STRUCTURE OF FIFTY-HOUR CHICKS IOI 



the neural fold with which it is associated before the closure 

 of the neural tube. The margin of the neural fold involves 

 cells which go into the superficial ectoderm and into the neural 

 tube, as well as those which are concerned in the formation of 

 the neural crest. 



When first established the neural crest is continuous antero- 

 posteriorly. As development proceeds, the cells of the neural 

 crest migrate ventro-laterally on either side of the spinal cord 

 (Fig. 37, C), and at the same time become segmen tally clus- 

 tered. The segmentally arranged cell groups thus derived from 

 the neural crest give rise to the dorsal root ganglia of the spinal 

 nerves, and in the head region to the ganglia of the sensory 

 cranial nerves. (For a later stage of the dorsal root ganglia see 

 Figure 44.) 



III. THE DIGESTIVE TRACT 



The Fore-gut. The manner in which the three primary 

 regions of the gut-tract are established has already been con- 

 sidered in a general way (see Chapter XI and Fig. 31). In 

 50 to 55-hour chicks the fore-gut has acquired considerable 

 length. It extends from the anterior intestinal portal cephalad 

 almost to the infundibulum (Fig. 35). 



As the first region of the tract to be established, the fore-gut 

 is naturally the most advanced in differentiation. We can 

 already recognize a pharyngeal and an cesophageal portion. 

 The pharyngeal region lies ventral to the myelencephalon and 

 is encircled by the aortic arches (Fig. 35). The pharynx is 

 somewhat flattened dorso-ventrally and has a considerably 

 larger lumen than the cesophageal part of the fore-gut (Cf . Fig. 

 36, B and C). 



The Stomodaeum. There is at this stage no mouth opening 

 into the pharynx. However, the location where the opening 

 will be formed is indicated by the approximation of a ventral 

 outpocketing near the anterior end of the pharynx, to a depres- 

 sion formed in the adjacent ectoderm of the ventral surface of 

 the head (Fig. 35). The ectodermal depression, known as the 

 stomodaeum, deepens until its floor lies in contact with the ento- 

 derm of the pharyngeal out-pocketing (Fig. 35). The thin 

 layer of tissue formed by the apposition of the stomodaeal ecto- 

 derm to the pharyngeal entoderm is known as the oral plate. 



