Tadpole of 12.0 mm. Cross-section of the Pronephric Region.


nch, Notochord. m, Muscles. /, Pronephric funnel, v, Blood-vessel. Ec, Ectoderm. /, Pronephric tubule.


I;/, Glomus. L11, Lung. X 9° diams. — (.After M. Furbringer.)


Further details are given in the practical part in connection with the study of the embryo pig and chick.


The third of the excretory organs is termed the metanephros or true kidney. It exists in all adult amniota, but only in them. In development and in structure it differs very much from the other excretory organs. For an account of its origin in mammals, see page 257.


It is essential that the student of embryology should have a clear preliminary notion of these organs, for without such he will be unable to comprehend an important series of embryological phenomena.


The Allantois

The allantois is a diverticulum of the entodermal canal, and is, therefore, lined by entodermal epithelium. It arises on the ventral side of the caudal end of the embryo in proximity to the anal plate. In its development we can distinguish two main types. The first type is illustrated by the sauropsida and the ungulates. In them it grows out and rapidly enlarges so as to form a vesicle of considerable size and connected with the embryo by means of a narrow, hollow stalk. When the allantois develops according to this type, it is spoken of as free, because it has no connection with the extra-embryonic somatopleure (chorion and amnion). This form of the allantois may be readily observed in chicken embryos, for by the fourth day it has become a considerable rounded vesicle which lies in the extra-embryonic coelom between the yolk-sac and the extraembryonic somatopleure or membrana serosa. During the fifth day it rapidly enlarges, and at the beginning of the sixth day is nearly or quite as large as the head of the embryo. In ungulates the growth of the free allantois begins very early and becomes enormous. Its principal expansion is sideways, that is to say, at right angles to the axis of the embryo, and it becomes a large sac, very much larger, indeed, than the entire embryo.


The second type of allantois occurs in the placental mammals of the unguiculate series and is not known to occur in any species of the ungulate type. In probably all unguiculates the posterior end of the body has a prolongation which is known as the body-stalk. Into this body-stalk the diverticulum constituting the allantois extends. The entoderm of the allantois is surrounded by mesoderm , which is present in the body-stalk in considerable volume. On the outer surface there extends a layer of ectoderm, so that the three germ-layers enter into the formation of the body-stalk as they do into the formation of the embryo. These relations are illustrated by the diagram (Fig. 48). By means of the body-stalk a connection is established between the embryo and the extra-embryonic somatopleure or primitive chorion, Cho. Later, when the formation of the amnion is completed, the essential relations are found to be as illustrated by the diagram (Fig. 48, B). The amnion arises from the distal end of the body-stalk, but the body-stalk retains its connection with the chorion. When the allantois becomes free, the connection with the chorion is entirely lost. The maintenance of that primitive connection in the unguiculates is to be regarded as a new modification of the relations of the embryonic appendages, evolved only in the higher animalsThe maintenance of that connection makes possible the modification in the structure of the chorion, which is of the greatest morphological importance. This modification is the development of the blood-vessels in the chorion. The anlages of these blood-vessels are outgrowths of the embryonic angioblast. They appear so as to form four vessels which grow through the length of the body-stalk in the neighborhood of the allantoic diverticulum. Two of these vessels are veins and two are arteries. They are termed the umbilical vessels. The veins at the embryonic end of the body-stalk enter the somatopleure of the embryo, through which they make their way toward the heart. The umbilical arteries, where they join the embryo, are found to unite and join the main aorta, so that they maybe termed the terminal branches of the embryonic aorta. In early stages they are the largest branches which the aorta has. At the distal end of the body-stalk the four vessels enter the mesoderm of the chorion, there branch abundantly, and produce a rich network of blood-vessels throughout the entire membrane. The unguiculate mammals, therefore, are characterized by this special feature, the possession of the body-stalk which contains the allantoic diverticulum and gives access for the blood-vessels, and therefore also, of course, for the blood, to the chorion, which thus becomes vascular. In all other amniota the chorion is without blood-vessels.


Fig. 48. — Diagrams illustrating the Relations of the Allantois in Unguiculate Mammals.


