518 ADAM SEDGWICK. 



as well as in the deeper parts {vide description of segmentation, 

 p. 179, Part II). By the continued division of the nuclei at 

 the edge of the blastoderm the embryo acquires an external 

 layer of nuclei, which are absent only at one point — the future 

 blastopore. 



Of all the forms of Arthropodan cleavage that I know of, this 

 process seems to resemble most nearly that of the mite Tetrany- 

 chus telarius, as described by Claparede (No. 6). In this form, 

 as in Peripatus, the first segmentation nucleus divides at the 

 periphery of the ovum, and not in its centre, as in most centro- 

 lecithal ova. There is, however, the greatest possible variety 

 in the position of the first segmentation nucleus in Arthropoda^ 

 and the matter does not seem immediately important. The 

 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. 25) ; 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. 25, 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. 7, 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. 



