32 Vertebrate Embryology 



The ectoblast, formed, as has been described, 

 by the gradual spreading of the pigmented 

 cells over the entire egg, soon shows two more 

 or less distinct layers, an outer or epidermal, 

 and an inner or nervous layer (Fig. 9). 



At about the time of the closure of the blas- 

 topore, when the embryo is still almost spher- 

 ical in shape (Fig. n, /?), the nervous layer 

 thickens to form the neural plate, which extends 

 along the dorsal side of the embryo, and causes 

 it to be slightly flattened. The neural plate 

 at its posterior end, which is just above the 

 blastopore, is narrow, but it gradually widens 

 as it extends forward for about one third of 

 the circumference of the embryo. The edges 

 of the neural plate soon begin to thicken and 

 to be elevated slightly on all sides, forming 

 the neural folds ; and the neural groove is 

 formed as a shallow furrow, extending forward 

 from the blastopore along the middle of the 

 neural plate (Fig. i, / and /, and Fig. n, B 

 and C). 



The neural folds (Fig. 13, NF) become 

 more and more elevated until they meet and 

 fuse along the mid-dorsal line, converting the 

 neural groove into a closed tube, the neural 

 canal (Fig. 15, NS). The fusion of the neural 



