6Q ARTHUR DENDY. 



branous invLSlinent of the head uiuloubtedly consists really of 

 two layers, epibiast and hypoblast, which have become so 

 stretched and flattened that they can be distinguished from 

 one another in parts only, especially behind (figs. 24 — 27, 64), 

 while there is no trace of mesoblast, at any rate in the more 

 anterior portion of the pro-amnion. The pro-amnion is, 

 perhaps, best shown in the diagrammatic longitudinal sections 

 of somewhat later stages represented in figs. 35 and 50. 



Thus the pro-amnion in Sphenodon forms a far more con- 

 spicuous feature than it does in the chick, owing to the much 

 greater extent to which the front end of the embryo sinks down 

 into the yolk. It is much more nearly approached by the 

 condition of the rabbit, as described and figured by Marshall (7), 

 but even here the pro-amnion does not appear to form nearly 

 so prominent a feature as it does in the Tuatara. 



According to Mitsukuri (14), as I learn from an abstract in 

 the ' Journal of the Royal Microscopical Society,^ in Chelonia 

 also the head-fold sinks in the yolk below, but as I have not 

 been able to see the original memoir I do not know how far 

 this process agrees with what takes place in Sphenodon. 



In Sphenodon it is only at a comparatively late stage in 

 the development (Stage O) that the pro-amnion ceases to exist. 

 At this stage the front end of the embryo, enveloped in the 

 true amnion (inner part of the pro-amnion), withdraws from 

 the pocket in the yolk-sac formed by the outer part of the pro- 

 amnion, and thus comes to lie entirely above the yolk-sac. 



The Amnion. — In the anterior part of the body the true 

 amnion is formed, as we have just seen, by separation and 

 withdrawal of the somatopleuric portion of the pro-amnion from 

 the splanchnopleuric portion. From about the region of the 

 shoulders backwards it is formed by the uprising of two folds of 

 sotnatopleure alone, accompanied by a downsinking of the body 

 of the embryo. These two somatopleuric folds (figs. 30 and 55, 

 Am.) meet and coalesce in the mid-dorsal line above theembryo 

 in a perfectly normal manner. This process of amnion forma- 

 tion, however, does not cease at the hinder extremity of the 

 embryo, but is continued backwards for some distance behind 



