212 



EMBRYOLOGICAL TYPES 



on, however, more and more of the floor of the gut and ventral 

 wall of the embryo is formed, and the intestinal portals come 

 close together leaving only a narrow opening between the 

 cavity of the gut in the embryo and the yolk — the umbilicus. 



eec. 



Fig. 99. — Gallus : transverse sections through a chick embryo after three 



days' incubation through A the hinder, B, the middle, and C, the 



anterior regions of the trunk ; showing stages in the development of 



the amnion. 



a, amnion ; ac, amniotic cavity ; af, amniotic fold ; c, coelom ; ch, 



chorion ; da, dorsal aorta ; ec, ectoderm ; eec, extra-embryonic coelom ; 



en, endoderm ; m, dorsal mesentery ; my, myotome ; n, notochord ; nc, 



nerve-cord ; pc, posterior cardinal vein ; sac, sero-amniotic connexion ; 



som, somatopleur ; spm, splanchnopleur ; vv, vitelline vein ; Wd, Wolffian 



duct. 



Meanwhile, the extra-embryonic portion of the blastoderm 

 has been extending down over the surface of the yolk, and 

 eventually covers it completely except for a small aperture 

 which is left open, and through which the yolk is separated 

 from the albumen only by the vitelline membrane. The 



