THE SOMITES OF THE CHICK 89 



In an embryo of fifty-two segments (H. E. C. no. 344, 5 days 

 16 hours) which, however, is considerably older than the one of 

 fifty-one segments, the mesenchymal mass (fig. 18, S) formed by 

 the fusion of the sclerotomes with the hypomerous mesenchyma 

 almost surrounds the dermomyotomes of the forty-fourth seg- 

 ment and meets above the neural tube. The dermomyotome is 

 still intact notwithstanding the conversion of the central part of 

 the cutis plate into mesenchyma. The lower edge of the dermo- 

 myotome and that of the myotome are somewthat enlarged. The 

 upper edges of both are thin. 



In transverse sections we find, in the upper part of the sections, 

 four layers of tissue between the ectoderm and the notochord 

 namely, a layer of mesenchyma (fig. 18, S) which extends upward 

 between the dermatome (D) and the ectoderm nearly or quite to 

 the top of the dermomyotome ; a second layer of mesenchyma rep- 

 resenting the central part of the cutis plate (D) ; the myotome (M) ; 

 and a thick layer of sclerotomic tissue which is divided into a 

 lateral zone, of dense and a medial zone of looser tissue. 



In a somewhat older embryo (H. E. C. no. 485, 5 days, 18 hours) 

 the dermal mesenchyma has fused with that between the dermo- 

 myotome and the ectoderm, and the resulting mass of tissue is 

 supplied with a rich plexus of blood vessels. The lower edge of 

 the myotome bears a small epithelioid cap which represents a 

 much reduced ventral zone of growth. The upper edge of the 

 myotome bears a somewhat rounded zone of growth. 



SUMMARY AND CONCLUSIONS 



Although the somites have the same fundamental structure 

 in all parts of the body, they differ greatly in many respects. 



The first somite at its origin is the smallest of the entire series, 

 with the possible exception of some of the degenerate caudal 

 somites, and each succeeding somite of the neck and trunk is 

 larger than the preceding somite. 



Every somite, owing to the rapid growth of the anterior part 

 of the embryo, is larger at any one time than any of the following 

 somites. 



