478 



EMBRYOLOGY OF THE CHICK 



are formed at their bases so that they now form optic stalks, which bend 

 downward and backward. 



The cranial nerves can be seen developing at this period also. 



It is at this time that the- first bend or flexure takes place in the 

 brain, cephalad to the notochord (Fig. 282). This is the cephalic flexure. 



If the neural plate be examined at the end of the first day, eleven 

 enlargements (Fig. 278) will be seen with definite constrictions between 



metacoele 

 ( ventricle V 

 thin roof of rnyelcncephalon 



myelococlc 



IV) 



ventral cephalic fold 



ecnetencei>halic fold 



mesocoele 

 (Sylvi.n.quducl) 



location of 

 posterior commissure 

 meso-diencephalic-fold 



lateral telencephalic 

 vesicle 



B 



Fig. 282. 



Diagrams of brain of 4-day chick embryo. Dotted lines show arbitrary 

 boundaries between vesicles. A, longitudinal ; B, right side ; C, horizontal sec- 

 tion. (From Patten, after V. Kupffer.) 



them. These enlargements are known as neuromeres and are really an 

 uncompleted segmentation. 



The literature is filled with many varying and unsatisfactory theories 

 as to what becomes of each neuromere, but as yet nothing can be demon- 

 strated satisfactorily. It is conceivable, however, that as in the crayfish, 

 for example, where we assume that each separate appendage or pair of 

 appendages bespeak an embryological segment, so in vertebrates, where 

 optic vesicles grow from the fore-brain, we may assume a fusion of sev- 

 eral segments. 



At about thirty-three hours, the floor of the prosencephalon has a 



