334 



Vertehrata. 



capsule, the secondary optic vesiclcj whilst the stalk 

 elongates ; then the cavity of the stalk diminishes as does also the 

 cavity between the two layers of the vesicle {i.e., the cavity of the 

 primary optic vesicle). The stalk forms the optic nerve, and 

 the wall of the vesicle forms the retina ; the outer layer becomes 

 very thin and forms a sheath of strongly pigmented cells (the 

 pigmented layer of the retina, tapetum nigrum), whilst the 

 rest of the retina is formed from the thick inner layer. At the 



i.eji 



Fig. 275. Diagrammatic representation of the development of the vertebrate eye. 

 1 section through the head at an early stage. ' 2 somewhat later stage : the incipient lens, 

 and the invagination of the optic vesicle may be seen. 3 — 4 further development : con- 

 striction of the lens, formation of the secondary optic vesicle. 5 the other chief parts of 

 the eye are formed, ch choroid, ep epidermis, ep' epidermal portion of the cornea, g vitreous 

 humour, h cornea, hj brain, i iris, I lens, m mesoderm, n optic nerve, " primary optic 

 vesicle, r retina, S sclerotic, t pigment layer (external sheath of the retina). — Orig. 



time when the primary optic vesicle is becoming capsular, the lens 

 begins to form. It develops as an invagination, which is finally 

 completely cut off from the rest of the epidermis and takes up its 

 position below this and opposite to the optic capsule. The trans- 

 parent lens develops from this epithelial sac by further modification 

 of the cells. The remaining portions of the vertebrate eye then 

 form round this essential part : the connective tissue between the 

 lens and the retina forms the vitreous humour, the covering to 

 the lens forms the cornea; between the latter and the lens there 



