48fi EMBRYOLOGY OF THE CHICK 



marcation known as the lateral limiting sulci. 



In this chapter we are concerned with the extra-embryonic mem- 

 branes which are developed from the various layers in the extra-embry- 

 onic region. The membranes themselves are four in number: the yolk- 

 sac, the amnion, the serosa, and the allantois (Fig. 270). 



THE YOLK-SAC 



This is the first of the extra-embryonic membranes to appear. It 

 must be remembered, that as the splanchnopleure grows outward from 

 the embryo, it surrounds the yolk, thus forming the yolk-sac. The yolk 

 itself forms the floor of the primitive gut. 



As the underfolding in the head-region separates the head from the 

 remaining blastoderm, it grows caudally and forms an entodermal floor 

 to the primitive gut, and the part which thus obtains this entodermal 

 floor is called the fore-gut. So, too, in the tail region a little later (about 

 the third day), the tail folds under the posterior end of the embryo and 

 the part which thus obtains an entodermal floor in that region is called 

 the hind-gut. The portion between fore-gut and hind-gut is the mid- 

 gut, which is, of course, that region where the yolk is still the floor. As 

 the fore-gut and hind-gut become larger and extend toward each other, 

 the mid-gut occupies less and less area, until there is merely a little duct 

 something like the small end of a funnel, the larger end of the funnel 

 being the extended splanchnopleure surrounding the yolk. In other 

 words, the mid-gut consists only of the opening of the yolk-stalk, which 

 latter is made up in turn of the walls of the splanchnopleure drawn to- 

 gether at this point. 



As the neck of the yolk-sac is thus constricted, the omphalomesen- 

 teric arteries and veins, which extend throughout the region where the 

 constriction takes place, have likewise been drawn into the constricted 

 area, and pass to and from the embryo through the yolk-stalk, side by 

 side. 



The yolk is now covered with a vascular network spreading through- 

 out the splanchnopleure of the yolk-sac, so that the entire food supply of 

 the embryo comes to lie in a sac with this circulation of its own definitely 

 attached to the mid-body region, though as far as we know, no yolk 

 granules pass directly into the embryo, all of it being absorbed by the 

 vascular network. In older embryos, the yolk-sac even folds consid- 

 erably, so that a still greater expanse of vascular area is established. 



The white albuminous portion of the egg rapidly loses the water it 

 contains, and is absorbed by the extra-embryonic membranes. 



Ultimately (about the nineteenth day) the yolk-sac is completely 

 enclosed within the embryo, and then rapidly disappears until it is en- 

 tirely gone by the sixth day after hatching. 



