254 The Nature of Biological Diversity 



the pupal host transforms into an adult, the cyst casts off its pupal 

 cuticle and secretes an adult cuticle covered with scales and hairs. 

 Here again, it is necessary to conclude that each epithelial cell is 

 de-repressing a fresh assortment of genetic information. 



This experiment teaches us two things. The metamorphosis of the 

 implant shows that each cell possesses a definite repertoire of morpho- 

 genetic information — a sequential program which it is able to act out 

 even in the semi-isolated situation. And in the synchronization be- 

 tween the behavior of implant and host, we are reminded once again 

 of the control and coordination which is exercised by hormones. 



Endocrine control of genetic information 



On the basis of present knowledge, the programming and prepat- 

 terning of the cellular community is little short of incomprehensible. 

 The endocrine control of metamorphosis is, by contrast, a far simpler 

 problem which has begun to clarify itself. 



We have seen that the prothoracic gland hormone, ecdyson, is the 

 growth hormone of insects. In its absence, the insect lapses into a 

 state of developmental standstill. When it is again secreted, diapause 

 is terminated and growth is resumed. 



When ecdyson is secreted and acts with little or no opposition, it 

 causes the cells to undertake synthetic acts accompanied by the de- 

 repression and utilization of fresh genetic information (Williams, 

 1961a, b). We have illustrated this action in the case of the giant 

 chromosomes of Chironomus (Clever and Karlson, 1960). The larval 

 cells pupate; the pupal cells undergo adult differentiation. So, to re- 

 vert to an earlier analogy, it is ecdyson that prompts the cells to pro- 

 ceed from one chapter to the next. Therefore, it is necessary to 

 conclude that ecdyson acts directly or indirectly on the nucleus to 

 cause the de-repression of genetic information which can then be 

 implemented by the cytoplasm in new synthetic acts. 



At this point I call attention to a minor technicality — the successive 

 instars of the larval life. During the larval period the insect is some- 

 how stabilized as a larva. It grows and molts and grows and molts. The 

 Cecropia silkworm, as we have seen, increases its mass 5,000-fold 

 during the five larval instars. But it fails to metamorphose until the 

 end of the fifth instar. 



It is easy to show that ecdyson is the necessary stimulus for larval 

 growth and molting. But somehow there is at work a conservative 

 force which opposes change, which blocks '"growing up" without 

 interfering with growth in an unchanging state. 



