76 THK CHANGES PROM STAGE A TO F. 



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^ lying in close contact with the ecto- 

 derm results (cf. Plate VI^ 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 (Plate VI, 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 (Plate VI, 

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

 latter, i. e. in the posterior part of the somite (Plate VI, 

 fig. 9). 



The same arrangement is present in a more advanced state 

 in the later embryos of Stage d (fig. 29, Plate II), cf. Plate VII, 

 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 b shows 

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

 show 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 

 cavity appears amongst the cells at the base of this rudimen- 

 tary septum (fig. 17 d, b. lat.). Both these latter features 

 are more clearly shown at sep. and b. lat. in Plate VI, 

 fig. 13. 



In a slightly more advanced stage (intermediate between d 

 and e) the cells of this parietal mass, as I may call it, are more 

 numerous, and the contained cavity — b. lat. — extends forwards 

 to the anterior part of the somite (fig. 21 a, me., b. lat.) ; while 

 in the posterior part of the somite, in the region of the limb 

 outgrowth, the rudimentary septum has become more marked 



1 It is difficult to say whether these layers of cells arise in this manner or 

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



