ON DEVELOPMENT ETC. OF THOKONIS. 517 



[I.e., p. 6). It seems to me that Schulze lias fallen into the 

 same mistake as Metschnikoff. 



Lastly, Caldwell ('82) has entertained a view quite different 

 from those of other writers. According to him, the bodies in 

 (juestion are not present as sueh in the blastoeœle, l)ut are in 

 reality only the enl ends of l)lastoderm eells projecting into the 

 cavity and as such of course have nothing to do with the true 

 mesoblast. 



In the Phoronis studied by me, the plasmic corpuscles are 

 present only in the highly advanced blastula (fig. 25, pl.co.). 

 They are usually round in shape and very much smaller in size 

 than any of the l)lastoderm cells, but as to structure, they do not 

 show any deviation from the latter, except in the important res- 

 pect that they have no nucleus. Although I tried with them all 

 the available nuclear stains, the presence of any chromatic sul)- 

 stance in them could in no instance be detected. In an earlier 

 stage such a free floating sphere has never been met with. In- 

 stead of it, some unusually elongate Ijlastoderm cells (jol. co., fig. 

 24), such as were found by Caldwell, were discovered protruding 

 their inner end into the blastocœle. The nucleus of these cells 

 commonly lies in the periphery as in all other cells of the more 

 ordinary sha]^e. In my opinion, the proximal ends of the elong- 

 ate cells l)reak off from the main ce]l-l)ody and fall into the 

 blastocœle, where they undergo degenerati(ai, breaking into ever 

 smaller and smaller spheres. By examining serial sections of an 

 advanced stage like that of fig. 25, it is easy to convince one's 

 self that there exists no connection whatever l)et\veen the spheres 

 and the blastoderm cells. The spheres, or the plasm ie corpuscles, 

 are clearly distinct Ijodies and not mere ends of blastoderm cells 

 cut olf in the process of microtomizing as was supposed by 



