VI.] THE LACRYMAL DUCT. 155 



the choroidal fissure gives rise to the vitreous humour, 

 while at a later stage a definite process of this meso- 

 blast becomes the pecten. 



Of the walls of the optic cup, the thinner outer 

 (posterior) wall becomes, behind the line of the ora 

 serrata, the pigment-epithelium of the choroid, while 

 the thicker inner (anterior) wall supplies all the ele- 

 ments of the retina, including the rods and cones which 

 grow out from it into the pigment-epithelium. 



In front of the line of the ora serrata, both walls of 

 the optic cup, quite thin and wholly fused together, give 

 rise to the pigment- epithelium of the ciliary processes 

 and iris, the bodies of both these organs being formed 

 from the mesoblastic investment. 



Accessory Organs connected with the Eye. 



Eyelids. The most important accessory structures connected 

 with the eye are the eyelids. They are developed as simple folds 

 of the integument with a mesoblastic prolongation between their 

 two laminae. They are three in number, viz. an upper and lower, 

 and a lateral one the nictitating membrane springing from 

 the inner or anterior border of the eye. Their inner face is lined 

 by a prolongation of conjunctiva, which is the modified epiblast 

 covering the cornea and part of the sclerotic. 



The Lacrymal glands and Lacrymal duct. 



The lacrimal glands are formed as solid ingrowths of the 

 conjunctival epithelium. They appear on the eighth day of 

 incubation. 



The lacrymal duct begins as a solid ridge of the epidermis, 

 projecting inwards along the line of the so-called lacrymal groove, 

 from the eye to the nasal pit. 



At the end of the sixth day this ridge begins to be separated 

 from the epidermis, remaining however united with it on the 

 inner side of the lower eyelid. 



