Chap. VII.] THE GENESIS OF TISSUES AND ORGANS. 131 



this chamber will be seen the delicate transparent retina, ending 

 anteriorly by a jagged border, the ora serrata. Between the 

 retina and the sclerotic lies the choroid (ch.), a pigmented vascular 

 membrane, black for the most part, one portion, however, the 

 tapetum, being iridescent. At the point of origin of the iris the 

 choroid is continued into a ciliary fold, which bears a number of 

 thickenings known as the ciliary processes. The optic nerve 

 which enters the back of the eye-capsule is in direct continuity 

 with the retina. In its centre runs a retinal artery and vein. 

 In the fowl this artery does not run in the centre of the nerve ; 

 and in the fowl there is a structure which is not developed in 

 the mammal. This is a peculiar fold projecting inwards from 

 the choroid through the retina, and known as the pecten. 



Let us now turn to the development of the eye. From the 

 first vesicle of the brain there grows out on either side a hollow 

 outgrowth known as the optic vesicle. These optic vesicles come 

 into close relation with two depressions on the exterior of the 

 head, much resembling the olfactory pits, and known as the 

 optic pits, which soon, by the growing together of their epiblastic 

 edges, become shut sacs. Thus a hollow mass of epiblast is on 

 each side separated from the superficial epiblastic layer; and 

 this, by the growth of its cells, especially the more posterior, 

 becomes a fibrous transparent mass, the crystalline lens, the 

 cavity of which is ultimately obliterated. With this the optic 

 vesicle comes into relation in such a way as to give rise to in- 

 vagination in the manner shown diagrammatically in Fig. 42, 

 where A. shows the optic vesicle (o. v.) growing out from the 

 hollow first vesicle of the brain (1st v.) to meet the optic pit 

 (o. p.), while B. shows the embryonic lens (I.) lying in the optic 

 cup (0. c.) formed by the invagination of the optic vesicle. 



The optic cup is not symmetrical, and its edge is indented from 

 below in such a way that a slit, the choroidal fissure (C., ch. /.) 

 breaks the continuity of the cup. Through this cleft meso- 

 blastic tissue grows inwards and gives rise to the vitreous 

 humour (v. h.), while in the fowl the pecten passes inwards by 

 the same fissure. After this the fissure becomes more or less 

 completely obliterated. 



