THE DETERMINATION OF SEX 



99 



lished the discovery of an accessory chromosome in the guinea- 

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

 unquestionable accessory chromosome indicated by the letter 

 Guyer,*^ also an American, has described the accessory 



X. 



X- 



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.*^ Phyl- 

 loxera and Aphis lay eggs which develop parthenogeneticaUy. 



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. AU fertilized eggs develop into females. This phenom- 

 enon does not contradict the new doctrine of sex determina- 



