706 



Comparative Animal Physiology 



When the anterior lobe and the pars tuberaHs are extirpated, the melano- 

 phores become maximally dispersed and show no background response. When 

 the posterior lobe is removed, the animals are maximally pale. The source 

 of the darkening hormone appears definitely to reside in the intermediate 

 lobe.^' 1^- Removal of the whole gland leaves the pigment slightly dispersed 

 and non-responsive. All these facts fit the hypothesis of the existence of two 

 factors, with the pars tuberalis responsible for the W-substance and the pos- 

 terior or the intermediate lobe for the B-substance. 



Support for the presence of two factors also comes from the relative ef- 

 fects of injection of B-substance into completely hypophysectomized Xeno- 

 fus"'^ as compared with its effects in normal animals or animals with only 

 the posterior lobes removed (Fig. 271). As would be expected, a larger dose 



Total removal 



A 



Fig. 271. Responses of the melanophores of Xenopus on a white background to in- 

 jections of equivalent doses of pituitary extracts into totally hypophysectomized specimens, 

 specimens with only posterior lobe removed, and normal specimens. From Hogben and 

 Slome.'^ 



is required in the last two types of recipients to bring about a given re- 

 sponse, while in those with complete hypophysectomy a much smaller dose 

 has an equivalent action. This latter observation finds a most logical explana- 

 tion in terms of the resulting absence of an antagonist secreted by the pars 

 tuberalis. 



Fishes. The fishes, which have probably been more intensively investi- 

 gated than any other group, with respect to their chromatophore system 

 and color changes, have several types of chromatophores. The most common 

 and conspicuous type is the melanophore. Other common types contain yel- 

 low pigment (xanthophores), red pigment (erythrophores), and white pig- 

 ment (leucophores). In addition, many fishes possess chromatophores con- 

 taining small clusters of glistening plate-like crystals that impart a bluish 

 green coloration (iridosomes). 



The activities of the melanophores are the principal ones involved in the 

 conspicuous responses to light and darkness and to black and white back- 

 grounds in fishes. The mechanism of response in fishes, as with amphibians, 

 commonly shows a change from primary color responses to secondary during 

 their early development. Young Perca and Salmo,^^- ^^^ Macroyodus,^^- and 

 Hoplias^^^ show such a transition in response mechanism. On the other 

 hand, a number of other species appear not to pass through a phase of pri- 



