Metamorphosis 



655 



In this connection it is interesting to note 

 that in the neotenous Nccturus maculosus, 

 pituitaries of the adult were found potent 

 in thyrotrophic principle when tested by 

 implantation (Charipper and Corey, '30). 

 Presumably in this form the pituitary-thy- 

 roid relation remains normal in spite of the 

 loss of the metamorphic response of the 

 tissues to thyroid hormone. 



EFFECTS OF ENVIRONMENTAL 

 FACTORS IN AMPHIBIANS 



Considerable attention has been given to 

 the influence of environmental factors on 

 amphibian metamorphosis. From the vantage 

 point of our present knowledge of the hor- 

 monal mechanisms of metamorphosis the 

 effects of environmental factors must be 

 considered in terms of their possible influ- 

 ences on these mechanisms, particularly on 

 the activation of the pituitary. They may 

 therefore be appropriately considered at 

 this point. 



An external factor may influence the 

 sensitivity or capacity for response of the 

 tissues to thyroid and thus affect metamor- 

 phosis. It is clear that Roentgen rays act in 

 this way since Puckett ('37) found it pos- 

 sible to prevent growth processes by x-ray 

 dosage while resorption changes were per- 

 mitted to continue. 



Environmental factors may, on the other 

 hand, influence metamorphosis by their ef- 

 fects on the thyrotrophic activity of the 

 pituitary. Starvation inhibits the onset of 

 metamorphosis in tadpoles when it comes 

 before the beginning of metamorphosis, but 

 accelerates it thereafter (D'Angelo, Gordon 

 and Charipper, '41). The tissues of starved 

 animals retain their sensitivity to thyroxine 

 and their thyroids respond to anterior pitui- 

 tary injections. It is therefore on the thyroid- 

 activating mechanism of the pituitary that 

 starvation before metamorphosis must act. 

 The functional self-differentiation of the 

 pituitary may be supposed to be brought to 

 a halt in the absence of nourishment. Once 

 *he pituitary has been completely activated, 

 - n the other hand, the reduced food supply 

 s5ems to make the released thyroid hormone 

 more effective, perhaps because the cessation 

 of tissue growth permits more active tissue 

 differentiation. Crowding inhibits growth 

 more than it does metamorphosis (Adolph, 

 '31). This would also appear to be due to a 

 differential effect, greater on the growth 

 and development of the pituitary than on 

 that of the body as a whole. 



The effects of temperature on metamor- 

 phosis appear to operate both on tissue sensi- 

 tivity and on the endocrine mechanism. A 

 number of workers report that the failure 

 of metamorphosis which occurs in the cold 

 (below 10° C.) is accompanied by loss of 

 tissue sensitivity to thyroid (Huxley, '29; 

 Hartwig, '36). Yet an effect of low temper- 

 ature also on the endocrine mechanism is 

 indicated by the failure of hibernating tad- 

 poles to continue their metamorphosis when 

 restored to warm conditions (Fosi, '35), and 

 by the fact that manipulation of the tem- 

 perature can differentially affect metamor- 

 phosis and growth (Adler, '16). At 16°C. 

 tadpoles grow larger before metamorphosis 

 than at 24°C. (Etkin, unpublished data). 



ACTIVATION OF INSECT MECHANISMS 



The release of the hormonal mechanism of 

 molt and metamorphosis in Rhodnivis is 

 clearly dependent upon the feeding stimulus, 

 since metamorphosis always follows feeding 

 by a definite number of days, 15 to 20 

 according to stage (Wigglesworth, '34). If 

 the ventral nerve cord is cut molting is 

 prevented. Presumably feeding provides a 

 stimulus that reaches the brain through the 

 nerve cord. Wigglesworth maintains that 

 the stimulus arises from the stretching of 

 the body wall. 



Clearly the mechanism of activation of 

 the brain in Rhodnius cannot apply to in- 

 sects which do not feed in single huge meals. 

 But we have very little indication of the 

 mechanism governing hormone production 

 or release in other insects. The effects of 

 starvation on insect metamorphosis are sim- 

 ilar to those on amphibians. Before a critical 

 period which is near the time of metamor- 

 phosis, starvation delays metamorphosis; 

 after the critical period it may have no 

 effect or may even accelerate the process 

 (Bounhiol, '38). As explained above, Wil- 

 liams found that chilling was necessary for 

 the activation of the brain in the diapausing 

 larva of Platysamia cecropia. 



A comparison of the insect and amphibian 

 reveals a highly suggestive similarity in the 

 relation of the primary endocrine apparatus 

 (i.e., neurosecretory brain cells in insects 

 and pituitary in amphibia) to environmental 

 factors. The vertebrate anterior pituitary is 

 anatomically closely related to the brain 

 both in position and by way of the infundib- 

 ulum. There appear, however, to be no 

 secretory nerves from the brain to the ante- 

 rior lobe. However, in recent years the blood 



