98 MARINE BIOLOGICAL LABORATORY. 



A large section of the blastula wall migrates at once, 

 and it takes some time for the opening so made to be 

 obliterated. This obliteration is the '* closure of the 

 blastopore." The stage succeeding this is the Paren- 

 chymella stage, though owing to caenogenetic modifica- 

 tions and acceleration in the development of organs, the 

 comparison is not perfect, but the embryo consists of an 

 ectodermal layer with a central mass of cells in which 

 differentiation has commenced. The blastopore or 

 prostoma of the Turbellarians and Gasteropods has no 

 exact counterpart in the Cnidaria ; it is the result of 

 the method of enclosure of the central cells. It is the 

 later formed mouth, or mouth and anus, which corre- 

 sponds to the Coelenterate mouth, and just as the mouth 

 forms in the Coelenterates at the pole where the immi- 

 gration occurred, so in higher forms the mouth, or 

 mouth and anus, appears at the pole of the embryo 

 formerly occupied by the larger yolk bearing cells. In 

 embolic gastrulas, where the invaginated inner layer of 

 cells forms a hollow sack, the archenteron, it may be 

 to the advantage of the embryo, little or no food-yolk 

 being present, for the blastopore to persist, complete 

 closure never taking place, a portion of the blastopore 

 being converted into the mouth or anus as the case 

 may be. Even in the Echinodermata, however, in 

 which as as a rule the blastopore persists as the anus, 

 a complete closure of it occurs in the Crinoids. 



This view of the relation of the blastopore to the 

 mouth and anus does not necessarily conflict with the 

 theory first advanced by Biitschli, and later supported by 

 Adam Sedgwick and E. B. Wilson, to the effect that the 

 mouth and anus of the higher Metazoa correspond to 



