342 ZOOLOGY FOR MEDICAL STUDENTS chap. 



words it was originally part of the outer surface of the body, whose 

 normal characteristic it is to be provided with sensory cells pointing 

 outwards. The outer wall of the optic cup becomes the pigment layer 

 (p.I) : its cells degenerate and become laden with pigment, and the 

 same applies to both layers of the cup in the inturned portion of its rim 

 which lines the iris. 



While these developmental processes are taking place the lens has 

 made its appearance. It arises not from the brain-wall but from the 

 external ectoderm. The portion of this in contact with the outer end 

 of the optic rudiment becomes slightly thickened (Fig. 142, B, Z). As 

 the optic rudiment becomes converted into the optic cup this thickened 

 piece of ectoderm sinks below the surface, lining a cup-shaped cavity the 

 opening of which to the exterior becomes gradually narrower (Fig. 142, 

 C, I) until it finally disappears and a closed vesicle is formed, isolated 

 from the external ectoderm. This vesicle is the lens rudiment. With 

 further development it grows rapidly in size and its cavity becomes 

 obliterated by the cells that form its inner wall growing out into a long 

 slender columnar shape. It is these elongated cells, their protoplasm 

 converted into transparent material, that form practically the whole of 

 the lens, the outer wall persisting merely as a thin layer of epithelium 

 coating its outer surface (Fig. 142, D and E). 



While the two primary constituents of the eye. Retina and Lens, 

 arise in the way indicated, the outer layers of the eyeball arise simply 

 from the mesenchyme or embryonic connective tissue which becomes 

 concentrated round them. The non-cellular contents of the eyeball — the 

 aqueous humour and the vitreous body — are apparently secreted by the 

 surrounding cells, in the case of the latter probably by the retinal cells. 



It will be remembered that in early stages a gap was present in the 

 wall of the optic cup — the choroid fissure. The nerve-fibres which 

 develop on the inner surface of the retina are continued along this sur- 

 face by way of the gap mentioned on to the involuted portion of the 

 surface of the optic stalk and along it to the brain. As development 

 goes on the choroid fissure becomes a narrow slit and eventually its 

 edges undergo complete fusion. This is how the optic nerve in the 

 adult eye plunges through the retina : it is situated in the position 

 formerly occupied by the bottom of the choroid fissure. 



Otocyst.i — The otocyst of the Dogfish, as of vertebrates in general, 



arises in much the same fashion as the olfactory organ. A piece of the 



outer ectoderm on each side of the medulla oblongata becomes thickened 



and depressed below the general surface, its opening becoming relatively 



1 " Membranous labyrinth " of human anatomy. 



