142 EYE 



development of the syncytial system of nuclear-free protoplasmic threads which 

 Szily has shown exist between all epithelial formations as they draw apart in the 

 course of development. In most situations, as has already been indicated, 

 this system is the basis of the mesenchymatous syncytial network when the 

 free cells have wandered into it, but in the case of the vitreous it remains 

 largely cell-free. The lens capsule belongs primarily to the same category, but 

 in mammals, in which alone a rete vasculosum lentis is developed, mesoderm cells 

 and blood-vessels enter into its formation. 



We have already mentioned the existence of primary protoplasmic connexions 

 between the lens and the future retinal epithelium (fig. 183). When the lens-pit 

 closes in, similar connexions are formed between the outer wall of the vesicle and 

 the surface epithelium. When the retinal layer draws away from the lens, and the 



lens ves 



FIG. 184. SECTION OF THE DEVELOPING EYE OF TROUT. (Szily.) 



lens, lena; ret, retina; hp, outer layer of optic cup; p-v., primitive vitreous; ves, blood-vessel. 

 Mesenchyme-cells are seen passing into the space between the surface-ectoderm, the optic cup, and lens. 



lens from the surface epithelium, these protoplasmic threads are drawn out into a 

 mesh- work which fills the optic cup and surrounds the lens-vesicle (fig. 184). The 

 mesenchyme surrounding the cup does not at first extend beyond its mouth, but in 

 mammals mesenchyme- cells soon extend round the lens and form a layer on the 

 back of that body, so that here the primitive syncytial mesh work is replaced by a 

 lamella of typical mesenchyme. In this vessels appear, and these are supplied 

 by a vascular loop which extends into the cup through the choroidal fissure. It 

 ramifies among the vitreous threads, and later becomes the central artery of the 

 retina and its hyaloid branch. As the optic cup expands this mesenchymatous 

 lamella clings close to the lens (fig. 182), and the space behind it is seen to be 

 filled with protoplasmic strands connected with every part of the retinal 

 epithelium (Eabl, Kolliker, and others). The connexion of the fibrillfp with the 

 pars optica is lost, but in the ciliary region the attachment persists, and the 



