102 EARLY EMBRYOLOGY OF THE CHICK 



Later in development the oral plate breaks thiough bringing 

 the stomodaeum and the pharynx into open communication. 

 Growth of surrounding structures deepens the original stomodaeal 

 depression, and it becomes the oral cavity. The region of the 

 oral plate in the embryo becomes, in the adult, the region of 

 transition from oral cavity to pharynx. 



The Pre-oral Gut. It will be noted by reference to Figure 

 35 that the oral opening is not established at the extreme 

 cephalic end of the pharynx. The part of the pharynx which 

 extends cephalic to the mouth opening is known as the pre-oral 

 gut. After the rupture of the oral plate, the pre-oral gut 

 eventually disappears, but an indication of it persists for a time 

 as a small diverticulum termed Seessell's pocket(Cf.Figs. 35 

 and 43). 



The Mid-gut. Although the mid-gut is still the most ex- 

 tensive of the three primary divisions of the digestive tract, 

 it presents little of interest. It is nothing more than a region 

 where the gut still lies open to the yolk. It does not have 

 even a fixed identity. As fast as any part of the mid-gut 

 acquires a ventral wall by the closing-in process involved in 

 the progress of the subcephalic and subcaudal folds it ceases to 

 be mid-gut and becomes fore-gut or hind-gut. Differentiation 

 and local specializations appear in the digestive tract only in 

 regions which have ceased to be mid-gut. 



The Hind-gut. The hind-gut first appears in embryos of 

 about 55 hours (Fig. 35). The method of its formation is 

 similar to that by which the fore-gut was established. The 

 sub-caudal fold undercuts the tail region and walls off a gut 

 pocket. The hind-gut is lengthened at the expense of the 

 mid-gut as the sub-caudal fold progresses cephalad and is 

 also lengthened by its own growth caudad. It shows no local 

 specializations until later in development. 



IV. THE VISCERAL CLEFTS AND VISCERAL ARCHES 



At this stage the chick embryo has unmistakable visceral 

 arches and visceral clefts. Although only transitory, they are 

 morphologically of great importance not only from the com- 

 parative view point, and because of their significance as struc- 

 tures exemplifying recapitulation, but also because of their 



