486 A. J. MASTERMAN. 



mouth may be accepted as general. But when we come to 

 the question of mesoblastic origin we are met with difficulties. 

 Ikeda finds^ like Caldwell and myself (my work on the early 

 development, in this Journal, was not ia Ikeda's hands till 

 after going to press), that certain bodies, which he calls 

 "plasmic corpuscles," are present in the blastoccele cavity, 

 but are non-nucleated, and hence cannot be regarded as 

 mesoblast cells. At the commencement of invagination he 

 finds that certain hypoblast cells migrate singly into the 

 blastoccele space. These contribute later to the formation of 

 mesoblastic organs. After invagination has been largely 

 completed he finds further mesoblast cells are derived from 

 a pair of anterior diverticula of the hypoblast, lateral to the 

 blastopore. These agree exactly in appearance and position 

 with the similarly named structures of Caldwell. The 

 presence of these " collar somites " has been independently 

 verified by myself, and we are also in agreement that the 

 mesoblast cells produced from them do not at first contain 

 any cavity, as maintained by Caldwell. The presence of 

 these collar somites has been so forcibly denied by both 

 Roule and Schultze that this corroboration is the more 

 gratifying. 



A third source of mesoblast cells he finds in the ventral 

 groove, which consists of cells not yet invaginated to form 

 hypoblast, lying along the mid-ventral line of the embryo. 

 These are in the nature of single cells set free into the 

 blastoccelic cavity. Lastly, he corroborates the presence ol: 

 a posterior invagination (CaldAvell's posterior diverticula), 

 but claims that this is merely an ectoblastic nephridial pit 

 which gives rise to the pair of nephridia. For these posterior 

 diverticula I searched in P. Buskii without success, but I 

 have since been enabled to find them in this species and in P. 

 hippocrepia. I am inclined to accept Ikeda's view that they 

 are the " anlage " of the nephridia, the development of which 

 I did not follow in P. Buskii. In passing we may say that 

 Ikeda does not recognise the presence of my " posterior 

 somites," but he figures without comment certain indications 



