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, 1s 
supposed by some to be a point of great morphological signifi- 
cance, and to mark the extreme anterior end of the original neural 
4 “ 
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f 
: _ VAo. On d/. 
a 
ye A 
Fig. 55. — Transverse section through the head immediately behind the 
optic vesicles; stage, 10s. 
Ao., Aorta. ax. Mes., Axial mesoblast. Eet., Ectoderm. Ent., Entoderm. 
F. B., Fore-brain. Mes., Mesoderm. or. pl., 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 until after the disap- 
pearance of the primitive streak. 
ec 
Le 
Fic. 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. 
Eet., Eetoderm. Ent., Entoderm. n. F., Neural fold. N’ch., Noto- 
chord. med. pl., 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 
