CHAPTER XVI 



THE EMBRYONIC MEMBRANES 



The Yolk-sac. — In those forms in which the quantity of yolk 

 contained in the egg is large, the embryo is formed from a 

 blastoderm on the surface of the yolk and does not wait for 

 the latter to be enclosed. So it comes about that the yolk is 

 not situated within the embryo, as, for example, it is in the frog ; 

 indeed, in the chick it would be manifestly impossible. In 

 the heavily-yolked forms, then, the yolk is outside the embryo, 

 and it becomes surrounded by a layer of cells which are 

 endodermal and continuous with those of the gut-wall inside 

 the embryo. The yolk then finds itself inside a " yolk-sac," 

 which may be regarded as temporarily extra-embryonic gut. 

 This sac carries a layer of mesoderm outside its (endodermal) 

 wall, and blood-vessels passing between the mesoderm and 

 endoderm absorb the yolk (which has been digested) and 

 convey it into the embryo. Chief among these vessels are the 

 vitelline arteries and veins. Indeed, in most groups of verte- 

 brates, the wall of the yolk-sac is the site of origin of the blood 

 in the form of blood-islands. 



In the fish, the function of the yolk-sac circulation is not 

 only to convey digested yolk, but also to oxygenate the blood in 

 its many capillaries, at the early stages of development before 

 the gills have become functional. 



The yolk-sac reaches the height of its development in 

 reptiles and birds ; and in the Monotremes which, although 

 mammals are oviparous, yolk is present and the yolk-sac is 

 large. What yolk there is in the egg of the Marsupials is 



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