64 ARTHUR BENDY. 



be enclosed, at any rate in part, in the alimentary canal (cf. 

 fig. 35). 



A primitive groove may be recognised both in front of ami 

 behind the blastopore (figs. 11, 34, P. G.). 



The hypoblastic lining of the alimentary canal is probably 

 formed in Sphenodon from flattened cells of the original 

 lower layer of the blastoderm. These cells, at first absent, at 

 any rate in the middle line of the embryo (fig. 4), appear to 

 gradually spread inwards from each side and meet beneath the 

 notochord (figs. 8 — 14). The layer thus formed corresponds 

 to the '' Darm-Entoblast" of Hertwig and Mitsukuri and 

 Ishikawa (5). The latter authors state that in Chelonia 

 (Trionyx) also it passes gradually under the so-called ''chorda- 

 entoblast," and fuses in the middle line to complete the upper 

 wall of the digestive cavity. 



Mitsukuri and Ishikawa (5) make much of the fact that in 

 Chelonia the mesoblast in front of the blastopore arises as a 

 paired mass from the junction of the '^ chorda-entoblast'' with 

 the "Darm-Entoblast"' on each side. I have seen nothing of 

 such a mode of origin in Sphenodon, where, as already noted, 

 it seems to spread forward from the primitive streak as a broad 

 and perfectly continuous sheet (vide fig. 11), the axial por- 

 tion of which presently separates as the notochord from the 

 lateral portions (vide fig. 33). 



As in other cases, however, the mesoblast in Sphenodon 

 certainly has two distinct origins; (1) from outgrowth of the 

 compact rounded cells of the primitive streak, and (2) from 

 loose stellate cells of the original lower layer left between 

 epiblast and hypoblast after the formation of the latter. The 

 first mode of origin is especially conspicuous in the posterior 

 part of the body, but the exact share taken by each in the 

 subsequent development of the embryo I have not determined. 



The hypoblastic lining of the digestive cavity, at first con- 

 sisting of flattened cells, gradually takes on a columnar cha- 

 racter. It is noteworthy that this change commences at the 

 anterior end of the body, — in fact, in front of the embryo alto- 

 gether (fig. 8), and gradually extends backwards (compare 



