THE EYES OF THE BLIND VERTEBRATES OF NORTH AMERICA. 195 



old. It was obtained from the caves at a time when it was ready to swim about 

 freely, that is, when the eye was already fully formed. There was no difference in 

 the gross anatomy of the eye of this individual as compared with that of others. The 

 minute anatomy, as the result of an accident, was not available for study. The 

 others examined in earlier stages have not been reared beyond a length of a few 

 millimetres, and the effect of the light, if any, was not appreciable. From the 

 observations on the development of the eyes which show that some processes are 

 arrested very early it would seem that the only rational way to determine the 

 effect of light on the total development is to colonize the adults in an outdoor pool 

 where the young can be reared, from the fertilization on, in normally lighted waters. 

 Such a colony has been planted, but so far without success in rearing young.* 



The lack of development of the eye not being chargeable to any factor in the 

 environment, is there any factor within the fish that inhibits its development, or 

 whose absence fails to furnish the stimulus necessary to the development? If so, 

 this factor must be present or absent at the time the retardation begins or some time 

 before. 



The inhibition, if any, might operate through a mechanical crowding on the part 

 of a neighboring organ or the greater selective power in eliminating the food requisite 

 for the development of the eye. The first may be eliminated, for there is no evidence 

 whatever of crowding other than that found in normal eyes; in fact in all stages 

 beyond the earliest the eye is much smaller than the optic sockets can easily 

 accommodate. 



The question of selective food elimination is not so readily disposed of. The 

 ophthalmic artery provides the eyes abundantly with blood, so it is not an absence 

 of this that causes the supposed starving. Indeed if the retardation were due to a 

 lack of blood-supply we would be removing the problem one step from the eye with- 

 out solving it. Besides, Loeb's ('93) experiments have shown that the action of the 

 heart may be greatly diminished without affecting the rate of growth of the larval 

 fish. The blood-supply being abundant, is there any other organ that may drain it 

 of the nutriment necessary for the proper growth of the eye? Leaving aside the 

 question whether an organ can be starved by having the nutriment requisite for 

 development withdrawn from the blood by another organ, I can think of no organ 

 or set of organs that attain an unusual growth aside from the tactile organs of the 

 skin. This system of organs is undoubtedly very highly developed in the adult and 

 has also attained a remarkable degree of development at the time the fish is 10 rnilli- 



* Since writing the above the whole colony has been destroyed. 



