312 ALFRED C. REDFIELD 



Suvwiary 



1. The melanophore pigment of the horned toad is contracted 

 by the direct action of nerves as well as by the action of adrenin. 



2. The spinal cord contains, between the eighth and thirteenth 

 vertebrae, nervous structures through which pass the impulses 

 which cause the contraction of the melanophore pigment. 



3. Impulses pass from this part of the cord directly to the adre- 

 nal glands, 



4. Impulses also pass from this part of the * cord posteriorly, 

 and perhaps anteriorily, within the cord to segmentally arranged 

 peripheral nerves which connect directly with the melanophores. 



0. The peripheral fibers are a part of the sympathetic division 

 of the autonomic nervous system. 



Vr. DISCUSSION 



/. The nature of the contraction of melanophore pigment 



Before attempting to make a unified presentation of the me- 

 chanism of color change in the horned toad, it will be valuable to 

 examine the nature of the changes occuring in the melanophore 

 which bring about a migration of the pigment granules. The 

 most recent consideration of this subject is by Spaeth, who says 

 ('16a p. 210): 



Considered as a physical-chemical system the melanophore consists 



essentially of a colloidal suspension of melanin granules 



When the melanophores are stimulated to contract, we observe the first 

 step in a reversible aggregation or coagulation process, i.e., the aggrega- 

 tion of the melanin granules. Is it possible to consider this phenomenon 

 a reversible coagulation such as occurs commonly in emulsoids? 



This question is answered in the affirmative. Spaeth concludes 

 in addition (p. 213) that 



In the contraction of the melanophore there is an aggregation of 

 melanin granules which is to be considered the visible counterpart of an 

 aggregation of colloidal particles that occurs during the contraction in 

 smooth, and possibly striated muscle. 



In attempting to elucidate the nature of tonus in muscle, 

 Sherrington ('15, pp. 229 to 230) presents a conception which 

 harmonizes well with the conclusion of Spaeth: 



