Paper - Description of two young twin embryos with 17-19 paired somites (1915)

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Watt JC. Description of two young twin embryos with 17-19 paired somites. (1915) Contrib. Embryol., Carnegie Inst. Wash. 2: 15-54.

Online Editor Note 
Mark Hill.jpg
This historic 1915 paper by Watt is an early description of the early development of twin human embryos. With 17-19 paired somites makes these twin embryos Carnegie stage 11.

PDF version

Week: 1 2 3 4 5 6 7 8
Carnegie stage: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

Historic Embryology - Twinning 
1915 Twin embryos 17-19 somites | 1916 Conjoined Twins | 1922 Monozygotic origin identical twins | 1927 Separate amnions | 1942 Twin chorion | 1955 Twins and multiple birth
Carnegie Embryos: 8505a 8505b 7170a 7545 9009a and b 9123 5542B 5935A 5621A

Modern Notes: twinning | Week 2

Stage 11 Links: Week 4 | Somitogenesis | Placodes | Lecture - Mesoderm | Lecture - Ectoderm | Lecture - Early Vascular | Science Practical | Carnegie Embryos | Category:Carnegie Stage 11 | Next Stage 12
  Historic Papers: 1908 | 1920 | 1923 somites 20 | 1927 Heart | 1928 somites 17-23 | 1959 stage 11 | 1964 dysraphism

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Pages where the terms "Historic" (textbooks, papers, people, recommendations) 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, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Description of Two Young Twin Embryos with 17-19 Paired Somites

Contributions to Embryology, No. 2.

By James Crawford Watt, B. A., M. B.

Lecturer in Anatomy and in Topographical Anatomy, at the University of Toronto.

With 7 text figures and 4 plates.


The human embryos herein described are Embryos V and VI in the private collection of Professor J. Playfair McMurrich. They were sent from Denver, Colorado, in 1908, by Dr. Mary Hawes, who found them discharged by the uterus of a patient who had called her during a miscarriage. They are specially valuable, as they occur midway in the interval between embryos of 13-14 paired somites and those of 22-23 paired somites described in Keibel and Elze's Normentafel (1908). For the filling in of this large interval there has hitherto been a great scarcity of material, so these embryos are described in detail, to serve as an example of at least one stage in bridging the gap.

The following facts are recorded concerning these embryos. The father was a German Jew, 31 years old, tailor by occupation, and affected by tuberculosis for the last 5 years. The mother was a German Jewess, 30 years old, robust and healthy, and had borne four children previously, all living, the youngest being three years old and delicate in health. Previous to this abortion the mother had experienced no trouble during her pregnancies.

As regards this abortion, the last menstruation began on December 3, 1907, the flow being perfectly normal and lasting three days. Coitus first occurred after this on December 20, and the next menstruation did not appear on the expected date. Three days later, however, on January 3, 1908, there occurred a slight bloody flow, followed by other similar ones on the 11th, 12th, and 13th of the month, the intervals between these discharges being entirely free. On the morning of January 14 there were three severe pains, followed in the afternoon by hemorrhage, and ending in the night by the expulsion of twin sacs from the uterus.

Preservation And Mounting

These, sacs and their contents were, immediately after their extrusion from the uterus. placed in 10 per cent formalin solution, followed by 70, 80, and 95 percent alcohol. The sacs were then cut away, except where the embryos were attached, and measurements and drawings of each embryo were made. Each was then cut transversely into serial sections 10yu thick, which were stained on the slides with hemalum followed by erythrosin. Both embryos, upon examination, appeared very similar in all features, so Embryo VI, which was somewhat the clearer of the two, was selected for the most thorough study, the results being supplemented by the findings in Embryo V.

Methods Of Study

By the aid of the camera lucida each section was carefully drawn at a magnification of 200 diameters and from these drawings, following Born's wax-plate method of reconstruction, a model of the embryo was made. As there were 335 sections in the embryo, each 10 micron thick, the model measured 670 mm. or 27 inches long, each plate being 2 mm. thick. The left side of the model shows the external appearance of the embryo, while on the right side the skin and mesenchyme were not reconstructed, so giving a view of the internal structure.

The model was also left in blocks of varying thickness, so that the structures not seen from the outside might be followed, and it is possible thus to see the main points of the nervous, alimentary, circulatory, and nephrogenic systems, and the mesodermic somites.

For closer study, and also for use in illustrating this paper, special wax models were made of the brain, heart, pharynx, and cloaca. Graphic reconstructions were made of the nephrogenic system from the dorsal, medial, and lateral surfaces, using His's method of plotting points on squared paper, and also a new and very instructive method described by Buist (1913), known as contour reconstruction, in which an element of perspective is introduced. Owing to the nature of this latter method there is, however, a considerable oblique distortion which must be taken into account in interpreting the results.

In addition to the above, the sections were gone over many times under the microscope and the results of all methods have been correlated and incorporated as one in this paper. The supplementary results appended in the description of each part, concerning Embryo V, were all obtained by microscopic study of the sections and comparison of them with sections of the same region in its twin, Embryo VI.

Age Of The Embryos

From the data given above it is difficult to determine accurately the age of these embryos. Counting from the first coition after the last menstrual period, which gives the earliest possible date of conception, December 20, to the last possible day of the embryo's life, January 13, we get 24 days as the greatest age, and if conception were later and death earlier the age will be several days less. The total length of Embryo VI, determined from the serial sections, is 3.35 mm. and by applying Mall's (1903) rule that the age of the embryo in days is equal to the square root of 100 times the length in millimeters, we get 18 days as the result. His (1880) estimates embryos of 3 to 4.5 mm. given length as being 2\ to 3 weeks old, and the ages for most embryos since described are based on these estimates. It will be seen that the age of these embryos agrees with the estimates of His and Mall fairly well.

On the other hand, Eternod (1895) states the age of an early embryo described by him, which measured only 1.3 mm., as 21 days, and he had very definite data upon which to figure. The embryo described here is 3.35 mm. long, and of a degree of development far advanced beyond Eternod's; yet, counting from the coition of December 20, it must be accepted as 24 days or less in age, that is, at the most, only 3 days older than Eternod's. The results are entirely contradictory as they stand, but it is quite possible that Embryo VI is really much older than appears from the above facts, and if so, they will entirely agree. It is a well-known fact that spermatozoa can live for weeks in the Fallopian tubes, which practically act as receptacula seminis, and this embryo may be the product of fertilization of an ovum from about the last menstruation by a spermatozoon from a coitus previous to this period, and which would have been in the tube some days. If fertilization occurred immediately after menstruation the embryo would be 5 weeks old; if during or just before the flow, another half week would be added to the age. It would be quite possible for the ovum to be fertilized just before menstruation and not be lost, as it would probably not leave the tube and enter the uterus before the flow was over. Menstruation during early pregnancy also is not unknown. If 5 weeks be the correct age it corresponds accurately with figures given by Professor McMurrich (1913) in the last edition of "The Development of the Human Body." Taking the ages of a few embryos where very exact information is available, he says the ages of all early embryos have evidently been underestimated; an embryo 2 to 3 mm. in length belongs to the fourth week, one 5 to 6 mm. in length to the end of the fifth, so that one 3 to 4 mm. long should belong to the early part of the fifth week.

In Keibel and Mall's Human Embryology (1912) it is also stated by Mall that the ages of early human embryos have all been underestimated previously by about 10 days, so that adding this amount to the results arrived at by comparing this embryo with His's results, would give almost 5 weeks as its age. The range of variation in the age of this embryo lies thus between 18 days on one hand and 30 to 35 on the other, according to the authority chosen, and it becomes impossible to state which of two coitions is responsible for fertilization, even though they are separated by an interval of 3 weeks and have a menstrual period intervening.

The length of Embryo V throws no light on this question. Indeed, at first glance, it seems to offer a contradiction. This embryo measures 2.75 mm., according to the serial sections, being 0.60 mm. shorter than its twin. It is of exactly the same stage of development, however, and the discrepancy in length is accounted for by the presence of a very deep concave dorsal bend, while the corresponding bend in Embryo VI is very shallow, measurements thus including almost the whole extent of the back in the latter but not in the former. Embryo V is almost identical in development with Embryo VI, but is not actually an identical twin, as the two embryos are the products of two separate ova, as shown by the presence of a separate complete sac for each.

External Appearance

In external appearance Embryo VI resembles most closely embryo 6 (figs. Vr and Vv) in Keibel and Elze's Normentafel (1908). This is embryo Pfannenstiel III, described by Low. There is also a close resemblance shown to figure 4 of His's Normentafeln (1880). The head of the embryo is rounded and small, and shows on each side a slight swelling marking the position of the optic vesicles. Between these is the anterior neuropore, still a fairly wide opening. The head is sharply flexed on the long axis of the boch r , forming a well-marked cephalic bend, and lies immediately in front of the very prominent, bulging heart region. Between these two parts of the body lies a wide slit, which narrows rapidly as it passes inwards and forms the primitive mouth cavity or stomodseum. Between it and the pharynx is a distinct quadrangular membrane, the buccopharyngeal membrane, which is still complete, except for 3 small, circular perforations, the membrane evidently just beginning to rupture. In embryo Pfannenstiel III, of 14 somites, described by Low (1908), it is still complete, and also in embryo Bulle, of 14 somites, described by Kollmann (1890). In the embryo with 23 somites described by Peter Thompson (1907) it has just torn completely, so that this embryo of 18-19 somites, with perforations just beginning, fits exactly into the series.

For two-fifths of its entire length the embryo lies spread out wide open over a large yolk sac, and is much flattened dorsoventrally. The yolk sac, at its junction with the body of the embryo, shows no constriction whatever laterally, but only in front and behind, so that the yolk stalk apparently is only beginning to be differentiated. Over the posterior part of the yolk sac there is a shallow but distinct concave dorsal bend in the body of the embryo, and immediately posterior to this the body is rounded and bulging and exhibits a convex sacral bend as it runs rapidly to its termination in a small, blunt, conical tail. The wide yolk stalk, concave bend, and short, blunt, posterior portion of the body are almost identical in appearance with those of the embryo described by Low (1908). Other similarities are to be seen in an appreciable interval between the yolk stalk and belly stalk, and in the exocoelom being very large and communicating with the endocoelom by a wide gap extending along the side of the embryo, beginning immediately behind the heart and measuring about half the total length of the embryo.

It is interesting to note that up to the 14-somite stage most early embryos exhibit the concave dorsal bend, but beyond this is a gap up to the embryo of 23 somites described by Thompson (1907) and two, of 22 and 23 respectively, described by Van den Broek (1911). None of these latter show the bend, but instead a rounded, convex back. The twin embryos here described occur in the middle of this interval, with 18-19 somites, and both exhibit this concave bend. The flexure is regarded usually as an artifact and is probably produced as follows : The posterior end of the body is round and solid, and firmly attached to the belly stalk. The anterior end is large, round, and heavy with the immense relative size of brain, pharynx, and heart. The middle connecting part of the body between these two heavy ends is fairly thin and therefore weak, and has the yolk sac suspended ventrally and dragging upon it. In handling the embryo for fixing and embedding, there must be dragging of yolk sac and belly stalk upon the body when the embryo is moved, and this pull in one direction, with the heavy ends sagging in the opposite, practically breaks the back, bending it severely. This appearance is of value, then, only as an indication of the strength of the middle region of the back, disappearing when this part of the body has become sufficiently strong, and because of the relative diminution in size of the umbilical opening and the passage of the amnion out around the whole umbilical cord, binding belly stalk and yolk stalk together and thus strengthening the middle region of the body.

Coming from the ventral surface of the body, just behind the yolk sac, is the large belly stalk, flattened on its dorsal surface but rounded on the others. It has a course of 1 millimeter from embryo to chorion, being thus about one-third the length of the embryo. The embryo lies somewhat twisted upon its long axis, the head to the right over the heart, the tail to the left over the belly stalk. The side to which the tajl turns in the spiral seems to be variable, being to the left here and in embryos described by Wallin (1913), Low (1908), and Mrs. Gage (1905), but to the right in cases recorded by Bremer (1906), Van den Broek (1911), and Ingalls (1907). Mall (1891) describes a case going to the left and says that the bend to the right is more usual.

The heart forms a very large, prominent bulging occupying the whole ventral body wall between yolk sac and stomoda?um. It agrees with that of the Peter Thompson and Van den Broek embryos in that it projects much farther forward on the right side than on the left. It is interesting to note that in the Ingalls (1907) embryo, 4.9 mm. in length, the heart lies mostly to the left. The position of the chambers of the heart is indicated externally by shallow grooves on the surface, bounding gently rounded prominences over-lying the chambers.

Starting at the sides of the stomodseum, and passing on each side caudally, is a well-marked groove, or sulcus, which marks off the heart region ventrally from the rest of the body lying dorsal to it. This sulcus extends back to meet the line of reflection of the amnion, where it is lost. Just caudad, and also somewhat dorsal to the stomodseum, is a shallow groove, or depression, running dorsoventrally, its lower end meeting the sulcus bounding the heart. This marks the position of the first gill cleft. A short distance directly caudad, and lying parallel to the first, is the depression of the second gill cleft, about equal in extent but shallower than the first. It also reaches ventrally the limiting sulcus of the heart. Immediately dorsal to the second gill cleft lies the otocyst, a deep, oval depression of the ectoderm, still opening widely to the outside. In Low's embryo, with 13-14 somites, 3 gill clefts are visible externally, while in this embryo, with 18-19 somites, there are only 2. Only 2 are to be seen externally in Janosik's ( 1887) embryo of 3 mm. and Wallin's (1913) of 2.3 mm. Again, in the 2 described by Van den Broek, with 22 and 23, and 1 by Thompson with 23 somites, there are 3 clefts, the same number as in Low's embryo, which thus seems to show a precocious formation of the third cleft. For if 3 were normally present at the 14-somite stage, at a stage with 9 somites increase we would expect the appearance of a fourth cleft, which is not found until embryos are reached such as Bremer's (4 mm. in length), Mrs. Gage's (4.3 mm. long with 28 somites), or Ingalls's (4.9 mm. long with 35 somites).

As in all embryos described in the early stages, this embryo does not show externally the position of the anterior mesodermic somites, but in the region of the concave dorsal bend and the rump they are indicated by noticeable protuberances.

There is no indication whatever of the limbs.

