THE SOMITES OF THE CHICK 81 



with the ectoderm of the dorsal surface of the embryo an angle of 

 76.° This part of the sclerotome is thus carried forward one-third 

 or one-fourth the length of the somite. The succeeding interseg- 

 mental fissures are more and more vertical, the fifteenth being 

 the first that is without any inclination forward. The sclero- 

 tomes of the first fifteen somites, except the first which, it will be 

 remembered, is not separated from the unsegmented mesoderm 

 of the head, differ from those of the remaining somites in having 

 this peculiar forward prolongation which, although it can be ob- 

 served readily in transverse and sagittal and often in frontal sec- 

 tions, has never before been described. The obliquity of the 

 sclerotomes of the first, and presumably the most primitive seg- 

 ments coupled with the well-known obliquity of the first one or 

 two intersegmental clefts gives color to the suggestion that the 

 segments of the chick are typically oblique rather than trans- 

 verse structures and, consequently, that each vertebra arises 

 from a single segment. 



The lower myotomic groove deepens rapidly and, in embryos 

 of twenty segments, appears on the posterior wall of the somites 

 of the tenth segment. Neural crest cells, migrating downward, 

 gather in the deep anterior part of this groove. The right and left 

 aortae are now moving toward the median plane and consequently 

 separate the notochord more widely from the entoderm. 



The cutis plate of the tenth somite of embryos of twenty-five 

 segments has nearly the same structure as that of the second so- 

 mite of embryos of eighteen segments, except that, being composed 

 of a greater number of cells, it more closely resembles a stratified 

 columnar epithelium. 



The deepening of the lower myotomic groove has now reduced 

 the connection between the dermomyotome and the sclerotome 

 to a slender stalk. This process accompanies the differentiation 

 of the scelerotome into two conspicuous but not sharply defined 

 regions. The medial portion of the sclerotome becomes less 

 dense owing in part at least to its expansion into the space beside 

 and below the notochord; its lateral part, on the other hand, 

 becomes denser. The right and left aortae have united in this 

 segment forming the median cylindrical aorta, which forces the 



The American Joohxaf. of Axatomy, Vol. II, No. 1. 



