DEVELOPMENT OF GALLUS 



207 



of the ectoderm along the axis of the embryo, in front of the 

 primitive knot. At its sides are the neural folds which rise 

 up and meet forming the neural tube. The first part of the 

 neural tube to form is the brain, which is clearly marked out 



3 



Fig. 95. — Transverse sections through an embryo chick after 30 hours' 

 incubation. (From Jenkinson.) 

 Taken in the planes shown by lines 1, 2, and 3 on Fig. 94. / is posterior. 

 For lettering see Fig. 96. 



into the regions which will become the fore-, mid-, and hind- 

 brain. As the primitive knot and streak move back, the neural 

 folds follow them and cover up the spots which they formerly 

 occupied, and so the neural tube comes to be formed 

 immediately above the notochord. On each side of the neural 



