XXXVI. 



THE DIFFERENTIATION OF THE SOMITES 



The somites have already been described as almost solid triangular 

 blocks of cells derived from the dorsal mesoderm. There is a tiny open- 

 ing in the center running horizontally through the somite. Oftentimes 

 the opening cannot be seen at all. This opening is called the myocoele. 

 As the embryo continues to increase in size, the triangular block 

 becomes more or less circular and there are two layers of cells, an outer 



epithelial layer and an inner 

 portion (Fig. 298). The inner 

 portion has its cells irregularly 

 arranged. 



It is this ill-defined group 

 of cells which is known as the 

 sclerotome. The cells are 

 mesenchymal. 



The sclerotomes of each 

 side now grow still farther 

 toward the notochord and sur- 

 round it. Later they develop 

 into the vertebrae. 



The dorsal portion of the 

 outer cell mass whose more 

 medial portion became the 

 sclerotome, has retained its 

 definite outlines and epithelial 

 characteristics. This portion, 

 now called the dermatome, is to 

 become the deeper layer of the 

 integument. It is important to 

 remember at this point that the ectoderm gives rise to the epithelial layer 

 of the integument only. 



The portion of the cell mass which lies medial and slightly ventral 

 to the dermatome is called the myotome. The myocoele now lies between 

 the dermatome and the myotome. It is from the myotome that the 

 entire skeletal musculature is developed by the ventral walls of the 

 myotome becoming converted into longitudinal muscle fibers. These 

 bands of fibres then remain divided into blocks which correspond to the 

 original somites. Here again we have a metameric arrangement of mus- 

 cles in the embryo of the chick which corresponds to the segmental ar- 

 rangement of muscles in the adult fish. 



Fig. 298. 



Diagram of Myotome and Nerve Development. 

 The more dorsal portion of the somatopleure is 

 known as the dermatome while the dorsal portion of 

 the splanchnopleure lying closest to the dermatome 

 forms the sclerotome. 



