GENERAL TOPICS IN INHERITANCE. 91 



but it was duplicated by Mr. Tegetmeier. This experiment certainl)' should 

 be repeated, and I have arranged to repeat it next season. 



One of the best cases of reversion is the gray coat of a hybrid between a 

 white and a black mouse. We now know, however, that even a "pure 

 race" of white mice may carry gray as a latent characteristic that first be- 

 comes patent ou crossing. In view of such facts cases of " reversion " to a 

 r^Kt"/^ ancestor must be critically examined. If the "reversion" be not a 

 neomorph, it must have been handed down without break in the germ 

 plasm from an ancestor possessing the characteristic. 



PURITY OH GAMETKS. 



The dogma of purity of the gametes, the second corner-stone of Mendelism, 

 asserts that while the unripe germ cells of a hybrid having antagonistic or 

 alternative characteristics A and A' contain representatives of both A and 

 A', yet the npe germ cells of such a hybrid contain representatives of either 

 A or A', and not of both. Thus the ripe germ cells (gametes) are pure in 

 respect to a given characteristic. They gain this purity, it is supposed, dur- 

 ing the maturation period, the period when the reduction division of the 

 chromosomes occurs, and when in each cell division one-half of each chro- 

 mosome moves bodily to one daughter cell and one half to the other. The 

 theory assumes, of course, that characteristics A and A', being derived from 

 different parents, inhere in different chromo'omes. Let us assume that our 

 hybrid has eight chromosomes, four derived from each parent, thus : 



in which the black dots represent chromosomes of maternal origin ; the 

 circles chromosomes of paternal origin. If all maternal chromosomes contain 

 the determinant a then purity of the gametes demands that all such go to 

 one gamete and all of the chromosomes of paternal origin go to the other, 

 and that such is their behavior has in fact been assumed by Cannon (1902). 

 But that would result in the extracted pure individuals of the second hybrid 

 generation being like their grandmother or their grandfather in all charac- 

 teristics, which is not the case. If we assume that some only of the maternal 

 chromosomes, such as are represented by the small dots, contain the deter- 

 minant a, then these may be associated with any of the paternal chromo- 

 somes excepting those that contain the determinant a! . Such a selection of 

 chromosomes so as to exclude from the ripe gamete chromosomes containing 

 both the alternative characteristics is quite possible, owing to the fact of 

 synapsis, in which the homologous chromosomes from the two parents unite 

 in pairs, as shown in the figure, in such a way that both can not pass to the 

 .same gamete. 



The foregoing hypothesis of Sutton (1902, 1903) and Boveri (1502) would 

 account for perfect purity of gametes. But it is clear that gametes are not 



