MESOTHELIAL STRUCTURES. lOI 



tion along either side of the notochord and the central nervous 

 system, while below the pouches grow until they meet in the 

 mid ventral line, below the entoderm. In each coelomic pouch 

 three horizontal zones are to be distinguished, — a dorsal muscle- 

 plate or myotome zone (epimere), a ventral lateral plate zone 

 (hypomere), and between these a much narrower middle zone 

 (mesomere). 



Fig. I lo. Diagram of the mesothelial pouch and the beginning of segmenta- 

 tion, based upon Aiiiblystoma. a, anus ; c, coelom ; e, epimere ; h, hypomere ; hp, 

 hypophysis; m, mesomere; mt, mouth; n, notochord; o, eye; s, spinal cord. 



By a series of constrictions not easily described, but readily 

 made out from the figure, epimere and mesomere become divided 

 transversely to the body axis into a series of cubical bodies, the 

 protovertebrae of older authors, the myotomes of recent embry- 

 ology. These divisions do not extend into the hypomere, and 

 so do not divide the ventral part of the coelom. As a result we 

 have below a single coelomic space extending the length of the 

 trunk, which connects with a number of dorsal coelomic diver- 

 ticula, extending, one into each myotome. Later, horizontal con- 

 strictions cut the epimeral portions off from the rest, so that from 

 this region there arises, on either side of the body, a series of 

 completely closed cavities with epithelial walls, — the myotomes. 

 To avoid confusion with that portion of the primitive body cavity 

 (metacoele or splanchnocoele) which remains between the lateral 

 plates, and to which the term coelom as usually applied is given, 

 the coelomic pouches in the myotomes have been called the 

 myocoeles. The myotomes give rise to the voluntary muscles 

 cf the body in a manner shortly to be described ; the modifi- 



