MUSEUM OF COMPARATIVE ZOOLOGY. 125 



of the embryo has two layers, the lower third is one-layered. The 

 cavity of the lower third contains some scattered cells, which, the au- 

 thor hints, may be representatives of the mesoderm, while the cavity in 

 which they lie may represent an enterocoel. The author regards the in- 

 ner layer of the upper two-thirds as true entoderm. The method of its 

 formation recalls that of the entoderm of some Ccelenterata, as demon- 

 strated by Metschnikoff. There is no epithelial invagination, such as 

 Kraepelin maintained, and therefore the cavity which the inner layer 

 lines cannot be regarded, says Korotnetf, as an enterocoel. Later, the 

 entire embryo becomes two-layered by an extension of the inner layer. 

 The two polypides arise from two distinct invaginations of the double- 

 layered wall. 



Unfortunately, Korotneff does not demonstrate by figures the method 

 of origin of the alimentary tracts of the first polypides ; but there is 

 little reason to doubt that it is essentially like that in other buds. If 

 it is admitted that the inner layer is entoderm, as Korotneff maintains, 

 then the entoderm takes no part in forming the digestive epithelium ; 

 but the latter is derived solely from ectoderm. 



In his discussion of the theoretical bearing of his results (p. 404), the 

 author seems to niaintain that the polypide is to be regarded neither as 

 an individual (Nitsche's view), nor, on the other hand, as an assemblage 

 of organs homologous with organs of the same name in other groups; 

 but rather as a new structure, developed upon the cystid, to aid in its 

 nutrition. 



In criticism of Korotneff s view, that the loose cells given off from 

 one pole of the blastula are entoderm, I may point out that this process 

 bears quite as much resemblance to the process of " mesenchyme " 

 formation (as described by Korschelt for the Echinoids), as it does to 

 the origin of the entoderm in some Coelenterates. Compare Figs. 13 E 

 and 182, in Korschelt und Heider's Lehrbuch der Vergleicheuden Ent- 

 wicklungsgeschichte. 



Braem ('89^ pp. 676, 677) has shown that the primaiy polypide of 

 the statoblast arises from the cell layers of the statoblast, exactly as the 

 primary polypide of the egg embryo does from those of the ** cystid," 

 and the alimentary tract is formed as in buds of Cristatella. 



To sum up : The outer layer of the colony-wall is ectodermal in ori- 

 gin ; the inner layer arises by an embolic (]) invagination of the blas- 

 tula, and would therefore appear to be entoderm, although the possibility 

 of its being homologous with the mesoderm in other forms is perhaps 

 not excluded. The first polypides so arise that their inner layers are 



