96 



THE DEVELOPMENT OF THE CHICK 



tive streak, but this part becomes distinct as the i^rimitive 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 



a.o. 



re. 



n.F. 





— SJ. 



-s.z. 











-^.3. 



-pr.gr. 



Fig. 50. — Embryo of 3 s from above, drawn in bal- 

 sam with transmitted light. 



a. c. v., Amnio-cardiac vesicle, a. o., inner margin 

 of Area opaca. F. G., Fore-gut. N'ch., Notochord. 

 n. F., Neural fold. pr. gr., Primitive groove, s. l,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 



