MORGAN, COAT COLORS IN MICE 95 



is incontestible that this latter conception of albinism is better in accord 

 with the facts. 



The hypothetical interpretation that I offered involved primarily the 

 idea, as stated above, that segregation may be a dynamic function of 

 division in the germ-cells in exactly the same sense that specification of 

 the cells to produce the organs and tissues of the embryo takes place dur- 

 ing embryonic development. In this process, the results of experimental 

 embryology had seemed' to show that specification (segregation during 

 cleavage) is not due to a separation of particles of chromatin, but to the 

 dynamic action of the cells on each other at the time of or just before the 

 division stages. Such a conception of embryonic segregation is still held 

 by most embryologists, and I am still of the opinion, that if this is true 

 for the ontogeny, it is true for the segregation that takes place in the 

 formation of the gametes. My hypothesis was complicated, however, by 

 the further supposition that such a dynamic segregation in the gametes 

 carries with it the idea of impurity of the gametes in the sense that it 

 allows as a possibility that the extracted recessive may under certain con- 

 ditions give rise to the dominant, and conversely that the dominant may at 

 times produce the recessive type. One especial condition that I assumed 

 to call forth the latent character in the recessive form was the process of 

 hybridization. I pointed out how this interpretation could be tested and 

 its truth or falsity established. If, for example, we cross an extracted 

 dominant gray mouse (one that has had white in its ancestry) with a 

 pure black mouse, the offspring in the first generation should all be gray ; 

 but if the presence of the black can call forth the latent white condition, 

 some white mice should appear in subsequent generations of the gray 

 hybrids. 



The gray mouse used in my experiment was from a race of extracted 

 dominants that produced only grays. The black mouse was given to me 

 by B. B. Horton, and was from black stock that he had formed which gave 

 only blacks. In the first generation grays only were produced. These 

 inbred produced grays and blacks. The third and fourth generation of 

 some of these mice were bred in several combinations and continued to 

 produce only grays and blacks (Plate VIII, figs. 2, 3). It is evident that 

 the hypothesis failed when tested and must therefore be abandoned. 



Crosses between the Spotted and the Uniform Coat 



The experiment was undertaken originally to examine the inheritance 

 of spotted coat versus uniform coat. Owing to the difficulties in obtain- 

 ing crosses in large numbers between waltzers and other races, this side 



