492 HEREDITY AND SEX 



has been disproved in rats by Copeman. Moreover, as Doncaster 

 and Marshall point out, it is known to stock-breeders that bulls 

 from which one testicle has been removed continue to give 

 calves of both sexes. 



The Accessory Chromosome. — 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 chromosome 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 chromo- 

 some which half have and half have not is called the X-element 

 or accessory chromosome. There are facts which go to show 

 that fertilisation 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. 



In the squash-bug, Anasa tristis, studied by Wilson, the eggs 

 have ii chromosomes and the sperms 10 or n. Egg n + sperm 

 ii produces a fertilised egg with 22 (2N) which develops into 

 a female. Egg 11+ sperm 10 produces a fertilised egg with 21 

 (2N — 1) which develops into a male. 



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, especi- 

 ally without diagrams, and as the facts have been repeatedly 

 summed up in the last few years (e.g. by Wilson, who has con- 

 tributed 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 



