GENIC CONTROL OF 

 DEVELOPMENT 



Development consists of a series of steps of morphological, physiologi- 

 cal, and biochemical diversification which are exactly timed, graded, 

 attimed to each other, and properly placed within the whole. All this 

 is certainly controlled by the action of the genie material, as follows 

 from the fact that mutational changes may aflFect every stage of 

 development, though the minor changes at or near the end of develop- 

 ment are the favorite materials of genetic research. We hear frequently 

 that orderly development is the result of an interplay between genie 

 activity and environment. This, in my opinion, is a completely mislead- 

 ing statement in spite of its apparent truth. If a silkworm egg, 

 immersed in sulfuric acid, does develop parthenogenetically, as it 

 otherwise would not do, this is an environmental efiFect. If an egg 

 reared in calcium-free water changes its type of cleavage, this is an 

 environmental effect. An axolotl injected with thyroxin metamorphoses 

 as a result of the internal environment. But in all these cases and a 

 thousand others, the new environment is an unnatural condition to 

 which the normally developing organism is not exposed. It is just like 

 changing the environment of a motor by pouring acid into the gaso- 

 line. But if we consider the organism within the environment to which 

 it is adapted, development is completely controlled by the genie mate- 

 rial. Millions of sea urchin eggs grown in their proper sea water de- 

 velop one like the other; millions of Drosophila eggs in their natural 

 food and temperature do the same; and also millions of rabbits in the 

 uteri of healthy does. All this means that the experimental zoologists 

 interested in interfering with genie actions by changing the internal or 

 external environment may speak of the collaboration of both in con- 

 trolling development. However, the geneticist as a rule deals with de- 

 velopment under normal, optimal conditions and therefore only with 

 genie control. If he finds a mutant which has a different phenotype 

 at different temperatures (including also temperature-sensitive bio- 

 chemical synthesis as known in Neurospora), this does not mean to 

 him a proof for environmental cooperation in control of development 

 but rather a definite type of genie action which has a norm of reaction 

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