Chapter XII 



THE HEREDITARY FACTORS AND 

 DIFFERENTIATION 



§1 

 One of the most important resuks obtained from the experimental 

 study of development is the fact that all the evidence points to the 

 equality of nuclear division as being the general rule. Also, many 

 of the results of regeneration would be unintelligible except on this 

 idea. Now, genetic research has revealed the existence of unit 

 hereditary factors or genes, whose only visible effect is upon some 

 local characteristic of the organism. For instance, in Drosophila, 

 there exist genes whose primary effect is to modify the colour of 

 the eye, while other genes are more particularly concerned with the 

 shape of the wing. But since the factors which control the forma- 

 tion of an eye are present not only in the cells of the eye but also in 

 the cells of the wing and everywhere else in the body, the question 

 immediately arises as to why the genetic effects are localised in 

 particular regions. It is useless to appeal to other hereditary factors 

 in order to account for this phenomenon, for such factors, on the 

 same evidence, must be present in all cells, and therefore will be . 

 unable by themselves to establish a differential anywhere. 



The answer to this question has already been provided. It is that 

 primary differentiation is not an effect of the hereditary factors, but 

 of external factors. Their first effect is to establish a system of 

 gradients, as a result of which the various regions of the developing 

 egg come to exhibit differences of a quantitative nature, both in 

 respect of the activity of their processes, and of the proportion of 

 materials such as yolk which they contain. There are several gradi- 

 ent-systems in the pre-mosaic stage of development of a newt's egg 

 — the primary apico-basal (animal-vegetative, or future antero- 

 posterior) with high point at the animal pole; the dorso-ventral 

 gradient with high point at the grey crescent ; the exterior-interior 

 gradient, presumably with low point at the centre of the egg ; and, 



