THE DETERMINATION OF SEX 



99 



lished the discovery of an accessory chromosome in the guinea- 

 pig. 47 Her pictures are reproduced in Fig. 52, and show the 

 unquestionable accessory chromosome indicated by the letter 

 x. Guyer, 48 also an American, has described the accessory 



FIG. 52. Guinea-pig. Spermatocyte nucleus in the preparatory stage (imme- 

 diately after the synizesis), in which the accessory chromosome becomes distinct. 

 After Miss Stevens. 



chromosome in birds and in man, and it has been found in 

 other animals also. 



That the spermatozoa really determine sex has been con- 

 firmed by a capital investigation of T. H. Morgan. 49 Phyl- 

 loxera"and Aphis lay eggs which develop parthenogenetically. 



FIG. 53. The unequal spermatocyte division, a-c, in Phylloxera; d-f, in Aphis 

 solicola. After T. H. Morgan. 



After several generations, and under conditions which are in 

 part known to us, the females deposit eggs, which are fertil- 

 ized. All fertilized eggs develop into females. This phenom- 

 enon does not contradict the new doctrine of sex determina- 



