124 LESLIE B. AREY 



2. MATERIAL AND TECHNIQUE 



Tlio work of Chiarini ('04a, '06), in which he systematically 

 investigated the effect of light on the retinas of representative 

 vertebrates, led him to the conclusion that the maximal changes 

 in the position of the pigment and of the cone cells occur in the 

 lower vertebrates and particularly in the fishes, whereas in the 

 highest vertebrates these changes are extremely hmited or may 

 even be imperceptible. For experimentation involving varia- 

 ble temperature poikilothermous animals are necessary, and no 

 vertebrates are more easily available for this purpose than are 

 fishes and amphibians. When to availability is added the ad- 

 vantages which they offer by the possession of exceptionally 

 well developed visual and pigment cells, which are capable of 

 undergoing the greatest dimensional changes found in any of 

 the vertebrates, it will be seen that these animals are particularly 

 favorable for such physiological studies as were conducted in 

 the present investigation. 



The following fishes were used: the common horned pout 

 (Ameiurus nebulosus Lesueur); the common killifish (Fundulus 

 heteroclitus Linn.) ; the shiner (Abramis crysoleucas Mitchill) ; 

 and the goldfish (Carassius auratus Linn.). Of these, all but 

 Fundulus are fresh water forms, while Fundulus is found in 

 brackish water, and especially at the mouths of fresh water 

 streams. Abramis and Carassius belong to the same family, 

 the others to different families. Goldfish were obtained for 

 the most part from dealers although a number of feral animals 

 were used. Many of the streams and ponds about Cambridge 

 are stocked with feral goldfish, liberated by accident or design. 

 The majority of such animals have lost their gold coloration 

 and have returned almost completel}^ to the original olivaceous 

 type. 



The amphibians used were as follows: the leopard frog (Rana 

 pipiens Schreber) ; the green frog (Rana clamitans Latreille) ; 

 the bull frog (Rana catesbiana Shaw) ; and the mud puppy 

 (Necturus maculatus Raf.). Of the three species of frogs, the 

 leopard frog was used almost exclusively for experimentation 

 on adult animals; the larvae only of the bull frog were employed. 



