41)0 



THE AMERICAN NATURALIST 



[Vol. L 



if the mutation is one of loss the character lost must be singleness. 



But a more important line of argument is that dealing with the 

 question as to when the elimination of singleness takes place. 

 At what stage in the sequence of cell divisions occurring in the 

 direct line between the fertilization of the egg cell at the begin- 

 ning of the cycle and the formation of the male sperm by the 

 factors, and is the disappearance of the single character from the 

 male germs illusory? In other words, is it the case that both 

 single- and double-carrying pollen grains are formed, but that 

 those carrying singleness always fail to achieve their purpose — 

 fertilization? This brings us to a consideration of the question 

 whether, if elimination occurs, it takes place at some point in the 

 series of cell-divisions which culminate in the formation of the 

 pollen mother cells, or at the reduction division, or at one of the 

 succeeding divisions which give rise successively to the complete 

 sporetetrad, the vegetative and generative cells of the pollen 

 grain, and the twin sperms formed from the generative cell. The 

 first alternative Frost dismisses on the ground that we have no 

 decisive evidence in support of somatic segregation and ' ' an over- 

 whelming convergence of probabilities against it." That we 

 have as yet no actual evidence of such segregation in stocks 

 themselves is true but in the light of known facts in regard to 

 bud variation it seems difficult to escape from the view that so- 

 matic segregation not only might, but does sometimes occur, and 

 that this possibility is not excluded in the present case. 



It appears to be rather generally assumed, though perhaps 

 without sufficient reason, that the segregation of all allelo- 

 morphs must necessarily occur simultaneously. Those who hold 

 that the evidence justifies the belief that segregation takes place 

 at the reduction division are naturally committed to the view of 

 simultaneous separation of all allelomorphs. But it must be ac- 

 knowledged for the reasons given above that there is a certain 

 difficulty in the way of accepting the view that the reduction 

 division of the germ cells constitutes the sole sorting mechanism 

 for the allelomorphs. May it not be, even if the majority of the 

 allelomorphs segregate as a rule at some particular point, that 

 occasionally or even regularly, segregation of one or another pair 

 of allelomorphs may occur prematurely or again may be post- 

 poned ? All that we certainly know is that by the time the 

 gametes are formed the sorting has been completed. As regards 

 the possibility of elimination at a later stage than the formation 



