42 AETHUR DENDY. 



No. 46 exhibits an extremely interesting jiecnliarity with 

 regard to the development of the amnion. Towards the hinder 

 end of the body, but in front of the neurenteric canal, a longi- 

 tudinal slit-like opening in the serous envelope was clearly 

 visible in the unstained embryo viewed as an opaque object 

 (fig. 54, A?n.S.). Transverse sections (fig. 55) demonstrate 

 that this opening leads into the true amniotic cavity above the 

 embryo, and is due to the two uprising folds of the somato- 

 pleure not having yet united in the middle line, although the 

 amnion is completely formed both in front of and behind the 

 slit, and the posterior amniotic canal is also present behind the 

 embryo, and no longer communicates with the true amniotic 

 cavity. A similar opening into the amniotic cavity in this 

 region is, of course, a feature very characteristic of the chick at 

 a certain stage in its development, but it appears to be abnormal 

 in Sphenodon, as I have seen it in no other embryo. It is in- 

 teresting to notice just within the lips of the slit the sudden 

 transition from the large epiblastic cells of the serous envelope, 

 which become markedly columnar at the lips of the slit, to 

 the flattened epiblast cells lining the amniotic cavity (fig. 55). 



The downward curvature of the fore-brain may be greater 

 than in No. 44, as in No. 60, where the fore-brain is inclined 

 downwards and backwards, so that it comes to lie very close 

 to the heart. In some embryos the cleft, where the two 

 halves of the fore-brain have not yet united in the middle 

 line, is still clearly visible externally. 



The development of the mesoblastic somites does not appear 

 to be nearly so definite and regular as in the chick, so that 

 these are of comparatively little use for the purpose of deter- 

 mining stages in the development. It appears that a number of 

 mesoblastic somites are differentiated almost or quite simulta- 

 neously, but very indistinctly. In No. 60 only about four pairs 

 Avere recognisable in the stained embryo viewed as a trans- 

 parent object, and in No. 73 they were very indistinct. 



I have already stated that some embryos (e. g. 46 and 60, 

 figs. 54 and 53) show much more clearly than No. 44 the 



