ORIGIN OP THE METAZOA. 109 



to notice one or two points in which Metschnikoff seems to 

 have misunderstood Balfour's views on this subject. Bal- 

 four, as is well known, was inclined to the view that 

 the gastrsea was preceded by a solid form, such a form as 

 Metschnikoff terms parenchymella, and Metschnikoff himself 

 quotes (No. 39) passages from the ' Corap. Embryology ' 

 which show this ; and yet Metschnikoff represents Balfour as 

 being opposed to the parenchymella theory. It is not quite 

 clear to me what exactly Metschnikoff means by the paren- 

 chymella theory ; but if this theory merely postulates the 

 existence of a beast with an outer ectodermal layer and an 

 internal mass of amoeboid cells, then I have no hesitation in 

 saying that Metschnikoff is mistaken in regarding Balfour as 

 having been actively opposed to it. It is true that Balfour 

 thought that Metschnikoff's view as to the method of origin 

 of the parenchymella was improbable ; but surely one may 

 accept the parenchymella without holding the precise views of 

 Metschnikoff as to its origin, just as one may accept the 

 gastraea theory without pinning one's faith to any particular 

 view of the mode of origin of the gastrsea. It appears to me 

 that Metschnikoff, in dealing with both the parenchymella and 

 gastrsea theories confuses two questions. 



(a) Was there an ancestral gastrula, with the characters 

 attributed to it ? 



(b) If so, how did this ancestral form itself arise ? 



If the answer to the latter question falls within the province 

 of the gastrsea theory, Balfour did not accept that theory and 

 Metschnikoff is wrong in saying that he was strongly inclined 

 towards it. If, on the other hand, the gastrsea theory simply 

 generalises from a large number of anatomical and embryo- 

 logical facts as to the past existence of an animal with a parti- 

 cular structure, and leaves the question of origin out of consi- 

 deration, then Balfour undoubtedly did accept the gastrsea 

 theory, but did not thereby, as Metschnikoff seems to think 

 must necessarily have been the case, reject the Parenchymella. 

 On the other hand, Balfour expressed himself distinctly in 

 favour of the latter, though he did not call it by that title. 