A, Before, Ii, after the formation of the amnion. All, Entodermal allantois. Am, Amnion. b.s, Bodystalk. Cho, Chorion. Ca, Extra-embryonic coelom. Emb, Anterior end of embryo. Yk, YolU-sac.



The size of the allantoic cavity in unguiculates varies considerably. In man it is minimal, forming only a long and very narrow tube (compare page 138). In rodents it expands somewhat, but it never becomes free in the sense that it is separated from the body-stalk, although it may acquire a partial independence. In this case it may also become more or less vascular by the development of branches from the umbilical arteries and veins around the allantois.


In those animals in which the allantois is free, the umbilical arteries and veins have all their branches in the allantois, there being no body-stalk. The embryo is without connection with the chorion, and, therefore, these vessels in their ramifications are restricted to the allantois.


Relations of the Allantois to the Chorion in Ungulates. — Since the true chorion is the outermost of the foetal envelopes, it alone can come in contact with the walls of the uterus. All placental developments, therefore, necessarily depend upon the chorion. Now, in ungulates, where the chorion is without blood-vessels, there is no physiological apparatus to transfer any nutritive material which may be taken up by the chorion from the uterus to the embryo until a second union takes place between the vascularized allantois and the chorion. The inner surface of the chorion and the outer surface of the allantois are both mesodermic. The two mesodermic layers come into contact with one another and unite loosely. The vessels of the allantoic mesoderm are thus brought into physiological union with the chorion, but, being allantoic vessels, they are, of course, morphologically -^ different from the chorionic vessels of unguiculate mammals. These considerations demonstrate that the ungui^uJate placenta is allantoic rather than chorionic, and is, morphologically speaking, essentially different from the true chorionic placenta, which can be developed only in those animals and embryos which have a permanent body-stalk.


The simple relations of the chorion in the ungulata to the uterine wall is illustrated by the accompanying figure 49, which shows a portion of the chorion of a pig embryo of 1 5 mm., together with the surface of the uterus to which it was fitted. The two membranes were accidentally separated in the preparation. The chorion consists of a layer of cylinder epithelial cells, Ec, each of which can be distinctly made out, and of a layer of mesoderm, Mes, containing only few cells and blood-vessels, two of which, Ve, are shown in the section; the mesodermic cells are a little more crowded near the epithelium. Each ectodermal cell is distinctly marked off from its neighbors by a line. The protoplasm stains somewhat, the nuclei are slightly oval and granular, and are situated near the middle of the cells. The top of each cell is concave. The uterine epithelium, L't. Ep, resembles in the general form of its cells and in the character of its protoplasm the chorionic ectoderm, but differs from it in that each cell has a convex free end, and, further, in that the nuclei of the cells are situated near the top of the layer. When the relations of the two epithelia have not been disturbed, it is readily observed that the concavity of each chorionic ectodermal cell receives the convex end of the uterine epithelial cell, so that the two layers are closely fitted together, cell for cell.



The Trophoblast

The trophoblast is the name applied to the special layer of cells developed on the outer surface of the ectoderm of the mammalian blastodermic vesicle. It has as yet beert observed only in unguiculates. The trophoblastic layer may be developed over the entire surface of the ovum, or over only a portion thereof. Its principal known function is to destroy the tissues of the uterus of the mother with which it comes in contact. The destruction of the tissue is supposed to serve two purposes: First, to supply nutrition to the embryo. It is from this supposed function that the layer derives its name of trophoblast. Second, to secure the attachment of the ovum to the wall of the uterus. This preliminary attachment is called the implantation of the ovum. In some cases the trophoblast is developed very early over the surface of the ovum, appearing almost as soon as the stage of the blastodermic vesicle is reached, and while the vesicle is very small. In such cases theovum makes a cavity for itself by dissolving away the epithelium and connective tissue at a small spot on the uterine surface, making a cavity in which it lodges itself. In other cases the trophoblast is developed later and does not appear over the whole of the blastodermic vesicle. The area over which it exists in such cases is called the placental area (compare pages 1 2 1 and 122). The trophoblast in these forms unites very closely indeed with the surface of the uterus, and the uterine tissues undergo degeneration and resorption. We may regard as the first step toward the production of the placenta proper the disappearance of the trophoblast. Our knowledge of its disappearance is incomplete, but it is probable that it is due to a transformation of the cells of the trophoblast, associated with contemporaneous modifications of the chorionic membrane, of which the general result may be said to be formation of the chorionic villi which constitute the foetal portion of the placenta. The modified trophoblastic cells are supposed to enter into the formation of the ectodermal covering of these villi.


