184 SECTIONAL ADDRESSES. 



and y in about 12 per cent, of the gametes, the occurrence of such 

 ' cross-overs ' would fulfil the required conditions. But the case of the 

 pollen presents a distinct difficulty on this latter view. This Stock is 

 distinguished both from the Drosophila and the Abraxas type by the 

 fact that none of the male germs carry either of the dominant charac- 

 ters. In place of the XX — XY form of sex-linked inheritance in 

 the former type and the WZ — ZZ in the latter, we should need to 

 regard this form as constituting a new class, which we might represent 

 as DE — EE, thus indicating that both members of the bivalent chromo- 

 some on the male side appear to be inert and able to carry only the 

 recessive characters, and hence are represented as EE, in contrast with 

 the DE pair of the female side. By this formula we can indicate the 

 behaviour of the several double-throwing strains. It is, besides, becom- 

 ing clear, I think, from recent results that there is no ' crossing over ' of 

 these factors on the male side in the Fj cross-breds. But the real 

 difficulty is to explain why these factors are confined to the female side 

 in the ever-sporting individual. This may result from abeirant 

 behaviom- or loss of chromosomes at some point in poUen development. 

 On this point I hope that evidence will shortly be available. Failing- 

 such evidence the presumption is that the elimination of XY (and in one 

 strain of W) must have taken place prior to, and not at, the 

 moment of the maturation division. Morgan's proposal to fit 

 the pollen into his scheme for Drosophila by having recoui'se to hypo- 

 thetical lethal factors does not appeal to the observer, who finds the 

 pollen all unifonnly good and every ovule set. Zygotic lethals are 

 clearly not in question under these circumstances. The supposition of 

 gametic lethals confined to the pollen appears far-fetched, seeing 

 that of the missing combinations two, viz. : single white, the double 

 dominant, and double white a dom.inant-recessive, occur in the 

 ovules, and the third, the single cream, the other dominant-recessive, 

 exists as a pure strain, so that the homozygous condition is evidently 

 not in itself a cause of non-development. Other examples suggesting 

 premeiotic segregation can be quoted, notably cases among variegated 

 plants and plants showing bud sports, where somatic segregation appears 

 to be of regular occun-ence. Among the Musciniae the present evidence 

 appears to show that the sex potentiality segregates in some fonns at 

 the division of the spore mother cells, so that already the spores possess 

 a sex character; while in oflier species this separation takes place later, 

 during the development of the gametophyte, the spores being then all 

 alike and undifferentiated in this resi^ect. In Fimaria hygrometrica. 

 an example of the latter class, an attempt has l^een made by E. J. 

 Collins" to ascertain the stage at which sex segregation takes place 

 by inducing the gi'owth of new individuals from isolated jxtrtions of the 

 vegetative tissues of the gametophyte. No doubt when the evidence 

 is derived from experiments in which a portion of the plant has been 

 severed from the rest, it is possible to urge that the result obtained 

 is not necessarily indicative of the potenti?;lity in the intact organism. 

 Phenotypic appearance is the product of a reaction system, in which 

 the internal as well as the external environment plays its part. We 



^' Journal of Genetics, vol. viii., 1919. 



