

THE ORGAN OF SIGHT 



1003 



I 



margin of the cup, and becomes separated from the overlying ectoderm by mes- 

 oderm. The cells forming the posterior wall of the vesicle lengthen and are con- 

 erted into the lens fibers, which grow forward and fill up the cavity of the vesicle 

 (Fig. 866) . The cells forming the anterior wall retain their cellular character, and 

 form the epithelium on the anterior surface of the adult lens. By the second month 

 the lens is invested by a vascular mesodermal capsule, the capsula vasculosa lentis ; 

 the bloodvessels supplying the posterior part of this capsule are derived from the 

 hyaloid artery; those for the anterior part from the anterior ciliary arteries; the 

 portion of the capsule which covers the front of the lens is named the pupillary 

 membrane. By the sixth month all the vessels of the capsule are atrophied except 

 the hyaloid artery, which disappears during the ninth month; the position of this 

 artery is indicated in the adult by the hyaloid canal, which reaches from the optic 

 disk to the posterior surface of the lens. With the loss of its bloodvessels the cap- 

 sula vasculosa lentis disappears, but sometimes the pupillary membrane persists 

 at birth, giving rise to the condition termed congenital atresia of the pupil. 



Rudiment of choroid 

 Rectus muscle 



Optic nerve 



Retina 

 J igmented layer 

 Vitreous body 

 (shrunken) 



Cornea 

 Membrana pupillans 



Eyelid 



Iris 



Pars ciliaris and pars iridica retinae 

 FIG. 866. Horizontal section through the eye of an eighteen days' embryo rabbit. 



X 30. (Kolliker.) 



The vitreous body is developed between the lens and the optic cup. The lens 

 rudiment and the optic vesicle are at first in contact with each other, but after the 

 closure of the lens vesicle and the formation of the optic cup the former withdraws 

 itself from the retinal layer of the cup; the two, however, remain connected by a net- 

 work of delicate protoplasmic processes. This network, derived partly from the cells 

 of the lens and partly from those of the retinal layer of the cup, constitutes the 

 primitive vitreous body (Figs. 867, 868). At first these protoplasmic processes 

 spring from the whole of the retinal layer of the cup, but later are limited to the 

 ciliary region, where by a process of condensation they appear to form the zonula 

 ciliaris. The mesoderm w r hich enters the cup through the choroidal fissure and 

 around the equator of the lens becomes intimately united with this reticular tissue, 

 and contributes to form the vitreous body, which is therefore derived partly from 

 the ectoderm and partly from the mesoderm. 



