MALE REPRODUCTIVE CELLS 



451 



the smaller member of which has disappeared. This suggestion is help- 

 ful in the further development of the theory of sex determination by 

 the accessory chromosome. Again the elaboration and extension of this 

 point has been made by Wilson. It was originally believed byMcClung 

 and Paulmier that, since the male germ cell carried the accessory body, 

 eggs fertilized by spermatozoa containing this element should produce 

 males. This conclusion was based upon an erroneous observation that 

 the male somatic cells had more chromosomes than the female. Wilson 

 has demonstrated in the case of Anasa tristis that exactly the reverse is 

 true, and the same fact holds for Aplopus and other forms of insects. 



Applying the same line of argument to Aplopus, and making similar 

 assumptions to those suggested by Castle for an interpretation of sex along 

 Mendelian lines: (i) two kinds of eggs (male and female) as also two 

 kinds of spermatozoa which have been actually frequently observed; 

 (2) selective fertilization or infertility of gametic unions of like sex 

 chromosomes, i.e. an egg with a female determinant must be fertilized 

 by a sperm with a male determinant, 

 and vice versa; (3) the dominance of 

 femaleness ; also the further assump- 

 tion suggested by Wilson; (4) that 

 the accessory chromosome repre- 

 sents the larger member of a pair of 

 idiochromosomes, the smaller mem- 

 ber of the pair having been lost it 

 is then seen that, since a mature 

 egg of 1 8 chromosomes fertilized by 

 a spermatozoon lacking the acces- 

 sory gives origin to a male, the male 

 determinant must have been intro- 

 duced by the egg and that the miss- 

 ing mate to the accessory was a 

 female determinant. Furthermore, 

 since when such an egg is fertilized 

 by a spermatozoon possessing the 

 accessory chromosomes a female 

 arises, the egg must have contributed 

 a female determinant to which the 

 accessory or male determinant is re- 

 cessive ; it thus also appears that the 

 accessory chromosome alternates be- 

 tween the two sexes in successive generations, i.e. the male contributes the 

 accessory to the female in the production of the female, and the female in 

 the ensuing generation contributes the accessory to the male in the pro- 



FIG. 415. Diagram to illustrate the part 

 played by the accessory chromosome in the 

 determination of sex during maturation and 

 conjugation. (After WILSON.) 



