OH. vm DEVELOPMENT OF AMNION 46*7 



In <'li. -Ionia the lirst indical ion <!' aiiiiiinn | '..nnat ion appears at 

 a stag*- like that represented in Fig. 212, A. The future body of 

 th<- rinlirvn. indiraird by the medullary folds, lies flat on the surface 

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

 si^ii of the amnion is produced by the front end of tin*, medullary 

 plate coming to dip downwards so as to form a df-j. slit <>i groove 

 IMLJS. I'll' and iU3, c.g) curving tailwards on each side as seen from 

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

 the head of the embryo \\liilc 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, 



ect. 



FIG. 213. Sagittal section through the head end of a Cheloniau embryo. 

 (After Mitsukuri, 1891.) 



iinnintii- (!-, : <.'/. cc] ilialic groove; <-<(, 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 



