Endocrine Mechanisms 765 



sinus glands.^'*^" More recently evidence has been presented to support the 

 view that a hormone from the sinus glands contributes at least in part to the 

 control of the distal retinal pigment of Leander .'^•^^' 



The retinal pigments of Camharns show different thresholds of response 

 to RPH.^*"'-* In low concentrations of the hormone only the distal pigment 

 is influenced; with higher concentration both distal and proximal retinal 

 pigments move to the light-adapted state. 



The origin of RPH is the sinus gland of the eyestalk. This organ, when 

 extracted alone, is able to induce a strong retinal pigment response. No other 

 tissue or organ of the evestalk approaches it in effectiveness. 



The retinal pigment hormone of the sinus gland will withstand boiling, 

 as in the case of the sinus gland chromatophorotropins, but nothing else is 

 yet known of its properties. There is ample reason to believe that it is not 

 identical with any of the dominant chromatophorotropins, inasmuch as the 

 pigmentary system of the integument ordinarily undergoes its complete gam- 

 ut of activities in color changes in response to illuminated backgrounds while 

 the eye remains continuously light-adapted.^^ This latter is true despite the 

 fact that the threshold of response of the retinal pigments to eyestalk ex- 

 tract is substantially higher than the threshold of response to the body 

 chromatophores. Such a situation obviously could not obtain were RPH 

 identical with one of the dominant chromatophorotropins. 



A search for a possible comparable endocrine influence on the state of the 

 retinal pigments of the insect, Ephestia, disclosed no evidence of such an en- 

 docrine activity. ^-^ Injection of extracts of the heads of light-adapted moths 

 into either dark- or light-adapted specimens produced no modification in the 

 state of the pigments. Extracts of crustacean sinus glands also showed no ac- 

 tivity on the retinal pigments of the moth. 



The movement of pigments and the changes in lengths of the contractile 

 fibrils (myoids) of the rods and cones of the eyes of lower vertebrates have 

 long been known, but there was no reason to suspect that they were other 

 than direct responses to light until it was shown that the lengths and posi- 

 tions of the rods and cones of the eyes of catfishes kept in constant darkness 

 still undergo a diurnally rhythmic change. ^"^^ It is not yet known whether 

 the control of these changes involves chemical coordinators or only nervous 

 activity. 



Coloration and Seasonal Color Changes of Birds and Mammals. Among 

 the numerous species of birds which show differentiation of hen and cock 

 plumages a wide variety of mechanisms are involved.^-- ^^ In the English 

 sparrow the control of plumage type is exclusively genetic. Among the pheas- 

 ants, the plumage type is determined by simultaneous action of genes and 

 hormones. In perhaps the majority of birds, however, the plumage typical 

 of one sex is neutral; that of the other sex is determined by blood-borne hor 

 mones. In the common domestic fowl the neutral type appears to be the cock 

 plumage, with the hen type the result of action of estrogens. In African 

 weaver finches, the neutral tvpe is the hen plumage; the cock plumage is 

 the result of action of hypophyseal hormones. Only in the herring gull, 

 among the birds thus far studied, does the cock type of plumage depend on 

 action of androgens.^- 



In the majority of common birds the adults undergo a rather complete 



