OUIGIN OF THE METAZOA. Ill 



large cells produced by it, persisting, and eventually, after the 

 appearance of a central cavity, forming the endoderm. 



I do not mean to say that the facts are more in favour of 

 this view than in that of Metschuikoff, but I think they 

 favour the one as much as the other. 



But granting the hollow blastula as a possible animal, I 

 agree with Biitschli in thinking that there are great physio- 

 logical difficulties in the way of accepting the process by which, 

 according to Metschuikoff, it may have become transformed 

 into a solid form. Surely a hollow sphere is in. a much more 

 advantageous position with regard to nutrition than a solid 

 onej and yet MetschnikoflF supposes that the transformation 

 into the solid form was due to the migration of surface cells 

 into the interior for nutritive purposes. The central part of 

 the animal being empty would require no nutriment, and even 

 if it did, what more convenient arrangement could there be 

 than a layer of cells to pass prepared nutritive substances into 

 it ? The ectoderm of Hydra or of Coelenterates in general is 

 not fed by the migration of cells from the endoderm, and in 

 short we know of no instance in the animal kingdom of food 

 being carried from one part of an organism to another in 

 actively migrating cells. It seems to me much more likely, 

 if the ancestral Protozoon was a hollow blastula, that the first 

 differentiation would have been into locomotive sentient cells 

 and cells for acquiring food, i. e. the differentiation supposed 

 by the invaginate gastrula hypothesis. For it is clearly neces- 

 sary that the nutritive cells should be in direct contact with 

 the external medium, and this they are not on Metschnikoff's 

 view. 



Then again, what justification do we find in the animal 

 kingdom for the hollow blastula hypothesis ? With the pos- 

 sible exception of Volvox, I know of no form at all approach- 

 ing a hollow blastula in structure. 



These difficulties are avoided by the hypothesis that the 

 primitive form was solid, which also suits the facts quite as 

 well. (1) It is no longer necessary to suppose the migration 

 inwards of cells. (2) There are a considerable number of 



