DEVELOPMENT OF THE CAPE SPECIES OF PERIPATUS. 499 



is practically attained. The remaining somites conform, on 

 the whole, to the type described. They do, however, present 

 certain differences, of which, perhaps, the most important are 

 found in the posterior part of the body. In the posterior 

 somites the dorsal divisions do not become obliterated, but 

 persist and give rise to the generative glands (Pis. XXXVI, 

 XXXVII, figs. 41, 43, d. s.=ffen. o.). It will be more 

 convenient, however, to defer the detailed consideration of 

 this and other peculiar features of the remaining somites 

 until after the description of the changes by which the adult 

 body cavity, pericardial cavity, and heart are formed. 



The Body Cavity and Vascular System. 

 I have already described the first appearance of the body 

 cavity. It arises in Stage d as a space between the dorsal 

 ectoderm and the endoderm (PL XXXIV, fig. 13 b. h.), and 

 between the ventral ectoderm and the endoderm {b. be), There 

 also appears at the same stage a space in the parietal thickening 

 of the walls of the somites (PI. XXXIV, fig. 13, b. lat.). In 

 later embryos of Stage d (PI. XXXV, figs. 17c; — d), these spaces 

 are all more marked, and cells — apparently amoeboid warf- 

 derers from the walls of the somites — have made their appear- 

 ance in the two former (PL XXXV, fig. 17 a — d.). These cells 

 apply themselves to the ectodermal and endodermal walls of the 

 chambers in which they are contained, and so form the founda- 

 tion of the mesodermic investment by which the body cavity 

 of the adult is lined. In the next stage the cavities b. h., b. be, 

 remain unchanged ; but the cavities in the parietal thickenings 

 become definitely established (PL XXXV, fig. 21 a — c, b. lat. 

 The latter at this stage appear to be segmentally arranged ; 

 each one beginning at the anterior end of a somite, and extend- 

 ing backwards to the level of the appendage, in the mesoderm 

 thickening of which it is lost. They are bounded internally 

 by the septum which runs from the ventral border of the somites 

 along the inner side of the nerve-cord to the ventral body wall, 

 and externally by a mass of mesoderm cells which project into 

 what I have called the limb-ridge (PL XXXV, fig. 21 a, I. r.), 



