. The development of the chick; an introduction to embryology . P^ici. 54 A. — Transverse section through the head of a 10 s embryo. The region of the section is near the center of the hind brain. Ac, Aorta. Endc, Endocardium. Endc. 8., Endocardial .septum. H. B., Hind brain. Myc, Myocardium, p. C, Parietal cavity. Ph., pharynx. Sopl., Somatopleure. Splpl., Splanchnopleure. v. M., \entral mesentery. The Neuropore. From the place where the neural folds firstmeet, the elevation and fusion proceed both forwards and back-wards in a continuous fashion (cf. Figs. 59, 61, 65, etc.). Althoughthe open
. The development of the chick; an introduction to embryology . P^ici. 54 A. — Transverse section through the head of a 10 s embryo. The region of the section is near the center of the hind brain. Ac, Aorta. Endc, Endocardium. Endc. 8., Endocardial .septum. H. B., Hind brain. Myc, Myocardium, p. C, Parietal cavity. Ph., pharynx. Sopl., Somatopleure. Splpl., Splanchnopleure. v. M., \entral mesentery. The Neuropore. From the place where the neural folds firstmeet, the elevation and fusion proceed both forwards and back-wards in a continuous fashion (cf. Figs. 59, 61, 65, etc.). Althoughthe open anterior stretch of the neural tube is very short in com-parison to the posterior open part, it is not until about the 12 s 102 THI] I)I:\KLOPMENT OF THE CHICK stage that the former closes eoni])letely (cf. I^ig. 64). The finalpoint of eh)sure at the anterioi- end, known as tlie neuro[)ore, issupposed by some to be a j)oint of great morphological signifi-cance, and to mark tiie extreme aiiterioi end of the original neural c/^t. Fig. bb. — Transverse section throiijj;li tlie head iiiiniediately l)ehiiKl the optic vesicles; stage, 10 s. Ao., Aorta, ax. Mes., Axial mesoblast. Ect., Ectoderm. Ent., Entoderm. F. B., Fore-brain. Mes., Mesoderm, or. Oral plate, pa. c, Periaxial cord. p. C Parietal cavity. Pra., Proamnion. Ph., v. Ao., Ventral aorta. axis. It is identified by these ^vriters with the permanent neuro-pore of Amphioxus. However, this is open to question. Poste-riorly the closure of the neural tube i)roceetls much more rapidly,though, of course, it is not fully completed until after the disap-pearance of the primitive streak.
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