276 



ZOOLOGY. 



along the stem. The whole structure is now called the 

 optic cup, and the hollow of the cup is the secondary optic 



vesicle. The 

 stem is the optic 

 stalk; and the 

 slit is called 

 the choroid 

 fissure. The 

 structure of the 

 optic cup can 

 be better under- 

 stood from figs. 

 144 and 145, 

 than from any 



Fig. 144. TRANSVERSE SECTION OF HEAD OF EMBRYO verbal descrip- 

 CHICK, TO SHOW DEVELOPING BYE. tion 



On the right side the section passes through the TJ-io IATIO io 



choroid fissure. * 



formed from the 



epiblast of the surface of the head, simultaneously with the 

 formation of the optic cup. There is first formed a pit 

 with thickened floor ; the edges curve over and close in the 

 pit, which becomes the 

 lens-vesicle. The lens then 

 separates from the epi- 

 dermis (fig. 146), and its 

 vesicle becomes obliterated 

 in time. 



The lens and retina thus 

 formed, the remainder of 

 the eye is formed from 

 mesoblast-cells, excepting 

 only the conjunctiva, which 

 is part of the epidermis 

 (epiblast). The retina at 

 this stage consists of two 



layers, with a narrow chink (the remains of the primary 

 optic vesicle) between them. The outer of these layers 

 becomes the pigment-layer of the retina (often counted 

 as part of the choroid). The primary optic vesicle becomes 

 completely obliterated. The inner layer is divided into 



Fig. 145. SIDE VIEW OF LEFT EYE 

 OF FIG. 144. 



(Diagrammatic.) 



