124 EARLY EMBRYOLOGY OF THE CHICK 



The first part of the gut to acquire a complete entodermic 

 lining is the fore-gut. Its floor is formed by the caudally 

 progressing concrescence of the entoderm which takes place as 

 the subcephalic and lateral body folds undercut the cephahc 

 part of the embryo (Figs. i6 and 31, B). At a considerably 

 later stage the hind-gut is formed by the progress of the sub- 

 caudad fold (Figs. 35 and 31, C). Between the fore-gut and the 

 hind-gut, the mid-gut remains open to the yolk ventrally. As 

 the embryo is more completely separated from the yolk the 

 fore-gut and hind-gut increase in extent at the expense of the 

 mid-gut. By the fourth day of incubation the mid-gut is re- 

 duced to the region where the yolk stalk opens into the enteric 

 tract (Figs. 31, D and 43). 



The Establishment of the Oral Opening. — When first estab- 

 lished the gut ends as a blind pocket both cephaHcally and 

 caudally. The mouth opening does not appear until the third 

 day, the cloacal opening is not estabhshed until much later in 

 incubation. In embryos of 55 hours the processes leading to- 

 ward the establishment of the oral opening are clearly indicated. 

 A mid-ventral evagination of the pharynx is established im- 

 mediately cephalic to the mandibular arch (Fig. 35). Opposite 

 this out-pocketing of the pharynx, and growing in to meet it, the 

 stomodeal depression is formed. The thin membrane formed 

 by the meeting of the pharyngeal entoderm with the stomodeal 

 ectoderm is known as the oral plate. The communication of the 

 fore-gut with the outside is finally estabhshed by the breaking 

 through of the oral plate. 



The formation of the mouth opening in the manner described 

 does not take place at the extreme anterior end of the fore-gut. 

 A small gut pocket extends cephahc to the mouth. This so- 

 called pre-oral gut rapidly becomes less conspicuous after the 

 breaking through of the oral plate. The small depression 

 which in older embryos marks its location is known as Sees- 

 sell's pocket (Fig. 43). Even this small depression eventually 

 disappears altogether. Its importance lies wholly in the fact 

 that it indicates for some time the place at which ectoderm 

 and entoderm originally became continuous in the formation 

 of the oral opening. 



The Phar3mgeal Derivatives. — Several structures arise in the 

 pharyngeal region which do not become parts of the digestive 



