&UBX1NGDOM METAZOA. 



55 



terou, which in this case is identical with the blastocoel, in 

 communication with the exterior. This process is known as 

 delamination (Fig. 26, A). 



A third method also exists, occurring like delamination in 

 its most typical form among the lower Metazoa. This is the 

 immigration method (Fig. 26, B), certain cells of the blastula 

 leaving their position at the surface and passing into the 

 blastocoel. Here they undergo division, and, by the addition 

 of other cells by immigration, the blastocoel gradually be- 

 comes filled iip and a solid organism, consisting of an exter- 

 nal layer of cells surrounding a central more or less solid 



JJ 



FIG. 26. DIAGRAM ILLUSTRATING THE FORMATION OF THE DIPLOBLASTIC 

 STAGE (A) BY DELAMINATION, (B) BY IMMIGRATION. 



mass, results. This is known as the parenchymella or sterrula. 

 Later a cavity appears in the centre of the solid mass, whose 

 cells gradually are pushed towards the periphery, where they 

 form eventually a single layer, the endoderm. Finally a blas- 

 topore is formed and the embryo becomes a gastrula. 



It does not seem easy to bring the delamination and invagination 

 methods of gastrulation into direct relation with each other, or to derive 

 one from the other, but it is probable that both must be referred back to 

 the immigration method. In typical cases of immigration the cells which 

 migrate are situated irregularly at any part of the blastula, but frequently, 

 especially in free-swimming blastulas, the migrating cells are all located at 

 the posterior extremity. If in such cases of polar immigration the migrat- 

 ing cells were to pass into the blastoccel en masse instead of individually, 

 invagination would result. On the other hand, if a considerable amount 

 of yolk were present in all the cells of a blastula, it might happen that, in- 

 stead of migrating, the cell might undergo division, cutting off the yolk- 

 containing protoplasm from the yolkless, delamination thus taking place. 



