THE EARLY DEVELOPMENT OF THE CHICK 267 



cell mass. The somatopleure, splanchnopleure and coelorn, 

 later become separated into embryonic and extra-embryonic 

 regions, but during these early stages they form continuous 

 structures extending laterally out to the germ wall, and ante- 

 riorly into the head region. From the embryonic portions of 

 these structures develop, respectively, the body wall, the gut 

 wall and vascular organs, and the pericardial, pleural, and 

 peritoneal cavities: their extra-embryonic portions give rise 

 to the embryonic membranes and appendages, and to the 

 extra-embryonic portions of the vascular system and ccelom 

 (exocoelom). 



The history of the mesoderm and coelom in the region of the 

 head-fold deserves a special word. The coelom very early 

 enlarges, either side of the head region, forming a pair of large 

 spaces called the ammo-cardiac vesicles. These grow inward 

 toward the head-fold, and by the time this is well established 

 (4-6 pairs of somites) they push into the lower limb of the head- 

 fold, between its ectodermal and endodermal layers (Fig. 105). 

 Here they finally meet and fuse, forming a median coelomic 

 space bounded, of course, by a mesodermal wall. This is the 

 rudiment of the pericardial cavity, and its formation and subse- 

 quent enlargement, bring about a wide separation of the ecto- 

 derm and endoderm, or as we may now say, of the somato- 

 pleure and splanchnopleure, of the head-fold, the latter being 

 carried much the farther posteriorly (Figs. 98, 99). The later 

 development of this region and of the vascular system in 

 general, may more conveniently be postponed, until after an 

 account of the history of the ectodermal and endodermal layers 

 during these early stages. 



3. Endoderm 



We left the embryonic endoderm as a thin sheet of cells 

 extending forward from the primitive streak, and we had de- 

 scribed the important events connected with the formation of 

 the fore-gut from its anterior margin (Figs. 97, 98). In the 

 region between the fore-gut and the primitive streak, i.e., in the 



