EMBRYOLOGY 



395 



referred to as mesenchyme. During the period dealt with above, 

 the mesoderm in the middle region of the embryo along the side of 

 the notochord becomes more solid and thicker, and constitutes the 

 segmental plate or myotomal region. It is fairly sharply marked 

 off from the thinner lateral portion which is the lateral plate. A 

 cleft soon appears in this, thus forming the beginning of the dcelom, 

 or more strictly the splanchnoccel, and this divides the mesoderm 



M.R 



NX:. 



So 



FIG. 136. Rana sylvatica. Part of a section through the anterior 

 part of the body of a young embryo ; ectoderm and entoderm 

 represented in outline only. After Field. 



C., ccelom ; E., ectoderm ; E.G., enteric cavity ; En., entoderm ; M., mesoderm ; M.P., 

 medullary plate ; N., notochord ; N.C., neural crest rudiment ; So., somatic mesoderm ; Sp., 

 splanchnic mesoderm ; S.P., segmental plate. 



into somatic and visceral layers. A ccelom originating in this way as 

 a split in a mesodermal sheet is often referred to as a schizocoel as 

 opposed to an enteroccel as in Amphioxus. At first this cleft is con- 

 fined to the upper part of the lateral plate, and it does not extend 

 ventrally until considerably later, although it passes up into the 

 segmental plate giving rise to rudimentary myocoels. This stage 

 is reached as the neural folds begin to approximate, and as they do 

 so the longitudinal bands of mesoderm begin to get cut up into pairs of 



