494 ADAM SEDGWICK. 



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 PI. XXXIVj 

 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. 31 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 

 (fig. 21 c, Sep.), but the median portion of the coelom still com- 

 municates with the lateral or appendicular portion (PI. XXXV, 

 fig. 21 c). The latter (/. s. 3) has developed a ventral out- 

 growth, which lies along the outer side of the nerve-cord, and 

 reaches the ectoderm. The ectoderm becomes slightly in- 

 dented at the point of contact, where a perforation is soon 

 formed. 



In the next stage (e) four changes are noticeable (PI. 

 XXXV, fig. 23 a—c) : 



(1) The dorsal or median part of the somite has extended 

 itself dorsalwards (fig. 23 a). At the same time it does not 

 extend so far backwards as the lateral part (i. e. the part in the 

 appendage), so that the latter is overlapped by the median por- 

 tion of the somite behind (4) (PI. XXXV, fig. 23 d, l.s.S, s. 4). 

 (It must be remembered in this connection that the outgrowths 

 into the developing appendage takes place at the hinder part of 

 the somite.) 



(2) The space [b. lat.) in the parietal mass of mesoderm 

 (m. t.) has much increased (fig. 23 a), and has, at the same 

 time, become partly divided by a tongue of cells, which 

 eventually give rise to the muscles of the posterior internal 

 projection of the jaw. 



(3) There exists a short, anteriorly directed, blind diverti- 

 culum (or, may be, constricted-oflf groove), in the form of a 

 tube, from the neck connecting the median portion with the 

 lateral portion of the somite (fig. 23 b, c, a. v., and /. s. 3). 



(4) The lateral portion of the somite has acquired an opening 



