Embryology 285 



The blastula is not typical of that of an animal such as Aniphioxus. 

 Instead, there is a large blastocoel surrounded by a layer of cells, the 

 trophoblast, and a large inner cell mass present at one pole. This 

 peculiar mammalian blastula is called a blastocyst. 



Area opaca 

 vitellina 



Proamnion 



Mesenchyme 



Neural fold 



Neural groove' 



Nofochord 



Somite 



Ectoderm of head 



Border of fore-gut 



Subcephalic pocket 



Margin of ant. 

 intestinal portal 









Area pellucida 



Unsegmented 

 mesoderm 



Hensen's node 



,v:vrv^A=' ■ -.:' :'■. :>_.■.' '■'< l-: : .-::^y^^ja^-;^:B^f :_ PrImitive 

 ::-^^v^^^#-.".;r:^'v'-^'- ,;#.,•, :>^:3£fe*:^-i^i-:- streak 







Border 



of mesoderm ■»'-ti:-v;V-'rrg?^'-j(^ — 



-Blood island 



'Area vasculosa 





:v-fi>' 



Fig. 86. — A dorsal view of a twenty-four hour chick embryo. (From Patten: 

 Embryology of the Chick, The Blakiston Company.) 



The next stages in this development are related to the rapid for- 

 mation of embryonic membranes for the implantation of the embryo 

 into the uterus. This is brought about by splits in the inner cell mass. 

 First a group of cells separate off to form the entoderm surrounding 

 the developing yolk sac. Above this, another separation occurs which 

 gives rise to the amniotic cavity in which the embryo later develops. 

 Between these two cavities there is a plate of cells, and it is on this 

 disc that the embryo now develops. An attachment to the trophoblast 

 is retained by the inner cell mass. This is the body stalk which ulti- 

 mately becomes the umbilical cord. 



