78 Cellular Differential; 



A. Procedure: ,. i , , 



Nucleus trom 



Egg nucleus , Needle y Egg nucleus , /^embryonic cell 



oooo 



Fig. 39. Transplantafion of embryonic nuclei. 



and from their sisters. If these new cell types can be shown to play a causal 

 role in the construction of the multicellular being and/or its subsequent 

 functioning, such cells are said to be differentiated, and the processes by 

 which they came to be different are collectively called cellular differentia- 

 tion. 



The question arises, what kind of information do we have to provide 

 before we can be satisfied that a single act of cellular differentiation has 

 been adequately described and understood? The categories of necessary 

 information are outlined below. 



1. Describe the new phenotype. It is not enough to say that the 

 differentiated cell differs from its parent or sister cells. We must show 

 how it differs. Which enzymes are present in the new cell type and are 

 absent or greatly diminished in the old, and vice versa? Which specific 

 structures in the cell have been altered? This is one of the points of de- 