The nervous system is still open to the exterior in two places, the anterior and posterior neuropores. The anterior neuropore is a wide opening situated in a shallow depression at the extreme anterior aspect of the flexed head, and lies directly between the optic vesicles. The posterior neuropore begins in the rump, where the roof of the neural canal ends and the canal opens as a deep gutter, whose walls gradually decrease until the gutter form is lost and it forms only a flattened medullary plate extending out on the tail.

It will be seen, from the above description, that this embryo, while situated exactly half way between two well-known stages of development, resembles embryos younger than itself very much more than older ones, and although much greater in size, in many respects does not seem to be any further developed than the younger ones. The main differences from the above description of Embryo VI, which are shown by Embryo V, are in the possession by the latter of a very deep concave bend in the back, a yolk stalk that is definitely constricted at the sides as well as before and behind, and in the fact that the long axis of the embryo practically forms a straight line and not a spiral. The belly stalk passes directly backward under the tail.

The Amnion

There is a well-developed, completely closed amniotic cavity. The amnion itself is formed of two closely applied but quite distinct layers of flattened cells, the nuclei appearing at scattered intervals and forming swellings in the cell layer where they lie. The amniotic cavity is fairly extensive and an idea of it can best be given by describing the line of reflection of the amnion from the body of the embryo. The whole of the head and the anterior half of the, heart region are placed entirely within the cavity, the most anterior point of the origin of the amnion being about half way back on the ventral surface of the bulging heart region, in a line convex forward. From here it inclines obliquely upward on each side of the heart, until it crosses the septum transversum, where it immediately changes direction and runs directly caudad on the lateral body wall. The body in this region is so flattened dor so vent rally that only the upper surface is in the amniotic cavity. On reaching the region of the origin of the belly stalk, the line of reflection again crosses the rounded body wall obliquely, this time toward the ventral surface, and passes off on the belly stalk, from the borders of the flattened dorsal surface of which it is reflected right out to the chorion. The whole of that portion of the embryo posterior to the belly stalk thus projects, like the head, into the amniotic cavity, and a narrow, tapering, or funnel-shaped prolongation of the cavity is continued over the upper surface of the belly stalk, its apex being in contact with the chorion.

The above description shows this reflection of the amnion in this embryo to be essentially similar to that given by Low for embryo Pfannenstiel III. The embryo described by Thompson (1907) shows the nearest stage described in advance of this. Here the whole heart region projects into the cavity, the amnion being reflected from the ventral body wall on the line of the septum transversum. The amnion is also shown beginning to pass out over the yolk stalk, so that the large aperture of the umbilical vesicle of previous stages here begins to be constricted. Posteriorly the amnion only covers the narrow, dorsal aspect of the belly stalk, and still exhibits the narrow, conical prolongation of the cavity.

There are no differences to be noted regarding the amnion of Embryo V.

The Chorion

The chorion agrees in every respect with the description given of other embryos. It is from 0.09 to 0.18 mm. thick and the villi arising from it are 0.17 to 0.25 mm. in diameter and 1.0 mm. to 1.6 mm. in length. These measurements are not very much greater than those given by Dandy (1910) for an embryo about 2 mm. long, with only 7 pairs of somites. As most of the chorion had been removed before the embryo was received, it can not be stated whether the whole sac was equally covered by villi, or not,

The inner surface of the chorion and the interior of the villi are formed of loose mesenchyme consisting of branching spindle and stellate cells. The nuclei of the cells are oval and vesicular, the protoplasm stains very lightly and forms only a thin covering over the nucleus and the numerous fine branching processes. The intercellular tissue spaces are large, and contain a clear jelly-like substance. Numerous large blood-vessels are found everywhere in this tissue in the chorion and its villi.

The outer surface of the chorion and villi is formed of two epithelial layers of cells. The inner layer, the Langhans cells, lying next the mesenchyme, have moderately staining nuclei and protoplasm. In sections vertical to the plane of the cell layer, the cells appear cubical in form and are very little deeper than is just sufficient to contain the large round vesicular nuclei. Cell boundaries are evident and distinct, In sections parallel to the surface of this layer the cells appear large and polygonal in outline, the majority being 5-sided. The outer layer is a syncytium formed of a densely stained, granular protoplasm containing deeply stained, flattened, oval nuclei which lie with their long axis parallel to the surface of the layer. This layer is thinner than the Langhans layer and contains fewer nuclei, and I find, contrary to Dandy (1910), that the nuclei of the Langhans cells are much larger than the nuclei of the syncytium.

Integument and Epithelial Thickenings

The ectoderm is nowhere separated from the nervous system except over that part of the brain tube between the anterior neuropore and the buccopharyngeal membrane. At the anterior and posterior neuropores it is fused with the wall of the nervous system, and everywhere else is connected by the neural crest to the mid-dorsal line of the neural tube. The ectoderm is in general a 1-celled layer, but exhibits thickenings in certain definite regions. In the dorsal portions of the body it forms a single layer of flattened cells and presents a similar appearance ventrally over the heart region and at the sides of the body close to the line of attachment of the amnion. Over the mesodermic somites its cells are cubical, and this condition is maintained even in front of the somites, far up into the head. Over the gill arches the ectoderm is much thickened and is 2-layered. These conditions are exactly similar to those described and illustrated by Ingalls (1907) for an embryo 4.9 mm. long. He found thickenings over 4 arches, 4 gill pouches being present in his embryo; only 2 gill pouches are to be found here, yet thickenings are present over 3 arches. There is as yet no trace whatever of the formation of the lens of the eye or of the olfactory placode. In the condition of the cloacal membrane this embryo agrees with that described by Mrs. Gage (1905), who found the anal plate thickened in an embryo of 28 somites. Ingalls found it very thin and in places almost unrecognizable in one of 35 somites. In the present embryo, at the 18-19 somite stage, it is quite thickened and differs in appearance from the adjacent ectoderm. The endoderm, as in the case of the two embryos cited above, is in contact with and apparently fused to the ectoderm, and is also thickened.

Mesodermic Somites

These embryos stand mid-way between stages 5 and 6 in Keibel and Elze's Normentafel, where the embryos possess 13-14 somites, and stages 7 and 8, of 23 paired somites. Embryo VI possesses 18-19 paired somites. There are 18 fully formed and distinct pairs of somites and a nineteenth pair is well formed but not yet cut off from the unsegmented mesoderm behind (text fig. 3). The first somite lies at the level of the posterior portion of the heart. It is a peculiar coincidence that the last somite occurs exactly at the level of the opening of the medullary canal into the medullary groove at the posterior neuropore. This also is the exact level of the fusion posteriorly of the somatopleure and splanchnopleure, cutting off ventral communication between the endocoelom and the exocoelom.

The general shape of the somites is that of a triangular prism with rounded anterior and posterior ends, and a ventral, a medial, and a dorsolateral wall. The more posterior somites are more quadrangular than prismatic. The fourth, fifth, and sixth somites lie somewhat obliquely to the long axis of the body, and between their posterior ends and the medullary canal the anterior ends of the succeeding somites are inserted. The eleventh to fifteenth pairs of somites inclusive lie in the region of the concave dorsal bend and the posterior portion of the dorsolateral surface of each of these is overlapped by the anterior end of the next.

In describing the various parts of the somite, dermatome is the term used to denote the flat plate forming the whole dorsolateral surface, myotome denotes the dorsal half of the medial surface, and sclerotome the lower half of the medial surface, the whole of the ventral surface, and the core of the somite.

The first somite is very small. Its ventromedial portion is of somewhat looser, more irregular tissue than the rest of the somite and suggests the beginning differentiation of the sclerotome. There is" a distinct solid core fused with this and almost obliterating the cavity of the somite, which appears only as a narrow slit. The dermatome and myotome are differentiated from each other only by the presence of the upper myotomic groove. The somite lies surrounded on all sides by dense mesenchyme with which it fuses anteriorly, so that there is no distinct anterior end to it. This fusion of mesenchyme and first somite is exactly similar to the condition described for the chick by Williams (1910), who found in a series of embryos that this somite is the first formed both in time and position, but develops extremely slowly compared to the others. The second somite shows the greatest degree of development of the whole series and each succeeding somite shows a retrogressive decrease in the degree to which it has attained. Each somite shows not only differences in degree of development but also individual differences in shape and appearance, so that no two exactly correspond. The second somite in this embryo shows almost exactly the same stage of development as the corresponding somite of a chick embryo of the 18-somite stage described by Williams. The dermatome is a flattened plate underlying the ectoderm and formed of the whole dorsolateral wall. The myotome is flexed under the dermatome medially and extends out about half the extent of the dermatome laterally, lying in contact with the latter and including between itself and the latter a small cut-off portion of the general cavity of the somite in the dorsomedial angle.

There is a deep cleft, named by Williams the lower myotomic groove, separating the myotome from the sclerotome below it. The sclerotome is formed of a mass of looser cells forming the ventromedial and ventral part of the wall of the somite, and from the ventral surface of this two long, curved, plate-like processes pass off. The inner one, the notochordal process, passes medially between the medullary canal and the dorsal aorta to the side of the notochord. The lateral one, the aortic process, passes ventrally around the lateral surface of the aorta toward the wall of the pharynx, its ventral end being indistinct and hard to differentiate from the surrounding mesoderm. The notochordal process arises from the whole extent of the ventral surface of the sclerotome, but the aortic process comes only from the anterior two-thirds, and there is a distinct interval between it and the next succeeding aortic process, except in the case of the second and third somites, in which these processes are fused. The cavity of this somite is distinct and irregular and is not very large. The core is fused with the sclerotome and is indistinct.

File:Watt1915 textfig01-02.jpg

Fig. 1. Transverse section through the middle of the fifth mesodermic somite on the right side in Embryo VI. Drawn with camera lucida. Magnification X 270. Fig. 2. Transverse section through the middle of the twelfth mesodermic somite on the right side in Embryo VI. Drawn with camera lucida. Magnification X 270. For list of abbreviations see page 40.

The appearance of each succeeding somite differs somewhat from the description given above for the second, the main difference being that proceeding caudad there is a retrogression in the amount of differentiation of the parts of the somite. The size of the somitic cavity increases greatly and is very large from the eighth to sixteenth somites and then decreases rapidly and is merely a slit in the nineteenth. There is apparently connection of some of these cavities with the coelom.

Only the myotomes of the second to fifth somites (text fig. 1) inclusive are flexed sufficiently to lie in contact with the under surface of the dermatome and so cut off a small portion of the somitic cavity. Proceeding caudally, they become less and less strongly flexed and the groove on the inner surface at the dorsal angle of the somite, named by Williams the upper myotomic groove, becomes more and more a rounded angle (text fig. 2), until beyond the thirteenth somite it is impossible to state that it is present. The lower myotomic groove is distinct as far back as the thirteenth segment, but after that it is hardly recognizable, although the distinction between myotome and sclerotome is clearly shown by the character and arrangement of the cells and by a slight chink, or split, in the wall at the point where this groove will develop.

The sclerotome is present in every somite, though in the first and last it is only distinguishable from the rest of the somite by the slightly looser arrangement of the tissue and more irregular outline. These two somites also are the only two possessing a well-marked core, which in each case fills up and almost obliterates the somitic cavity. There is evidence of a core in the other somites, but it has evidently fused with the sclerotome and passed ventrally with the latter, largely losing its identity.

The notochordal process of the sclerotome (text figs. 1 and 2) is present in all segment s from the second to the sixteenth, and in this series shows a marked decrease in development in each segment proceeding caudad. It comes off the whole ventral extent of the sclerotome and curves inward under the medullary canal to the notochord, but nowhere does it surround the latter, merely coming laterally into contact with it. As far back as the seventh segment the notochordal processes have fused and obliterated the lower portion of the intersegmental clefts originally present between them. These clefts still persist between every two processes posterior to this point.

The aortic process is present in each sclerotome (text fig. 1) as far back as the thirteenth, where it finally disappears. No two of these fuse or come into contact except in somites 2 and 3, and they are separated by intervals considerably larger than the inter-segmental clefts, as they are developed, not from the whole ventral extent of the sclerotome, but only from the anterior three-fourths to two-thirds of this extent. Beyond the the teenth segment there is no evidence whatever of the aortic process, which is thus a slightly later development of the sclerotome than is the notochordal process.

From the ninth segment to the end, the nephrogenic tissue, the intermediate cell mass, is connected to the lateral angles of the somites. In some instances it appears as if separation were just commencing and the wall of the somite is broken at this point and the cavit y is open. The attachment to the somite is on a line between dermatome and sclerotome (text fig. 2).

There is an intersegmental cleft between every two somites, but none cutting off the nineteenth posteriorly. This nineteenth somite has a dennomyotome, a core, a sclerotome, and a small cavity, and is almost the equal in size of the somite just ahead of it, but is still fused posteriorly with the unsegmented mesoderm. In each intersegmental cleft is a small amount of loose mesenchyme, which is also seen burrowing around the anterior and posterior rounded ends of a few of the most anterior somites, working between them and the ectoderm. In these somites mesenchyme is also burrowing in from the lateral edge between the dermatome and the ectoderm. No mesenchyme is found between the medullary canal and any somite (except the first, which is incomplete and fused with mesoderm anteriorly), and each somite lies in contact medially with the wall of the medullary canal. Also, no mesenchyme is found, except as stated above, between the dermatomes and the ectoderm.

The dermatomes and myotomes are epithelial in nature. The cell nuclei are huge. round, and vesicular, and in the dermatomes are crowded toward the wall of the cavity of the somite, leaving only the cell bodies to form the peripheral portion of the layer. The ends of the cell bodies anastomose freely immediately beneath the ectoderm, but no external limiting membrane is to be distinguished, such as Bardeen (1900) has described in the pig. Williams (1910) in the chick finds the dermatomes exactly similar to the above and is also unable to demonstrate any external limiting membrane. Regarding Bardeen's contention that the dermatome does not form the cutis layer of the skin and really gives rise to muscle tissue only, as he found in the pig, little light can be obtained here. It is significant, however, and partly confirmatory of Bardeen's results, that in the most advanced segments the dermatomes are beginning to recede from contact with the ectoderm and loose mesenchyme in finding its way between the two at the edges of the dermatome. It is to be expected that this loose mesenchymatous tissue would develop in situ into the cutis layer later on, and, if so, the so-called dermatome would be really all myotome.

File:Watt1915 textfig03-04.jpg

Fig 3. Graphic reconstruction to show the position and relations of the mesodermic somites and the nephrogenic system in Embryo VI. Fig 4. Graphic reconstruction to show the position and relations of the mesodermic somites and nephrogenic system in Embryo V. The numbers refer to the mesodermic somites. For abbreviations sec page Hi.

