580 



TEXT-BOOK OF EMBRYOLOGY. 



boundary of the pupil. The remainder of the two-walled optic cup becomes 



the retina. 



The Retina.— Of the two layers which form the wall of the optic cup (p. 579) , 

 the outer (away from the cavity) forms the pigmented layer, while the inner forms 

 the remainder of the retina (Figs. 501, 505). Soon after the formation of the 

 optic cup, it is possible to distinguish a boundary zone— the future or a serrata— 

 between the larger posterior part of the retina or nervous retina and the smaller 

 anterior non-nervous part which becomes the retinal portion of the ciliary body 



Vascular mesoderm 



.Remains of optic 

 vesicle cavity 



Pigmented layer of retina 

 (outer layer of optic cup) 



Vascular mesoderm 

 Wall of brain vesicle 



Ectoderm 



Lens anlage 

 Lens invagination 



Fig. 504. — Section through optic cup and lens invagination of chick of fifty-four 



hours' incubation. Lange. 



Between the lens anlage and the pigmented layer of the retina is the broad inner layer of the optic 



cup, the anlage of the remainder of the retina. 



and iris. While the optic cup is forming, its two layers are both rapidly in- 

 creasing in thickness by mitotic division of their celis. Especially is this true of 

 the inner layer over that region which is to become the nervous retina, and it is 

 the rather abrupt transition between the thicker nervous retina and the com- 

 paratively thin non-nervous anterior extension of the retina that forms the ora 

 serrata. 



The invagination which gives rise to the two-layered optic cup thus differen- 

 tiates what maybe called the two primary layers of the retina, the pigmented layer, 

 and a broad layer from which are to develop all the other layers of the retina. 



