REPRODUCTION AND SEX 529 



with one ovary completely removed, produced young of both sexes. 

 The theory has also been disproved in Amphibians by H. D. King. 

 That it cannot apply to birds is obvious, since they have only one 

 ovary. 



Two Kinds of Spermatozoa. — In about thirty different kinds 

 of animals there are two shapes of spermatozoa which differ 

 in certain details. It has been suggested that each kind is pre- 

 destined towards the development of one sex, but there is no definite 

 evidence that the dimorphism has this significance. 



The theory that in Vertebrates one testis yields male-producing 

 spermatozoa and the other female-producing spermatozoa, 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 give calves of both sexes. 

 The Accessory Chromosome. — Of great interest are the facts 

 that have come to light regarding what is called the accessory 



Fig. 78. 



The Chromosomes of a Single Nucleus, showing considerable variety in 

 shape and size. After Belar. 



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 Hkewise. 

 Now half 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 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. 



A fine corroboration of the importance of the chromosomes has 

 been recently afforded by the work of T. H. Morgan on Phylloxera 

 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 sperma 



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