REPRODUCTION AND LIFE CYCLES 431 



systems of organs. The modern experimental cmbryologist raises 

 the specter of the old controversy of rpigenesis or preformation, by 

 inquiring into the question of how much of the development is depend- 

 ent upon the contents of the fertilized egg and how much is due 

 to environmental factors. 



Genes 



All of the evidence which has been gathered to date indicates that 

 the development of an embryo is a highly complicated process. As 

 a starting point one might mention the character-controlling genes 

 of the chromosomes that are brought together in the formation of a 

 zygote. The vital part w^iich these play in altering developmental 

 patterns has been clearly demonstrated many times. 



Environment 



The second important factor is the environment. Changes in the 

 normal environment frequently result in abnormalities. It is well 

 known that temperature is a vital factor, since in all except viviparous, 

 warm-blooded forms, a change in temperature will affect the rate of 

 development. Under some conditions, for example when gastrula- 

 tion is occurring, atypical forms may result. Likewise variations in 

 temperature may produce apparent changes in the genes themselves. 

 When certain kinds of fruit flies are kept at a higher temperature, 

 there is a decrease in the number of ommatidia produced in each eye. 

 Subsequent breeding experiments and a lowering of the temperature, 

 however, result in a return to the original type. Another example of 

 the environmental influence which upsets the normal metabolism of 

 the embryo so that abnormalities result may be seen in the alter- 

 ation of the oxygen, or food supply. The introduction of poisons 

 also has similar effects. 



Changes in the metabolic rate of an organism are definitely cor- 

 related with environmental factors as shown by the work of Child 

 and his associates, who demonstrated the presence of definite "meta- 

 bolic gradients." The axial gradient theory accounts for differences in 

 dominance of certain areas in the developing organism, beginning 

 with the axis occurring between the two poles of an egg. The dorsal 

 lip of the blastopore soon becomes established as the region of greatest 

 metabolic activity and so determines the rate of development of the 

 other parts. It is at this region of highest metabolic activity that the 



