2 INTRODUCTION 



of a Starfish zygote (i.e., fertilized egg) that it becomes anything but 

 a starfish, or change the potentiaHties of a fertilized frog egg into 

 anything but a frog. This means that the nucleus and the cytoplasm 

 of the fertilized egg together possess certain potentialities, as well as 

 certain limitations, in development. Within the range of those limita- 

 tions the potentialities will inevitably become expressed, in any 

 reasonably normal environment. 



It is now quite clear that both the nucleus and the cytoplasm are 

 influential in development. The genetic influences are largely nuclear, 

 but cellular differentiation is cytoplasmic. With each of the cleavages 

 there is a synthesis of nuclear material (e.g., desoxyribose nucleic 

 acid) out of the cytoplasmic (ribonucleic acid), and cytoplasmic 

 from the yolk or other extrinsic food sources. Development is not 

 simply growth or increase in mass. It involves a constant synthesis or 

 the building-up of those elements so vital to the normal processes in 

 the development of the individual. Each stage is built upon the suc- 

 cessful completion of the preceding stage. 



Comparative embryologists have found that there is a somewhat 



Frog's egg and swollen jelly shortly after fertilization. 



