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ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



that fertilized it. If the sperm was one of those having the same 

 number of cliromosomesastheegg — i.e., one with an a'-chroniosome — 

 the new nucleus will have just twice the number of chromosomes 

 that was in the maturated egg nucleus, and, therefore, the same 

 number as that of the original female germ cells and the female 

 body cells of its species. The resulting individual will consequently 

 be a female. If, on the other hand, the fertilizing sperm happened 



to be one of those with no 

 a;-chromosome, the number 

 of chromosomes in the new 

 nucleus will be one less, 

 and will be the same as 

 that in the original male 

 germ cells and in the male 

 body cells. The new indi- 

 vidual will be a male. To 

 illustrate with our hypo- 

 thetical example used be- 

 fore: If a maturated egg 

 nucleus containing 9,-\-lx 

 chromosomes (fig. 9 A, &) 

 is fertilized by a sperm 

 nucleus {a) with 2-[-la? 

 chromosomes, the new nu- 

 cleus (c) will have 4+2aj 

 chromosomes, the female 

 formida; but if the same 

 egg, or another like it 

 (B,e), should be fertilized 

 by a sperm (c?) having only 

 two ordinary chromo- 

 somes, the new nucleus (/) 

 will have 4-1- la; chromo- 

 somes, the male formula 

 for the same species. 

 Sperms containing y-chromosomes give the same result, differentiating 

 the eggs by the chromosome formulas of 4+2a? and 4-|-xy. Therefore, 

 apparently according to the presence of two a;-chromosomes or of only 

 one, the number resulting by " chance " in the fertilization of the egg, 

 the new individual developed from the egg will be either female or male. 

 The reader must understand, now, that in spite of appearances, 

 it is not certain that sex is actually determined by the a?-chromo- 

 somes, or by any of the chromosomes. Though in the great 

 majority of animals the number of a;-chromosomes appears to be 



Fig. 9. — Diagram showing results of fertilization 

 of eggs by the two kinds of sperm (flg. c) 

 A, a maturated egg nucleus (b) containing 

 two orthochromosomes and one x-chromosome, 

 fertilized by a spermatozoon (a) having the 

 same number of chromosomes, producing a fer- 

 tilized ogg nucleus (c) with four orthochromo- 

 somes and two x-chromosomes, which now con- 

 tains the original female assortment of chromo- 

 somes (flg. 4A). B, a maturated egg nucleus (e) 

 of the same species fertilized by a sperm (d) 

 containing only two orthochromosomes, produc- 

 ing a fertilized egg nucleus (f) with four ortho- 

 chromosomes and only one x-chromosome, which 

 is the original male assortment of chromosomes 

 (flg. 4D). 



