THE CHICK. 



neural folds fuse to complete the neural tube. The optic vesicles 

 at first stand out at right angles to the head, but they soon 

 become constricted at their bases, and directed somewhat down- 

 wards and backwards (F ig. 1 1 1 , BO) . These constrictions rapidly 

 deepen, so that by the end of the second day the optic vesicles 



OL- 



Crl 



BH 



FIG. 121. Transverse section across the head of a Chick Embryo at the forty- 

 eighth hour of incubation. The section is taken along a line corresponding 

 to one joining the reference letters BI and OL in the three-day embryo shown 

 in Fig. 113. Owing to the cranial flexure, both fore-brain and mid-brain 

 are cut by the section. The right side of the section is slightly anterior in 

 position to the left side, x 60. 



A, aorta. AC, carotid artery. BF, cavity of fore-brain. BH, cavity of hind-brain. 

 CH, notochord. El, auditory pit. HM, hyo-inandibular cleft. M~N", mandibular 

 arch. OC, cavity of optic cup. OL, invagiiiation of epiblast to form the lens. OS. 

 optic stalk. PT, pituitary body. TP, pharynx. VII, facial nerve. 



are connected with the floor of the fore-brain by narrow tubular 

 stalks (Fig. 121, os). 



Towards the end of the second day a circular patch of the 

 external epiblast, opposite the outer wall of each optic vesicle, 

 becomes thickened, and shortly afterwards pitted in to form the 

 vesicle of the lens (Fig. 121, OL). The formation of this pit 

 is accompanied by an infolding of the outer wall of the optic 



