286 JAMES ROLLIN SLONAKER 



The cells in the thickest portion of the annular pad are longest. 

 The nuclei of these cells are arranged in a double row in the outer 

 ends near the lens capsule. From this region they become 

 gradually shorter, both anteriorly and posteriorly, and are 

 arranged in a single layer and finally disappear from view. 

 Toward the posterior portion of the annular pad the cells become 

 gradually shorter and less granular until they have completely 

 lost their identity as annular pad cells, at about the posterior 

 border of the lenticular space. The nuclei are conspicuous in 

 this region. A little farther back these cells gradually become 

 longer, their nuclei disappear, and they merge into the true 

 lens cells. There is, therefore, no sharp Une of division between 

 the cells forming the annular pad and those of the lenticular 

 portion of the lens. 



Judging from the behavior and appearance of the cells of the 

 annual pad, both in the developing lens and in the adult, I have 

 concluded ('18) that their function is that of nourishment. 

 They can certainly play no part in accommodation. 



By the time the young leaves the nest (X) the nuclei of the 

 anterior layer have disappeared and the layer has been reduced 

 in thickness, almost to the adult condition. The size of the 

 lens is, however, considerably less than that of the adult (Y). 

 All of the structures of the adult lens are well formed, though 

 not completely developed. They are, however, in such a state 

 of perfection as to give the bird distinct vision. 



THE VITREOUS BODY 



The manner of the formation of the vitreous body in the verte- 

 brates is one of the most difficult problems. There are a number 

 of opposing views in regard to its origin. Kessler ('77) considers 

 the vitreous body a transudate, secreted from the adjacent loops 

 of blood-vessels and that the cells are merely the migrated white 

 corpuscles. Kolhker ('83), Hertwig ('90), and others consider 

 it an exceedingly watery connective tissue which later becomes 

 surrounded by the hyaloid membrane. Keibel and Mall ('12) 

 claim that it is both ectodermal and mesodermal in origin; that 

 the retinal portion of the optic cup secretes the primitive portion 



