148 OUTLINES OF CHORDATE DEVELOPMENT 



it jigain becomes solid through the elongation of the cells of its 

 inner side: the outer side remains as a thin epithelial layer over 

 the distal surface of the solid lens. When the pupil begins to 

 narrow the lens moves into the opening and finally remains 

 included just within it, in the cavity of the optic cup. (In 

 other vertebrates, except the Teleosts, the lens forms as a hollow 

 vesicle resulting from an invagination of the surface ectoderm.) 

 After hatching the cells of the optic cup layers begin to 

 differentiate into the histological elements characteristic of the 

 adult eye. The outer layer becomes very thin and pigmented, 

 while in the thick inner layer the characteristic cell layers of 

 the retina appear (Fig. 50, D, E}. The cells in contact with 

 the pigment layer gradually differentiate as the rods and cones; 

 this process begins in the fundus region and spreads thence to 

 the more peripheral regions of the retina. Other cells of the 

 retina are neuroblasts and send out nerve processes; those from 

 the cells bordering the cavity of the optic cup (posterior 

 chamber) grow down, by way of the margins of the choroid 

 fissure, to the optic stalk. Entering the ventral and posterior 

 sides of this, they pass thence to the ventral wall of the fore- 

 brain, forming there the optic chiasma and optic thalami. The 

 cavity of the optic stalk is finally obliterated bv the collection 



/ l ** - - -i * 



pj_tese_fibersj in the brain the optic recess marks the region 

 of its original opening (Figs. 41, 42, 43). The component cells 

 of the stalk become largely non-nervous, forming the sup- 

 porting neuroglia cells. The optic stalks are commonly known 

 as the II pair of cranial nerves, but it is evident from their 

 development that they are tracts of the brain, without any 

 similarities to the true cranial nerves. 



The eye develops very slowly during the larval period after 

 hatching, but at the time of metamorphosis it is rapidly per- 

 fected. The details in the later development are not very well 

 known. The rudiment of the vitreous body, occupying the 

 posterior chamber of the eye, is of mixed origin, although 

 purely ectodermal. It arises from outgrowths of cells from 

 the inner surface of the lens placode and also from the retinal 

 layer of the cup. These elements intermingle and form the 



