DEVELOPMENT OP THE CAPE SPECIES OF PERIPATUS. 495 



to the exterior (fig. 23 e) . This happened in a slightly younger 

 embryo. The opening is already covered over by the lip (L.), 

 which is rapidly growing backwards (cf. Part I^ fig. 35 ; embryo 

 with commencing cerebral grooves). 



Briefly to recapitulate, the structure of the third somite at 

 this stage is as follows : — In front it is dorsally placed and 

 overlaps the second somite; its middle portion slopes ventral- 

 wards and communicates with the posterior part, which is con- 

 tained in the limb, and is peculiarly bent (fig. 23 c — e), and 

 opens to the exterior. The middle portion further sends a 

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

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

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

 the coelom, 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 a dorsal {b. h.), and the 

 other ventral to the gut (6. 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 ot 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 



