94 MARINE BIOLOGICAL LABORATORY. 



Invagination can readily be deduced from immigra- 

 tion. Metschnikoff points out that immigration may- 

 take place irregularly over the entire surface of the 

 blastula (multipolar immigration), or may be confined 

 to the posterior extremity, especially in free-swimming 

 blastulas (unipolar immigration). It is interesting to 

 note, in this connection, that according to recent ob- 

 servers there is a tendency towards unipolar immigra- 

 tion in the case of the sexual cells of Volvox. From 

 the polar immigration Metschnikoff deduces invagina- 

 tion. In the Hydroid Laodice a few cells at the poste- 

 rior extremity of the blastula are clearer than the rest 

 and gradually migrate into the interior. If these cells 

 should migrate en masse, instead of individually, we 

 should have such an invagination as occurs in Nansithoey 

 the alteration of the manner of migration being an abbre- 

 viation of the original process. 



The simplicity of this explanation recommends it, and 

 it serves to clear up the sporadic appearance of invagi- 

 nate gastrulae in the Sponges and Scyphomedusas. But 

 the occurrence of these gastrulse suggests the question, 

 when once the invaginate gastrula has been established, 

 does invagination continue to be the mode for endoderm 

 formation, other processes, such as epibole, being de- 

 rived from it } A negative answer to this question 

 involves the assertion that the invaginate gastrula may 

 have been developed several times independently. Is 

 this impossible t 



We have invagination occurring in the Sponges and 

 in the Discomedusae. The gastrula of Sycandra cannot 

 be regarded as having been the caenogenetic ancestor 

 of the gastrula of Oscarella, and neither of these that of 



