66 SYNOPTIC COLLECTION. 



and Volvox) and also by the delamination of the inner 

 ends of the ectoderm cells. The former process results 

 from the longitudinal division of the cells, the latter from 

 cross division. In Proterospongia and Volvox the migrat- 

 ing cells become reproductive, but with this differentia- 

 tion in function it is not difficult to conceive that other 

 cells might become digestive and pass to the interior, 

 leaving the locomotor whipped cells on the surface. 



The fact now demonstrated, that digestion in many of 

 the more generalized Metazoa is intra-cellular, or carried 

 on within the cells, and not in a stomach or archenteron, 

 strengthens the theory of Metschnikoff. In time the cells 

 within the solid embryo arrange themselves in a layer to 

 form the endoderm. Later an opening breaks through 

 the two layers, endoderm and ectoderm. The resultant 

 form is similar in appearance to the invaginated gastrula, 

 but in this case it is clear that the endoderm is not formed 

 as a bag-shaped invagination with a terminal opening. 



The parenchymella is in reality the primitive condition, 

 arising from the blastula, and the gastrula-like stage is 

 acquired later. 



It is not difficult to see how the process of immigration 

 might apparently give rise to invagination in certain cases, 

 since if the cells migrated en masse from one pole of the 

 blastula instead of individually from all points of the sur- 

 face, a form would appear resembling an invaginated 

 gastrula. The formation of the parenchymella and the 

 resultant gastrula-like embryo is the normal development 

 of most of the Porifera and Coelentera, the invaginated 

 gastrula occurring rarely, as for example in Sycandra 1 



1 Dr. Otto Maas (Zoologische Jahrbiicher, Anat., VII, Heft 2, 

 1893) maintains that the invagination of the "ciliated" cells in 

 Sycandra has nothing to do with the process of gastrulation, the 

 two layered embryo being already formed, according to this author, 

 before the occurrence of this invagination. A " fundamental simi- 

 larity 1 ' Dr. Maas finds between the development of the calcareous 

 and horny sponges, and he thinks that the apparent exceptions to 

 the rule (Sycandra, Oscarella, etc.) will be found to conform to it on 

 further study. 



