GENERAL FEATURES OF CHORDATE DEVELOPMENT 



43 



irregular shape wander away by amoeboid movements to more or less definite 

 regions where they give rise to certain tissues (see Fig. 9). Those parts of the 

 mesoderm which do not become mesenchyme but retain their epithelial charac- 

 teristics are called mesothelium. 



2. The fate of the epimeres. — The medial wall of each epimere transforms 

 into a mass of mesenchyme cells which migrate to a position around the noto- 

 chord and there give rise to the vertebral column. This mass of mesenchyme 

 is known as the sclerotome (see Fig. 10). The outer wall of each epimere trans- 

 forms into mesenchyme cells which migrate to the under side of the ectoderm 

 and there give rise to the inner layer (dermis) of the skin. This part of the 

 epimere is called the dermatome (Fig. 10). The remainder of the epimere persists 



neural tube 

 sclerotome / notochord neural tube 



myotome de ™atome \ / notochord 



Fig. 10. — Diagram of cross-sections of vertebrate embryos to show the differentiation of the epi- 

 mere into dermatome (skin-producer), myotome (muscle-producer), and sclerotome (skeleton-producer). 

 Dermatome and sclerotome consist of mesenchyme, myotome of mesothelium. In B and C the derma- 

 tome is seen spreading beneath the ectoderm to form the dermis of the skin; the myotomes are growing 

 ventrally to form the muscle layer of the body wall; the sclerotome is accumulating around the noto- 

 chord at x to form the vertebrae; and the hypomere incloses the intestine producing the dorsal and 

 ventral mesenteries and the mesoderm of the intestine. 



in place as mesothelium and is known as a myotome or muscle segment. Each 

 myotome becomes separated from the adjacent ones by a connective tissue 

 partition, the myocomma or myoseptum. The myotomes give rise to the volun- 

 tary muscles of the body (with certain exceptions). Each grows from its original 

 dorsal position ventrally between the ectoderm and the hypomere to the median 

 ventral line where it meets its fellow from the opposite side. There is thus 

 produced a complete muscular coat for the body (see Fig. 10; also K, pp. 15, 

 17; W,p. 64). 



3. The fate of the mesomere. — The mesomere gives rise to the kidneys, the 

 reproductive organs, and their ducts (the terminal portions of the urogenital 

 ducts may have ectodermal or entodermal linings) . 



