DEVELOPMENT OF THE CAPE SPECIES OF PEEIPATUS. 52^ 



did when they were written five years ago, the state of our 

 knowledge on this subject, and in my opinion neither Metsch- 

 nikofi'^s view, nor that which I have just put forward as to the 

 exact method of transition between the Protozoa and Metazoa, 

 can be regarded as anything more than a more or less plausible 

 suggestion without any strong basis of fact. 



The gastrEea theory^ in so far as it implies the existence of an 

 ancestral two-layered organism, is still in accordance with 

 known facts, and no discoveries have been made which deci- 

 sively settle the mode of transition between the Protozoa and 

 Metazoa. As, however, the subject is an interesting one it 

 seems worth while contrasting MetschnikofF's view with that 

 which I have just put forward. But before doing so, I am anxious 

 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 

 Metschnikofi' terms parenchymella, and Metschnikoff himself 

 quotes (No. 23) passages from the ' Comp. 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 

 gastrsea 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 ? 



