THE POSTERIOR SOMITES. 93 



out life as a vesicle lying in the leg compartment of the body 

 cavity (pseudoccele) and receiving the internal so-called funnel 

 of the nephridium, has hitherto escaped notice. 



The fate of the median compartments of the ccelom of the 

 jaws, oral papillae^ and legs 1 — 15, I have already described. 

 Except in the case of the second and third somites, in which they 

 coalesce in Stage r, they retain their segmentation until their 

 disappearance. This takes place in Stage f first at the level 

 of about the fifth leg, proceeding backwards and forwards. It is 

 preceded by the diminution in size of the cavity (Plate IX, 

 fig. 45), and the development of the pericardium. 



An indication of the somites remains for a short time as a 

 thickening in the floor of the pericardium, with which thickening 

 the ventral wall of the heart is fused (Plate IX, fig. 46, d. s.). 

 The floor of the pericardium soon, however, separates from 

 the ventral wall of the heart, so that the two halves of the 

 pericardial cavity become continuous. At the same time the 

 dorsal wall of the heart separates from the dorsal body wall, 

 and the heart then forms a tube lying quite freely in the peri- 

 cardium. The cardiac ostia seem first to appear in Stage f, 

 and are confined in the Cape Peripatus, at any rate, to the 

 posterior part of the heart. 



The Development of Somites 16 — 20 differs from that just 

 described only in so far as concerns the median divisions. The 

 lateral divisions proceed in exactly the same way as in the 

 anterior somites. The median divisions, however, contain in 

 their splanchnic walls the germinal nuclei, and persist through- 

 out life as the generative tubes. Their development therefore 

 requires a special description. 



The Generative Organs. 



During Stage d a number of especially large (as large as the 

 largest of the ordinary endoderm nuclei), round, granular 

 nuclei appear in clusters in the dorsal endoderm of the hind 

 end of the body. Their anterior limit is the sixteenth somite. 