Fig. 49. — Pig, 15.0 mm.. Series 135, Section 58, to Show the Relations ok the Chorion to the Uterus.

1,1111, Connective tissue of the uterus. /;V, Chorionic ectoderm. Mts, Chorionic mesoderm. Ut. Ep, Uterine epithelium. Fe, Chorionic blood-vessel. X 35° diams.


The Growth of the Embryo

In all vertebrates there is provision made for the nutrition of the embryo, the development being strictly of the embryonic type. In most cases this provision consists in a sort of yolk material, but in the placental mammals the provision is made by means of the placenta from the uterus of the mother. In either case the embryo has only to assimilate the food which is already more or less prepared for it, and we find that in all vertebrates there is an extremely rapid growth of the embryo. In amniota we have a marked distinction between the embryo proper and its so-called appendages, the yolk-sac, chorion, amnion, and allantois. These appendages are all ultimately sacrificed for the benefit of the embryo, and in mammals, except for a portion of the allantois which is retained within the body of the embryo as the anlage of the bladder, these appendages are ultimately cast off altogether, and take no part in the construction of the child after birth. We note, in fact, as we ascend the vertebrate series, an increasing tendency to give the embryo prominence and differentiate it more decisively from the embryonic appendages. This becomes so marked in the higher vertebrates that we speak of the growth of the embryo almost as a separate thing from the growth of the appendages.


The embryo, when its differentiation commences, lies as a small area upon the surface of the ovum. By the growth of the tissues of this embryonic region, the embryo at once begins to enlarge, and as it enlarges we see that it outstrips the extra-embrvonic structures with which it is associated, and first the head end of the embryo becomes so large as to rise up from the general surface of the ovum and then to project forward. A very little later a similar process occurs at the caudal end, and the whole body of the embryo rises now above the yolk, and the further growth results not only in a greater protrusion of the head forward and of the tail backward, but also of the body sideways, so that now the embryo appears to have a constricted connection with the rest of the ovum. The general character of the process may be readily understood by comparison of the three diagrammatic cross-sections (Figs. 34, 3 1 , A , and 3 1 , B), and also of the three diagrammatic longitudinal sections in figure 43. If we view the longitudinal sections, we see an increasing protuberance of the head and tail ends of the embryo, so that the embryo appears more and more separated from the yolk. The process has long been traditionally described as a folding-in of the germ-layers, but this traditional description is incorrect, for the separation of the embryo is reallv due to the expansion of the embryo, not to the constriction of this connection with the yolk. The diagrams referred to show at a glance how the original width of the communication is retained, while the intestinal canal or embryonic archenteron extends forward and backward. In figure 43, A, the archenteron is open to the yolk throughout its entire extent. In B the head has begun to be free, and with it the archenteric cavity has begun to extend forward and forms a distinct cephalic portion, which is entirely within the embryo and is not open directly to the yolk or, as it would be in mammals, into the entodermal cavity of the blastodermic vesicle. In C the tail also has grown forth from the yolk, and the archenteron with it, so that now we have a caudal embryonic digestive canal. By further development the embryo enlarges more and more, but the opening into the yolk-sac remains nearly the same absolute size.


Fig. 50.— Transverse Sectioh of an Embryo Catfish (Amiurus) ; Series 25, Section 43.


I . \orta. oas.g. Basal ganglion of mid-brain. Ec, Ectoderm, epett, Ependymal layer of mid-brain. //, Cavity of mid-brain. /..Lens. Mi, Meckel's cartilage. N.op, Optic nerve. Op.L, Optic lobe. Per. ea, Pericardial coelom. PA, Pharynx, pig, Pigment layer of the eye. R, Retina. To, Torus. Tin//, Trabecula cranii. x. Undetermined organ. Yk, Yolk. 40-diams.


