468 EMBKYOLOGY OF THE LOWER VEKTEBKATES < n. 



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 edire. 

 The whole of the amniotic roof however except these marginal parts 

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



ect 



end. 



FIG. 214. Diagrammatic transverse sections through Chelonian embryos ( CV< //< //<//*. A , stage 

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

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



a./, aiimiotic Map: <///. .-minion ; </, ectoderm; ///. iidnilt'nn :./.", falsr ainniuii ; nteo, 

 MViij.'iit -. A', imtorlinni ; n, siTu-aiiiniiitic .junction : *<>,, .somatoplriire : xy/. splam-hnoplrm 

 Hplanchnocoele. 



214, A, a.f). As development goes on the inesoderni ex tends 

 between ectoderm and endoderm and then splits into somatic ;ind 

 splanchnic layers. The result of this is that the endodcrm. \\itli its 

 covering of splanchnic, iiirsoderm. sinks dm\ n and no longer jmijcct^ 

 upwards on each Hide into the |,;isc of the ainnicm (Fig. 214, B). Tin- 

 somatic mesoderm on ih- other hand does continue to project into 

 the kise of the ainnion just as did the .-ndodenn previously Figj 

 214, B). The ( ,i-i;jiii.ill\ siiiijde rct<lcriiial roof of the ;iinniot ie cavit.y 

 ;i pi-nci- .,! putting fix>m il-s lateral niargin inwards ftJKJ 

 this split extends towards the mesial plane the amniotie fold of 



