180 THE EYES OF THE BLIND VERTEBRATES OF NORTH AMERICA. 



VII. THE EYE OF THE ADULT. 



The eyes of adult individuals from 25 to 75 millimetres long were fully described 

 in my first paper (Eigenmann, '99), and the eyes of very old individuals were men- 

 tioned briefly. The most highly developed eye found was that of an individual 75 

 millimetres long (PI. XII, Fig. 3). This eye is much above the average in the develop- 

 ment of its pigmented layer, etc. Perhaps 25 millimetres represents the stage at 

 which the eye as a whole reaches its maximum development (Fig. 4). It is not my 

 intention to redescribe the eyes of these stages, but I wish to make a few corrections in 

 the account of the adult eye given in my first paper.* It was stated (Eigenmann, '99, 

 p. 566) that blood-vessels did not enter the eye of the adult. I have since found 

 small vessels in the remnant of the vitreous cavity. The choroid slit in a few indi- 

 viduals of all stages up to the very oldest remains open. A detailed account was 

 given of the supposed lens (Eigenmann, '99, p. 569). From the evidence of embry- 

 ology it seems even more doubtful than when that account was written that the lens 

 ever persists till maturity. The structures described as the lens are probably some- 

 thing else. The eyes of senescent individuals will be sufficiently described under a 

 later head. 



VIII. AMOUNT AND RATE OF GROWTH OF THE EYE FROM THE 



TIME OF ITS APPEARANCE. 



The question of the rate and amount of growth of the eye from the time it 

 appears can best be answered by the following table of measurements of the 

 eyes of successive sizes of embryos. Attention should be called to the great varia- 

 bility of the size of the eye in any one stage or in successive stages of development. 



It is seen from this table that the eye reaches the full vertical and longitudinal 

 diameter of the adult when the embryo is only 2 millimetres in length. Since the eye 

 does not make its appearance till the embryo has reached a length of 1.5 millimetres 

 and the lens does not begin to develop until 1 millimetre has been added to the length 

 attained by the embryo after the eye has reached its full size, that is, not until it has 

 reached a length of 2.5 millimetres, it is apparent that from the beginning the eye 

 is in longitudinal and vertical diameter equal to the full adult eye. 



* The last paragraph on page 566 of this paper and the first two of 567 should be transferred to come imme- 

 diately after the first paragraph of page 565. 



