656 



Metamorphosis 



supply of the anterior lobe has been shown 

 to consist in part of a portal venous system 

 which brings blood from the hypothalamus 

 to the pituitary (Green, '51). It is believed 

 that through this venous system the neuro- 

 secretory apparatus in the hypothalamus is 

 able to exert some controlling influences on 

 the activity of the anterior lobe. It would 

 thus appear that the varied instances where 

 environmental factors influence the meta- 

 morphic process in insects and amphibians 

 operate through a basically similar mech- 

 anism. The stimuli are received by the 

 nervous system, which transmits them to 

 neurosecretory elements which, in turn, start 

 the endocrine chain mechanism into action. 

 The intimate association of the brain and 

 the primary endocrine apparatus thus per- 

 mits the development of some degree of 

 control of the endocrine system through 

 environmental factors. In this way the life 

 history of an animal can be brought into 

 synchrony with seasonal change. It wovild 

 be interesting to explore the influence of 

 cold on the pituitary of such amphibians as 

 hibernate in larval stages, along the lines 

 of the work of Williams on Platysamia.* 



In connection with the neurosecretory 

 pathway in insects Scharrer ('52a,b) has 

 suggested that the corpus cardiacum be con- 

 sidered part of the neuroendocrine complex 

 regulating insect development. She has dem- 

 onstrated that on an anatomical basis the 

 C.C. might be considered as a storage organ 

 for the secretion formed by the neurosecre- 

 tory cells of the brain [perhaps analagous 

 to the sinus gland of Crustacea (Passano, 

 '51)]. Yet experiments with the C.C. have 

 not yielded dramatic results. Pfeiffer ('39) 

 asserted that C.C. removal delays molting 

 in the grasshopper, which is consistent with 

 the above theory. Vogt ('46), on the other 

 hand, found in Drosophila that C.C. implants 

 delay puparium formation. Perhaps clear- 

 cut results are not to be expected until both 

 brain and C.C. are treated as a unit. 



CESSATION OF METAMORPHIC 

 TRANSFORMATION 



Amphibians. As a final step in the regula- 

 tion of metamorphosis, the processes involved 



* The posterior lobe of the pituitary (not involved 

 in metamorphosis) is innervated by neurosecretory 

 fibers from the hypothalamus. The extraordinary 

 morphological parallelism between this and the 

 neurosecretory system of insects has been com- 

 mented upon by Scharrer and Scharrer ('44). Per- 

 haps this, too, finds its ultimate significance in a 

 linking up of the endocrine gland with environ- 

 mental factors. 



must be brought to a halt. When resorption 

 of the tadpole's tail is complete, for example, 

 no further change is possible. When the 

 tympanic membrane is fully formed no 

 further development seems possible. Yet it 

 is apparent that such considerations do not 

 apply to all metamorphic events. Do the 

 legs of the tadpole cease their rapid growth 

 because they lose their capacity to respond 

 to thyroid or because the hormone itself 

 ceases? It would appear probable that the 

 sensitivitj^ of the leg tissues changes with 

 development, since the legs slow their growth 

 before the metamorphic climax is complete 

 and while the thyroid gland is still very 

 active (Etkin, '32, '36a). On the other hand, 

 skin shedding is a characteristic phenomenon 

 of metamorphosis in Amphibia (Etkin, '32). 

 Plainly the capacity of the skin for respond- 

 ing to thyroid by shedding is not lost at the 

 end of metamorphosis, since it is found in 

 the adult. It mvist be supposed, therefore, 

 that the cessation of skin shedding late in 

 metamorphosis is a consequence of the re- 

 duction in activity of the thyroid which, 

 according to histological studies, takes place 

 at this time (Etkin, '36a). 



The inactivation of the thyroid is itself 

 an aspect of metamorphosis that requires 

 explanation. No direct evidence is available 

 to indicate whether this is to be ascribed to 

 a change in sensitivity, as in the case of the 

 tadpole's legs, or to a cessation of pituitary 

 stimulation. From what is known of the 

 general physiology of the thyroid-pituitary 

 relation we might suppose that the second 

 of the above alternatives holds true, since 

 prolonged stimulation of the thyroid gland 

 by pituitary implants is possible. Further- 

 more, a mechanism for the suppression of 

 thyrotrophic function in the pituitary is 

 suggested by the observation in many ani- 

 mals that thyroid hormone itself, when it 

 reaches a high level in the blood, inhibits 

 the thyrotrophic activity of the pituitary (for 

 a summary of the evidence on this point see 

 Adams, '46). 



It can be seen from the above discussion 

 that, though several factors appear to con- 

 tribute to the mechanism bringing metamor- 

 phic change to an end, little direct experi- 

 mental knowledge in this field is available. 



Insects. In insects as in amphibians some 

 metamorphic changes have an inherent end 

 point, as in the resorption of larval organs. 

 This may also apply to constructive changes 

 at metamorphosis when the living cells of 

 the primordium are used up in the metamor- 

 phic change, as in the case of the formation 



