The President's Address. By J. Arthur Thomson. 151 



(c) The Accessory Chromosome. 



23. Of great interest are the facts that have recently come to 

 light regarding what is called the accessory chromosome. In a 

 number of Insects, Myriopods, and Arachnids, the females have 

 more chromosomes in their cells than the males have. In the 

 simplest cases (Anasa, Protenor) the female has one more chromo- 

 some than the male, and the egg has one more likewise. Now 

 half of the spermatozoa differ from their neighbours in having the 

 same number of chromosomes as the egg, while the others have 

 one fewer. This extra chromosome which half have and half have 

 not is called the X-element or accessory chromosome. There are 

 facts which go to show that fertilization of the eggs by one class of 

 spermatozoa results in males, by the other in females. When two 

 equal numbers come together the result is a female. 



The chromosomic dimorphism has been proved in about a 

 hundred species, but all are not equally convincing, and there are 

 many variations in detail. As the subject is difficult, especially 

 without diagrams, and as the facts have been repeatedly summed 

 up in the last few years (e.g. by Wilson, who has contributed more 

 than any other to the investigation), we do not propose to do more 

 than refer to two or three important points. 



(a) In many cases, instead of there being an accessory chromo- 

 some in one half of the spermatozoa and no corresponding body in 

 the other half, there is a " large idiochromosome " or X-element in one 

 half and a " small idiochromosome " or Y-element in the other half. 



(b) The evidence that the one set of spermatozoa induce male- 

 development and the other set female-development is indirect ; it 

 is obtained by an examination of the state of the chromosomes in 

 the body-cells of the offspring. The Y-element, for instance, is 

 found only in the males, while the X-element is found in both 

 sexes, but doubled in the female, single in the male. 



(c) Wilson gives the following formulae : — 



(a) In the absence of a Y-element 



Egg X -f spermatozoon X = zygote XX (female). 

 Egg X -f- spermatozoon no X = zygote X (male). 



(b) In the presence of a Y-element 



Egg X + spermatozoon X = zygote XX (female). 

 Egg X + spermatozoon Y = zygote XY (male). 



24. A fine corroboration of the importance of the chromosomes 

 has been recently afforded by the work of T. H. Morgan on Phyl- 

 loxera and of von Baehr on Aphis saliceti. In these forms half of 

 the spermatocytes degenerate (as Meves pointed out in the bee), 

 namely those without the accessory chromosome ; therefore all the 

 spermatozoa are female-producers, and every one knows that all 



