78 THE CHANGES FROM STAGE A TO ¥. 



narrow diverticulum forwards for a short distance (fig. 23 b, 

 a. v.). The parietal mass of cells is larger than ia the last 

 stage, and contains several cavities, which are not derived from 

 the coelora, but arise independently in it. 



As in the earlier stages, there is a sheet of cells closely applied 

 to the ectoderm, and extending from the dorsal end of the 

 somite to the middle dorsal line (d. s.), and from the ventral 

 corner {v. s.) along the parietal mass of mesoderm and the 

 inner surface of the nerve-cord to the ventral ectoderm. Fur- 

 ther, the endoderm has entirely separated from the ectoderm, 

 so that two large spaces are left, the one dorsal {b. h.), and the 

 other ventral to the gut {b. be). These spaces contain a few 

 scattered, more or less branched cells, which appear to be 

 derived from the splanchnic walls of the somites, and are there- 

 fore probably mesodermal in nature. 



There are therefore four distinct systems of spaces present 

 in the embryo at this stage (e) : 



1. The cavity of the gut derived from a vacuolation of the 

 endoderm mass of the gastrula stage. 



2. The cavities of the somites, derived from a vacuolation of 

 the protoplasm of the mesodermal bands. 



3. The spaces which appear independently of the other spaces 

 in the parietal masses derived from the somatic walls of the 

 somites. 



4. The spaces formed by the separation of the endoderm from 

 the dorsal and ventral ectoderm, and derived in all probability 

 from the vacuoles found in a corresponding position in the 

 earlier stages (Plate V, fig. 24 a — d). These latter spaces 

 are comparable with blastocoele spaces of other embryos, so 

 called because they present tbe relations of the segmentation 

 cavity of the earlier stages. Such a name is, however, obviously 

 out of place here, inasmuch as the segmentation cavity is never 

 present. 



In the next stage (f) the dorsal division of the somite has 

 entirely separated from the ventral, so that the two parts may 

 be considered separately. 



The ventral division of the third somite presents the 



