350 GENERAL EMBRYOLOGY 



of the chick, may also be said to exist only virtually, for it is 

 represented only by a shallow space left between the endoderm 

 and the yolk. 



The mesoderm and chorda are here more closely related with 

 the outer than with the inner layer. In the germ ring there is 

 little indication of separation of the germ layers, other than the 

 distinction of the endoderm, and when the primitive streak is 

 formed, it appears rather as a thickening of the ectoderm. The 

 mesoderm begins to be differentiated along the sides of the 

 primitive streak, and back as far as the region where this is 

 being formed by the fusion of the limbs of the germ ring. Hence 

 the mesoderm is more largely gastral, that is to say, it does not 

 become distinct, as a separate rudiment, until the establishment 

 of the primitive streak has occurred. In the germ ring there 

 is of course a region where ectoderm passes into endoderm, and 

 where the cells may be said to belong to either layer or neither 

 layer. This is the region where the primary " mesoderm" 

 forms and apparently special conditions may determine with 

 which of the primary layers it may seem to have the more inti- 

 mate relation. Little is gained by attempting to define germ 

 layers in the germ ring. 



In the pigeon or chick the rudiment of the notochord appears 

 in the deeper part of the primitive streak after its lateral parts 

 are cut off as mesoderm. The endoderm has therefore the value 

 of an enteroderm from the beginning, and has the form of a very 

 thin flat sheet of cells widely spreading over the yolk surface. 

 Ultimately the yolk mass becomes entirely enclosed in a layer 

 of endoderm as well as by the other germ layers, but this does 

 not occur until a comparatively late stage in the development 

 of the embryo. 



In Amphioxus and the frog we have seen that the embryo is 

 formed from the entire ovum, that is, the yolk-containing cells 

 become actually included within the wall of the gut. In the 

 Teleosts the yolk mass is so large, and so completely separated 

 from the embryogenic tissues, that the embryo may be said to 

 develop upon the surface of the yolk, which, enclosed within a 

 structure called the yolk sac, is only indirectly related to the 



