78 BLIND VERTEBRATES AND THEIR EYES. 



The chromatophores in Amblyopsis are differentiated and contain color before 

 the yolk is absorbed. The black chromatophores are minute granules, few (15 

 or thereabout) to the segment. In an older larva the pigment was much more 

 abundant. The eyes are pigmented early, shortly before hatching, and, owing to 

 their pigment, they soon become conspicuous and remain so till the fish has reached 

 50 mm. in length, when the overlying tissues have become thick. The pigment 

 of the body is lost, or, what amounts to the same thing, does not increase much with 

 age. There is an abundance of pigment cells in the adult, but they are very poor 

 in pigment, and, being in the dermis and covered by the thick layer of epidermis 

 rich in glands, are not apparent. Pigment cells are also abundant in deeper tis- 

 sues in the adult, so that, while no pigment is visible on the surface, an abundance 

 of chromatophores is present in deeper tissues. 



The pigment cells can not be made to show themselves, i.e. become greatly 

 pigmented, even by a prolonged stay in the light. The old, if kept in the light, will 

 not become darker; and a young one reared in the light until ten months old not 

 only showed no increase in the pigmentation, but lost its pigment, taking on 

 ihr exact pigmentless coloration of the adult. Pigment cells appear late in 

 Amblyopsis. When the young are two months old pigment is abundant. This 

 pigmented condition is evidently a hereditarily transmitted condition. It disappears 

 with age. In the first instance this disappearance was probably individual. But as 

 in the flounder, the depigmentation has also become hereditarily transmitted, for 

 even those individuals reared in the light lose the color. 



Numerous facts and experiments show that, while pigment may be and is devel- 

 oped in total darkness, the amount of color in an individual animal depends, other 

 things equal, directly on the amount of light to which it is habitually exposed. 



A number of apparently contradictory observations may be noted : 



(a) The absence of pigment in pelagic animals or their larvoe, which depend on 

 their colorless condition for their existence, is evidently due to causes entirely dif- 

 ferent from those preventing the formation of pigment in cave animals. Natural 

 selection has, in pelagic animals, eliminated the color. 



(6) The migration of pigment granules due to temperature or light and the 

 expanding of chromatophores, when an animal is over a dark background or in 

 the dark, and the contracting over a light background, which may take place at 

 once or at the expiration of several days, is evidently also a different question. 

 The i ibservations of Cunningham, Agassiz, and Semper along this line are of interest. 



(c) Fisch.e] (A. M. Anat, vol. XLVII, pp. 719-734, plate xxxvi, 1893) has 

 noticed that larva,- of salamanders reared in water at 6 to 7 are dark, while others 

 kept in water from 15 to 58 are light. 



(d) Flemming (A. M. Anat., vol. XLVIII, pp. 369-374, 1896) found that with 

 uniform temperature in two vessels side by side, the one dark, the other light, the 

 salamander larva; in the dark vessel develop pigment cells rich in color granules; 

 the larva- in the white vessels become pale, although the number and character of 

 the pigmenl cells is not otherwise changed. The difference is entirely due to the 

 character of the vessels, for if the larvae are taken from the dark to the light vessel, 

 they become light colored in a few days. 



(c) Semper ("Animal Life," p. 89) records that " * * * in the tadpoles of our 

 common toads and frogs the pigment is equally well developed in yellow, blue, or 



