232 SCIENCE PROGRESS. 



mass must be regarded as the true gastrulation. A pro- 

 cess can, however, hardly be termed gastrulation when the 

 preceding stage is a solid larva, as in siliceous sponges, 

 and when the future " ectoderm " is entirely internal. 

 An ectoderm containing the yolk and lying internally to 

 the endoderm is a paradox. 



Maas considers the sponges true Metazoa on account 

 of their reproduction by ova and spermatozoa and the 

 differentiation of their tissues. They are derived from 

 two-layered ancestors, of which the two layers are com- 

 parable to the ectoderm and endoderm of other Metazoa. 

 The difficult thing to explain, from a phylogenetic point 

 of view, is the complete reversal in position which their 

 layers have undergone. This must have been in some way 

 the result of a change in their mode of nutrition, whereby 

 the flagellated ectodermal cells retained their structure, but 

 changed their place and became carried into the interior to 

 become the collar cells of the sponge. Sponges are in no 

 case to be ranked with Ccelenterates, since their inner and 

 outer layers are not homologous with the corresponding 

 layers of Ccelenterates, and their canal system is of abso- 

 lutely different origin. 



Delage, on the other hand, regards the sponges as 

 descended from a colony of Protozoa, represented by the 

 blastula in ontogeny. They are descended from the 

 Protozoa entirely independently of other Metazoa, and their 

 layers are not to be compared with the germ layers of 

 Metazoa. They show a progressive differentiation of their 

 tissues, which, however, does not take place in the sense 

 of a germ-layer formation as in higher animals, but by 

 means of division of labour among the cells of the colony, 

 so that some become epithelial, others skeletal, and so 

 forth. 



These, however, are questions upon which at present, 

 at any rate, no final decision is possible, nor need one be 

 attempted here. 



