240 ESSENTIALS OF ZOOLOGY 



increases in length the folds in the forward region approximate 

 and later fuse. The fusion, however, is not terminal, a wide 

 but narrowing neuropore being left. 



The anterior end of the head is early during the first day 

 marked by a fold, the head fold, which extends as a wing on each 

 side of the developing brain. The groove in front of the fold 

 deepens, and as it deepens the head is rotated forwards, carrying 

 the brain with it. The fold occurs in the gastrula region in 

 front of the brain, and consists merely of ectoderm and endo- 

 derm, and both these layers are concerned. The head is now 

 marked as a process free from the underlying blastoderm, 

 and the head process contains an anterior diverticulum of the 

 endoderm. The process is exactly similar to that which takes 

 place in the Elasmobranchs. 



During the second day the closing in of the neural canal 

 proceeds posteriorly, but it remains open at the posterior end, 

 where the folds diverge and approach again on each side of 

 the primitive streak. The brain is now plain as a wide region 

 of the neural canal. The neuropore is closed, but the front 

 end of the brain and the ectoderm remain connected for a time. 

 The brain is being bent downwards, undergoing a mesencephalic 

 flexure. It is resolved into fore-, mid-, and hind-brains, the 

 latter passing insensibly into the spinal cord. The forebrain 

 is expanded laterally into large optic cups, and later into paired 

 hemispheres. Towards the end of the second day the sinus 

 at the posterior end of the neural canal closes to form a narrow 

 slit, and the head becomes still more flexed and is directed to 

 the right. 



During the third day the mesencephalic flexure is carried 

 so far as to bring the anterior end of the head into a backwardly 

 directed position, and the twisting of the head to the right side 

 to the extent that the plane of the head is parallel to the under- 

 lying blastoderm. The torsion is continued until finally the 

 rest of the body occupies this position. 



The posterior lips of the neural folds now meet and fuse, 

 with the result that the anterior portion of the primitive 

 groove is enclosed in the canal and the posterior is shut out. 

 The former is later in some birds, as has been said, converted 

 into the neurenteric canal ; and in the chick the medullary 



