ch. vni DEVELOPMENT OF AMNION 467 



In Chelonia the first indication of amnion formation appears at 

 a stage like that represented in Fig. 212, A. The future body of 

 the embryo, indicated by the medullary folds, lies flat on the surface 

 of the egg, extending out all round into the blastoderm. The first 

 sign of the amnion is produced by the front end of the medullary 

 plate coming to dip downwards so as to form a deep slit or groove 

 (Figs. 212 and 213, eg) curving tailwards on each side as seen i'rom 

 above. The posterior wall of this slit forms the anterior limit of 

 the head of the embryo while its anterior wall forms the rudiment 

 of the amnion (Fig. 213, a.e). The portion of blastoderm in front 

 of and to the side of the head of the embryo is as yet two-layered, 

 the mesoderm not yet having spread into it, and it follows that the 

 amniotic rudiment is also two-layered. This region of the blastoderm, 



eel. 



Fig. 213. — Sagittal section through the head end of a Chelonian embryo. 

 (After Mitsukuri, 1891.) 



a.e, amniotic edge ; c. g, cephalic groove ; ec(, ectoderm of medullary plate ; end, endoderm. 



which is still without mesoderm and which in this case forms the 

 amniotic rudiment, is termed the proamnion. As development 

 proceeds the head end of the embryo increases in size and as it does 

 so it dips more and more downwards so as to deepen the cephalic 

 groove or slit in front of it. While this is going on there takes 

 place active growth of the ectoderm along the sharp edge of the 

 amniotic rudiment (Fig. 213, a.e) in such a way that this edge 

 becomes prolonged backwards as a solid flap, covering over the 

 body of the embryo from before backwards. This amniotic flap 

 continues to grow tailwards, its growing edge concave and prolonged 

 backwards on each side (Fig. 212, B, a.e), until it reaches the tail 

 end of the embryo, so that the whole of the latter is covered in by 

 an amniotic roof. Nor does the process stop now : it goes on with 

 the result that there is formed a long tunnel (Fig. 212, C, a.f) 

 continuous in front with the amniotic cavity, i.e. the cavity between 



