THE THIRD SOMITE. 77 



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

 municates with the lateral or appendicular portion (Plate VII, 

 fig. 21 c). The latter {I. 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 (Plate 

 VII, 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 (Plate VI, fig. 23 d, /.s.3, «. 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-off groove), in the form of a 

 tube, from the neck connecting the median portion with the 

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



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

 to the exterior (fig. 23 e) . This happened in a slightly younger 

 embryo. The opening is already covered over by the lip {L.), 

 which is rapidly growing backwards (cf. Plate II, fig. 35; 

 embryo with commencing cerebral grooves). 



Briefly to recapitulate, the structure of the third somite at 

 this stage is as follows : — In front it is dorsally placed and 

 overlaps the second somite; its middle portion slopes ventral- 

 wards and communicates with the posterior part, which is con- 

 tained in the limb, and is peculiarly bent (fig. 23 c — e), and 

 opens to the exterior. The middle portion further sends a 



