Differentiation and Morphogenesis in Insects 257 



refrains from synthesizing those which are not needed. The decision 

 not to synthesize a new enzyme is passive in the sense that it involves 

 a continuation of the repressed state of the corresponding gene or 

 genes — a further withholding of the genetic information prerequisite 

 for the synthesis. An affirmative decision to undertake the synthesis 

 of a new enzyme is active in that it involves the de-repression of the 

 corresponding genes and a flow of the appropriate genetic information 

 to the synthetic centers of the cell. 



A bacterial cell is equipped, not so much with a "construction 

 manual" as with a "cookbook" — a collection of recipes for the man- 

 agement of any number of energy-rich molecules which it may en- 

 counter. Therefore, its genetic equipment is delicately tuned to the 

 exigencies of the environment. In a bacterial cell the countdown on 

 the synthesis of a new enzyme must be counted in milliseconds. 



Anyone familiar with the elegant prganization of animal cells is 

 entitled to the view that a bacterium is a nondescript object. The cells 

 of a louse are, by comparison, of outstanding beauty. But a far more 

 important distinction is that an animal cell, say, the egg of an insect, 

 has a vastly complicated charge on life. Its method of making another 

 egg is, in the least, a roundabout process. 



A central dogma of experimental embryology has been the concept 

 of the totipotent egg — a cell that starts out doing everything and in 

 the course of embryonic development gives rise to a progeny which 

 specializes and differentiates by doing less and less. The microbes 

 have disenchanted us from this point of view. It now appears that the 

 egg is a highly repressed and specialized cell. It is specialized for 

 reproduction, for cloning itself to establish the "critical mass" of the 

 blastoderm (Grobstein, 1959). Even the molecules of yolk seem, for 

 the most part, to have been synthesized elsewhere and "injected" into 

 the ovum prior to fertilization (Bonhag, 1958; Telfer, 1961; Hisaw, 

 1962). 



Unlike the bacterial cell, the egg can scarcely allow itself to be at 

 the disposal of the molecules of the environment. Therefore, it com- 

 monly envelops itself with one or more shells and membranes to be- 

 come a self-contained system. Meanwhile, the genetic plans for the 

 embryo must be speedily programmed. 



Here we trip over the phenomenon of "embryonic determination," 

 which is encountered in its extreme expression in the so-called 

 "mosaic eggs" of the Diptera and Lepidoptera. The cells of the very 

 early embryo are somehow programmed for a series of future differ- 

 entiations which they and their progeny will undertake, not only in 

 the embryo, but also during successive phases of metamorphosis. And 



