256 PROFESSOE F. M. BALFOUR. 



There can be no doubt that the mesoblast so formed gives 

 rise to the great majority of the mesoblastic somites. 



This posterior opacity is marked in Stage C by a slight 

 longitudinal groove extending backwards from the hind end 

 of the blastopore. This is difficult to see in surface views, 

 and has not been represented in the figure, but is easily seen 

 in sections. 



But in Stage D this groove has become very strongly marked 

 in surface views, and looks like a part of the original blasto- 

 pore of Stage C. 



Sections show that it does not lead into the archenteron, 

 but only into the mass of mesoblast which forms the posterior 

 opacity. It presents an extraordinary resemblance to the 

 primitive streak of vertebrates, and the ventral groove of 

 insect embryos. 



We think that there can be but little doubt that it is a part 

 of the original blastopore, which, on account of its late appear- 

 ance (this being due to the late development of the posterior 

 part of the body to which it belongs), does not acquire the 

 normal relations of a blastopore, but presents only those rudi- 

 mentary features (deep groove connected with origin of meso- 

 blast) which the whole blastopore of other tracheates presents. 



We think it probable that the larval anus eventually shifts 

 to the hind end of the body, and gives rise to the adult anus. 

 We reserve the account of the internal structure of these embryos 

 (Stages A — E) and of the later stages for a subsequent memoir. 



We may briefly summarise the more important facts of the 

 early development of Peripatus capensis, detailed in the 

 preceding account. 



1. The greater part of the mesoblast. is developed from the 

 walls of the archenteron, 



2. The embryonic mouth and anus are derived from the re- 

 spective ends of the original blastopore, the middle part of the 

 blastopore closing up. 



3. The embryonic mouth almost certainly becomes the adult 

 mouth, i. e. the aperture leading from the buccal cavity into 

 the pharynx, the two being in the same position. The era- 



