STRUCTURE OF FIFTY-HOUR CHICKS IO7 



arrangement. In the center of the somite a cavity is usually 

 discernible. This cavity is at first extremely minute. In 

 somites which have been recently formed it may be altogether 

 wanting. 



As the somite becomes more sharply marked off the radial 

 arrangement of the outer zone of cells appears more definitely 

 (Fig. 38, B). The boundaries of the central cavity are con- 

 siderably extended but its lumen is almost completely filled by 

 a core of irregularly arranged cells. In sections which pass 

 through the middle of the somite, this central core of cells is 

 seen to arise from the lateral wall of the somite where it is 

 continuous with the intermediate mesoderm. 



A little later in development the outer zone of cells on the 

 ventro-mesial face of the somite loses its originally definite 

 boundaries and becomes merged with the central core of cells. 

 This ill-defined cell aggregation, known as the sclerotome, be- 

 comes mesenchymal in characteristics, and extends ventro- 

 mesiad from the somite of either side toward the notochord 

 (Fig. 38, C and D). The cells of the sclerotomes of either side 

 continue to converge about the notochord and later take part 

 in the formation of the axial skeleton. 



During the formation of' the sclerotome the dorsal part of 

 the original outer cell-zone of the somite has maintained its 

 definite boundaries and epithelial characteristics. The part of 

 this outer zone which lies parallel to the ectoderm is known as 

 the dermatome (Fig. 38, C and D). It later becomes asso- 

 ciated with the ectoderm and forms the deeper layers of the 

 integument, the ectoderm giving rise to the epithelial layer 

 only. 



The dorso-mesial portion of the outer zone of the somite be- 

 comes the myotome. It is folded somewhat laterad from its 

 original position next to the neural tube (Fig. 2,^, C) and comes 

 to lie ventro-mesial to the dermatome and parallel to it (Fig. 

 38, D). (A later stage in the differentiation of the somite is 

 shown in Figure 44). The portion of the original cavity which 

 persists for a time between the dermatome and myotome 

 is termed the myocoele. The myotomes undergo the most 

 extensive growth of any of the parts of the somite, giv- 

 ing rise eventually to the entire skeletal musculature of the 

 body. 



