562 ANIMAL PHYSIOLOGY. 
apparatus for the protection of the eye as a whole, and for 
preserving exposed parts moist and clean, 
Embryological—_ We have already learned that the first indi- 
cation of the eye is the formation of the optic vesicle, an out- 
growth from the first cerebral vesicle. This optic vesicle be- 
comes more contracted at the base, and the optic stalk remains 
as the optic nerve. 
Fic. 406. 
oy 
Fie. 405. 
Hat Miblsst cndcreoig iiecouing tn fora lene; oc opele yealtie: ¥,, trek copeeeet emia! 
‘ a le ee vesicle. It will be observed that the retina is already distinctly in- 
Fic. 406.—Later stages in development of eye (after Cardiat). a, epiblast; v, developing 
lens ; 0, optic vesicle. 
At an early stage of development (second or third day in the 
chick) the outer portion of the optic vesicle is pushed inward, 
so that the cavity is almost obliterated; the anterior portion, 
becoming thickened, ultimately forms the retina proper, while 
the posterior is represented by the tesselated pigment layer of 
the choroid. 
As this retinal portion breaks away from the superficial epi- 
thelium, the latter forms an elliptical mass of cells, the future 
lens, the changes of which in the formation of the cells peculiar 
to the lens illustrate to how great lengths differentiation in 
structure is carried in the development of a single organ. It - 
will thus be seen that the most essential parts of the eye, the 
optic nerve, the retina, and the crystalline lens, are, according 
to a general law, the earliest marked out. The cornea, the iris, 
the choroid, the vascular supply, the sclerotic, etc., are all sec- 
ondary in importance and in formation to these, and are derived 
from the mesoblast. while the essential structures are traceable, 
