314 McMURRICH. [Vol. IV. 



An affirmative answer to the first part of this question formed 

 the basis of Lankester's planula theory (]"]), while the second 

 part of the interrogation is considered a correct statement of 

 affairs by the supporters of the Gastraea theory, invagination 

 being according to them the original phenomenon. It is diffi- 

 cult to understand how one process could have been converted 

 into the other, as is required by either theory. Lankester's 

 theory, too, finds little support from the facts of development in 

 the higher forms, whereas the Gastraea theory is strengthened 

 by them. On the other hand, the Gastraea theory weakens when 

 we come to study the developmental phenomena of the lower 

 Metazoa. 



I have already shown the improbability of the occurrence 

 of an invaginate gastrula in the Actinozoa. In the Scypho- 

 medusae, it is certain apparently that it occurs in Pclagia and 

 Naiisithoe, but these are certainly exceptions. Goette i^Z"]) has 

 shown that a gastrula-like structure may be produced in Anrelia 

 anrita without invagination, by the hollowing out of a Sterrula ; 

 and I have found this same structure to be formed by the 

 immigration of certain of the blastula cells in Cyanea artica. 

 Lucernaria, which is probably a very primitive Scyphomedusan, 

 likewise forms a Sterrula (Kowalewsky, '84), and, so far as is 

 known, an invaginate gastrula occurs in the Scyphomedusae 

 only in two isolated cases. The formation of a Sterrula is to 

 be regarded as typical for the Scyphomedusae. 



In the Hydrozoa not a single case of invagination is known ! 

 See then what a showing we have among the Cnidaria, only 

 two authentic cases of invagination for the formation of the 

 endoderm are known ! 



How then is the endoderm formed in this group ? Delami- 

 nation occurs in the Actinozoa and in certain Hydrozoa, but 

 there is a process back of this, from which it has been derived : 

 I mean immigration. The most suggestive and admirable work 

 of Metschnikoff (^6), has placed the occurrence of this process 

 beyond a doubt ; and that author has been led to regard the 

 Parenchymella (Sterrula) as an ancestral form of the Coelen- 

 terates, from which, as he shows, a gastrula by invagination 

 might readily be derived. With this opinion I desire to express 

 my full agreement, and I have elsewhere ('91) endeavored to 

 extend the idea somewhat. It will be unnecessary, therefore, 



