OUTLINES OF THE DEVELOPMENT OF THE TUATARA. 31 



wards to form the outer layer of the pro-amiiionj as shown in 

 fig. 50. Behind this point the serous envelope remains united 

 with the true amnion in the mid-dorsal line to form a single 

 membrane, composed of three layers, viz. cubical epiblast on 

 the outside, mesoblast in the middle, and flattened epiblast on 

 the inside (fig. 50, L.A. S.). The space developed elsewhere 

 beneath the serous envelope is, of course, the pleuro-peritoneal 

 space, and beneath this space the yolk-sac is not yet, at any rate 

 in most parts, clearly diflferentiated into mesoblast and hypoblast. 



The posterior amniotic canal is still clearly traceable in 

 sections ; though, owing perhaps to the crumpling of the 

 membranes in removal, I failed to observe it in the embryo 

 before cutting. It lies embedded in the thickness of the serous 

 envelope above the pleuro-peritoneal space, and it opens in 

 front into the hinder end of the much wider amniotic cavity 

 above the embryo, while behind it still opens to the exterior 

 as before. In this specimen it lies obliquely to the long axis 

 of the embryo, and its anterior opening lies to one side of the 

 middle line. It is, therefore, not shown at all in the median 

 longitudinal section represented in fig. 50. 



The alimentary canal, except as regards the development of 

 the thickened rim already noticed, has scarcely progressed 

 further than at Stage F. The short columnar or cubical 

 epithelium which lines it is well marked throughout the 

 greater part of its extent, but gradually thins out and becomes 

 flattened behind (fig. 50). 



The notochord also has progressed no further. On the con- 

 trary, it seems to be scarcely so far advanced as in embryo 61 

 (Stage F), appearing to end in a mass of undiff'erentiated meso- 

 blast cells beneath the hind brain (compare figs. 35 and 50). 



The pericardium is now again represented by a distinct 

 cavity lying beneath the anterior enclosed part of the alimen- 

 tary canal, just where the epiblast and hypoblast are reflected 

 upwards to join the pro-amnion around the head (fig. 50, P. C). 

 Just where the bend takes place the epiblast and hypoblast are 

 separated from one another by a considerable space. This 

 space I believe to be the pericardium, although in this embryo 



