DEVELOPMENT OF BIEDS AND MAMMALS 243 



third day, and in the fourth the fusion of the amnion folds is 

 completed. The result of the fusion is that an inner layer of 

 ectoderm, continuous with that of the embryo, forms a cavity 

 around the embryo the amniotic cavity while the outer layer 

 of the fold continuous with the ectoderm of the blastoderm is 

 cut off from the embryo. The first is called the amnion, or 

 the true amnion, and the second the false amnion, or serosa. 

 The amnion increases in size with the increase of the fluid 

 which it contains, the amniotic fluid. As the embryo becomes 

 constricted from the yolk, it is more and more surrounded by 

 the amnion and the amniotic fluid, and development pro- 

 ceeds therefore in what might be called aquatic conditions. 



Amnion 



fleural can&l \ Muscle, layer 

 Dermis 



Splanchnocoel \ Aorta 



Splanchnoplcure Proneph.duct 



Notochord' 

 FIG. 125. Transverse section of body of third-day fowl. 



Post. card. 

 Sclerotome 



Yolk Sac. The endoderm, to begin with, is a layer of 

 flattened cells next the yolk, and the primitive enteron lies 

 between the two. With the formation of the head and tail 

 folds the enteron becomes tubular in front and behind. It may 

 be said, therefore, to be resolved into three regions, fore-, mid-, 

 and hind-gut, the mid-gut being that part of the endoderm 

 which lies over the yolk and expands on each side as the 

 endodermal part of the blastoderm. Both the fore-gut and 

 the hind-gut, but especially the former, increase in length with 

 the growth of the embryo, and the mid-gut becomes reduced 

 to a small portion of the enteron. At this place, however, 

 its floor is continued as a tube-like outgrowth which rapidly 

 widens to form the endoderm of the yolk. The investment 



