SENSE ORGANS. 79 



this cup eventually becomes the retina, while the outer layer 

 forms the pigmented epithelium (pigment layer of the choroid 

 of older works) of the eye. 



Connected with the invagination of the retinal layer is 

 another phenomenon, an account of which is necessary for the 

 understanding of other features of the eye. This invagination 

 is not confined to the distal portion of the optic vesicle, but ex- 

 tends along its lower surface and continues upon the optic stalk 

 in a manner readily understood from Fig. 85, the result being" 

 a gap, the choroid fissure, in the ventral wall of the optic cup, 

 produced as a groove along the lower side of the optic stalk. 



Through this choroid fissure mesenchyme cells, and later 

 blood-v^essels, enter the optic cup. Later, when the fissure 

 closes, the walls of the stalk unite around the blood-vessels, 

 which hence, apparently, enter the optic cup through the centre 

 of the optic stalk. A loose watery tissue, the vitreous humor, 

 is developed from the immigrant mesenchyme cells, and fills the 

 optic cup, or as it is called in the adult, the posterior chamber, of 

 the eye. The blood-vessels serve to nourish the retina, etc. 



At first the retinal layer is thin ; but it gradually increases in 

 thickness by cell division so that it eventually consists of several 

 layers of cells, and finally these become differentiated so that 

 several strata can be distinguished. Those nearest the lens 

 become the ganglion cells, those farthest away the rod- and cone- 

 cells, and between these a so-called granular layer, this last 

 being separated from the other two by an inner and outer mo- 

 lecular layer. From some of the rod- and cone-cells, which are 

 the sensory strictures of the eyes, slender rods and cones (Fig. 

 66), grow out towards and into the pigmented epithelium, while 

 others of this layer develop into supporting or isolating cells. 

 From the other side of each rod cell and cone cell a nerve fibre 

 grows out towards the front of the eye, and breaks up into 

 dendrites which interlace with other dendrites coming from the 

 cells of the granular layer, their fibrillations producing the inner 

 molecular layer. The outer molecular layer is similarly an in- 

 terlacing of dendrites from the granular cells and from the gan- 

 glion cells, the minute granulations which occasioned the name 

 molecular layer being the sections of the nerve fibrillations. The 



