CHAPTER XXXIII. 



THE SECOND HALF OF THE SECOND DAY 



(Thirty-six to Forty-eight Hours) 



Fig. 280. 

 38 to 43 hour chick embryo. (15 somites.) 



It is at this period where the caudal end of the embryo becomes 

 definitely outlined by the formation of a tail fold and lateral folds similar 

 to the head fold. 



THE BRAIN 



Going on from where we left off in our discussion of the formation 

 of the three primary brain vesicles, we find that at this period the neural 

 canal has completely closed, even the rhomboidal sinus has fused. The 

 primary vesicles have enlarged, and their lines of demarcation have be- 

 come more definite. The fore-brain has grown forward as an unpaired 

 vesicle from which the cerebral hemispheres are to develop. The walls 

 of the brain itself lie under the ectoblast, while between the walls and 

 the ectoblast there can be seen a small amount of mesoblast which is to 

 form the skull. 



The optic vesicles have become elongated and definite constrictions 



