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Wen, I. C., The anatomy of human embryos with seventeen to twenty-three pairs of somites J. Comp. Neurol, 45, 301-376.

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The Anatomy of Human Embryos with Seventeen to Twenty-three Pairs of Somites

I Chuan Wen

Hull Laboratory of Anatomy, University of Chicago

Eight Heliotype Plates and Twelve Text Figures

Introduction

Many problems in early human development remain still unsolved. Studies of many more embryos from the period of somite formation will be necessary before accurate accounts of this stage of development can be written. It is important, accordingly, that every young specimen be described as fully as possible and its characteristic features be compared with those of the few similar embryos that are available for study, as well as those described in the literature. In the present paper an embryo with 17 somites (H951, Emb. Coll., Department. of Anatomy, University of Chicago) will be studied in detail and compared with embryos of 22 (H1903) and 23 (H984) somites of the same collection. The 17—somite specimen may also be compared with a 15—somite embryo of Giglio-Tos (’02), a 16-somite embryo of the Carnegie Embryological Collection (no. 470), twin embryos with 17—19 somites of Watt (’15), and the 20-somite embryo of Davis (’23). It differs, however, from all of the above mentioned in having certain features that throw light upon questions left unsettled in the others.

This work was carried on under the direction of Prof. G. W. Bartelmez. It is a pleasure to me to acknowledge my indebtedness to him for his constant aid, helpful criticism, and, above all, inspiring guidance. To Prof. R. R. Bensley and Prof. C. Judson Herrick my thanks are due for the training I received from them and for their courtesies shown to me during the course of this work. It is also a pleasure to me to express my appreciation of the courtesy of Prof. George L. Streeter for the privilege of studying the material belonging to the Carnegie Institution, Laboratory of Embryology. I have also received technical help from Miss Nixon, of Chicago, Mr. O. 0. Heard and Mr. J. F. Didusch, of Baltimore, to all of them I wish hereby to express my thanks.

Material and Methods

The embryos herein studied are listed in table 1.

Embryo H951 (fig. 1, A) was obtained from Drs. E. L. Whitney and J. T. Rocks, of Walla Walla, Washington. “The uterus was removed for fibrosis on the 13th day after the day on which menstruation was expected to begin,” opened and fixed in ‘4%’ formalin. No further clinical information could be obtained. The embryo was mordanted in aqueous chrom—sublimate, slowly dehydrated, double embedded in celloidin and paraffin, cut into transverse sections, and stained with iron hematoxylin (all but one slide being counterstained with eosin). During the mounting, five sections were slightly distorted and, unfortunately, three sections lost, so it is not easy to follow the series at the region between the 4th to 5th somites. A plaster model of the head region (figs. 2, 3), a wax model of the entire embryo, and one of the anterior portion of the central nervous system with the ventral pharyngeal wall were made from serial photomicrographs at the magnification of 150 times. Two other plaster models, one of the entire embryo and one of the head region alone showing the anatomy therewith, were made by O. O. Heard. Histologically, this specimen is in excellent condition.

Table 1

Number And Dbl:-E()Iirs1I1(Sii,{§ Source Or

Designation	Number of Somites	Greatest Length	Dimensions of Ovum (including villi)	Source of Specimen
H951 (University of Chicago Collection)	17	4.0 mm. in 80 per cent alcohol	336 sections at 10 µ	23.1 x 10.4 x circ.11 mm.	Hysterectomy
H1093 (University of Chicago Collection)	22	3.8 mm. (dors. ﬂex.)	323 sections at 10 µ	14.5 x 11 x 7 mm.	Tubal pregnancy
H984 (University of Chicago Collection)	23	2.8~2.9 mm. in 95 per cent alcohol	349 sections at 8 µ	26.7 x 22.7 x 17.3 mm.	Abortion

Embryo H1093 (fig. 1, B) was obtained by Dr. Garrette Van Sweringen and Dr. W. W. Duemling, of Fort Wayne, Indiana. The clinical record gives the following points: “The October period began on the 6th, 1924; next period from 10th to 14th or 15th. The expected period on December 10th did not appear. Flow supposed to be menstruation appeared on December 22nd, but bloody discharge continued until 31st.

Contraceptive measures were employed until after November period. Symptoms of ectopic pregnancy were observed on January 4, 1925, and operation was performed on the same day.” In cutting open the hematoma, the head of the embryo was severed from the trunk just caudal to the otocyst. Although no loss of tissue is apparently involved, yet the difference in the plane of sections of the two portions of the embryo makes it materially difficult to follow the series in the region of rhombomere VI. The specimen was fixed in neutral formalin, double embedded, cut transversely, and stained with iron hematoxylin. Histological differentiation is fair, but there is much shrinkage. Cells with coarse granules evidently representing necrobiosis are found abundant in the neural crest as well as the medullary tube. A wax model of the nervous system in front of the otocyst was made from serial photomicrographs at the magnification of 150 times.

