. The development of the chick; an introduction to embryology . Fig. 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. Ao., Aorta. Endc, Endocardium. Endc. S., Endocardial B., Hind brain. Myc, Myocardium, p. C, Parietal cavity. Ph., , Somatopleure. Splpl., Splanchnopleure. v. M., Ventral 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 ante
. The development of the chick; an introduction to embryology . Fig. 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. Ao., Aorta. Endc, Endocardium. Endc. S., Endocardial B., Hind brain. Myc, Myocardium, p. C, Parietal cavity. Ph., , Somatopleure. Splpl., Splanchnopleure. v. M., Ventral 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 THE DEVELOPMENT OF THE CHICK stage that the former closes completely (cf. Fig. 64). The finalpoint of closure at the anterior end, known as the neuropore, issupposed by some to be a point of great morphological signifi-cance, and to mark the extreme anterior end of the original neural ax: 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., Oralplate, pa. c, Periaxial cord. p. C, Parietal cavity. Pra., 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
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