DEVELOPMENT OP THE OAPE SPECIES OP PERIPATUS. 519 



a stage in which the body of the common ancestor consisted of 

 a mass of organically separate cells ? 



The answer to the first question must be undoubtedly a nega- 

 tative one. The cells arising from the segmentation in Peri- 

 patus capensis are at no period of development completely 

 isolated units, but retain a connection with one another 

 throughout life. It is true that some of them break away from 

 the rest, and form blood-corpuscles and generative cells, but 

 the greater number present in the adult a connection with 

 their neighbours — a connection which has been derived directly 

 from the connections between the cells of the segmenting; 

 ovum. The same fact, as has been shown by Heathcote 

 TNo. 11), holds good for Julus; and it seems to me highly 

 probable that the connections between the various adult cells 

 of other Arthropoda with centrolecithal eggs will be found to 

 be derived continuously from the connections between the cells 

 of the segmenting ovum. 



But while we must admit that there are cases in which the 

 cleavage is not complete, yet in a great many animals — in all 

 cases of small holoblastic eggs — it seems to be quite certain 

 that the cleavage is complete. It is true that the spheres 

 always touch one another, and there may be an organic con- 

 nection at the point of contact ; but assuming that there is no 

 such connection, the question naturally arises : which of these 

 two processes — the incomplete or the complete cleavage — is, 

 from a phylogenetic point of view, the more correct ? 



It has generally been supposed that the complete cleavage is 

 the most primitive process, and that the mass of organically 

 distinct and similar cells, such as is found in the morula of a 

 typical development, represents a colonial Protozoon-like 

 ancestor of the whole of the Metazoa. In short, the general 

 view seems to be that the immediate ancestor of the first 

 Metazoon was a multicellular Protozoon, the separate cells of 

 which were all distinct from one another. Can we find any 

 justification in the animal kingdom, as we know it, for this 

 view ? Is there any living form constituted in this manner ? 

 The answer is, it is hardly necessary to say, a negative one. 



VOL. XXVII, PART 4. NEW SER. () () 