Embryo H984 (fig. 1, C) was donated to the collection by Drs. E. W. Rawson and B. H. Foreman of Tacoma, Washington. It came from an abortion and after remaining in weak formalin for several years it was mordanted in aqueous chrom-sublimate, embedded in paraffin, cut into transverse sections, and stained with iron hematoxylin. The material was macerat.ed and considerable shrinkage and cracking occurred during dehydration. Histological differentiation is poor. Serial photomicrographs at the magnification of 125 times were made, but no model has been attempted.

External Form

The external form of the 17-somite embryo may be ‘seen in figure 1, A. The features which are of chief value for seriation at this stage of development are the cranial flexure, optic vesicles, Visceral arches, otic pits, and the pronephric ridges. All of these except the last named are recognizable in figure 2, which presents the appearance of the left-side head region. In opening the uterus before fixation, the embryo was somewhat compressed and twisted, but this side of the head shows little evidence of distortion. The pronephric ridges are, however, not recognizable in ﬁgure 1, A, for they are obscured by the twisting of the embryo’s trunk in the region involved.

The cranial ﬂexure appears to be an arc of a circle with a very gentle curvature involving obviously much more than the midbrain (fig. 1, A). According to the model (figs. 2, 3), which shows the shrinkage due to the technical procedure, the midbrain stands out clearly at the extremity of the embryo and the ﬂexure upon the surface is represented by a slightly elevated eminence. This relation of the midbrain forming the knee of the cranial ﬂexure was found by Bartelmez (’23) in a series of younger human embryos. Cephalad to the cranial ﬂexure, the dorsal contour is gently curved until a median ﬂattened area is reached. This is the site of the closing anterior neuropore. In the model the optic vesicle forms a distinct eminence on the side of the head. Caudal to the optic vesicle, we come to the branchial region. Two distinctly marked branchial arches — mandibular and hyoid - are recognizable. At its proximal border the mandibular arch shows an indication of the maxillary process, which in this stage of development is but a narrow and small swelling extending ventral to the optic vesicle from which it is marked off by a shallow groove. The arches are elevated considerably above the gill membrane, so that we may speak of a hyomandibular furrow. The mandibular arch is a bluntly wedge—shaped eminence with its ventrally directed apex resting upon the heart region. The hyoid arch is similar in shape, but much smaller and shorter. Its caudal border is separated from the third arch by a shallow and broad fossa — the second gill furrow. Like the first arch, the extremity of the second also reaches the cardiac eminence. As to the rest of the arches, very little can be said of them, since no boundaries are visible on the surface. In the external view the third arch is not delimited caudally. Although a clear-cut picture of this arch is not present on the external form, yet the third entodermal pouch can be recognized in the sections. Therefore, it would seem justified to warrant the assumption that the third arch is in the process of formation. A little caudal to this level but dorsally, the first somite makes its appearance on each side, occupying a position relatively more posterior than that of Wallin’s (’13) embryo (13 somites) and 0.0. 470 (16 somites; compare Bartelmez and Evans, ’26, figs. 16-17, pl. 7). The fourth arch, though appearing merely to be a tiny rounded eminence on the surface, is without any clear outline whatever. The entire branchial region is bounded ventrally by a limiting sulcus of the cardiac eminence marking the pericardial area from the rest of the body lying dorsal to it. Starting with the site of the oral sinus, this sulcus appears first as a deep groove curving on each side of the embryo between the extremities of the mandibular and hyoid arches and the heart swelling, then it gradually fades out at the cephalic border of the third arch.

From the above description of the development of the branchial region, the 17—somite embryo H951 seems to fit well the gap between the 16—somite embryo 0.0.470 and the 20—somite one described by Davis (’23). The head region of the former is sufficiently like that of H951; whereas in the latter, Davis recognized three definite gill arches, the presence of which undoubtedly marks a step further in development. Furthermore, H951 is also perfectly in harmony with other embryos of about the same age of development, so far as the visceral arches are concerned. In J anosik’s (’87) embryo of 3 mm. in length and Watt ’s (’15) twin embryos of 17-19 somites, only two gill furrows are visible externally. In the slightly older embryos, such as Van den Broek’s (’11) with 22—23, Thompson’s (’07) with 23, and Johnson’s (’17) with 24 somites, the presence of an additional gill groove, the third gill furrow, has been recorded. In this respect, the 17-somite embryo falls well in the first group rather than bridging over the gap between the two.

Turning to the dorsal aspect of the second visceral furrow, the otocyst in this stage of development is still wide open to the outside, with a definite ridge seen around the edge of the

Cite this page: Hill, M.A. (2024, April 30) Embryology Paper - The Anatomy of Human Embryos with Seventeen to Twenty-three Pairs of Somites. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_Anatomy_of_Human_Embryos_with_Seventeen_to_Twenty-three_Pairs_of_Somites

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