102 



THE DEVELOPMENT OF THE CHICK 



stage that the former closes completely (cf. Fig. 64). The final 

 point of closure at the anterior end, known as the neuropore, is 

 supposed by some to be a point of great morphological signifi- 

 cance, and to mark the extreme anterior end of the original neural 



ax: Mes. 



Fig. 55. — Transverse section through the head immediately behind the 



optic vesicles; stage, 10 s. 



Ao., Aorta, ax. Mes., Axial mesoblast. Ect., Ectoderm. Ent., Entoderm. 

 ]\I. A., Mandibular arch. M. B., Mid-biain. ]Mes., Mesoderm, or. pi., Oral 

 plate, p'a. c, Periaxial cord. p. C, Parietal cavity. Pr'a., Proamnion. Ph., 

 Pharynx, v. Ao., Ventral aorta. 



axis. It is identified by these writers with the permanent neuro- 

 pore of Amphioxus. However, this is open to question. Poste- 

 riorly the closure of the neural tube proceeds much more rapidly, 

 though, of course, it is not fully completed untd after the disap- 

 pearance of the primitive streak. 





-,_ medpl. 





^■F 





"^^Ec^. 



m^^^mmi^^^^smis^- 





Fig. 56. — Early stage of the neural folds. Transverse section through a 



4-5 s embryo between the last somite and the anterior end of the primitive 



streak. 



Ect., Ectoderm. Ent., Entoderm, n. F., Neural fold. N'ch., Noto- 

 chord. med. pi., Medullary plate. Mes., Mesoderm. 



The question as to the position of the anterior end of the 

 original neural axis is one of great morphological significance. 

 Accompanying the closure of the neural tube in this region the 



