1885.] 



The Development of Peripatus Capensis. 



359 



established by the spiral stage, when the primitive streak vanishes, and 

 the anus shifts to the hind end of the body. In the original account 

 of the early stages in Balfour's memoir, it was stated by the editors 

 (p. 256) that the greater portion of the mesoderm was formed from 

 the primitive streak. 



I am now in a position to state that the whole of the mesoderm 

 originates in this manner. 



On the view that the primitive streak is the remains of a portion of 

 the blastopore and the ingrowth of cells from it a modified develop- 

 ment of that part of the ancestral archenteron which gave rise in the 

 ancestor to the somites, the mesoderm of Peripatus is to be regarded 

 as developing from the walls of the archenteron.* 



The mesoblastic bands at once become segmented, a cavity appearing 

 in each somite as it becomes marked off. 



The anterior somite, which is the first to^be formed, is at first 

 placed some little distance behind the front end of the blastopore. It 

 soon shifts forward, and by stage E is placed in front of the embry- 

 onic mouth. 



The somites are from the first placed at the sides of the mesenteron. 

 When the appendages are formed they send prolongations into 

 them ; this remark applies to the antennas as well as to the other 

 appendages. 



The body cavity of the adult is very complicated, and divided into 

 several parts, viz. : (1) the main central division containing the gut : 

 (2) two lateral chambers separated from (1) by a longitudinal vertical 

 septum, and containing the nerve cords ; (3) the body cavities of the 

 legs, separated from (2) by a vertical septum, and containing the 

 segmental organs and crural glands ; (4) the pericardial cavity. Of 

 these (1) and (2) are derived, not from the cavity of the somites, but 

 from a space which is formed by the separation of the endoderm from 

 the ectoderm, and from the inner wall of the somites, these being at 

 first in contact. At first these divisions of the body cavity have no 

 special lining of cells, but subsequently migratory cells from the 

 somites apply themselves to the endoderm and ectoderm, and form 

 the very sparse lining found in the adult. 



The cavities in the legs are derived from the cavities of the somites, 

 the changes of which I will now shortly relate, so far as I have been 

 able to follow them. I regret to say that there are one or two points, 



* It was from this point of view that the editors of Balfour's memoir {op. 

 cit.) made the second statement at the bottom of p. 256 with regard to the meso- 

 derm. This statement, which runs thus, " The greater part of the mesoblast is 

 developed from the walls of the archenteron," is obYiously at variance with the 

 statement already referred to at the top of the same page, unless considered in the 

 light of the above hypothesis. It was meant to be so considered, and I regret that 

 the omission of the explanation now given has led to some misunderstanding. 



VOL- XXXVIII. 2 C 



