Metamorphosis 



657 



of the wings. This situation cannot account 

 entirely for the absence of further develop- 

 ment in the adult insect, for experiments 

 reveal that the hypodermis, at least, is po- 

 tentially capable of further molting and 

 can even revert to the production of larval 

 exoskeleton under appropriate stimulation 

 (Fururkawa, '35; Piepho, '39). The absence of 

 metamorphic change in the adult insect is at 

 least partly due to failure of the hormonal 

 pulses to continue. The disintegration of the 

 prothoracic glands at the end of metamorpho- 

 sis (Kaiser, '49; Rahm, '52) accounts for the 

 cessation of the hormonal stimulation there- 

 after. What determines this disintegration 

 or the cessation of C.A. activity at the pre- 

 vious stage in insects is not clear. 



In any case it is evident that the morpho- 

 genic processes involved in the cessation of 

 metamorphic activity involve both the loss 

 of sensitivity of the tissues and cessation of 

 hormonal activity in both insects and am- 

 phibians. It is also noteworthy that the 

 maintenance of the adult morphological 

 structures brought into existence under hor- 

 monal stimulation does not require the con- 

 tinued presence of that stimulation for their 

 maintenance. In this respect the morpho- 

 genic principles involved in metamorphosis 

 differ from those common, though not uni- 

 versal, in the sex system of vertebrates, 

 where most structures require the continued 

 presence of the hormone under which they 

 developed for their maintenance. This differ- 

 ence correlates, of course, with the cyclic 

 or seasonal nature of sex activity in contrast 

 to the stability of adult characters. A report 

 by Kollros and Pepernik ('52), however, 

 indicates that some metamorphic events in 

 amphibians may also depend for their main- 

 tenance upon continued thyroid stimulation. 

 They found that the neurones of the mesence- 

 phalic V nucleus regress if thyroid hormone 

 is withdrawn. 



SUMMARY 



The concept of the metamorphic mech- 

 anism operative in amphibian metamorpho- 

 sis may be stated as follows: 



The anterior pituitary begins its thyro- 

 trophic activity early in embryonic develop- 

 ment. However, it is not until late in larval 

 life that the rate of hormone production 

 reaches a level high enough to stimulate a 

 pulse of growth and secretion in the thyroid 

 gland. With the activation of the thyroid 

 the concentration of its hormone in the blood 

 rises according to a definite pattern. Each 



of the larval structures responds to thyroid 

 hormone in its own specific fashion and 

 each at its own rate. As a result of the differ- 

 ences in speed (not in thresholds) of re- 

 sponse of the different tissues metamorphic 

 changes occvir in a definite sequence of 

 appropriately spaced events. Metamorphic 

 changes are fvirther integrated by inductive 

 effects of one tissue upon others in its im- 

 mediate neighborhood. The pulse of thyroid 

 activity is brought to an end by inhibition 

 of pituitary activity by the high level of 

 thyroid hormone. Subsequent pulses of thy- 

 roid activity during the life of the amphibian 

 produce no metamorphic change because 

 the structures concerned have either com- 

 pleted the possibilities of change or lost 

 their tissue sensitivity. 



In insects the first known stimulus to 

 metamorphosis originates in the neurose- 

 cretory cells of the brain. These produce a 

 hormone acting upon the prothoracic glands, 

 which in turn produce a hormone acting 

 upon the tissues. Each tissvie responds to 

 the hormone of the prothoracic gland by its 

 own characteristic metamorphic transforma- 

 tion. This pulse of hormonal activity is the 

 last of a series of such pidses that occur 

 during larval life. The earlier pulses do not 

 eventuate in metamorphosis because each 

 is accompanied by the secretion of a hormone 

 by the corpus allatum. The response of the 

 tissues to the presence of both prothoracic 

 gland hormone and the hormone of the cor- 

 pus allatvim is to undergo a larval rather 

 than a metamorphic molt. 



In both the amphibian and the insect the 

 metamorphic pattern is the product of an 

 interaction of two basic factors, a pulse of 

 hormonal activity and an inherent pattern 

 of tissvie sensitivity. In both organisms the 

 hormonal pulse is produced by two glands. 

 The second gland depends for activation 

 upon the first and the first is intimately 

 related to the brain. 



The mode of activation of the primary 

 hormonal factor is bvit little known. Physio- 

 logical self-differentiation may be predomi- 

 nant in most cases. In others environmental 

 factors or the general body metabolism may 

 play a role by way of the brain. The nature 

 of the tissue response presumably rests on 

 an enzymatic basis, as work on insects is 

 beginning to elucidate. 



The remarkable parallelism between the 

 metamorphic mechanism in insect and am- 

 phibian can have no basis in homology but 

 must be ascribed to the fundamental similar- 

 ity of the life processes in all organisms. 



