96 THE DEVELOPMENT OF THE CHICK 
tive streak, but this part becomes distinct as the primitive streak 
splits into its derivatives. It is therefore only when the latter 
is entirely used up that the entire length of the medullary plate 
is established. However, long before this time the greater por- 
tion has become converted by folding into the neural tube, a 
process that proceeds in general from in front backwards. Thus 
o.0. 
Fic. 50. — Embryo of 3s from above, drawn in bal- 
sam with transmitted light. 
a.c. v., Amnio-ecardiae vesicle. a. o., inner margin 
of Area opaca. IF. G., Fore-gut. N’ch., Notochord. 
n. I’., Neural fold. pr. gr., Primitive groove. s.1,s. 2, 
s. 3, First, second and third somites. 
successive stages may be studied in serial sections of the same 
embryo; an anterior section, for instance, showing the completed 
tube, one farther back, the folded medullary plate, and yet more 
posteriorly the central part of the medullary plate disappears in 
