574 



TEXT-BOOK OF EMBRYOLOGY. 



The first indication of eye formation is found in the chick at the beginning 

 of the second day of incubation ; in the human embryo, at what has been estimated 

 as about the second or third week. At this stage the neural canal is not yet 

 completely closed in and its anterior end shows three primary brain vesicles 



Optic vesicle area 



Neural canal 

 Fig. 495. — Diagram showing location of areas shown in Fig. 494 after the formation of the 

 neural canal. Modified from Lange. 



(p. 480, Fig. 497). The anlagen of the eyes first appear as bilaterally sym- 

 metrical evaginations from the lateral walls of the fore-brain vesicle (Figs. 497 and 

 498) , and are at first large in proportion to the brain vesicle, itself. When 

 first formed, the optic evagination opens widely into the fore-brain vesicle (Fig. 

 498, right side), but as the distal part of the evagination expands more rapidly 



Retina 



H-b. 



Optic stalk 



Fig. 496. Fig. 497. 



Fig. 496.— Diagram showing location of the (dark) optic area (see Fig. 495) after the beginning of 

 the formation of the optic cup and optic stalk. Lange. 



FiG. 497.— Dorsal view of head of chick of 58 hours' incubation. Mihalkovks. 



Lam. term, lamina terminalis; Fb., fore-brain; Opt. v., optic vesicle; il.b., mid-brain; 



H.b., hind or rhombic brain; H., heart. 



than the proximal part, there soon results a spheroidal optic vesicle attached to 

 the fore-brain by the narrow optic stalk (Fig. 498, left side) . Through the latter 

 the cavity of the optic vesicle and the cavity of the fore-brain are in communi- 

 cation. With the development of the hemispheres, that part of the brain to 

 which the optic stalks are attached becomes the inter-brain (diencephalon). 



