152 Vertebrate Embryology 



1. The fusion of the medullary folds 

 throughout the greater part of their length to 

 form the neural canal. 



2. The dilation of the anterior end of the 

 neural canal to form the fore-brain, which, in 



Iff.C. 



F.So. 



FIG. 50 (A). DIAGRAMMATIC REPRESENTATION OF FIG. 50. 

 (After Foster and Balfour.) 



N.C.) neural canal. Ch.^ notochord. Z>, commencing fore-gut or front part of 

 the alimentary canal. F. So, somatopleure, raised up in its peripheral portion into 

 the amniotic fold, Am. Sp., splanchnopleure ; at Sp. it forms the under wall of the 

 fore-gut ; at F. Sp. it is turning round to run forward; just at its turning-point, 

 the heart, ///, is being developed, pp, pleuroperitoneal or body cavity. A, ecto- 

 derm. B, mesoderm. C, entoderm. 



turn, begins to push out on each side a hollow 

 diverticulum, the optic vesicle ; and the indica- 

 tion of the position of the mid- and hind-brain 

 as slight enlargements of the neural canal be- 

 hind the fore-brain. 



3. The increase of the head-fold, and the 

 elevation of the head above the level of the 

 blastoderm. 



4. The increase in the number of the mes- 

 oblastic somites. 



