DEVELOPMENT OP THE CAPE SPECIES OP PERIPATUS. 493 



Description of the Development op the Mesoderm with 

 ITS Cavities from Stages d — f. 



It will facilitate matters if I begin by describing the deve- 

 lopment of a particular pair of somites and of the mesodermal 

 structures around them, and I will take for this purpose the 

 third somite of the body (somite of the oral papillae) as one 

 which is typical of the rest and in which the changes are most 

 easily followed. 



During Stage d, at the time when the ectoderm separates 

 from the endoderm, the walls of the dorsal and ventral portions 

 of each somite come together; the cavity is thus obliterated 

 and a single layer of cells^ lyiog in close contact with the ecto- 

 derm results (cf. PL XXXIV, figs. 9 and 12, d. s., v. s.). 



At the same time some cells appear between the somatic 

 wall of the somite and the ectoderm (PL XXXIV, fig. 10, me.). 

 These cells, which are undoubtedly derived from the somatic 

 mesoderm, extend the whole length of the somite. In the 

 anterior part, i. e. in front of the limb outgrowth (PL XXXIV, 

 fig. 10), they are less numerous than in the region of the 

 latter, i. e. iu the posterior part of the somite (PL XXXIV, 

 fig. 9). 



The same arrangement is present in a more advanced state 

 in the later embryos of Stage d (fig. 29, Part I), cf. PL XXXV, 

 fig. 17, a—d. Fig. 17 a, me., shows the mass of cells in the 

 front part of the somite, anterior to its connection with the 

 somatic mesoderm. The same section also shows the anterior 

 part of the limb-ridge overlying this cell-mass. Fig 17 6 shows 

 the connection with the mesoderm, while fig. 17 c and d 

 shows this mass in the posterior part of the somite in the 

 region of the limb outgrowth. In the posterior part of the 

 somite some of the cells of this mass project into the cavity of 

 the somite in such a manner as to tend to separate the limb 

 portion from the portion in the body, and at the same time a 



> It is difBcult to say whether these layers of cells arise in this manner or 

 as outgrowths from the dorsal and ventral corners of the somites. 



