(24) 



have their origix. in the wall of the archentoron (Fig. 17) and 

 are quite di f f eront from the bodies found in th-s blastocoel of 

 the blastula. Thoy imve a definite nucleus and they seem to 

 be similar cells to those founci by Ikeaa in t^ie larva with one 

 pair of tentacles. They certainly do resejJDle the blooa cor- 

 puscles found in the older larvae only they are considerably 

 lar^^rir, Ikeda (9) carao to the conclusion that these cell^? 

 were the "jiother cells of blood corpuscles which aro founa as 

 corpuscle-masses in the collar cavity o "*^ the Ac tino troclia" . 

 Since the publication of his paper, Mr. Ikeda has written me 

 that he considers his theory concerning the fate of these cells 

 to be incorrect. 



These cells are easily di stinjjuishable from all other cells 

 by the fact that tr. ey are larger and that tiie cytoplasm does 

 not staixi. They have a nucleus which is rattier small. I 

 shall return to a consideration of these cells wlien I aescribe 

 the blood corpuscles of the Actinot ro cha. 



As th.e blastopore lips bei_:in to close up posterio rly ( Fij^s. 

 lS(d)-(e) ), the endoderm cells in that re^^ion be^^in to lose 

 the power o^ .jivir^: rise to mesoderm cells but they are still 

 ■^ound arisint; in a more anterior ret^iovi. 



At a little later stage in which the blastopore has become 

 circular again after tr.e f\jsion of the blastopore lips and the 



