OF PERIPATUS CAPENSIS. 911 



same position as that of the aclult ; but that the anus is consider- 

 ably in front of the hind end of the body in Stage E, while it is 

 terminal in the adult. 



If the embryonic mouth and anus do become the adult mouth 

 and anus, there would appear to be an entire absence of stomo- 

 dseum and proctodaeum in Peripatus, unless the buccal cavity 

 represents the stomodaeum. The latter is formed, as has been 

 shewn by Moseley, by a series of outgrowths round the simple 

 mouth-opening of the embryo, which enclosing the jaws give rise 

 to the tumid lips of the adult. 



For our determination of the posterior and anterior ends of 

 each of these embryos, Stage A to E, we depend upon the 

 opaque tissue seen in each case at one end of the blastopore. 



In Stage A it has the form of a band, extending backwards 

 from the blastopore. 



In Stages B D, it has the form of an opaque mass of tissue 

 occupying the whole hind end of the embryo, and extending a 

 short distance on either side of the posterior end of the blas- 

 topore. 



This opacity is due in each case to a proliferation of cells of 

 the hypoblast, and, perhaps, of the epiblast (?). 



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 blastopore 

 of Stage C. 



Sections shew 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 pri- 

 mitive 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 



