102 GENERAL CONSIDERATIONS. 



same cannot, however, be said about the continuity between 

 the segments. This seems to me a matter of the greatest im- 

 portance at the present moment. It has long been known that 

 the segments of many centrolecithal eggs are at first connected 

 with one another. In proof of this I need only refer to Balfour's 

 summary of the cleavage of centrolecithal ova in vol. i of the 

 ' Comparative Embryology/ and to any of the recent works on 

 Arthropod development (e. g. Patten, No. 41) ; that is to 

 say, it has for some time been known that the segmentation of 

 centrolecithal eggs is not a complete cleavage, and, indeed, 

 sometimes does not deserve the name of cleavage at all (e. g. 

 most Insecta). But it has generally been supposed that 

 this continuity is soon lost, and that the final result of seg- 

 mentation is in all cases a mass of completely separate cells 

 (vide Patten, No. 41, pp. 565, 567). According to this 

 view the connections which undoubtedly exist between the 

 majority of cells of the adult is purely secondary {vide Flem- 

 ming. No. 1, p. 74). 



Two questions now present themselves : (1) Is this view true 

 in fact ? (2) Is it genetically true ? 



In other words, (1) Is it universally true that there is a stage 

 in the embryonic development of the Metazoa in which all the 

 cells of the body are isolated from one another? (2) Has 

 there been such a stage in the evolution of the Metazoa, i. e. 

 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 M-ith 

 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 

 (No. 27), holds good for Julus; and it seems to me highly 



