MORGAN, COAT COLORS IN MICE 115 



the units or factors, and most of those who have written on the subject 

 give the impression that this sorting involves the actual materials out of 

 which each character is built up. I do not think that we are justified in 

 this interpretation of the factor hypothesis, for all the facts can be equally 

 well explained, if the material particles sorted in maturation represent 

 only parts of the material basis, whose entire activity is essential for the 

 formation of parts of the organism. The difference in point of view is, 

 I believe, fundamental, even if the practical outcome is the same, for our 

 entire conception of the mechanism of heredity and development is 

 involved. 



On another occasion 7 I have discussed the view as to how far the mech- 

 anism of segregation in the germ cells is comparable to the processes that 

 take place in the later development of the egg itself, when different re- 

 gions develop into different parts at the time of the specification of the 

 organs and tissues. I pointed out that there is a good deal of analogy to 

 support the view that the two processes are the same. The Weismannian 

 conception practically identifies the two processes, for the biophores are 

 supposed to be sorted out in the maturation process in the same way that 

 they are sorted out as the development of the embryo proceeds. 



On the one hand, if the facts lead us to interpret the process of segre- 

 gation as due directly to a separation during the process of maturation 

 of material particles (factors) essential for development of particular 

 organs, on the other hand the facts of experimental embryology and of 

 regeneration have seemed to experimental embryologists to speak equally 

 emphatically in favor of the view that the localization factors are essen- 

 tially different from those assumed for Mendelian segregation. 



An examination of the facts of sex-limited inheritance has seemed to 

 me to show with some probability that the simplest conception of the 

 mechanism of segregation is that material particles carried in the chro- 

 mosomes are separated at these divisions and lead to the Mendelian classes 

 of gametes. I do not deny that the facts may be conceived as due to 

 other kinds of processes that lead to the formation of two classes of 

 gametes for each pair of allelomorphs, but when we take into account the 

 evident random segregation of the factors any other hypothesis than that 

 based on particulate separation seems to me far more improbable than the 

 one here suggested. If this is admitted, the next logical step would seem 

 to require a similar process in development of the embryo. I need only 

 mention by way of example the occasional appearance in heterozygous 

 individuals of the dominant and the recessive characters in different parts 



7 American Breeders' Association, V, 1909. 



