Endocrine Mechanisms 729 



inducing factor, and hence normally do not metamorphose but can be made 

 to do so bv large doses of thyroxin; others, such as NecUinis, appear never 

 to develop the reactivity and never metamorphose. That the failure of meta- 

 morphosis in Necturus is the result of failure of tissue response rather than 

 of the absence of an appropriate thyroid principle is shown by the fact that 

 Necturus thyroid will accelerate metamorphosis in Rana clamitans.^-'^ 



In metamorphosis there is an orderly sequence of changes. This sequence 

 appears due, at least in part, to a graded responsiveness of the various tis- 

 sues of the organism to the hormone.'"- -^- ^^ 



The capacity of amphibians to metamorphose is also lost after hypophysect- 

 Qmy 5. 7, 148 y}^J5 activity has been traced to the anterior lobe;" the prin- 

 ciple involved is ineffective in inducing metamorphosis after complete thy- 

 roidectomy.^ The thyroids are also known not to accumulate any colloid in 

 the absence of the hypophysis. This demonstrates that the production and 

 liberation of the thyroid hormone in amphibians, and hence metamorphosis, 

 is under the control of the hypophysis. 



Among teleost fishes it is known that the thyroid gland is concerned in 

 metamorphosis in eels and in certain flatfishes,''"^- ^" and that changes in the 

 amount of thvroxin available will, in other species, result in alterations in 

 growth rates and in body form.^^- '-• ^'-'^ Furthermore, as in higher verte- 

 brates, the pituitary yields a thyrotropic principle on whose activity the de- 

 velopment and secretion of the thyroid depends.^- ^'^ It is interesting that, 

 despite these clearly evident influences of the thyroid on teleost growth and 

 development, within this group of animals the thyroid appears to have no in- 

 fluence on the general metabolic rate.^"- ^^^- ^^^ 



Invertebrates: Gene Hormones. The color of wild-type eye of Drosofhila 

 is due to the presence of two types of colored granules: purplish red and 

 brownish yellow. The chemical composition of these pigments is unknown, 

 but they are considered to belong to a new group of substances named om- 

 matins, of low molecular weight.-*^ The reddish pigment is called erythrom- 

 matin; the brownish, phaeommatin. In the eye these pigment molecules are 

 believed to be conjugated with proteins. These retinal pigments differ greatly 

 in composition among the insects thus far investigated. The absence of either 

 one or both of these pigments in Drosophila is observed in certain mutant 

 stocks,^''^ the vermilion group of mutants possessing only the red, the mutant 

 brown possessing only the brownish, and the mutant white possessing neither 

 of these two pigments. 



In Drosophila the eyes differentiate during pupal development from the 

 optic imaginal discs. When the differentiation of the eye is nearly complete, 

 first the brov.'nish pigment makes its appearance and later the red. The de- 

 veloping eyes of the wild-type flies thus pass through a brownish phase in 

 their development. The stimulus for development of the eye discs, as with 

 all other aspects of pupal development, appears to require the presence of the 

 general growth and differentiation hormone,^" which is treated in the sec- 

 tion on molt, pupation, and metamorphosis in insects. 



It has been clearly established that the development of brown pigment in 

 the eye depends on the presence of a blood-born factor which has been 

 named the V-\- hormone.^^- ^"^ Eye discs from a vermilion larva, normally pro- 

 ducing only red pigment, when transplanted into a wild-type larva develop 