The undifferentiated mesoderm back of the somites is very solid and condensed, and lies in contact with the medullary canal and the ectoderm, no loose tissue whatever intervening. The intermediate cell mass at first is cut off from it, forming the nephrogenic cord, but very soon is seen to be fused with the somitic plate on one hand and the splanchnic and somatic mesoderm on the other. This common mass of mesoderm passes into the tail to fuse with the primitive streak.

For some distance caudad of the last somite, a peculiar narrow process passes off ventrally from the undifferentiated mesoderm on each side and, running between the dorsal aorta and the medullary groove, comes into contact with the notochord. It is very suggestive of a precocious formation of the notochordal process of the sclerotome in somites yet to be differentiated, and is perfectly similar in shape and position with the aotochordal processes already formed.

Embryo V is almost identical with Embryo VI. There are 17 pairs of somites, complete and distinct, and an eighteenth pair is to be seen forming in the undifferentiated mesoderm. The aortic processes of the second, third, and fourth somites, and the notochordal processes also of the same, have fused into common masses. Back of this, all processes remain separate from those preceding and succeeding. There is thus fusion of one more aortic process, but of three less notochordal processes than in its twin. The aortic processes end in the twelfth segment, there being thus one less than in Embryo VI.

The Notochord

The notochord extends continuously from just behind the hypophysis cerebri to the caudal region (plate 1) and is clearly distinguishable throughout the whole course, although varying greatly in its degree of development in different regions. It will be necessary, in order to obtain a proper conception of it, to trace its course and describe its appearance in the various regions.

The notochord first appears immediately posterior to the hypophysis and is here formed by a heaping up of cells around a median groove on the dorsal wall of the foregut. Bremer, Broman, Van den Broek, and Ingalls all describe it as beginning immediately behind the hypophysis, just as is found here. Peter Thompson places it at the flexure between midbrain and forebrain, and Janosik just under the midbrain. Evidently back of the hypophysis is the normal point. In this embryo this point is the extreme cephalic end of the dorsal wall of the gut (plate 2, fig. 1). The groove disappears by the time the region of the first gill arch is reached (plate 2, fig. 2), but the heaping up of cells persists and is somewhat higher and narrower than anteriorly. Over the region of the second gill cleft the median groove again appears in the roof of the pharynx and the notochord is here seen in its most rudimentary condition in this embryo (plate 2, fig. 3), being formed merely of a flattened double layer of endodermal cells lining the groove. Mrs. Gage ( 1906 1 has shown that long after the notochord has separated from the endoderm elsewhere it retains connection with it in this region, both in man and the pig, and Huber (1912) has proved that it is in this region that the pharyngeal bursa is developed, due to the evagination of the wall of the pharynx by the pull of the notochord attached here. Neither of the above described the condition in embryos younger than stages showing complete formation of the rod-like notochord, but it is interesting to note that although the notochord is nut yet separated from the endoderm in any of this anterior region, yet this point, corresponding to the region where the pharyngeal bursa later appears, can be determined as the place of the very least development of the notochord, and therefore the place where separation will last occur, so giving the conditions described by Huber and Mrs. Gage. Proceeding backward, the notochord becomes thicker, more heaped up, and gradually assumes a spherical form on section, and forms a round rod (plate 2, fig. 4) merely lying in contact with the endoderm. Over the anterior portion of the yolk sac this rod begins to separate from the endoderm and for a short interval mesoderm passes between the two. and then the two dorsal aortse, which have been rapidly approaching each other, fuse immediately over the gut and directly beneath the notochord (plate 2, fig. 5). Over the posterior portion of the yolk sac the aorta again becomes paired and the notochord immediately sinks to come into contact with the endoderm, with which it is connected by a narrow neck of cells (plate 2, fig. 6). It is in this region that the notochord attains its largest diameter.

In the dorsal wall of the hindgut a median longitudinal groove now develops along the line of attachment of the neck of cells from the notochord and coincidently with the deepening of this groove the notochord loses its spherical condition, becomes elongated dorsoventrally and compressed laterally into a flask shape (plate 2, figs. 7 and 8), and then passes rapidly into a condition similar to that at the anterior end, forming a heap of cells surrounding the groove. In descriptions of embryos in these early stages, I have found no mention of any condition of delayed development such as this. The rod-like form, once attained behind the pharyngeal region, has been retained to the caudal termination. This condition is small in extent, however, and by gradual transition the notochord is again found spherical in section (plate 2, fig. 9) at the level of origin of the allantois and from this point back to its termination it forms a round rod, separated from contact with the endoderm by the second fusion of the two dorsal aortae between it and the gut. It ends abruptly in the tail (plate 2, fig. 11) just beyond the termination of the dorsal aorta, at the same level as the end of the postanal gut, and here lies in contact with the medullary plate and widely separated by mesoderm from the postanal gut.

The notochord was found by Low (1908) not to extend as far caudad in embryo Pfannenstiel III as the gut; it did not pass into the tail, but ended over the cloaca. All other authors unite in stating that it passes into the tail, to fuse there with the tissues of the primitive streak — exactly what has been found in this case.

From the foregoing description it will be noticed that the stage of least development of the notochord is exhibited over the anterior part of the pharynx, the next stage over the hindgut, while the greatest is over the posterior portion of the yolk sac. The complete separation of the notochord from the endoderm is evident only in two regions, the first over the posterior two-thirds of the yolk sac, the second over the whole of the cloaca and postanal gut. In each case the two dorsal aortae have fused between notochord and alimentary canal. Mesenchyme only passes under the notochord in one place, namely, just in front of the first fusion of the dorsal aortae. The notochord lies in direct contact with the under surface of the nervous system throughout its whole course, with the exception of two very small intervals. The first is at the extreme anterior end, where a small amount of mesenchyme separates it from the floor of the brain. The second is where the cells of the first notochordal process are just beginning to burrow in around the notochord. All other notochordal processes of the sclerotomes come in contact only with the sides of the notochord.

In the cylindrical portions of the notochord the cells are large, fairly clear, and are arranged as a concentric layer with their nuclei near the periphery. The nuclei are large, round, and vesicular. A thin but definite cuticular membrane surrounds the notochord, except in those parts where it has still the form of an endodermal plate and is not yet cylindrical. Low also found no cuticular membrane in his embryo of 13-14 somites, in which the notochord was altogether in the form of a plate, except at the posterior end, where it was free.

In the embryos described by Van den Broek (1911) there is a cuticular membrane. These embryos are of 21 and 22 paired somites and the only place where the notochord is not free from the endoderm is over the pharynx. The absence of a membrane in this region is not mentioned. The embryo described by me is midway between these two and, as shown, the membrane is only present in the cylindrical portions.

I find confirmation, in this embryo, of the lumen found in the notoehord by His (1880) in embryo L u 2.4 mm. long, and demonstrated by Eternod (1899) in three embryos of lengths 1.3 mm., 2.11 mm., and the third slightly larger but length unknown. In several places (plate 2, figs. 1, 4, and 9) there are distinct spaces in the very center of the cylindrical portions of the notoehord. They are not very large, and do not extend over more than :i few consecutive sections in any region, but they are perfectly distinct and appear like isolated parts of a lumen not yet completely occluded. In other regions the cells extend right in to the center, occluding the lumen, and frequently one or two extra cells are found in the center of the notoehord. No recent investigators seem to have found evidence of this lumen except Grosser (1913), in the case of a very young embryo where the canal is very distinct. Some authors definitely state, indeed, that it is entirely absent, even in embryos not nearly as large as this one; such, for instance, being the case in the embryo of only 8 paired somites described by Dandy (1910).

At its posterior free rounded extremity (plate 2, fig. 11) the notoehord is not of exactly the same structure as farther forward. Over the cloaca the radial arrangement of cells with a single peripheral layer of nuclei is lost. The cells become much more crowded and are irregular in disposition, with nuclei lying everywhere closely packed. This resembles very much the illustration of the posterior end of the notoehord given by Low (1908). It ends by fusing with the tissues of the primitive streak, 2 sections later than the fusion of the postanal gut and 3 sections previous to the fusion of the medullary groove with the primitive streak. It here lies immediately under the medullary groove, but quite independent of it. This posterior portion has been described as ending in three different ways: First, by Low, it is said to end freely midway between medullary tube and cloaca, embedded in the mesoderm. Second, as exemplified by Ingalls and by Thompson, it runs in contact with the under surface of the medullary tube, but fuses posteriorly in the tail with the solid mass of mesoderm of the primitive streak. Third, as described by Bremer and Janosik, it fuses in the tail with the under surface of the nervous system, and this in turn with the primitive streak. The ending described by Low seems to be true of embryos up to the 13-somite stage, as two of 7-8 somites confirm this, one described by Dandy and one by Eternod (2.11 mm.), each having the notoehord, as shown in the illustrations, not extending so far caudad as the gut. The second plan described is the normal one, as it is confirmed by the great majority of those who have described young embryos. Fusion with the nervous system into a common mass is really a fusion of each of these structures at a common level with the primitive streak.

Eternod (1899) is practically the only author who mentions the presence of a neurenteric canal in embryos of 2 mm. or more. In many descriptions of early embryos it is not mentioned, and it may therefore be inferred that it was not present, and Kollmann ( 1890) remarks positively, in an article on the notoehord, that it had entirely disappeared in an embryo of 14 paired somites. Eternod found it well developed in an embryo of 1.3 mm., another of 2.11 mm., and says that in a third (larger again, but measurements omitted) appearances were practically the same as in the 2.11 mm. embryo. This last embryo had 2 gill clefts developed. Reckoning thus, Embryo VI, here described, is of the same stage of development. It is possessed of 18-19 segments, a distinct advance over the one mentioned by Kollmann, and yet it shows distinctly the neurenteric canal. This is, as far as I can ascertain, the oldest embryo in which this canal has yet been demonstrated. The location of this canal (plate 2, fig. 10) is in 4 sections immediately succeeding the caudal edge of the cloaca! membrane, and it is represented by a solid rod-like connection between the upper surface of the notochord and the under surface of the medullary groove in the median plane. There is now no lumen either in this neurenteric canal or in this portion of the notochord, it having been obliterated by the crowded, densely packed cells forming these structures. There is no doubt of this representing the neurenteric canal, as the structure is perfectly evident and distinct, and there is no solid tissue adjoining, with which it might be confused. The notochord extends caudad of the neurenteric canal for a distance of 8 sect ions (80m) before fusing with the primitive streak. This is in accord with the findings of Eternod, for the notochord in his embryo of 2.1 1 mm. also passed caudad to the canal. In his case the only difference was the possession of a lumen by the structures, and thus of communication with the underlying gut. No communication with the gut occurs in Embryo VI, owing to the presence of the dorsal aorta between it and the notochord.

The close correspondence of Embryo V to its twin is nowhere better exemplified than in the development of the notochord. The description given above for Embryo VI will serve accurately for the account of the stages seen in Embryo V. It agrees in having the same extent, starting and ending at the same points as in its twin, in having fusions of the same degree with the endoderm and in exactly the same regions, both over the pharynx and over the hindgut. The main difference is that the dorsal aortae are smaller and the areas of their fusions, one over the yolk sac, the other over the cloaca, are less in extent, so that not so long a stretch of the notochord is separated from contact with the endoderm. There is not as distinct an appearance of a lumen in this case, although there is a central space found twice, for two sections in each case, one just behind the pharyngeal region, one where the notochord becomes free from the gut over the cloaca. The neurenteric canal is equally as distinct here (plate 2, fig. 12) as in the other embryo and occurs at exactly the same level. In this case the dorsal aorta ends over the cloaca just before this level is reached and the notochord is very near the gut, and a solid cord of cells connects the notochord with the endoderm, exactly in line with the solid cord connecting the notochord with Hi,, medullary groove. The presence of such a distinct neurenteric canal in both of these embryos is a point of especial interest. The notochord ends by fusing with the primitive streak independently of either the nervous system or postanal gut.

Alimentary System

The stomodseum here is merely the cleft (plate I) between the head and the heart region. It IS fairly wide and shallow and is closed off from the pharynx by the buccopharyngeal membrane. This membrane is perforated by three small apertures, marking the beginning of the rupture and disappearance of the structure. In Low's (DOS) embryo of 13 11 somites, which is the nearest younger to this, the buccopharyngeal membrane is complete, while in the nearest older, Van den Brock's ( I'll 1) of 22 somites, only the remains of the membrane arc to be found. In the two embryos last mentioned there is a distinct pouch of Rathke found, but no signs of this pouch are to be distinguished in Embryo VI, unless a small Ihickc 1 portion of the ectoderm of the roof of the stomodseum, just in front of the buccopharyngeal membrane, be interpreted as such.

The pharynx (plate )i, figs. 1 and 2), just back of the pharyngeal membrane, is prismatic, appearing triangular on section, with the apex forming the notochordal groove dorsally. No pouch Of Seessel is recognizable. As the branchial region is reached it widens nut considerably and becomes flattened dorsoventrally. The gill pouches, .'* in number on each side, open widely off the cavity of the pharynx, there being practically no constriction of the openings except a slight one for the first. The first pouch is very large and runs laterally and dorsally out to come into contact with the ectoderm, with which it is fused in two areas for a very short distance. The cephalocaudal diameter of the pouch is much greater than the dorsoventral. and its cavity is wide and capacious, hut as yet undivided into dorsal and ventral portions. There is a very peculiar condition found here, however, for just before the lateral edge of the pouch is reached, a narrow villus-like ridge, filled with mesoderm, rises from the ventral wall of the pouch into the cavity, cutting off a narrow gutter in the extreme lateral part of the pouch. From the appearance of the sections this ridge is evidently not due to an infolding of the wall as the result of sectioning or mounting.

File:Watt1915 textfig06-07.jpg

6. Diagram of nephrogenic system of Embryo VI. 7a. Diagram of nephrogenic system of Embryo V. Tho numbers in text d^ures 5 and indicate the level of the somites. 7 A, lamera-lucida outline of the alimentary canal in Embryo VI. just cephalad of th. 130.

7b. lamera-lucida outline of the cloaca in Embryo VI. at the level of the cloacal membrane. Magnification X 120. 7c. (.'amera-lucida outline of the cloaca in Embryo V. at the level of the cloacal membrane. Magnification x 120. For list of abbreviations see page 40.

