62 I The Process of Evolution 



The details of how selection operates and has operated to produce 

 these systems will become clear only when the mechanics of the 

 systems themselves are elucidated. Therefore it will be necessary to 

 consider briefly developmental systems. 



DIFFERENTIATION 

 AND MORPHOGENESIS 



Mitosis has been described as a means of ensuring the equal alloca- 

 tion of genetic information to the daughter cells in the course of cell 

 division. That mitosis can accomplish this is easily demonstrated in 

 a number of ways, as previously discussed. For example, if the 

 zygotic nucleus of the dragonfly Platijcnemis is permitted to divide 

 seven times (to the 128-cell stage) and then all but one daughter 

 cell are killed with a narrow beam of ultraviolet light, a complete 

 embryo still will develop. Obviously all the necessary genetic in- 

 formation has been passed on from the original nucleus to its de- 

 scendants. In view of the complex mechanism that seems to exist for 

 the purpose of ensuring this successful transfer of necessary genetic 

 information (and considering the demonstrable success of this sys- 

 tem), it is pertinent to ask how cells and tissues become differenti- 

 ated and arranged into a functional organism. Why is a nerve cell so 

 diflFerent from an erythrocyte when both are descended from the 

 same zygote? 



One answer might be that the two cells were exposed to difi^erent 

 environments during development. Even in very early cleavage 

 stages, when few cells are present, the differences in cellular en- 

 vironment may be striking. DiflFerentiation of animal cells may be 

 influenced by such things as their positions relative to the animal 

 and vegetal poles, the outside or inside of the blastula, and proximity 

 to the blastopore in the gastrula. Position may aflFect the amounts of 

 vital nutrients reaching the cell, the amount of oxygen available, 

 the rate of accumulation of excess metabolites, etc. Once differentia- 

 tion has begun, the effects multiplv exponentially. Various combina- 

 tions of differentiated elements add to the heterogeneity of the cellu- 

 lar environment, and complex interactions could provide the basis 

 for the development of the entire organism. (The complexity of the 

 developmental system, of course, varies greatly from organism to 

 organism.) It can thus be said that development of the organism is 

 controlled entirely by interactions within the cluster of dividing and 

 growing cells. Each cell possesses the same information but uses it 

 differently because it is operating in a difiFerent physiological en- 

 vironment. 



