THE ORGANS OF SPECIAL SENSE. 585 



With the development of the cells of the inner nuclear layer and their proc- 

 esses, there differentiates the inner molecular layer which separates the inner 

 nuclear layer and the layer of ganglion cells. It consists mainly of ramifica- 

 tions of the dendrites and axones of cells the bodies of which lie in the inner 

 nuclear layer and in the layer of ganglion cells. (Fig. 508.) 



The Chorioid and Sclera.— These develop wholly from the mesoderm. 

 The way in which the mesoderm grows in between the lens and the surface and 

 surrounds the optic cup has been described (p. 576). That part of the meso- 

 derm lying immediately external to the retina develops very early a close- 

 meshed capillary network. This appears before there is any definitely limited 

 sclera and may be considered the anlage of the chorioid. Somewhat later the 

 mesoderm which lies just to the outside of the chorioid takes definite shape as the 

 external fibrous tunic of the eye or sclera. 



The Vitreous. — The manner in which the vitreous humor is formed has 

 been the subject of much controversy arid remains still undetermined. As 

 already noted in describing the development of the lens (p. 595), the latter is at 

 first in direct contact with the inner layer of the retina (Fig. 504) . The lens and 

 the retina separate as the vitreous forms between them. During the develop- 

 ment of the lens the arteria centralis retinae does not stop, as in the adult, 

 with its retinal branches, but continues across the optic cup as the hyaloid 

 artery to end in the vessels of the tunica vasculosa lentis. Some investigators 

 consider the vitreous a transudate from these blood vessels. As the chorioidal 

 fissure closes, some mesodermic tissue is enclosed with the artery, and some 

 investigators consider the vitreous a derivative of this mesoderm. In Birds 

 the formation of the vitreous humor begins before either mesoderm or blood 

 vessels have penetrated the optic cup, and Rabl suggests that the vitreous may 

 be a secretion of the retinal cells. Bonnet describes a double origin of the 

 vitreous, differentiating between a retinal vitreous and a mesoderm vitreous. 

 According to Bonnet, the primary vitreous body begins its formation before the 

 closure of the chorioidal fissure. This primary vitreous appears at the time 

 of formation of the optic cup, is a fibrillated secretion of the retinal cells, and 

 fills in the vitreous space with a feltwork of fine fibrils. With the formation of 

 the optic cup and the closure of the chorioidal fissure this type of vitreous forma- 

 tion ceases and a secondary vitreous body formation takes place from the cells 

 of the pars ciliaris retinae. This is also fibrillated and there develops at this 

 time the so-called hyaloid membrane which closely invests the vitreous. Among 

 the fibers of the vitreous body appears the vitreous humor. Up to this point the 

 vitreous is entirely non-cellular. There next grow into it mesodermal cells 

 which have reached the vitreous through the chorioidal fissure along with the 

 hyaloid artery. To what extent these cells are used up in the formation of the 

 blood vessels of the vitreous and to what extent they remain as connective tissue 



