60 



SCIENCE 



[N. S. Vol. XXIX. No. 732 



■whatever be its particular composition 

 (whether a single chromosome or more than 

 one) it is present as a single unit in the 

 male, while in the female it is doubled. 

 The explanation of this fact is as follows: 

 It is true of organisms generally that in 

 maturation each pair of chromosomes is 

 reduced to a single chromosome. The 

 X-pair in the female must, therefore, be 

 reduced to' a single X-element, which is 

 present in all the eggs when ready for 

 fertilization,^ while in the male it is present 

 in only half the spermatozoa. The char- 

 acteristic female combination can, there- 

 fore, only arise by fertilization of the egg 

 by spermatozoa that contain the X-element, 

 as is shown by the following formulas: 



{a) In the absence of a Y-element 

 Egg X -}- spermatozoon X = zygote XX (female) 

 Egg X -j- spermatozoon no X = zygote X (male) 



(6) In the presence of a Y-element 

 Egg X + spermatozoon X = zygote XX (female) 

 Egg X + spermatozoon Y = zygote XY (male) 



In either case it is evident that the X- 

 element of the male zygote is derived from 

 the egg. The significance of the Y-element 

 is not known; but since it is often alto- 

 gether absent, it apparently does not play 

 a necessary role in sex-production, and 

 may, for the present, be left out of account. 



This general result has not been attained 

 without many false steps which have con- 

 fused the simple principle to which the 

 phenomena conform; but so many of the 

 contradictions have disappeared upon more 

 exact later studies that we may now confi- 

 dently expect to see the few remaining ones 

 cleared away. 



It is clear from these facts that the sexes 

 often differ in the number of the chromo- 

 somes; and in this case it is always the 

 female that has the larger number. When 



" This part of the conclusion, at first based on 

 indirect evidence only, has recently been shown to 

 be true by observations, still unpuljlished, by Mr. 

 C. V. Morrill. 



the X-element is a single unpaired chromo- 

 some in t^e male (accessory chromosome) 

 the female has one more chromosome than 

 the male (Anasa, Protenor). When the 

 X-element in the male is double, but with- 

 out a Y-element, the female has two more 

 chromosomes than the male (Syromastes) .^ 

 When a Y-element is present the numerical 

 relations are modified accordingly (since 

 this element is present in the male, absent 

 in the female). Thus, in Gelastocoris the 

 X-element is represented by four chromo- 

 somes in the male, while the female has 

 eight ; but since the male has in addition a 

 single Y-element the net difference between 

 male and female is but three. In the com- 

 mon case where the male contains a single 

 X-element (large "idiochromosome") and 

 a single Y-element ("small idiochromo- 

 some") the sexes have the same number, 

 the female containing X -}- X and the male 

 X + Y. In this case the Y-element may 

 often be distinguished by its size and the 

 male and female chromosome-groups are 

 visibly different. In some cases, however, 

 the Y-element is as large as the X-element 

 {Nezara, Oncopeltus) and no visible differ- 

 ence between the sexes appears to the eye; 

 but this case is connected by intermediate 

 gradations in other species with cases in 

 which the difference plainly appears, and 

 we have every reason to believe that the 

 same principle applies to all. The general 

 formulas X -|- X = female and X -|- Y = 

 male may, therefore, apply to many forms 

 in which no nuclear differences between the 

 sexes are visible. 



The general conclusion reached by the 

 study of the purely sexual forms has re- 

 cently received a most convincing confirma- 

 tion through the brilliant discoveries of 

 Morgan and von Baehr on some of the 



' The female number in this ease, which I at 

 first inferred only, has recently been demonstrated 

 by direct observation. 



