THE MESOI3LASTIC SOMITES. 243 



wedge-shaped, being thickest next to the notochord and gradu- 

 ally thinning as it passes outwards towards the margin of the 

 blastoderm (cf. Fig. 117, M). 



About the twenty-first hour of incubation, the mesoblast 

 cells become arranged more or less clearly in two layers, upper and 

 lower, with a slight space between them. This splitting of the 

 mesoblast, as it is termed, first appears in the part of the meso- 

 blast beyond the embryo, but soon spreads inwards to the em- 

 bryonic region, extending almost up to the notochord. 



The cavity, formed in this way, by splitting of the mesoblast, 

 becomes the ccelom or body cavity of the chick. Of the two layers 

 into which the mesoblast is split, the upper or outer is spoken of 

 as the somatic layer, and the lower or inner as the splanchnic 

 layer. From a very early period the somatic layer (Fig. 129, ME) 

 becomes closely connected with the surface epiblast, forming with 

 this the somatopleure or body wall ; while the splanchnic layer 

 becomes similarly related to the hypoblast, and forms with this the 

 splanchnopleure or wall of the alimentary canal (Fig. 129, MH). 



A body cavity that is formed in this way, by splitting of the 

 mesoblast into somatic and splanchnic layers, is spoken of as a 

 schizoccel, in contradistinction to the enterocoel of Amphioxus, 

 which arises as a series of hollow outgrowths from the enteron 

 or primitive alimentary canal. Inasmuch as the mesoblast of 

 the embryo is derived almost entirely from the hypoblast, as 

 described above, the distinction between an enteroccel and a 

 schizoccel may be said to consist in this : in the enteroccel the 

 mesoblast arises as hollow outgrowths from the hypoblast, which 

 subsequently become shut off from the gut, while the cavities of 

 the outgrowths open into one another and become the ccelom of 

 the adult. In the schizoccel, on the other hand, the mesoblast 

 arises as two solid sheets, budded off from the hypoblast, in 

 which the ccelom is formed at a later stage by splitting of the 

 sheet into two layers, with a space between them. Of these two 

 methods of formation of the ccelom there can be little doubt that 

 the enteroccelic is the more primitive one, the schizoccelic the 

 more modified. 



Almost immediately after the splitting of the mesoblast is 

 effected, about the twenty-second hour, a series of clear trans- 

 verse lines, really vertical clefts through the mesoblast, appear in 

 the embryo, extending outwards a short distance each side of the 



H 2 