The second pouch is not nearly so large as the first, but also reaches the ectoderm, with which it is fused over two small area-, with a narrow interval between. This pouch is wide for some distance as it leaves the pharynx, but only the most dorsal part reaches the ectoderm, appearing as a tapering, conical process of the larger portion. The third pouch is differentiated in front from the second by the presence oi the intervening branchial arch, but can not be delimited behind, where it merges into the pharynx. This pouch does not extend far laterally, and so does not reach the ectoderm. It is somewhat pointed, as was the case with the second. Wallin (1913] describes only "J pairs oi gill pouches in an embryo of 13 somites, but Low describes 4 in one of 14 somites, while here only 3 are present. Again, Van den Broek and Thompson, in embryos of 22 and 23 somites, find only 4, the fourth being small, so that evidently the fourth in Low's embryo is precocious in its appearance.

The median thyreoid anlage (plate 1 and plate 3, fig. 2) is present in this embryo and is very similar to that described by Low, being a rather wide shallow depression in the median line ventrally between the first and second gill pouches. In front of this is the large elevation of the floor of the pharynx caused by the underlying ventral aorta as it turns forward and divides into the paired aortae. Wallin also found the median thyreoid anlage just caudad of the ventral aorta.

The relation of the notochord to the pharynx has been already fully described.

In the young embryos both older and younger than Embryo VI, the alimentary canal caudal to the pharynx becomes laterally compressed, forming a narrow dorsoventral passage giving rise to esophagus and stomach. This is not so in this embryo, where the gut slightly narrows laterally from the third gill pouch backward, still retaining a considerable lateral diameter, but showing the remarkable condition of being very markedly compressed dorsoventrally (text fig. 7 a), so that the lumen is almost slit-like. Over the sinus venosus this condition changes, as the gut begins to widen dorsoventrally again, and then it opens rapidly into communication with the yolk sac. On the ventral wall as it slopes toward the yolk sac, just behind the sinus venosus, a shallow depression runs forward, forming the liver bay, which is in about the same stage as seen in Low's embryo (1908).

No tracheal groove or lung anlage can be distinguished.

The opening of the yolk sac (plate 1) comprises one-third the total length of the embryo, and here the embryo is flattened out and there are no lateral constrictions and no way of locating the dividing line between yolk sac and body of the embryo. Back of this passes the hindgut, at first rounded in cross-section, but rapidly becoming ovoid or pear-shaped (plate 3, fig. 3), with the small end dorsally. As the cloaca (plate 3, fig. 3) is reached, a groove (text fig. 7 b) appears on each side between the large and small ends of the ovoid and almost cuts the cloaca into two separate chambers throughout its whole length. There is, however, a very narrow chink throughout the extent of the cloaca, where the two parts of the lumen (the rectal bay and the bladder bay) are in continuity. The rectal part of the cloaca is thus much compressed laterally and has a small oval lumen. The bladder portion is lozenge- or diamond-shaped, with dorsal, ventral, and two lateral angles, and the lumen of this part is very large. From this embryo it appears that the separation of the ventral part of the cloaca from the rectum is accomplished by the cutting in of two lateral grooves between them, and not by the extension caudally, as claimed by Felix (1912), of a gradually deepening saddle fissure starting just cephalad of the origin of the allantois. It also occurs much earlier than stated by Felix, who says the separation begins in embryos of 4.9 mm. nape length or 5.3 mm. greatest length. There is actual contact of the sides and every possibility of fusion along the lines of lateral constriction. The saddle-like constriction described by Felix, starting above the allantois, is not yet here indicated on the ventral wall of the gut, but the allantois quite clearly comes off the most cephalad portion of the ventral part of the cloaca. It runs ventrally (plate 1 and plate 3, fig. 3) in a gentle curve out into the belly stalk, with an umbilical artery on either side of it. The arteries unite in a blood reservoir above it in the stalk, where it at first lies ventral to all other structures. The common artery soon splits, however, and then reunites, forming a loop through which the allantois passes to a position dorsal to the artery and ventral to the umbilical veins. A similar course has been noted for the allantois by Wallin (1913) in an embryo of 13 somites. This position it retains throughout the rest of its course to its termination, which is just before the chorion is reached. The allantois possesses a lumen throughout, although it is almost obliterated in some parts of its course in the belly stalk.

On the ventral surface of the body in the median line, just behind the belly stalk, is a shallow depression (plate 1 and plate 3, tig. 3) marking the position of the cloaca! membrane, where the cloaca fuses with the ectoderm. The cloacal membrane is more than 2 cell layers in thickness, but the cells forming it lose their regular order, and are so irregular in shape and position that no definite number of layers can be recognized. The cloaca fuses with the ectoderm in two areas, both situated in the median line, with a free interval between, in which mesoderm is found. Just behind the level of the second area of fusion the grooves on the cloaca disappear and both bladder and intestinal bay open into a full rounded cavity contained in the postanal gut. This part of the gut is very short and ends in a broad, blunt surface, where the cells fuse with and are indistinguishable from the rest of the tissues in the tail region, here forming the primitive streak.

The alimentary system of Embryo V is different enough to merit a separate description and throws some very valuable light on appearances not very clear in its twin. The stomodseum and buccopharyngeal membrane of Embryo V are similar to those of Embryo VI, except that there is only one perforation of the membrane. As in the latter embryo, no pouch of Rathke is present and no distinguishable pouch of Seessel. The first branchial pouch is large and reaches to the ectoderm, with which it is only just coming into contact, and there is thus only a very small area of fusion. The entrance into this pouch from the pharynx is distinctly constricted. Near the distal end of the pouch 2 ridge-like villosities, lying on opposite walls, project into the lumen and cut off a very small terminal chamber in the pouch, communicating by the narrowed opening with the larger medial part of the pouch. These villosities are similar to that mentioned in describing the first pouch* in Embryo VI. There is also a single one present in the second pouch of Embryo V. These villosities I interpret as being identical with the structure described by Grosser, in Keibel and Mall's Embiyology, as being constantly present in the first gill pouch of all young embryos of about this age, and which he interprets as a rudimentary internal gill. He found this structure only in the first pouch, and evidently single, which is true here in the case of Embryo VI, but in Embryo V there are 2 ridges on opposite walls in the first pouch and 1 in the second pouch. If these represent internal gills, we would expect in favorable cases to find two lying opposite in a pouch, and would also expect indications of them in more than one pouch, so that the conditions found here ought to add considerable proof to this suggestion as to their significance.

The second branchial pouch is small and pointed and does not yet reach the ectoderm, as in Embryo VI, but is still considerably separated from it. The third pouch is small, pointed, and indistinct, and merged with the posterior part of the pharynx. In the median line ventrally between the first and second pouches is seen the anlage of the thyreoid gland. This is much more developed than in Embryo VI, and forms a large, broad pouch with a slightly narrowed neck, but has still a large lumen. The epithelium is somewhat thickened at the bottom of the pouch. This anlage lies directly caudad of the median ventral aorta and projects down deeply behind it.

Immediately in front of the thyreoid anlage the ventral aorta causes, as in Embryo VI, an immense median bulging of the pharyngeal floor. A deep groove lies to each side of this, while a shallow one lies in the median line on the crest of the elevation. This third groove disappears, when followed caudally, on the down slope of the elevation, but the lateral grooves unite behind the elevation and then become, suddenly the deep median groove or trough, forming the thyreoid, with its opening into the pharynx slightly constricted by the lips of the groove. These 2 lateral grooves have never been described in any other embryo, but their presence here is not an abnormal phenomenon. It is due to the immense size of the aortic stem that the floor of the pharynx is heaved up, forming a temporary stage of no special consequence, and this elevation and its consequent lateral grooves will quickly disappear, so that in stages such as shown by Van den Broek's and Thompson's embryos, the pharynx does not exhibit these peculiarities, and these grooves, being only incidental to the formation of the median elevation, are no longer found converging to form the beginning of the thyreoid gland.

The gut back of the gill pouches, as far as the yolk sac, is much compressed dorso-ventrally, just as in Embryo VI, and consequently no esophageal and gastric region, which is in these early stages laterally compressed, can be recognized. In this portion of the gut the dorsal surface is smooth, the sides pointed, and the ventral surface wavy in outline on section. This latter condition is due to 3 or 4 longitudinal grooves on the wall. As the yolk sac is approached and the ventral wall begins to slope down behind the sinus venosus, a distinct median bay or outpouching forward is seen, which is the anlage of the liver (the liver bay). The communication between yolk sac and intestine is somewhat constricted to form a yolk stalk, so that this embryo does not lie wide open over the sac, as Embryo VI does. The hindgut is oval in cross-section, and passes insensibly into the cloaca, which is not quite so well developed as in Embryo VI, the main difference being that the cavity is not yet divided into two, although indications are present of an approaching differentiation. The stages here are valuable in showing the condition just previous to division. Here the lumen is large and capacious and on section appears triangular, showing a broad dorsal area and a ventral pointed portion (text fig. 7 c). On the lateral wall, in the broad portion, is a shallow furrow on each side, forming a small ridge projecting into the lumen. It is by the deepening of this furrow that the condition in Embryo VI is obtained, with the narrow dorsal rectal bay and the lozenge-shaped ventral bay almost completely separated. Fusion along the line of these opposing ridges would completely separate the rectum from the ventral part of the cloaca. The furrows outside, and consequent ridges inside the cloaca, are not well developed in Embryo V, but start about the level of the origin of the allantois and extend back beyond the level of the cloacal membrane, where they disappear. The postanal gut is very short and it very soon fuses with the tissues of the primitive streak. The cloacal membrane is marked by a depression on the outside of the body and the endoderm of the gut comes into contact and fuses with it for a short distance, and there are thickening and change in the epithelial layers of both ectoderm and endoderm.

The allantois, arising from the cloaca, runs cephalad, then ventrally into the belly stalk, where it lies for a long time ventral to the umbilical arteries, but finally, as in Embryo VI, passes through a loop formed by the arteries to lie dorsal to them. It is formed of a single layer of endoderm. The lumen is almost obliterated just after entering the stalk, and then becomes very large as the allantois lies under the umbilical artery, but becomes small again as it goes through the loop. The termination of the allantois is just at the chorion.

The remains of the neurenteric canal, as described under the notochord, fuse with the cloaca dorsally.

It will be seen from the foregoing thai while the exact stage and degree of development of the various parts of the alimentary system in these twin embryos are not the same, yet in general they agree perfectly, showing the same plan of structure, and each helps to explain appearances found in the other. Taken together, they give valuable information as to the stage of the gut in the 18-19 somite stage of development, especially in regard to the gill pouches and the cloaca.

The Circulatory System

The Heart

The heart occupies the whole of the body ventral to the pharynx, and between the stomodaeum in front and the septum transversum and yolk sac behind. This region is very prominent and bulging and the chambers of the heart are indicated on the outside by distinct protuberances. The pericardial cavity (plate 1) is very spacious and in the sections appears triangular, with walls nearly equal and angles all well-rounded, with a heart chamber lying in each angle, the atrium and atrioventricular canal dorsally and to the left, the ventricle ventrally, and the bulbus cordis to the right dorsally. The heart is still in the form of a simple tube, but is very strongly S-shaped, with abrupt flexures. At two of these flexures are the constrictions of the tube which mark the limits of the various chambers, visible both on the inside and outside of the muscular tube and also in the endothelial tube.

The muscular wall of the heart (plate 3, fig. 4; plate 4, figs. 1 and 2) is several layers thick, and forms a large, wide tube. The endothelium is 1 layer thick and forms a very small tube (plate 4, figs. 3 and 4) lying in the very center of the muscular tube and everywhere separated from the muscle by a considerable interval, which is entirely devoid of nuclei but shows many fine fibrillar processes and appears to contain a very clear, homogeneous matrix. In the sinus venosus and the atrium, however, the endothelium is closely applied to the muscular wall. Mall (1912), in an article on the development of the heart, states that that part of the tube forming the atrium can be very early identified by this disposition of the endothelium, the apposition of the latter to the muscle being complete in an embryo of 3.5 mm., while in the rest of the heart tube they are widely separated. The contact is complete here also, and there is an abrupt change at the atrial canal, where the endothelial tube becomes quite narrow and lies free from the muscle.

The heart as a whole resembles very closely that of embryo Pfannenstiel III, modeled by Low (1908), except that it possesses a much larger bulbus cordis, resembling in this respect the Meyer embryo modeled by Thompson (1907). Mall (1912) shows an illustration of the heart of an embryo of 3.5 mm. very similar in shape and appearance (both of the muscular and endothelial tubes) to the one here described.

The duct of Cuvier on each side passes into the septum transversum just lateral to the pleuroperitoneal passage (plate 1 ; plate 3, fig. 4; and plate 4, fig. 2) and is immediately joined by the immense trunk of the united umbilical and vitelline veins of each side, to form the horns of the sinus venosus. From here the main portion of the sinus venosus runs transversely, embedded in the septum transversum and joining the two horns. It is convex forward and its most forward portion issues from the septum, and has a very short piece of the dorsal mesocardium (plate 4, fig. 2) suspending it to the roof of the pericardial cavity. This ends at the beginning of the atrium. The atrium consists of a large chamber slightly subdivided on its surface by shallow dorsal and ventral longitudinal grooves, which unite at the anterior border. The atrial canal lies to the left, while the atrium runs forward a short distance in the median line, to end between the ventricle and bulbus cordis.

From the atrial canal the ventricle continues on at the left and runs far forward in the pericardial cavity, where it is strongly flexed ventrally and turns caudad as it reaches the median line. It is here attached to the pericardial wall by a short stretch of ventral mesocardium, the only portion of this structure which is still present. Wallin (1913) finds a similar attachment of the ventricle in an embryo of 13 somites. The lower portion of the ventricle runs caudad in the median line ventrally, until it lies under the atrium and end of the sinus venosus, when it turns very sharply dorsally to the left. There is a constriction at this flexure, and the cavity into which the ventricle here opens, which is almost as capacious as the ventricle itself, is the bulbus cordis. The bulbus now turns forward, running on the right side and parallel to the atrioventricular canal and the upper portion of the ventricle. Opposite where the ventricle begins to sink at the approach to its anterior flexure, the bulbus cordis begins to narrow in a funnel-shaped fashion, and at the same time this narrowed trunk turns medially and, reaching the middle line, goes suddenly up toward the roof of the pericardial cavity with a little twist backward, and then is immediately flexed cephalad outside the pericardial cavity and directly under the pharynx, in the floor of which it makes a very extensive elevation ; this runs forward a very short distance only as the unpaired ventral aorta, and bifurcates just behind the second branchial pouch. The peculiar loop formed at the end by the bulbus cordis in the embryos of 22 and 23 somites, described by Van den Broek (1911), is not seen here, but it would not take a great deal of displacement or increased flexure of the funnel-shaped portion to produce something like it. The bulbus cordis forms the part of the heart which reaches farthest cephalad and so the heart prominence is much greater and farther forward on the right than the left, as is the case in the Van den Broek embryos and in the one described by Thompson. It will be seen that the heart of this embryo is intermediate between those of stages already mentioned as either just earlier or just older.

