54 



EARLY EMBRYOLOGY OF THE CHICK 



walls and dilated lumen mark the region which will develop 

 into the brain. The undilated posterior part of the neural tube 

 gives rise to the spinal cord. Three divisions, the three primary 

 brain vesicles, can be distinguished in the enlarged cephalic 

 region of the neural tube (Fig. i8). Occupying most of the 

 anterior-part of the head is a conspicuous dilation known from 

 its position as the fore-brain or prosencephalon. Posterior to 



proamnion 



ectoderm of head 



anterior horn of mesoderm 

 Si^^>-V-'>,'J. .-, anterior neuropore 



infundibulum 



cephaHc 

 mesenchyme 



line of endocardial 

 fusion 



margin of anterior 

 intestinal portal 



extra-embryonic, 

 vascular plexus 



Pig. 19. — Ventral view ( X 45) of head and heart region of chick embryo of 9 

 somites (about 29-30 hours incubation). 



the prosencephalon and marked off from it by a constriction is 

 the mid-brain or mesencephalon. Posterior to the mesenceph- 

 alon with only a very slight constriction marking the boundary 

 is the hind-brain or rhombencephalon. The rhombencephalon 

 is continuous posteriorly with the cord region of the neural tube 

 without any definite point of transition. 



In somewhat older embryos (Fig. 19) the lateral walls of the 

 prosencephalon become out-pocketed to form a pair of rounded 

 dilations known as the primary optic vesicles. When the 



