MENDELISM 93 



Fj generation are of two different kinds in each sex ; thus 

 the Fi females have two kinds of ova, equal numbers of 

 each : one kind containing the factor for blackness only, 

 the other that for whiteness only. Similarly the sper- 

 matozoa of the males are of two kinds, containing either 

 factor, and again equal numbers of each. In the inbreed- 

 ing of the Fj generation these germ-cells may be supposed 

 to come together at random, and the result will be the 

 formation of equal numbers of four kinds of fertilised 

 egg-cells : *' black " ovum and " black " spermatozoon, 

 '' black " and " white," " white " and " black," *' white" 

 and " white." The second and third of these are identical, 

 and the animals developing from them will be identical 

 in appearance with those developing from the first kind 

 of fertilised ovum ; only by continued breeding will the 

 presence of the recessive " white " factor become manifest. 

 In short, if the constitution of the F^ generation is expressed 

 as BW, the germ-cells are either J5 or Win the female, B or 

 W in the male, and the following combinations result : 

 BB, BW, WB, WW, all the forms in which the dominant 

 character is present being apparently identical. 



When Mendel's work was rediscovered it was seen that 

 precision could be given to the theory by the assumption, 

 since then abundantly justified, that the hereditary " factors " 

 were carried in some way by the chromosornes. For we 

 know that in the cells of the F^ generation the chromo- 

 somes are half of paternal, half of maternal origin ; and 

 that in the development of the spermatozoa a reducing 

 division takes place, in which the chromosomes are sorted 

 out into two lots, and the paternal chromosome which 

 bears the factor for colour is separated from the corre- 

 sponding one of maternal origin, so that there will be 

 two kinds of spermatozoa, differing in this particular ; 

 and that a similar reducing division in the maturation of 

 the ovum will have a similar effect, producing equal 

 numbers of two kinds of ova, if it is a matter of chance 

 whether it is the paternal or the maternal chromosome 

 that is thrown out in the first polar body. The factor 

 for a given character carried by a chromosome is now often 

 called a '* gene " and regarded as a specific material 

 substance, but speculations as to its nature or mode of 



