474 



EMBRYOLOGY OF THE CHICK 



From the lateral walls of the prosencephalon the primary optic vesi- 

 cles push out as a pair of rounded pockets, the lumen of each being di- 

 rectly continuous with that of the fore-brain. 



The notochord extends as far as the infundibulum (Fig. 282, A), 

 (a depression in the floor of the fore-brain), so that all regions of the 

 brain lying anterior to it are called pre-chordal, while the rhomben- 

 cephalon, mesencephalon, and the part of the prosencephalon posterior 

 to the infundibulum, which lie dorsal to the notochord are called epi- 

 chordal. 



Fig. 279. 

 33 hour chick embryo (12 somites). 



As has been noted some time back, the most anterior region where 

 the neural tube closes is called the neuropore. The neuropore is still 

 open at this time and remains so, although gradually becoming smaller 

 until after the thirty-third hour period, but even then, there is a scar-like 

 fissure. As we -know of no structure arising from the neuropore, it is 

 important only as a sort of landmark in describing the location of brain 

 structures. 



At this time the neural tube is closed back as far as the somites, and 

 it is of nearly uniform diameter, although, posterior to the last formed 

 somites, the neural tube is still open, and the neural folds can be seen 

 to diverge on either side of Hensen's node (Fig. 279). 



It will be remembered that the first four somites formed are also a 

 part of the head region. Therefore it can be understood that as the 

 neural folds approach each other in the midline, in the region of Hen- 



