250 The Nature of Biological Diversity 



secreting cells showed no correlation with the peculiar features of 

 the synthetic product and, in fact, did not depart from those encoun- 

 tered in the other tissues that were examined. 



As Yeas and Vincent (1960) point ovit, this implies that the "bulk 

 RNA" of the ribosomes does not contain the genetic information 

 which codes the amino acid sequence in silk. This finding is com- 

 pletely in line with that obtained on microbial systems where, as pre- 

 viously mentioned, genetic information is continuously transmitted to 

 the ribosomal "factories" by a certain short-lived messenger RNA 

 which makes up only a tiny fraction of the total RNA. 



Metamorphosis of the Cecropia silkworm 



In the case of giant chromosomes of Rhynchosciara, we considered 

 the evidence for gene repression and de-repression as well as the cyto- 

 logical picture of the synthesis and coding of what appears to be 

 messenger RNA. Then from a study of Epanaphe we learned that the 

 ribosomes are "ignorant" synthetic centers to which genetic informa- 

 tion is apparently conveyed in the form of messenger RNA. I now 

 propose to consider the harnessing of genetic information to specific 

 morphogenetic acts. And for this purpose I hasten to introduce our 

 final performer. 



In fields and forests of eastern North America there occurs a hand- 

 some silkworm whose outstanding properties as an experimental ob- 

 ject entitle it to rank alongside Rhynchosciara and Epanaphe. Tax- 

 onomists continue to amuse themselves by changing the scientific name 

 of this insect ; therefore, let us simply call it the Cecropia silkworm. 



The eggs are oviposited in early summer on any of a number of 

 common trees and shrubs. Ten days later they hatch into little spinose 

 caterpillars which grow rapidly, punctuating this growth by a series 

 of four larval molts. By transforming leaves into silkworm, they in- 

 crease their mass some 5,000-fold and finally become mature fifth- 

 instar larvae. 



Through the workings of hormonal factors which we shall shortly 

 consider, there comes a day when the behavior of the mature silk- 

 worm suddenly changes. It ceases to feed, having already built up 

 within its own tissues a stockpile of molecules to sustain the 10 re- 

 maining months of life. After a period of random locomotor activity, 

 it begins to spin a cocoon around itself. Work on the structure con- 

 tinues for 2 days, during which time a silk thread, nearly a mile long, 

 is transformed into a complicated edifice. 



