MELANOPHORES OF THE HORNED TOAD 317 



Although the foregoing are the only direct observations upon 

 the subject, there are well-known color phenomena in many 

 vertebrates in which the adrenal secretion may play a part.^ 

 Adrenin is secreted in mammals during emotional excitement. 

 It would not be surprising, therefore, to find the secretion of the 

 adrenal glands responsible for the emotional or 'psychic' color 

 changes of fishes, amphibians, and reptiles which have been fre- 

 quently described. Since it is recognized that hormones play 

 an important role in the development of secondary sexual char- 

 acters, it is quite possible that the striking color changes which 

 occur in the breeding season or during courtship are, as Fuchs 

 ('06) has suggested, produced by these agents. 



b. The coordination of melanophores and smooth muscle. There 

 is a certain resemblance between the mechanism coordinating the 

 smooth muscles of mammals and that which coordinates the 

 melanophores of many vertebrates. Both are under the control 

 of the sympathetic nervous system and both are influenced by 

 adrenal secretion during nervous excitement. The smooth 

 muscles, however, are known to be innervated by antagonistic 

 fibers belonging to two morphologically distinct parts of the 

 autonomic nervous system: It may be asked, are the melano- 

 phores also under a double innervation? 



Bert ('75) suggested that the melanophores of Chameleon are 

 innervated by two sets of nerves analogous to those controlling 

 the blood-vessels. Carnot ('96) and Sollaud ('08) applied a 

 similar explanation to their observations upon the frog. Babak 

 ('10) was forced to conclude that the expansion as well as the 

 contraction of the melanophore pigment of Amblystoma larvae 

 is produced by nervous impulses arising in the retina, because the 

 pigment of tadpoles is expanded in the dark, while that of blinded 

 larvae is contracted in the absence of photic stimuli. Other 



' It must not be supposed that all vertebrates will be found to exhibit the 

 adrenal control of melanophores to the extent which the horned toad does. One 

 may judge from the experiments performed on the chameleon, the frog, and many 

 fishes that these forms do not possess so sensitive a mechanism, for if they did it 

 would be impossible to execute such experiments as the stimulation of nerves 

 without producing a secretion of the hormone. 



