468 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



the body of the embryo and the amnion, and terminating behind in 

 an opening bounded above by a concave free edge (a.e). 



An important point to realize is the relation of the amnion to 

 the cell layers. The first rudiment, as has been indicated, is com- 

 posed of the two primary layers ectoderm and endoderm, and this 

 applies also to the lateral prolongations backwards of the free edge. 

 The whole of the amniotic roof however except these 1 marginal parts 

 is formed at first of solid ectoderm and of ectoderm alone (Fig. 



Fig. 214. — Diagrammatic transverse sections through Chelonian embryos (Clemmys. A, stage 

 with. 2-3 mesoderm segments ; B, 6-7 segments) illustrating the relations of the 

 amnion. (Based on figures by Mitsukuri, 1891.) 



a.f, amniotic flap ; am, amnion ; net, ectoderm ; end, endoderm ; /.a, false amnion ; mes, mesoderm 

 segment ; N, notochord ; sa, sero-amniotic junction ; soni, somatopleure ; spl, splanchnopleure ; splc, 

 splanchnocoele. 



214, A, a.f). As development goes on the mesoderm extends 

 between ectoderm and endoderm and then splits into somatic and 

 splanchnic layers. The result of .this is that the endoderm, with its 

 covering of splanchnic mesoderm, sinks down and no longer projects 

 upwards on each side into the base of the amnion (Fig. 214, B). The 

 somatic mesoderm on the other hand does continue to project into 

 the base of the amnion just as did the endoderm previously (Fig. 

 214, B). The originally simple ectodermal roof of the amniotic cavity 

 undergoes a process of splitting from its lateral margin inwards and 

 as this split extends towards the mesial plane the amniotic fold of 



