524 ADAM SEDGWICK. 



that the animal of which ^the gastrula is the embryonic repro- 

 duction — if indeed it ever existed — was preceded by a form in 

 which the endoderm consisted of a vacuolated mass of proto- 

 plasm without any definite enteric cavity. So far most 

 embryologists of the present day are with him. But he goes 

 further ; he considers that this parenchymatous gastrula or 

 blastula^ — or as he calls it^ Phagocytella — was preceded by 

 a hollow blastula-like Protozoon form from which it arose by 

 the migration inwards of certain of the cells of the wall of 

 the blastula. This suggestion as to the origin of the gastrula 

 and the form of the ancestral Protozoon has often been 

 criticised, and it seems to me that the facts suggest, with 

 equal strength, quite another view of the matter, viz. that 

 which I have just hinted at. 



There are several ways suggested by embryology in which 

 the passage from the Protozoa to the Metazoa may have 

 been effected ; and a most admirable and profound analysis of 

 each of these, and a critical review of our knowledge on this 

 subject, is to be found in chap, xiii, vol. ii, of the ' Comparative 

 Embryology.^ I cannot do better than quote the words in 

 which Balfour sums up this review of the facts — "Considering 

 the almost indisputable fact that both the processes above 

 dealt with [delamination and invagination] have in many 

 instances had a purely secondary origin, no valid arguments 

 can be produced to show that either of them reproduces the 

 mode of passage between the Protozoa and the ancestral two- 

 layered Metazoa. These conclusions do not, however, throw 

 any doubt upon the fact that the g a s t rula, however evolved, 

 was a primitive form of the Metazoa; since this 

 conclusion is founded upon the actual existence of 

 adult gastrula forms independently of their occur- 

 rence in development " (' Comp. Emb.,^ vol. ii, p. 283 ; the 

 italics are mine). 



These words seem to me to express as clearly now, as they 



1 I am not quite sure whether he considers that the cortical layer (kyno- 

 blast) of his Phagocytella was interrupted at any point for the protrusion 

 of the central mass (phagocytoblast). 