The relations of the embryo to the yolk in the anamniota are illustrated by the accompanying figure 50, which represents a transverse section through a young stage of the catfish (Amiurus). The section passes through the head of the embryo and shows both eyes and the slender optic nerves, N .op, almost symmetrically cut on both sides. The yolk, Yk, is a large mass heavily laden with yolkgranules. Between the tissues of the embryo proper and of the yolk-sac there is a direct continuity. Not only can the ectoderm, Ec, be followed around from the embryo over the yolk-sac, but also a layer of mesoderm. The part of the volk-sac which carries the yolk-grains is, as above stated, a modification of the entoderm. There is no amnion.


The Umbilical Cord

The umbilical cord may be best defined as the tissues connecting the body proper of the embryo with the amnion. It accordingly includes a portion of the body-stalk and a certain extent of the body-wall or somatopleure. In early stages we can hardly speak of an umbilical cord, because the amnion arises close to the embryo (Fig. 69). As development progresses the body-stalk lengthens out (Fig. 40), and the amnion arising from it recedes further and further from the embryo, this recession being assisted by a growth of the somatopleure which leads to the formation of the umbilical cord, Urn, proper. By this means a tubular structure is produced, the cavity of the tube being a prolongation of the coelom of the embryo. During the first development of the umbilical cord the neck of the yolk-sac, Vi, becomes constricted and very much lengthened out, forming the yolk or vitelline stalk, Vi. s. The yolk-stalk springs within the embryo from the wall of the intestine, runs through the coelom of the umbilical cord, and makes its exit beyond the amnion, as shown in the figure. The yolk-sac proper still occupies its original position between the amnion and chorion. The student should note carefully that the umbilical cord is never covered by the amnion, for it has unfortunately been often stated that it is so covered. An idea of the relations can be gathered from cross-sections (Fig. 51 ). The coelom, Cir, is a large cavity and contains the yolk-stalk, Y, with two blood-vessels, but with its entodermal cavity entirely obliterated. Above the body-cavity is the duct of the allantois, All, lined by entodermal epithelium, and in its neighborhood are two arteries and a single vein. In yet earlier stages there


Fig;. 51. — Sections of Two Human Umbilical Cords.


A, From an embryo of 21 mm. ; B, from an embryo of sixty-four to sixty-nine days. All, Allantois.


h. Umbilical artery. Ca, coelom. v, Umbilical vein. )', Yolk-stalk.


are two veins. The outer surface of the section is bounded by ectoderm. The further development of the cord depends upon the growth of the connective tissue and blood-vessels, the abortion first of the coelom, later of the yolk-stalk, and lastly of the allantoic duct. Remnants of the allantoic epithelium are, however, often found in the umbilical cord even at birth. There occurs also a further differentiation of the connective tissue and of the entoderm.


The umbilical cord is characteristic of mammals. It varies greatly in length. In the pig it is very short and in man it attains great length and size, becoming at full term about 55 cm. in length, and 12 mm. in thickness. When fully developed, it has a whitish color and presents a twisted appearance somewhat like a loop. Its surface is smooth and glistening. The attachment of the cord to the embryo is known as the umbilicus. This attachment to the chorion is in the placental region.


The twisting of the cord is well marked externally at the time of birth by the spiral ridges, within each of which a large blood-vessel runs. The number of spirals varies from 3 to 32, the turns beginning at the embryo, though usually from left to right, but sometimes from right to left. The twisting begins about the middle of the second month. Its cause is unknown, but there is no reason to assume that it is due to revolutions of the embryo. The cord is covered by a layer of epithelium which is continuous at the distal end with the epithelium of the amnion, and at the proximal end with the epidermis of the embryo. It contains typically no capillaries, and, except in the immediate neighborhood of the embryo, no nerve-fibers.



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A Laboratory Text-Book of Embryology: 1. General Conceptions | 2. Early Development of Mammals | 3. Human Embryo | 4. Pig Embryos | 5. Chick Embryos | 6. Blastodermic Vesicle and Ovum Segmentation | 7. Uterus and the Foetal Appendages in Man | 8. Methods | Figures | Second edition | Category:Charles Minot

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