There is a small piece of dorsal mesocardium (plate 3, fig. 4, and plate 4, fig. 2) extending to the roof of the pericardial cavity from the dorsal surface of the bulbus cordis. This membrane is much vacuolated and evidently in process of degeneration. A similar remnant of dorsal mesocardium in this location has also been described by Wallin (1913).

The heart of Embryo V is so similar to that of Embryo VI as to merit no mention except in two particulars. The chambers, flexures, and constrictions of the tube are an exact replica of those already described, but the bulbus cordis projects considerably more cephalad of the rest of the heart than in Embryo VI, and the anterior part of the pericardial cavity for some distance contains only the bulbus cordis and aortic stem. The only other difference is the presence, on the atrium, of a much deeper and more distinct groove dorsally, so disposed that it does not lie in the median plane, but, starting just in front of the left horn of the sinus venosus, runs obliquely toward the middle line, dividing this part of the heart very distinctly, so that to the right of the groove lie the combined sinus venosus and atrium, and to the left only a portion of the atrium.

The correspondence of the hearts of these two embryos to the heart of embryo Hal 2, modeled by Weintraub, and figured in Keibel and Mall's Human Embryology, is even closer than to the hearts of the Low and Thompson embryos. Of course there is a certain individual difference in general appearance, but the disposition, relations, and sizes of the various chambers and the amount of flexure show a most remarkable similarity. This embryo is 3 nun. long and possesses 15 paired somites, and so is the nearest in general development of all those quoted to the two here described. In all three embryos the ventricle is by far the largest portion of the heart, and the bulbus cordis is next in size.

The ventricle resembles a letter U laid on its side, so as to have dorsal and ventral limbs and a flexure looking cephalad, and with the atrium lying over the open end.

The Arteries

From the unpaired ventral aorta, where it bulges up under the floor of the pharynx, arise the right and left ventral aorta? (plate 4, fig. 1), just back of the second branchial pouch. They diverge like the limbs of a letter V, and as they run forward form a prominent ridge in the floor of each side of the pharynx. There is no definite end to them, since each passes imperceptibly into the first branchial arch vessel, which is (plate 1 and plate 3, fig. 2) almost as large as the ventral aorta from which it springs, and runs cephalad and dorsally in front of the first gill pouch, and then arching caudally over the pouch turns into the dorsal aorta. From the outer side of each ventral aorta arises the second branchial arch vessel (plate 1 and plate 3, fig. 2), which is only half the caliber of the first, but is complete, possesses a lumen, and communicates with the dorsal aorta by running dorsally between the first and second gill pouches, in an arch convex laterally, while the first arch was convex forward. There is a slight bud posterior to this, which is the first indication of the third branchial artery. This embryo fits in perfectly in the series of early embryos, in development of the aortic arches. Dandy's embryo (1910) of 7 somites possesses one completely formed, the first arch, with small buds of others posterior to this. Low (1908) in the embryo of 14 somites finds the first arch complete, the second being formed. Then comes the embryo of 18-19 somites here described with two arches, the second just complete, and immediately following is the embryo of 23 somites described by Thompson (1907), which shows two complete arches.

The two dorsal aorta? begin anteriorly, directly continuous with the first branchial arch vessel, and are situated far out at the sides of the pharynx. As they run caudally they very gradually approach each other, and finally, about half way back over the yolk sac (plate 1) they fuse beneath the notochord, presenting an hour-glass shape as seen in the cross-sections, showing that fusion has only just occurred. This area of fusion extends from the eighth to the thirteenth somite, inclusive, and then the two aortse again separate, but lie in close contact with the gut and notochord. At the level of the succeeding 4 somites are the roots of origin of the right and left umbilical arteries (plate 1) from the aorta?. These arise from a series of large ventral branches of the aorta?, which lie alongside of the gut, from the end of the yolk stalk to the beginning of the allantois. These all anastomose and give rise to the umbilical artery. This origin is in accord with the latest investigations of Felix (1912) and others. It will be seen that at this stage the umbilical artery arises opposite the third to sixth thoracic segments. In two embryos studied by Felix, one younger, one older, the origin of the umbilical artery is respectively cranial and caudal to this. In the younger embryo, Pfannenstiel III, of 14 somites, the origin of the umbilical artery is opposite somites 12, 13, and 14, then in this embryo of 18-19 somites it is opposite the fourteenth to the seventeenth, and in the Meyer embryo of 23 somites is opposite the unsegmented mesoderm posterior to the somites. At the commencement of the unsegmented mesoderm in Embryo VI, over the beginning of the cloaca, the two dorsal aorta? fuse a second tune, and form a vessel in the shape of an inverted letter U from here to their termination, which is coincident with the end of the gut. This description applies to the muscular tubes of the arteries. The endothelial tubes are very small, often indistinct, and as far as can be ascertained do not fuse similarly to the muscular ones, so that the endothelial aorta? are separate throughout their entire extent.

This embryo is unique in two particulars. First, it is the youngest embryo recorded in which there is any fusion of the dorsal aorta?. Those nearest to it and younger, such as that described by Low, have still no fusion whatever, while in those just older, such as described by Thompson and Van den Broek. the dorsal aortse are fused over the yolk sac. In Thompson's case the somites at the level of fusion are given, and include only one more than in Embryo VI, in his case being from the seventh to the thirteenth, in this case the eighth to the thirteenth. The second unique feature is the second fusion of the aortse in the caudal region, where even in older embryos paired aortse are found. It will be noticed also that in the region between fusions the aortse lie very close together, only the narrow notochord lying between them. The aortse show their largest caliber around the origin of the umbilical arteries and just posterior to this.

The branches of the aortse are numerous. There is no sign of forward extension of any artery into the head as the internal carotid on the right side, but there is a short branch on the left (plate 1; plate 3, fig. 1) continuing forward in the line of the dorsal aorta to a point just in front of the hypophysis cerebri. There is a dorsal intersegmental artery (plate 1) between every two somites back to the depth of the concave dorsal bend. Posterior to this point the sections cut the body in a plane unfavorable for finding these arteries, and it can not be stated whether they are present or not. There are many primitive vitelline arteries (plate 1) given off as ventrolateral branches both of the paired and unpaired aortse, and these ramify over the wall of the yolk sac. The roots of the umbilical arteries are in series with these. Dorsolateral branches of the aorta run to the pronephros and cranial half of the mesonephros. Arteries to the gut and urinogenital system were not counted, as they were hard to detect and in some cases, where the shape of the aorta indicated the origin of a branch, the branch could not be followed. Counting would thus not give any accurate number and so was abandoned.

The umbilical arteries run out into the belly stalk, one on each side of the allantois, and unite dorsal to the allantois, as they all turn into the stalk. This common vessel is of immense size and filled with blood, forming a blood reservoir (plate 1), as in the embryo described by Dandy (1910). A short distance out in the belly stalk the artery divides, and then immediately unites again, forming a loop through which the allantois passes as it proceeds from a position ventral to the artery to one dorsal to it and ventral to the veins. The artery is now single all the way out to the chorion, where it divides into two, each of which ramifies over one-half of the chorion, supplying the villi.

The arteries in Embryo V are somewhat smaller and are much harder to follow than in Embryo VI, part of the difficulty being due to their being so filled with blood in many cases as to be almost indistinguishable from the surrounding mesoderm. There are two branchial arch vessels on each side, the second of which is only just complete. The dorsal aortse are fused from the ninth to the thirteenth segments, one less than in Embryo VI. The origin of the umbilical artery on each side is from the fourteenth to seventeenth segments and the caudal aortse are fused over the cloaca as in the other embryo. The aorta terminates here shortly before the end of the gut, allowing the persistence of that part of the neurenteric canal between the notochord and the gut, as already described.

VEINS. The only veins as yet developed are the primitive embryonic trunks. The vena capitis medialis (plate 1) is present on each side, beginning in the region of the optic vesicles. It can be traced caudad, without any interruption, to just in front of the first somite, whore it turns out laterally into the anterior cardinal. The vena capitis medialis lies throughout its course close in against the side of the brain and passes ventromedially to the trigeminal, acusticofacial, glossopharyngeal, and vagus ganglia and the otocyst. It lies so close against the trigeminal ganglion, whose under surface is irregular, as to give the appearance of penetrating it. In embryos described by Ingalls (1907), Broman (1896), and Mrs. Gage (1905), which are somewhat older than this embryo, the vein lies lateral to the acusticofacial ganglion. Mall (1905) states that the first position of the vein is medial to the ganglia, but that loops form around them and the medial branch disappears, leaving the vein lateral to all but the trigeminal. This process has not yet begun in this embryo of 18-19 somites, but is completed in Mrs. Gage's of 27 somites.

The anterior cardinal vein (plate 1 and plate 3, fig. 4) is the direct continuation of the vena capitis medialis, where the latter turns out in front of the first somite. The anterior cardinal vein lies lateral to the somites and on the right side extends to the middle of the third, on the left to the beginning of the third somite, where it ends in the duct of Cuvier. The finding of the duct of Cuvier at this level is evidently normal for early stages, as Evans, in the account of the vascular system in Keibel and Mall's Human Embryology, places it here, and Williams (1910), in chick embryos of 15 and 18 somites, also found it at this level.

The posterior cardinal vein (plate 1 and plate 3, fig. 4) is as yet very short, and can only be traced back from the duct of Cuvier to the sixth somite. Beyond this it is not recognizable, though isolated spaces, apparently vascular, occur along the line of its future course. This is the youngest embryo in which the posterior cardinal vein has yet been found. It is only a very short trunk in the embryo of 23 somites described by Thompson, and in Bremer's embryo of 4 mm. it is only a very small bud situated on the duct of Cuvier.

The vitelline veins (plate 1) commence in a plexus over the yolk sac and run forward in the wall of the yolk sac near the body wall and parallel to the long axis of the body. At the anterior surface of the yolk stalk they turn upward into the body, being situated in the septum transversum and immediately lateral to the pleuroperitoneal passage. Just in front of the opening of this passage into the coelom each unites (plate 3, fig. 4) with the umbilical vein of the corresponding side. There are no veins or anastomosing channels uniting the vitelline veins of the two sides anteriorly; their only communication here is through the sinus venosus, but such communications occur, Low (1908) describing one in an embryo of 14 somites.

The umbilical veins, two in number, start in the chorion and enter the belly stalk, running in its dorsal portion (plate 1), and as the belly stalk broadens out, they lie in its dorsal angles just under the reflection of the amnion from it. In the belly stalk the veins show frequent anastomoses, forming a blood sinus similar to that described by Dandy (1910), and finally separate to enter the body, one on each side. They run forward in the somatopleure, forming a bulging ridge under the line of reflection of the amnion. On reaching the septum transversum each unites with its respective vitelline vein into a large common trunk (plate 3, fig. 4) which receives the diminutive duct of Cuvier, as they all pass into the sinus venosus.

The description of the veins of Embryo V is essentially similar to the above. The posterior cardinal vein can not be so distinctly followed, however, and there is an interval in front of the first somite where the vena capitis medialis can not be distinguished. These veins, however, may be present, for vessels in this embryo are hard In follow, and serial sections, it is well known, do not give the best results unless the vessels have been previously injected.

The Urinogenital System

The urinogenital system is in a very early stage of its development. There is a definite area of the coelomic wall, the middle plate of the body, between splanchnopleure and somatopleure, which extends back as far as the sixteenth somite on each side, forming the Wolffian or urinogenital fold. Beyond the sixteenth somite this portion of the wall is not formed, the splanchnopleure and somatopleure meeting at a narrow angle. All the nephrogenic tissue is found in the urinogenital fold or located at the angle behind it. This fold begins gradually anteriorly, increasing in size as it proceeds caudad and then decreasing again rapidly in the last few segments.

The description following is for the right side (text figs. 3 and 5) in Embryo VI, which shows the greatest development in this embryo. The structures exhibit a close correspondence to those described by Felix in Keibel and Mall's Human Embryology.

In the first three somitic segments no trace whatever of nephric tissue can be identified, although tubules have been described in these segments, Dandy (1910) finding them in the first two in an embryo of 7 segments. If they were ever present in Embryo VI they have completely degenerated. In segments 4 and 5 there are small, compact masses of cells embedded in loose tissue in the nephric region, their appearance very much suggesting parts of tubules, and in the sixth, seventh, and eighth segments remains of tubules even more distinct are to be found, and in these segments is also to be found a portion of the intermediate cell mass from which these tubules arose. In the sixth segment this is attached only to the coelomic epithelium, and in all the remaining segments it is connected with both the somite and the coelomic epithelium. Mrs. Gage (1905) , describing an embryo of 4.3 mm. with 28-29 somites, says: "The connection which probably existed between the myotome and the coelom through the intermediate cell mass which gives rise to the blastema forming the tubules is of an earlier stage than these here considered." She says she has observed this in the chick, and it has been found in the rabbit. I can confirm her supposition of its presence in the human embryo of early stages, as it is quite evident in a few of the segments in this present embryo and also in its twin.

At the cephalic boundary of the ninth segment the Wolffian duct begins and also a well-developed pronephric tubule, which arises from the dorsal or lateral wall of the intermediate cell mass, passes dorsally, and then turns caudad and opens into the Wolffian duct. Starting from the intermediate cell mass of the tenth segment are two large pronephric tubules, one beginning at the cephalic edge of the segment, the other in the caudal half. Projecting into the coelom in this region, just ventral to the attachment of the intermediate cell mass to the coelomic epithelium, is a fairly solid, club-shaped protuberance with a slightly narrowed stalk, in which can be seen a blood-vessel. I think that this may unhesitatingly be called an external glomerulus. It is the only one occurring in this embryo. Janosik (1887) also found a single external glomerulus in a 3 mm. embryo. In the eleventh segment there is a pronephric tubule, not so large as those in the tenth, and in the twelfth is a still smaller one. These tubules all run caudally under the Wolffian duct, which lies dorsal and lateral to them, and they open into the duct in the segment behind the one in which they arise. The Wolffian duct and the tubules, and the intermediate cell masses of these segments, all contain lumina.

