104 MARINE BIOLOGICAL LABORATORY. 



Other cases might be given having a more or less 

 direct bearing on this question, but the time at my dis- 

 posal prevents a fuller treatment of the subject. I 

 would like, however, to refer to one case, namely, Grob- 

 ben's account of the development of the Phyllopod 

 Moina. We have in this form three distinct invagina- 

 tions : (i) an invagination of certain cells to form the 

 endodermal midgut ; (2) of certain cells to form the 

 general mesoderm ; and (3) of four cells to form the re- 

 productive organs. Only one of these, however, the 

 first, can be considered a true invagination ; the others 

 are more correctly immigrations. The entire process 

 can, I think, be referred to the formation of a parenchy- 

 mella in which there has been a precocious segregation 

 of certain important organs. The germ plasma has been 

 early segregated into a certain spherule of the develop- 

 ing ovum and accordingly immigrates independently of 

 the general mesoderm cells, whose perfect segregation, 

 like that of the endoderm, is postponed to a slightly 

 later period, these two last-named structures likewise 

 immigrating independently. 



There are some cases, however, which seem to throw 

 serious obstacles in the way of the view as to the origin 

 of the endoderm and the reproductive elements which I 

 have advanced. In the Hydrozoa and Ctenophora, the 

 reproductive cells have been shown by Weismann, and 

 the Hertwigs especially, to be derived from the ectoderm. 

 The latter authors endeavored, on this account, to asso- 

 ciate the Hydrozoa and Ctenophores together as dis- 

 tinct from the Scyphozoa, in which the reproductive 

 cells have an endodermal origin. Other structural pecu- 

 liarities, such as the presence of an ectodermal stomato- 



