THE LATER DEVELOPMENT OF THE CHICK 353 



(c) the sclerotome. The somites form as solid segmental cell 

 masses; their superficial cells are arranged as a rather dense 

 wall, epithelial in character, which encloses a loosely arranged 

 central mass of mesenchymal nature. The densely arranged 

 cells soon become limited to the dorsal and dorso-lateral 

 regions of the somites. The dorsal portion, in particular the 

 region toward the nerve cord, forms the rudiment of the 

 myotome or muscle plate, while the dorso-lateral region gives 

 rise to the cutis plate. In the more loosely arranged core, the 

 formation of intercellular substance begins very early, pro- 

 ducing a truly mesenchymatous structure. This part of the 

 somite then extends over toward the notochord and nerve 

 cord, as the rudiment of the sclerotome. 



The myotome becomes thin and turns under the thicker 

 cutis plate, finally extending downward and outward entirely 

 beneath it, occupying a position between the spinal ganglion 

 and the cutis plate. The cells of the myotome elongate antero- 

 posteriorly, through the whole extent of the segment, and each 

 becomes converted into a striated muscle fiber. Later the 

 myotomes enlarge, as their component cells multiply and grow, 

 and each extends down into the body wall. Opposite the limb- 

 buds, outgrowths of the myotomes extend into these, forming 

 their musculature. The entire voluntary musculature of the 

 chick develops from the myotomes; the involuntary muscula- 

 ture is mesenchymal in origin, chiefly splanchnic. The cutis 

 plate extends laterally, as the embryo grows, and after thinning 

 considerably, breaks up into a mesenchyme which spreads 

 underneath the ectoderm, forming the foundation of the thin 

 dermis layer of the integument. 



The sclerotomal cells, multiplying and continuing the for- 

 mation of intercellular substance, extend dorsally, between 

 the nerve cord and the myotome, and ventrally, around the 

 notochord and dorsal aorta, and finally fill all the spaces around 

 these axial structures. Later the sclerotomes acquire a secon- 

 dary segmentation, in that each becomes transversely divided 

 opposite the middle of the somite; the posterior half of one 

 sclerotome then unites with the anterior half of the succeeding, 