Behind the twelfth segment there is a sudden change in the nephric system. The Wolffian duct (text fig. 5) receives no tubules and rapidly decreases in size, loses its lumen, and becomes flattened and crescentic in outline, lying applied to the dorsolateral surface of the nephrogenic cord. It approaches the ectoderm and ends just beneath and in contact with the latter at the beginning of the sixteenth segment. Whether there is actual fusion can not be stated, but it may be emphasized that immediately in this vicinity, around the end of the Wolffian duct, the ectoderm shows absolutely no change and no preparation for further formation of the Wolffian duct and its growth caudad. The evidence here corresponds exactly with that obtained by Felix (1912) and supports his view of the formation of the Wolffian duct by fusion of the ends of pronephric tubules anteriorly, and the caudal extension of it to the cloaca by growth of the tip, or splitting off of cells from the nephrogenic cord; it also strengthens materially his statement that he would deny any participation of ectoderm in the formation of the duct. This embryo comes midway between two described by him, one of 13-14 segments, having no duct yet formed, and one of 23 somites, where the duct nearly reaches the cloaca.

From the thirteenth to eighteenth segment, inclusive, the character of the nephrogenic tissues is similar, forming a large rounded mass of cells in each segment, connected on one side by a narrow string of cells to the somite, on the other side to the coelomic epithelium. On them are small protuberances in the thirteenth and fourteenth segments, suggesting the origin of tubules from them. These masses (text figs. 3 and 5) are all continuous, forming an unbroken nephrogenic cord, but there are at intervals small central lumina, so that the cord is practically a fused series of vesicles. It is rather difficult to determine the exact number of mesonephric vesicles present, as the lumina are not all distinct, but there are either 10 or 11. There is thus no numerical agreement with the number of segments in which they lie, a fact universally true in all human embiyos described. Fusion with the coelomic epithelium is very extensive, beginning at the seventeenth somite, and the line of division between the epithelium and nephrogenic cord is marked by a furrow which gradually disappears by the time the unsegmented mesoderm is reached. Marked fusion also occurs with the nineteenth somite, which is not entirely formed, and the line of division between somite and nephrogenic cord is very soon lost, and the intermediate cell mass, which here represents the nephrogenic cord, very soon also loses its identity — the somitic plate, intermediate cell mass, and lateral plate all blending in one common, undifferentiated mass of mesoderm.

The fully developed tubules of the ninth to twelfth segments are called pronephric, principally because the Wolffian duct is just in process of formation and is being formed by their union — according to Felix (1912) mesonephric tubules never appear until after the formation of the Wolffian duct — and also because other authors have found pronephric tubules at this same level in young embryos possessing 23 paired somites or less. The abrupt change from pronephric tubules developed as far as the twelfth segment to mesonephric vesicles beginning in the thirteenth is in thorough accord with the emphatic statement of Felix that in the human embryo pronephric tubules are never developed caudal to the twelfth segment, which is the first thoracic.

As to the presence of openings of the nephric tubules to the coelom (the nephrostomes or trichter) there is some doubt, but it may be mentioned that in several segments there is a cleft in the coelomic epithelium at the point where the intermediate cell mass fuses with it, and in some cases this even opens into the cavity of the intermediate cell mass. But this may be an artifact, and indeed it is simulated by occasional breaks in the coelomic epithelium in other places, so giving rise to doubt as to its actual significance. There are, however, two undoubted openings to the coelom, each situated in a depression of the coelomic wall. These openings (text fig. 5) are the nephrostomes of the first two pronephric tubules, one in the ninth segment, the other in the tenth. The connection of the intermediate cell mass in segments caudad to this occurs at the point of a slight depression in the wall of the coelom, but there is no definite, funnel-shaped opening present.

The left side of this embryo (text figs. 3 and 5) is very similar to the right, but slightly less developed. In the first 5 somitic segments tubal remains are very few and are small and hard to distinguish from the surrounding mesenchyme. The intermediate cell mass is recognizable from the sixth segment back to the unsegmented mesoderm. The first recognizable tubule is in the caudal portion of the ninth segment, starting from the coelomic epithelium, and at this point there is a funnel-shaped depression forming the nephrostome. The tubule soon separates from the epithelium and runs dorsally and then caudally, forming the beginning of the Wolffian duct, there being no little blind end of the duct in front of it, as on the right side. There are 5 pronephric tubules in all, the first being the only one possessed of a nephrostome. The others are found, 2 in the tenth segment, 1 each in the eleventh and twelfth; each has a lumen and unites with the Wolffian duct. The duct is very large here, but rapidly decreases in size, becoming crescentic and solid, as it runs caudally, and ending as a rapidly narrowing string of cells. There are three cells in the section before the end, and one cell finally at the end, in contact with the ectoderm, at exactly the same level as on the right side, namely, between the fifteenth and sixteenth somites.

Embryo V shows even much better than Embryo VI the development of the pronephros, which is nearly double the size of that found in Embryo VI, while the mesonephric anlage contains only about half the number of vesicles found in the latter. This surprising difference is the more remarkable because in all other organs the two embryos present an almost completely equal and parallel development. In the matter of somites formed it will be remembered Embryo VI has 18-19 and Embryo V has 17-18, and it would not be expected that a difference in development of only one somite would mean so great a difference in the urinogenital system as is here manifest.

The urinogenital folds extend from the third somitic segment, on the dorsal wall of the pleuroperitoneal passage, to the caudal end of the sixteenth segment. The intermediate cell mass is present from the fourth segment on the left and the fifth on the right, back to the unsegmented mesoderm, and is connected to the somite and to the coelomic epithelium in each segment. Remains of tubules in front of the pronephros can not be seen on the right side. The pronephros (text figs. 4 and 6) begins on this side with two fully formed tubules in the seventh segment. Following this tubules occur as follows: 1 in the eighth, 2 in the ninth, 1 in the tenth, 2 in the eleventh, 2 in the twelfth, and 1 at the beginning of the thirteenth segment, making a total of 11 tubules. In each case there is a distinct trichter or nephrostome by which the tubule communicates with the coelom. Each tubule is budded dorsally from the intermediate cell mass, forming the principal tubule as described by Felix (1912), the intermediate cell mass forming the supplementary tubule, and by the fusion of the ends of the principal tubules the Wolffian duct is formed, which therefore extends from the seventh segment. It receives no connections beyond the last pronephric tubule, but extending caudally, overlying the nephrogenic cord, ends at exactly the same point as the duct of each side in Embryo VI, namely, between the fifteenth and sixteenth segments.

Posterior to the pronephros the intermediate cell masses have fused to form a solid string, the nephrogenic cord, which contains small cavities at intervals, forming mesonephric vesicles, of which there are 5, the last being in the seventeenth segment.

On the left side, as in Embryo VI, development is not quite the same as the right, being a little less (text figs. 4 and 6). There are 7 pronephric tubules, the first appearing in the ninth segment, 2 caudad of the first on the right. Following it are 1 in the tenth, 2 each in the eleventh and twelfth, 1 in the beginning of the thirteenth. There is a aephrostome present for all except the last tubule. The Wolffian duct is similar to that of the right side and extends caudally to exactly the same level. In front of the first tubule, for two segments, doubtful remains of tubules are seen. The nephrogenic cord beginning in the thirteenth segment contains 7 mesonephric vesicles, 2 more than on the right, the first being in the thirteenth segment, the last in the eighteenth.

The embryos standing nearest to these two in development of the urinogenital system show many interesting points of comparison. Embryo Pfannenstiel III, modeled by Low, the urinogenital system of which is specially described by Felix (1912), possesses 13-14 somites and the pronephros is just beginning its development. There are rudiments of it in the second, fourth, and sixth segments, a separate tubule in the seventh, 2 united in the eighth and ninth, and two pronephric ridges on each side of the body, united end to end and connected to the tubules as far as the fourteenth somite. There is no Wolffian duct. The presence of this pronephric anlage back as far as the fourteenth somite is interesting and leads to the question whether pronephric tubules may not be found caudad of the twelfth segment, which Felix has stated to be the extreme limit of their occurrence. This would give a reasonable explanation of the conditions found by me, for although Embryo VI has no tubules beyond the twelfth segment, Embryo V has one on each side in the thirteenth. -There is a gap in the series described by Felix, in the very middle of which come the two embryos here described, and then comes Felix's second example, the Meyer embryo of 23 paired somites, modeled by Peter Thompson, Felix studying specially the urinogenital system. Here there are 7 pronephric tubules, lying opposite the ninth to twelfth somites, the first separate, the others united, forming the Wolffian duct, which extends caudad to the twenty-second somite. From the thirteenth to the beginning of the sixteenth somite lie 8 mesonephric vesicles in a chain, continuous from the latter point to the unsegmented mesoderm with the nephrogenic cord.

The above-mentioned embryo, although advanced 4-5 somites over the two described in this paper, shows a nephric system intermediate between the two, more tubules and fewer vesicles than Embryo VI, fewer tubules and more vesicles than Embryo V. The most noticeable difference is the caudal extension of the Wolffian duct over 6 additional segments, it reaching only to the sixteenth segment in both the twins. It will be noticed that from a condition with no Wolffian duct at the 14-somite stage we pass to one in the 18-19 somite stage with a duct extending over from 7 to 9 segments and in the 23-somite stage extending over 14 segments. There is thus a very rapid caudal growth of the free end of the Wolffian duct, it soon reaching the unsegmented mesoderm, and from now on, as records show, it keeps pace with the formation of the somites and reaches the cloaca at the 27-somite stage, although it does not penetrate until later.

The urinogenital system has been described by MacCallum (1902) for an embryo of 19 somites, length 3.5 mm., which is thus almost exactly comparable in its general development to the two here described. In this embryo MacCallum found a single duct lying in the sixth to ninth segments, connected to the coelom at a depression opposite the sixth somite. There was then a break, and opposite the tenth segment the Wolffian body began and extended to the unsegmented mesoderm, having 13 thickenings on it, regarded as developing mesonephric tubules. MacCallum interprets the small anterior mass as a rudimentary pronephros. Evidently there is here one pronephric tubule and a short -tret eh of Wolffian duct. This Wolffian duct has not half the length shown in the two embryos described by me, although all three embryos are at the same stage reckoned by the number of somites. In an embryo of V. B. length 4.5 mm., N. B. length 5 mm., he finds a short stretch of duct with one tubule on it, opposite the seventh somite, then beginning at the eighth somite, the Wolffian duct with 15 mesonephric tubules on the right, 17 on the left. Mrs. Gage (1905) found one isolated tubule in the eleventh segment, separate from and just cranial to the beginning of the mesonephros, in embryo 148, three weeks old and possessing 27-28 somites. In two others of similar age she found similar isolated tubules. Tandler (1907), in an embryo with 38 paired somites, found isolated tubules and one glomerulus (presumably external) in front of the mesonephros, but does not state in what segments. Janosik (1887) in a 3 nun. embryo found two isolated pronephric tubules with nephrostomes, and an external glomerulus in front of the mesonephros. Bremer (1906), in a 4 mm. embryo, found cranial to the mesonephros 3 isolated tubules, the first opening into the coelom, at the level of the liver, by a nephrostome. The mesonephros in all these cases is well developed. Pronephric tubules have thus been found persisting for a long time and always in the region of the seventh to twelfth somites, shown to be that of the greatest development of the pronephros in the two embryos here described and also in those of Felix. If this region degenerated with anything like the rapidity of the parts of the pronephros cranial to the seventh somite (and pronephros is present here also, as shown by the tubules in Dandy's embryo and remains in older ones), these tubules enumerated above would disappear long before they do, so that their persistence in this region is rather an interesting fact.

Regarding the external glomerulus, Felix states that it does not appear until the pronephros is in full process of degeneration. This agrees with its occurrence in Tandler's and Janosik's embryos, and if the statement is true the presence of a glomerulus in Embryo VI and none in Embryo V will explain why the former exhibits only half the number of pronephric tubules found in the latter; Embryo VI therefore shows an advance in the nephric system over that in Embryo V more than commensurate with the possession of only one extra pair of somites, and this advance is further shown by the possession of nearly double the number of mesonephric vesicles that occur in Embryo V.

Felix (1912) remarks as to the Meyer embryo: "Whether this concentration of six pronephric tubules within the limits of 3| body segments indicates a primary dysmetamerism or has resulted from the approximation of originally more separated anlagen can not be determined." This suggestion of primary dysmetamerism in the formation of the pronephric tubules is very interesting, as it will be remembered in both Embryos V and VI several of the segments gave rise to 2 tubules each. There are 10 tubules in all in Embryo VI, 18 in Embryo V, of which respectively 4 and 12 are found in pairs (2 opposite 1 somite), making a total of 16 out of 28 tubules showing evidence of dysmetamerism. In the mesonephric anlage no definite arrangement of vesicles according to segments is to be found.

Regarding the genital cells and ridges there is little to say. Between the cloaca and the mesoderm of the body, in a few instances, were masses which might have been either multinucleate cells or very solid clumps formed by a few mesenchyme cells. They resemble the cells known as wandering or primary genital cells, 2 of which, from embryo Pfannenstiel III, are shown in a figure in Keibel and Mall's Embryology. Whether the cells I have seen are actually genital I am not prepared to state, owing to a certain amount of maceration of mesoderm in this region. No genital ridge is yet present. The coelomic epithelium is 2-layered on the medial side of the urinogenital fold and around the angle from this on to the root of the mesentery. It is thinner in the region of the nephric tissues and 2-layered again at the reflection on to the lateral body wall, so that evidently its thickening at the root of the mesentery is not significant, although it is heavier than the other thickened portions and occasionally is irregularly heaped up in more than 2 layers at points just medial to the pronephros.

The Septum Transversum and the Coelom

The septum transversum is very much like the one described by Peter Thompson (1908) for the 23-somite stage, but is situated (plate 1) further cephalad, being opposite the first 2 somites in this case. It is also not inclined obliquely from the dorsal surface caudally and ventrally, but is nearly horizontal. It is convex ventrally and immediately behind it is the yolk sac. In the dorsal portion the liver bay lies embedded. The septum is completely attached to the body wall laterally and ventrally, but not dorsally, for to each side of the gut lies the pleuroperitoneal passage of the coelom, passing dorsal to the septum. To the dorsal body wall the septum is attached by two lateral horns, one lateral to each pleuroperitoneal passage, and in the median line it fuses with the mesoblast surrounding the gut. Its cephalic surface dorsal to the sinus venosus has attached to it the dorsal mesocardium, which latter is prolonged forward from it in the median line, over the small free portion of the sinus venosus.

