DEVELOPMENT OF THIRD DAY 



493 



tral to the myelencephalon and encircled by aortic arches, and an 

 oesophageal portion, lying posterior to the pharyngeal with a much 

 smaller lumen than the pharynx. 



At this time there is an outpushing of the ventral portion of the 

 pharynx and an inpushing from the ectoderm close to this region, which 

 will soon meet and form the mouth-opening. The ectodermal inpushing 



Spinal con! 



Spina! ganglion 



Ventral root 



Mixed spinal nerve 

 Myotome 



Sympathetic ganglion -- 



Fig. 290. 



Developing nerve roots in a chick embryo of 4% days. (After 

 Neumayer. ) 



is known as the stomodaeum and the thin layer of tissue between the 

 inpushing and outpushing which is later to break through to complete 

 the mouth-opening is called the oral plate. (Fig. 301, I, Seessel's 

 pocket.) It is this oral plate region in the adult which separates the 

 oral cavity from the pharynx. 



The fore-gut extends into the head region cephalad to the 

 stomodaeum, and this portion is called the pre-oral gut. This pre-oral 

 gut, however, disappears shortly after the oral plate breaks through, 

 leaving only a small diverticulum which is then called Seessel's pocket. 



The digestive tract has been lying close to the notochord up to this 

 time, being separated from the notochord and the aortae by a broad thin 

 layer of mesoderm. Now it begins to draw ventrad from this position, 

 remaining attached, however, by the mesentery, a constantly narrowing 

 band of tissue. 



This mesentery is composed of mesoblast continuous with that 

 which surrounds the entoderm of the digestive canal. The mesoblast 

 consists of an undifferentiated middle layer (Fig. 291, b), in which blood 



