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THE AMEBIC AN NATURALIST [Vol. XL VI 



selection in dihybrids. Lutz's results seem to be further compli- 

 cated by the fact, to which he calls attention, that the great 

 variability of the character in question gave apparent normals 

 that were really abnormals genetically, and this very fact ren- 

 dered the establishment of really pure lines a matter of great 

 difficulty, if indeed not an impossibility. A line is not necessar- 

 ily pure (genetically) because it has descended through many 

 generations of the closest possible inbreeding from a single orig- 

 inal pair. It is only pure in a Mendelian sense when all its alle- 

 lomorphs are duplex and the members of each pair of them are 

 identical in character. An individual might be pure in this 

 sense even though both its parents were complexly bred mon- 

 grels. On the other hand, if Lutz's original pair were not thus 

 pure, their descendants would only by chance constitute a pure 

 line. It is very doubtful if Lutz has really proved the effect of 

 selection in modifying a pure line, for it is not established that 

 he worked with a pure line, in the sense in which that term 

 is now used. 



Professor E. B. Wilson has recently published an excellent 

 review of cytological investigations relating to sex inheritance. 11 

 There appears now to be no doubt of the relation of sex to cer- 

 tain chromosomes in dicecious organisms. This relation is also 

 interpretable from the Mendelian viewpoint, so that we may say 

 that sex inheritance belongs in the category of Mendelian phe- 

 nomena. In nearly all organisms thus far studied the female is 

 to be regarded as homozygous and the male heterozygous for 

 sex. Hence the male produces two kinds of spermatozoa, while 

 the females produce only one kind of egg. When an egg is fer- 

 tilized by the one type of spermatozoa the zygote is a female ; if 

 by the other, the zygote is a male. In general, the female dip- 

 loid nuclei contain two of the sex chromosomes or two groups 

 of them when the sex chromatin element is compound. The 

 nuclei of the male contain only one of these elements, with or 

 without a synaptic mate different from it in character. In sea 

 urchins it has been shown that it is the female that is heterozy- 

 gous. But in this case the female possesses one of the sex ele- 

 ments and the male none. In both cases the female is charac- 

 terized by an excess in the number of the sex elements as com- 

 pared with the male. 



A large number of ordinary Mendelian characters have been 

 11 "The Sex Chromosomes," Arch. f. Mikrosko. Anat., Bd. 77, 1911, 

 pp. 249-271. 