Contained in the tissues of the septum transversum are posteriorly the vitelline and umbilical veins of each side, which enter it from the lateral body walls, while passing laterally over the pleuroperitoneal passage and running ventrally into the septum, lateral to the passage, is the duct of Cuvier of each side. The vitelline and umbilical veins of each side unite into an immense common trunk in the septum, and with this trunk unites the duct of Cuvier, the total union forming in the septum the horns of the sinus venosus. These horns are united by the median, transverse, crescentic part of the sinus, which is embedded in the anterior part of the septum, except a small portion which escapes from the septum just before opening into the atrium.

The pericardial portion of the coelom has been described in connection with the heart. It is large and capacious and forms one large chamber, except dorsally over the atrium and again over part of the bulbus cordis and over the aortic stem, where the dorsal mesocardium subdivides it into two. The most cephalic part of the cavity is that surrounding the bulbus cordis, where the cavity reaches a level just in front of the beginning of the otocyst. This corresponds exactly with the findings of Mrs. Gage (1905). The pericardial cavity rapidly narrows posteriorly to each side of the dorsal mesocardium, and from this portion the pleuroperitoneal passage proceeds caudad, dorsal to the septum transversum. This passage lies on a level with the gut and is at first hook-shaped, curving round the dorsal and medial surface of the horn of the sinus venosus. As it passes back it comes to lie altogether on the median side of the vitello-umbilical venous trunk, and is very much compressed laterally, the long axis of the lumen being dorsoventral. At the level where the vitello-umbilical trunk is split into its components, the vitelline and umbilical veins, which level is also the point of transition of these veins to the septum transversum from the body wall, the pleuroperitoneal passage becomes rounded and immediately opens into both the endocoelom and exocoelom. The endocoelom and exocoelom are in wide communication for over one-third the total length of the embryo, from the level of the posterior edge of the septum transversum to the beginning of the belly stalk.

At the level of origin of the belly stalk the splanchnopleure and somatopleure unite to cut off communication between the endocoelom and exocoelom. The endocoelom lies to each side of the gut and is roughly oval in section, with its long axis dorsoventral. It rapidly decreases in size and ends at the sides of the cloaca at the level of the cloacal membrane.

There is thus, just as Dandy (1910) and Wallin (1913) have described in younger embryos, a continuous circuitous route through the body of the embryo from the exocoelom of one side to that of the other, by way of the endocoelom, pleuroperitoneal passages, and pericardial cavity. The question as to whether the cavities of the somites communicate with the endocoelom is doubtful in many cases, owing to a slight maceration of the lateral parts of the somites in the most favorable region for answering this. There is, however, in a few cases in Embryo V, direct continuity of the cavity of the somite with the lumen of the intermediate cell mass, which in turn opens through the nephrostome into the endocoelom.

The septum transversum and coelom of Embryo V are exactly similar in all respects to the above.

Sense Organs

The Internal Ear

The only sense organ yet in evidence is the internal ear, excepting of course the optic vesicles. No lens thickening and no olfactory placode are yet to be found.

The internal ear is in the stage of the auditory cup, still widely open to the exterior. It lies directly dorsal to the region of the second gill cleft. It is an egg-shaped invagination of ectoderm, 3 to 4 layers of cells thick and surrounded, except on its medial side, by mesoderm of the head. Medially it lies to the side of the hindbrain, and between it and the brain is the hinder part of the acusticofacial ganglion, while the rest of the ganglion extends out directly in front of it. The shape of the cavity throughout is rounded, with no indications of the approaching division into pars superior and inferior, already beginning to show in Van den Broek's (1911) embryo, only 4 somites in advance of this in development. In Low's (1908) embryo, 4 somites less than Embryo VI, there is only the slightest depression, the thickened ectoderm mainly indicating this organ, and Wallin (1913) describes an embryo of 13 somites with no evidence whatever of the invaginations. In Van den Broek's embryo the opening from the outside is not equal in extent to the cup, but is much constricted, being only 40m long, while the cephalocaudal diameter of the cup is 184/x. In Embryo VI the opening comprises almost the whole cephalocaudal extent of the cup and so even the extremities of the cup are not as yet constricted off from the ectoderm. The auditory cups of Embryo VI are slightly larger, but those of Embryo V are entirely similar in every way to the others. The main measurements are appended for both. These measurements were not taken in the plane of the principal axes of the body of the embryo , but in the plane of the principal axes of the auditory cup alone, which, lying on the sloping body wall, has its axes oblique to those of the body.

Outside diameters "I auditory cup.

( lephalocaudal Mediolateral


Diameters of opening into cup

( !ephalocaudal


Embryo VI.

Embryo V.

Van den

Brook's embryo.









120 100


140 70

170 M 150





110 120

120 100

140 120




The above measurements show how much alike in size and shape are these auditory pits. Measurements given by Van den Broek for the embryos he describes, of 22 somites, show little increase in length or width of the cups, but there is a remarkable increase in depth and a very much more constricted opening to the exterior, showing how rapidly the cup is becoming converted into a vesicle.

The Nervous System

The nervous system is in the stage where closure has just occurred and is complete, except for the anterior and posterior neuropores. The fusion of the lips of the neural groove is not very heavy and over most of the hindbrain and again over a large extent of the yolk sac the seam has opened in both of the embryos studied. Careful study of the sections shows, however, that there had been fusion, ragged edges being evidence of this, as is also the fact that the ectoderm is broken, and not continuous with the walls of the neural canal, as it would be if fusion had not yet occurred.

The anterior neuropore (plate 1 and plate 3, fig. 5) is not situated exactly at the most anterior point of the head, but on the surface just ventral to this, so that it looks vent rally as well as forward. It is situated in a shallow depression and is very wide open and looks directly into the cavities of the optic vesicles as well as into the lumen of the rest of the forebrain.

The posterior neuropore represents the still unformed, or at least undeveloped, portion of the spinal cord, and is wide open. It will be described later.

It will be seen thus that the nervous system is still open both in front and behind at the neuropores, but closed everywhere else. In Low's (1908) embryo, Pfannenstiel III, of 14 somites, a large part of the nervous system is still open in front, so that quite an advance in closure is shown by the embryos here described. Van den Broek's (1911) embryo of 22 somites and Thompson's (1907) of 23 somites show complete closure of the anterior neuropore. Embryo V and VI thus show a degree of closure which is coincident with their stage of development.

The nervous system conforms to the general contour of the dorsal surface of the embryo and exhibits one important flexure, the cephalic. The anterior part of the brain is flexed to form exactly a right angle with the rest, the flexure occurring in the region of the midbrain. In general contour and appearance the brain shows a remarkable correspondence to figs. 26 and 27 in the account of the nervous system in Keibel and Mall's Embryology. To make their figure serve here it would be necessary only to make a wider neuropore and indicate the neuromeres.

The anterior end of the nervous system, forming the brain, is divided distinctly into the three primary vesicles, each of which in turn shows a series of secondary divisions, forming the anlagen of many future structures in the brain (plate 3, fig. 5). These anlagen are in the form of the total folds, as described by Broman (1896) and Mrs. Gage (1905). By a total fold I mean one involving the whole brain wall and indicated by a bulging of the exterior coincident with an enlargement of the lumen, and bounded by furrows externally, which form corresponding ridges on the interior of the wall.

The forebrain (plate 3, fig. 5) as a whole is narrow from side to side and very deep in its dorsoventral diameter. Its lumen is narrow and slit-like. There are five distinct regions delimited in the forebrain, as follows :

The first is the optic evagination. This involves nearly the whole dorsoventral extent of the lateral wall of the brain just behind the anterior neuropore. There is yet no differentiation into optic stalk and optic cup, and the whole evagination is flattened so that its lumen is a vertical cleft, which opens in its whole dorsoventral extent into the lumen of the rest of the forebrain. The optic evagination does not reach quite to the ectoderm and there is no evidence of any lens thickening.

The second forms the cerebral hemispheres. Above and behind the origin of the optic evagination, and involving the roof of the forebrain, is a small but distinct expansion, which is the beginning of the cerebral hemispheres. No evidence of the olfactory lobes are to be seen yet in front of the cerebral lobes, and this is probably to be associated with the presence of the still open neuropore, as this lobe is demonstrated by Mrs. Gage (1905) in a somewhat older embryo where the neuropore is closed.

The third division of the forebrain corresponds with what Mrs. Gage calls the infundibular fold. It is in exactly the same position as figured by her and is distinctly cut off by furrows from adjacent parts. It is situated immediately below and extends behind the optic evagination, involving thus the floor and lower part of the lateral wall of the forebrain. Also involving the floor is the fourth fold, the hypophyseal, forming that part of the pituitary body derived from the brain. It occurs immediately in front of the cephalic flexure and is thus the furthest caudal portion of the floor of the forebrain. It is a distinct median evagination containing a small tubular lumen and projects directly caudad, to lie just in front of the anterior end of the notochord (plate 1) and against the front end of the gut. No pouch of Rathke yet reaches it.

The fifth fold in the forebrain differs from the condition found by Mrs. Gage in this respect, that one fold here extends from the posterior part of the roof of the forebrain, obliquely downward and forward across the lateral wall, ending just above the hypophyseal and infundibular folds. This includes all the region which is found in the albicantial and the two diencephalic folds in the embryo described by Mrs. Gage. This fold is large and well marked — in fact is more developed than any other except the optic.

The midbrain (plate 1 and plate 3, fig. 5) is quite distinctly marked out and is quite in accord with the descriptions of it as found in other young embryos. It occurs at the region of the cephalic flexure and so is wedge-shaped, the apex being the floor or ventral surface and the base of the wedge being the dorsal surface of the brain. The midbrain is incompletely subdivided into two portions, an anterior and posterior on each side, by a shallow groove starting on the roof and passing half way down the sides, where it is lost. The folds formed thus foreshadow the formation of the corpora quadrigemina. In Mrs. Gage's case this division of the mesencephalon is much more marked.

The isthmus is not well marked and no constriction or lessened development are in evidence at this stage, as shown by the diameters.

The hindbrain (plate 1 and plate 3, fig. 5) or rhombencephalon is well developed and shows a marked degree of differentiation for an embryo so young, all the total folds described by Broman and by Mrs. Gage being readily identified. Four cranial ganglia on each side are also to be found, representing 5 nerves, the trigeminal, facial, auditory, glossopharyngeal, and vagus.

Of the cerebellum there is little evidence yet, that part of the hindbrain from which it arises being hardly differentiated. The fourth ventricle, however, is indicated, for the dorsal part of the lumen, especially in the posterior portion, is expanded, appearing lozenge-shaped on section, while the ventral part of the lumen is cleft-like throughout.

On the side wall of the rhombencephalon and involving the floor are found 7 neuromeres (plate 3, fig. 5) which agree accurately with those found in this region of the brain of early human embryos by Mrs. Gage and by Broman.

The first neuromere, immediately behind the isthmus, is small and only slightly marked out. It is wedge-shaped, the apex being placed ventrally. It includes in the apex a slight extent of the brain floor, not having been yet forced entirely on to the side of the brain by development of surrounding parts, as in Broman's case.

The second neuromere is very large and bulging and causes a marked swelling of the floor of the brain in this region, where it gives the first indication of the location of the future pons. This neuromere is wider ventrally than it is above, to accommodate itself to the neuromere on each side of it, which is wedge-shaped, with the broad end dorsal. Associated with this second neuromere is the Gasserian ganglion, which is quite large. It is connected to the neuromere, but peripherally does not give indication of the three divisions of the nerve.

The third neuromere is large, though not so large as the second. It still includes a share of the brain floor, while in the stage described by Broman this neuromere has been forced wholly on to the lateral wall. It agrees with Broman's case in being wedge-shaped, with the narrow end directed ventrally.

The fourth neuromere is very large and causes a bulging of the floor of the brain below the level of the third. Associated with this neuromere is the acusticofacial ganglion, the two parts of which are readily recognizable. The anterior portion, forming the facial ganglion, lies just in front and almost in contact with the anterior end of the otocyst. It is connected to the brain and peripherally extends (plate 1) right out into the second branchial arch, which is rather remarkable for an embryo of this age. The acustic ganglion lies directly behind and in contact with the facial and is fitted in between the otocyst and the wall of the brain. It is quite small.

The fifth neuromere is similar in shape to the fourth, being rather rhomboidal. No structures are connected with it, but the posterior half of the otocyst lies opposite to it, the anterior half being opposite the fourth neuromere.

The roots of the seventh and eighth cranial nerves are invariably found opposite and attached to the fourth neuromere, as described here, but the position of the otocyst seems to vary, being usually opposite the fifth neuromere, as seen in the embryos described by Broman (1896) and by Mrs. Gage (1905). In this case, however, approximately half of the otocyst is anterior to the fifth neuromere, being opposite the fourth, and in intimate contact with both the auditory and facial portions of the acusticofacial ganglion. In Keibel and Mall's Embryology (1912) the location of the auditory vesicle is placed opposite the fifth neuromere only. Of course the embryos I have referred to are all considerably older than the ones here described, so that there is plenty of opportunity afforded for a change in the relations from those found at this time.

The sixth neuromere and those back of it resemble very much the neuromeres of the spinal cord, and this posterior part of the medulla is indistinguishable from the cord except for one feature, which is the commencing formation of the fourth ventricle, as shown by the expanded dorsal portion of the lumen. Each neuromere from here back is similar and includes a section of the whole nervous tube, but is not marked on the roof, as this is not yet separated from the ectoderm. The sixth neuromere has connected with it on each side the ganglion of the glossopharyngeal nerve. This ganglion is fairly large and extends about half the distance toward the branchial arch with which it is to be associated.

The seventh neuromere has connected with it the ganglion of the vagus. This is not nearly as large or as distinct as the ganglion of the glossopharyngeal nerve, but is quite recognizable. The sixth and seventh neuromeres have been constantly associated with the ninth and tenth cranial nerves, respectively, so that these findings are in full accord with those of other investigators.

The next two neuromeres are included by Mrs. Gage as folds 8 and 9 of the medulla oblongata. She finds them as dorsal pockets only of the neural tube, but in Embryo VI they include the whole tube. These two folds lie in the region of the first three pairs of somites, the neuromeres being opposite the intersegmental clefts. The origin of the accessory portion of the eleventh nerve is stated to arise from these neuromeres, but no evidence of this nerve is here present. The ganglion crest of the neuromeres is continuous with the ganglion of the vagus in front. There is no trace of the hypoglossal nerve, whose fibers must come from these neuromeres, as they lie opposite the segments later included in the occipital region of the skull.

