144 A HUMAN EMBRYO OF TWENTY-FOUR PAIRS OF SOMITES. 



end of the medial wall is again distinct. This somite is similar in appearance to 

 Kollman's 24 plate 1, figure 1, the myotome of a human embryo of three weeks, 

 but its dermatome contains fewer layers of cells than pictured by Kollman. 



In the sixth somite (fourth cervical) the inner lamella is thicker, particularly 

 at its anterior end. The dermatome is also slightly thicker and larger. Its cells, 

 of which there are from one to two layers, are distinctly columnar. Mitotic figures 

 are numerous and again confined to the upper ends of the cells. The myoccele is 

 reduced to a small cleft between the medial and lateral lamellse. Caudally and ven- 

 t rally the intervertebral cleft is again seen distinctly. This somite is probably 

 similar to those of the lower thoracic region of Ingalls's embryo. 



The fourth somite (second cervical) is not so far developed as the first thoracic 

 as described by Ingalls. It is interesting to note, however, that it is quite similar 

 in structure to the second somite of a 25-segment chick as described by Williams 49 . 

 The dermo-myotome is a flattened quadrilateral body lying just beneath the 

 ectoderm. Its lateral and medial lamellae are closely approximated, there being 

 no evidence of a myoccele. The breaking-up of the dermatome, as described 

 by many writers, is now beginning, as is indicated by the sending out of a few 

 protoplasmic processes of the outer portion of the dermatome to the covering 

 ectoderm. 



The third somite (first cervical) is similar to the fourth, but shows a somewhat 

 more broken-up condition. Its dermotome lies almost in contact with the outer 

 ectoderm and its cells are beginning to send out processes. The cells of the 

 myotome are also beginning to undergo further differentiation, for their spindle- 

 like forms can be made out. 



The second and first somites (occipital) are not definitely marked off from 

 each other. The second shows a slightly more advanced condition than the third. 

 The first is small. Its dermo-myotome is distinct, but the outlines of the sclero- 

 tome are lost. It is also not definitely separated from the mesenchyma in front 

 of it. 



CHORDA DORSALIS. 



Throughout its whole extent, the chorda dorsalis lies just ventral to the 

 medullary tube, the curvatures of which it closely follows. Its anterior end, 

 which begins opposite the point at which the remnants of the oral membrane are 

 attached to the roof of the mouth, is flattened dorso-ventrally and makes a slight 

 bend to the right. Caudal to this flattened portion, the chorda assumes in general 

 a cylindrical shape, although in some places it is flattened either dorso-ventrally 

 or laterally, while in other places it is triangular in cross-section. It terminates 

 caudally in the tail by joining the undifferentiated cells of the primitive-streak 

 region (text-fig. 9). 



An examination of the chorda dorsalis shows that it is not everywhere of 

 uniform size, but that it is alternately expanded and constricted. In order to 

 determine whether the expanded portions are arranged in any way with reference 

 to the body segments, a wax reconstruction of a portion of the chorda was made; 

 but owing to its irregular shape it is difficult to determine from the reconstruction 



