THE EYES OF THE BLIND VERTEBRATES OF NORTH AMERICA. 



181 



TABLE OF MEASUREMENTS OF THE EYE FROM THE TIME OF ITS FIRST APPEARANCE TO MATURITY. 

 All measurements are given in micra except lengths of embryos which are in millimetres. 



* The individual measurements of the eyes of the seven specimens whose average is here given is: 



Number of the specimen 1 



Longitudinal diameter 176 



Vertical diameter 144 



Lens 48 



t Or none. 



IX. THE HISTORY OF THE LENS. 



The lens begins to develop when the embryo is about 2.5 millimetres long (PI. 

 XIV, Fig. 25). It forms as a thickening of the skin where the optic vesicle is in con- 

 tact with it. It is still connected with the skin when the embryo has reached a length 

 of 4.5 millimetres (Fig. 32). (Compare Figures 25, 27, 32, 33, and 43 with Figures 28, 

 29, and 30, the latter representing the development of a normal lens.) The history of 

 the lens after this stage is somewhat uncertain. It is well established that the cells 

 composing it never lose their embryonic condition, that they are never differentiated 

 into fibres. In many eyes, certainly in all in which a lens could be detected in later 

 stages, the lens becomes separated from the skin (Fig. 33). The separation is com- 

 pleted when the larva has reached a length of 5 millimetres (PL XV, Fig. 36). From 

 this stage on, the lens begins to be resorbed; in some 6-millimetre larvae it could no 



