338 E. W. MACBRIDE. 



Metschnikoff starts, like Haeckel, from the blastosphere or 

 blastula as an ancestral form ; he supposed it, however, to 

 have become filled up by cells -wandering in from the periphery. 

 In the midst of these a digestive cavity was later developed, 

 and finally the mouth was formed by the specialisation of the 

 area through which food was taken in. Lankester, starting 

 from the same form, supposes the inner ends of the cells of the 

 blastula to have become differentiated so as to be specially 

 digestive in function, and later that they became separated off 

 as a special layer. The cavity of the blastosphere was thus the 

 digestive cavity, and food at first taken in over the whole surface, 

 was latertakeuinonly atone point, and thus a mouth was formed. 



Sedgwick, on the other hand, is inclined to start from a 

 protozoon in which cell territories were non-existent, though 

 many nuclei were present. He supposes that the gut originated 

 as a digestive vacuole, and that the nuclei acquired a definite 

 arrangement with regard to this vacuole and other organs, and 

 thus tissues were constituted. Cell territories, in so far as they 

 exist in the adult, he regards as due to secondary rearrangement 

 of the protoplasm. 



We have already pointed out that Echinoderm development 

 tells strongly in favour of the view supported by Haeckel and 

 Korschelt and Heider, and that Echinoderm development, 

 from its almost exclusively larval character, is of the very 

 greatest importance in deciding such a question. Its evidence 

 is by no means solitary ; the statement may be made that, in 

 all Coelomata without exception, when the yolk is feebly 

 developed and evenly distributed we find the embryo pass 

 through a blastula stage which is converted into a gastrula 

 by invagination (cf. Leucifer among Crustacea, Polygordius 

 and Serpula and many others in Annelids, Paludina and 

 Chiton in Mollusca, Amphioxus and Cyclostomes in Verte- 

 brata, &c.). The groups which constitute the chief support 

 of Metschnikoff's theory are Sponges and Coelenterata. We 

 may leave the first entirely out of account, as it is quite pos- 

 sible that they constitute a distinct phylum to the rest of the 

 Metazoa. In many Coelenterates we start from a blasto- 



