SENSE OF SIGHT. 177 



3 anterior wall. Each elongated cell is transformed into a 

 is fibre. 



The cells of the anterior wall retain their primitive form 

 ig. 144). New lens fibres are added by the cells at the margin 

 mator) becoming multiplied and elongated. The lens reaches 

 full size in the 1st year of life and then no more fibres are 

 •med. It will thus be seen that the lens is an area of modified 

 idermis. Like the epidermis, it shows a tendency in the aged 

 be transformed into keratin. The oldest cells (the central or 

 clear fibres) alter first ; . hence the central position of the 

 taract which occurs so frequently in old people. 



(b) The cornea. — The epithelial covering of the cornea is 

 ntinuous with the epidermis and in some animals (snakes, etc.) 

 is shed with that structure, rendering the animal blind for the 

 ne being. It becomes transparent. The mesoblast which 

 3ws in between the lens vesicle and epiblast forms the con- 

 ctive-tissue basis of the cornea and also the capsule of the lens 

 ig. 148). 



(c) It is probable, although not yet verified, that certain cells 

 >m the epiblast grow into the optic vesicle and afterwards form 

 e sensory epithelium of the retina — the rods and cones (G-askell). 

 ifore the incursion of the mesoblast separates them, the optic 

 side and epiblast are in contact. If this is so, then the rods 

 d cones — the sensory cells of the retina, the olfactory cells, the 

 rte cells, the acoustic cells, are all of similar origin — epiblastic. 



2. Structures formed from the Optic Vesicles (neuroblastic 

 jment). — (a) The optic nerve is formed out of the stalk of the 

 tic vesicle. The vesicle is well developed at the commence- 

 mt of the third week (see Fig. 141) ; even before the medullary 

 ites have quite met to enclose the cavity of the fore-brain the 

 tic vesicles have commenced as evaginations of those plates. 

 iey form a great lateral diverticulum on each side of the 

 ■e-brain — a cavity which becomes the third ventricle in the adult, 

 e condition of the optic nerves at the commencement of the 

 :ond month is shown diagrammatically in Fig. 145. The stalk 

 neck remains constricted while the vesicle enlarges. 

 Invagination of the optic vesicle. — Almost as soon as it begins 

 grow out the optic vesicle becomes invaginated, one half 



M 



