MELANOPHORES OF THE HORNED TOAD 293 



attempting to discover the mechanism through which a noxious 

 stimulus, locally applied, is able to produce a general contraction 

 of pigment of the entire body, it became apparent that the melano- 

 phores are coordinated by means of a hormone (Redfield, '16). 



When the nerves supplying a region of the skin are cut, and the 

 surface of the mouth or cloaca is stimulated with a weak faradic 

 current, the melanophore pigment of the entire surface of the 

 skin contracts. There is no difference between the behavior of 

 the melanophores in the part which has been denervated and the 

 rest of the skin. For example, the sciatic nerve may be cut close 

 to the junction of one hind leg with the trunk. Upon applying 

 faradic stimulation to the mouth, both hind legs become equally 

 pale as the melanophore pigment of the entire skin contracts. 

 Again, the series of horned toads operated upon in the manner 

 described on page 288 may be subjected to a noxious stimulation. 

 In none of these animals does the operation, which in some cases 

 at least must have cut the nerves to the portion of skin under 

 observation, interfere with the contraction of the pigment cells 

 upon this region of the body. It is clear that the reaction may be 

 completed without the aid of nerves which connect directly with 

 the melanophores. A hormone, set free through the action of 

 the noxious stimulus, is the natural alternative to account for this 

 result. 



To test the hypothesis that the pigment is contracted by a 

 hormone, carried to it by the circulation, the following experiment 

 was tried: 



August 30, 1915. Tied a ligature firmly about the right hind leg of a 

 very dark horned toad. Movements of the lower part of this leg indi- 

 cated that pressure of the ligature had not blocked the nervous impulses 

 to the leg. 



12.20 Started stimulating mouth with weak faradic current. 



12.25 Color of skin of right leg unchanged. Skin of left leg has 

 become much paler, in striking contrast to the right leg (fig. 8). Mel- 

 anophore pigment of the other parts of the body has contracted. 

 Stopped stimulation. 



12.27 Removed ligature. 



12.36. Right leg has become as pale as the left. 



