DEVELOPMENT OF THE CAPE SPECIES OP PERIPATUS. 487 



bands of nuclei placed in the vacuolated protoplasm inter- 

 vening between the ectoderm and endoderm. 



The mesoblastic bands gradually acquire a greater extension 

 forwards as the embryo and the blastopore increase in length. 

 They very soon become segmented — before they have reached 

 the level of the front end of the blastopore. This fact is 

 clearly shown by PI. XXXII, figs. 23 and 24, Part I. The seg- 

 ments so formed do not at first (PL XXXII, fig. 23, Part I) 

 possess any distinct central cavity around which the nuclei are 

 arranged. By the stage figured in fig. 24, Part I, in which 

 five mesoblastic somites are present, the cavities have appeared. 

 The appearances presented by surface views are confirmed by 

 an inspection of sections. I have sections of embryos, in 

 which three solid somites could be made out on each side. 

 PI. XXXIV, fig. 2, represents a transverse section through a 

 stage with one solid somite. The section is slightly oblique 

 and passes through the somite of one side only ; on the other 

 side the section passes in front of the mesoderm. Fig. 5 a—f 

 represents sections through an embryo of the stage of fig. 25, 

 Part I, in which the somites had reached the level of the 

 anterior end of the blastopore. A distinct cavity, around which 

 the nuclei of the somites are arranged, is present in the somites. 

 By the close of Stage b (fig. 25, Part I) the somites of the 

 anterior pair have reached their permanent position in front of 

 the blastopore. They are separated from one another by the 

 alimentary canal, which at this stage extends in front of the 

 mouth (PL XXXIV, figs. 4, 6 a). 



With regard to this development the following questions, 

 "which can only be settled, if they can be settled at all, by a 

 study of transverse sections, present themselves: — (1) How do 

 the mesoblastic bands grow forward before they have aug- 

 mented into the somites ? and (2) How do the somites, which 

 are established before the bands have reached the front end of 

 the blastopore, reach their final position ? 



1. Do the bands grow forward independently of surrounding 

 structures? or do they receive nuclei from the adjacent ecto- 



VOL. XXVIIj PART 4. NEW SliK. M M 



