8S0 THE BABBIT. 



two eyes cross one another in the floor of the third ventricle, 

 to form the optic chiasma (Fig. 154, ot), and then continue 

 their growth upwards and backwards, round the walls of the 

 fore-brain, to the mid-brain. 



The lens. During the tenth day the deeper part of the 

 surface epiblast, opposite the optic vesicle, becomes thickened, 

 forming the first rudiment of the lens. On the eleventh day 

 this thickened patch becomes invaginated, forming the vesicle 

 of the lens ; the mouth of the pit narrows, and by the end of the 

 twelfth day or early on the thirteenth day it closes, completing 

 the vesicle, which soon separates from the surface epiblast. 



From the first, the inner or deeper wall of the lens vesicle is 

 much thicker than the outer wall. The inner wall continues to 

 increase in thickness, through elongation of the cells composing 

 it, until by the end of the fourteenth day (Fig. 155, OL), the 

 cavity of the vesicle is almost completely obliterated. 



The lens continues to grow rapidly, and throughout the 

 later stages of development is of large proportionate size. Its 

 structure and relations on the twenty-first day are shown in Fig. 

 156, OL, where its inner surface is seen to be very strongly 

 convex, and the outer surface less markedly so. The axial cells 

 of the lens remain straight or nearly so, while the more marginal 

 ones are curved in the direction indicated by the lines crossing 

 the lens in the figure. The first formed part of the lens acts as 

 a nucleus, round which successive layers of cells are added on in 

 the later stages of development. These arise at the equator of 

 the lens, and, increasing rapidly in length, spread on to the faces 

 of the lens, over the ends of the first formed cells. 



The lens, during the time of its formation, is invested by a 

 sheath of mesoblast. According to Kolliker, this is present 

 from the first as a thin layer, between the epiblastic thickening, 

 which gives rise to the lens, and the optic vesicle ; and is carried 

 in by the lens as this becomes invaginated. Most other in- 

 vestigators maintain that it arises from mesoblast which gains 

 admittance into the globe of the eye through the choroidal 

 fissure. This mesoblastic investment of the lens is very vascular, 

 the blood being brought to it by a branch of the central artery 

 of the retina (Fig. 155, ao). This divides into very numerous 

 branches on the inner or deeper surface of the lens, which 