Accepting the number of neuromeres in the rhombencephalon as 9, there are to be found 11 (plate 1) in the spinal cord, the last one being opposite the thirteenth intersegmental cleft. There is no line of demarcation between brain and cord. The condition of the cord changes slowly and very gradually, becoming less in its various diameters as it is followed caudad. Back of the last neuromere it gradually becomes circular in section. In front of this it is much elongated dorsoventrally. At the level of the last somite the cord becomes open dorsally, forming the posterior neuropore. The walls are at first close together and the gutter formed is narrow and deep, but this gutter gradually becomes shallower and wider and finally, in the tail region, all trace of the lips of the neural groove are lost and the nervous system terminates as a flat plate which rapidly blends with the tissues of the primitive streak. Shortly before this fusion, from the under surface of the medullary groove to the notochord, the remains of the neurenteric canal pass, evident as a solid cord of cells, already described (plate 1 and plate 2, fig. 10).

The whole extent of the nervous system on its dorsal surface is still fused to the ectoderm; nowhere is there as yet any separation. In all the closed portion of the tube in the region of the spinal cord the neural crest is evident and from it there passes a spinal ganglion (plate 1) between every two somites as far back as the last neuromere. Back of this point no ganglia are yet evident. There is a neural crest also in the posterior part of the hindbrain, from which the ganglia of the glossopharyngeal and vagus nerves are formed. This crest seems to be directly continuous with that of the spinal cord and would thus agree with the views of Dohrn (1901), who found them continuous.

No motor nerves are yet in evidence in any portion of the nervous system. Histologically the whole closed portion of the nervous system appears similar. There is an internal limiting membrane lying next the lumen, then the layer of ependymal cells. External to this is the mantle zone, in which the cells are apparently in the indifferent stage. Outside of this again is the marginal zone, formed of protoplasmic processes of the cells and containing absolutely no nuclei. This zone is not developed in the dorsal and ventral portions of the tube, but is well marked in the lateral regions. There is no division yet into the dorsal (or alar) and the ventral zones, corresponding to the sensory and motor regions of the cord. Inclosing the whole nervous system throughout is a very definite, thin, structureless, external limiting membrane. The nuclear layers in the walls of the neural tube vary from 4 to 6 in number.

The nervous system of Embryo V is so similar in every way to that of its twin. Embryo VI, that no separate description of any part is necessary, and it can be dismissed with only a few remarks regarding some slight individual differences. The appearance on section of the various parts of the brain and cord is almost identical in the two embryos. Embryo V, however, does not exhibit quite as well developed a neural crest and series of spinal ganglia. It does possess trigeminal, acusticofacial, and glossopharyngeal ganglia of approximately equal development to its twin, and also exhibits the extremely large extension of the facial ganglia into the second gill arch. No vagus ganglion can be distinguished in Embryo V, and it will be remembered that this ganglion in Embryo VI was small, and though quite recognizable would easily be overlooked without careful search, so that this difference is not of great significance. The only other difference worth mentioning is that shown in the combined fold representing the albicantial and diencephalic folds as described by .Mrs. Gage. As in Embryo VI, one fold only is found, in place of the three Mrs. Gage mentions, and this fold is most prominent. It stands out in a winged fashion on each side of the forebrain, and is even much more conspicuous than in its twin. Its posterior boundary is formed by a very deep groove, and this is the main factor in making the fold so prominent, as the posterior or caudal wall stands out perpendicularly to the wall of the rest of the forebrain. This fold is not so sharply delimited in front, because from the crest of the fold the anterior or cephalic wall slopes gently forward and is much longer than the other.

Apart from the differences mentioned above, there is complete agreement between the stage of development and the appearance of the nervous system of these two embryos, and no further description is necessary for the second.

Regarding the question as to what point is the morphological anterior end of the brain, I find valuable evidence here in support of the views advanced by Johnston (1910) that it is situated at the recessus prseopticus. For a discussion of the various views on this question I refer the reader to the descriptions of embryos by Airs. Gage (1905) and by Van den Broek (1911). It is sufficient to state that His (1893) and others have placed the extreme cephalic end of the ventrimesal line or basilar axis of the brain at a point on the anterior wall of the recessus infundibuli. The dorsimesal line is marked, of course, by the fusion of the lips of the neural groove. According to His, the side walls of the brain unite in front, forming a closing seam between the ends of the dorsimesal and ventrimesal lines. This occurs by the closure of the anterior neuropore. Most investigators have discarded the view of His regarding this closing seam and Mrs. Gage says "it logically follows the cephalic end is the point where the dorsimesal and ventrimesal lines meet." All writers seem to agree that this point does not coincide with the location of the anterior neuropore and of the part of it which longest retains its connection with the ectoderm. Stress is laid, however, on the presence of a seam as one means of identifying the dorsal surface. Now. in both of the embryos here described, the nervous system is nowhere separated yet dorsally from the ectoderm, so that there is no doubt of the location and extent of the dorsal seam. At the anterior end of the body the anterior neuropore lying between the optic vesicles is still wide open and is, as shown by the rolling of its lips out into the adjacent ectoderm, the still open portion of the dorsal seam. Nowhere have I seen any account of the closure of the nervous system in which it is stated to begin in any place except over the hindbrain and upper part of the spinal cord, whence it extends both caudad and cephalad. No closure begins at the cephalic end of the brain to meet the main closure, although the anterior neuropore does not always close regularly from behind forward. It is reasonable, however, to take the extreme cephalic end of the neuropore as the extreme front end of the dorsimesal line. This point, then, will mark the extreme, front end also of the ventrimesal line or basilar axis, and it is situated in front of the origin of the optic vesicles and forms the preoptic recess. According to His and to Mrs. Gage and others, that part of the brain wall from the preoptic recess back to the infundibular recess is dorsal. If so. in these young embryos it ought to give evidence of a seam, but no evidence whatever of any seam or fusion is to be found in this part of the brain. It does not seem reasonable to consider this region a part of the dorsal wall of the brain, when in all its relations it is so different from all the rest of the dorsal portion of the nervous system, and at this stage of development it also seems to me quite rational to take the presence of the seam at the line of fusion of the lips of the neural groove and the connection with the ectoderm as a criterion for determining what is the dorsal surface. Otherwise one is forced to the conclusion that the extreme anterior end of the brain closes very early and independently of the ordinary closure, the anterior neuropore being the space temporarily existing until these two regions of closure fuse. If this is the case this anterior end must close very much earlier, indeed, than the rest, for it is already separated by mesoderm from the ectoderm. The conclusion does not seem a reasonable one to me, and I also do not see any reason why, at this early stage of development, any part of the roof of the brain should be rotated down until it lies in line with the floor. The brain at this stage is a simple tube with dilatations representing the anlagen of its various parts, but there are no structural peculiarities that would indicate a displacement of the anterior end of the roof to the ventral surface. I think there is every ground to assert with Johnston (1910) : "Rather, as the neural plate rolls up, the neural tube tapers to a point in the preoptic recess, and the lamina terminalis is the anterior part of the seam of closure along the mid-dorsal line."


In conclusion I wish to briefly call attention again to the following points: These embryos are twins and show appearances and degrees of development which are almost identical throughout. Yet these embryos are not identical or "real" twins, as shown by the fact that they were not contained in one sac, but each embryo had its own individual membranes. These twins, therefore, are the product of the fertilization of two ova simultaneously or with only a very small interval between. This gives an opportunity for more variation than is possible with twins which are the product of one ovum, and produced by independent development of each cell of the 2-celled stage into an embryo. Yet in spite of this they show a remarkable correspondence. Two embryos more alike would be hard to find.

It is to be emphasized that these embryos are extremely valuable in bridging the large gap existing between embryos described to date, with 14-15 pairs of somites, and those of 22-23 pairs. That there may be no hesitation in accepting the work embodied in this paper, I can positively assert that there is no evidence or hint of any pathological condition in these embryos. They are in excellent condition for topographical work, but minute histological work on the cells can not be pursued on account of the slight damage resulting from the section cutting and from the unavoidable beginning of maceration before the embryos were extruded from the uterus. This damage, however, in no way interfered with the study of the form and relations of the various organs and parts of the body.

As my final word I wish to tender to Professor J. Playfair McMurrich my most sincere thanks for the loan of these valuable embryos and for furnishing me with material and all available facilities for prosecuting this study. It would be unfitting to close without also referring to his live, keen interest in the progress of this work and his pleasure in the results it has been my fortune to attain.

June 1, 1914.


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List Of Abbreviations used In Illustrations

List Of Abbreviations used In Illustrations
A. B. S Arterial blood sinus.

Ac. Fac. Gang. Acusticofacial ganglion.

All Allantois.

Ant. Neur. Anterior neuropore.

Ao Dorsal aorta.

Ao. P Aortic process of sclerotome.

At Atrium of heart.

A.V. C Atrioventricular canal.

B.A. 1 First branchial artery.

B.A. 2 Second branchial artery.

B.A. 3 Third branchial artery.

B.B Bladder bay of cloaca.

B.C Bulbus cordis of heart.

B. P. 1 First branchial pouch.

B. P. 2. Second branchial pouch.

B. P. 3 Third branchial pouch.

B. Ph. M Buccopharyngeal membrane, with perforations.

Br. U. A Branch of umbilical artery.

B. S Belly stalk.

Cav. Cavity of somite.

Cer. Hem Cerebral hemisphere.

Cl Cloaca.

CM.. Cloacal membrane.

Coel. Coelom.

D.A Dorsal aorta.

D.C Ductus Cuvieri.

Derm. Dermatome of somite.

Dien. Dieneephalon.

D.M. Dorsal mesocardium.

D.R Degenerative remains of tubules.

Ect. Ectoderm.

En. Endoderm.

Fus.E Fusion of ectoderm to branchial pouch.

Fus. N Fusion of notochord with cloaca.

Gloss. Gang. Glossopharyngeal ganglion.

Gr Groove on cloaca marking rectum from bladder.

H. G Hind gut.

Hyp. Hypophysis cerebri.

I.C.A Internal carotid artery.

I.C.M Intermediate cell mass.

Infund. Infundibulum.

I.S. A Intersegmental artery.

L.B Liver bay.

L.H. Left horn of sinus venosus.

L.M.G Lower myotomic groove.

L.U.A Left umbilical artery.

L.U.V Left umbilical vein.

MB Midbrain.

Mes Mesenchyme.

M. T Median thyreoid.

M. V Mesonephric vesicles.

My Myotome of somite.

N. C Remains of neurenteric canal.

N. Cd Nephrogenic cord.

Neur. l-'J Nine neuromeres of hindbrain.

Not Notochord.

N. P Notoehordal process of sclerotome.

Nr. Cr Neural crest.

Nr. S. C Neuromeres of spinal eord.

N. S Wall of central nervous system.

Nt. Cn Notoehordal canal.

Op. Ves Optic vesicle.

O. V Otic vesicle.

P. A. G Postanal gut.

Pc Pericardium.

Ph Pharynx.

P. N Posterior neuropore.

Pn. T Pronephric tubules.

P. S Primitive streak.

P. V. A Paired ventral aort*.

R Remains of pronephric tubules.

R.B Rectal bay of cloaca.

R.H Right horn of sinus venosus.

Rt. U. A Roots of umbilical artery.

R.U. A Right umbilical artery.

R.U.V Right umbilical vein.

R.V.A Right ventral aorta.

Scl. Sclerotome of somite.

Som. Wall of mesodermic somite.

Sp. G Spinal ganglion.

St Stomodseum.

S.V. Sinus venosus.

Tr Trichter or nephrostome.

Trigem. Gang. Trigeminal ganglion.

U. M. G Upper myotomic groove.

U. V Umbilical vein.

V. A Median ventral aorta.

Vag. Gang Vagus ganglion.

V. B. S Venous blood sinus.

V. C. A Vena cardinalis anterior.

V. C. M Vena capitis medialis.

V. C. P Vena cardinalis posterior.

Vent. Ventricle of heart.

Vit. A Vitelline artery.

V.U.T Vitello-umbilical trunk.

V.V. Vitelline vein.

W.D. Wolffian duct.

Y.S. Yolk sac.

A graphic transparent reconstruction of the embryo \'I except the nephrogenic system and mesodermic somites which are shown in text figure 3.

Arterial system is coloured red, venous system blue, alimentary system violet, notochord green, central nervous system light yellow, all ganglia and the ange yellow.

Figs. 1 to 11 are from Embryo VI; fig. 12 from Embryo V. All wore drawn with the aid of the camera lucida. Magnification X 300.

File:Watt1915 fig01.jpg

Fig. 1. Notochord at its cephalic end. Notochordal canal is evident.

Fig. 2. Notochord at level of first branchial pouch.

Fig. 3. Notochord over middle of yolk sac. Dorsal aorta is shown under the notochord.

Fig. 4. Notochord at level of sinus venosus. Notochordal canal is evident.

Fig. 5. Notochord at level of second branchial pouch.

Fig. 6. Notochord at posterior end of yolk sac, connected by neck of cells to endoderm.

Fig. 7. Notochord over caudal end of hind gut.

Fig. 8. Notochord over beginning of cloaca.

Fig. 9. Notochord over cloaca at level of allantois.

Fig. 10. Notochord over level of caudal edge of cl :al membrane, showing remains of ncurenterie canal.

Fig. 11. Notochord two sections before its fusion with the primitive streak.

Fig. 12. Notochord over cloaca in Embryo V, to show the. remains of the neurenteric canal. ( !ompare with fig. in. from Embryo VI.

On page 40 will be found a list of abbreviations applicable to all the plates in this paper.

1. Dorsal view of wax-plate reconstruction of the pharynx in Embryo VI.

2. Lateral view of wax-plate reconstruction of the pharynx in Embryo VI.

3. Lateral view of wax-plate reconstruction of the cloaca in Embryo VI.

4. Lateral view, from the left, of wax-plate reconstruction of the muscular tube of the heart in Embryo VI.

5. Lateral view, from the rij<ht, of wax-plate reconstruction of the brain in Embryo VI.

1. Ventral view of wax-plate reconstruction of the muscular tube of the heart in Embryo VI.

2. Dorsal view of wax-plate reconstruction of the muscular tube of the heart in Embryo VI.

8. Dorsal view of wax-plate reconstruction of the endothelial tube of the heart in Embryo VI.

4. Lateral view, from the left, of wax-plate reconstruction of the endothelial tube of the heart in Embryo VI.

Cite this page: Hill, M.A. (2024, April 22) Embryology Paper - Description of two young twin embryos with 17-19 paired somites (1915). Retrieved from

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