THE DETERMINATION OF SEX 287 



Explanation to Figure 90 



"Non-disjunction and its results in Drosophila. The two large 

 circles in the first row represent male and female flies producing 

 sperms and eggs respectively. Non-disjunction in the female gives 

 2 kinds of eggs, with XX- and no sex-chromosomes, instead of 

 the normal single kind with one X. At fertilization there are 

 possible 4 combinations rather than 2, as shown in the large circles 

 of the second row. Owing to the several ways in which her 3 sex- 

 chromosomes may be distributed at maturation, the female repre- 

 sented by the third circle produces 4 kinds of eggs. When mated 

 to a normal male (below the horizontal line) with two kinds of 

 sperms, 8 combinations are possible (last row). Numbers 1, 4 

 and 5 are normal flies and give the usual type of progeny. Num- 

 bers 2, 6 and 7, owing to the presence of 3 sex-chromosomes, give 

 exceptional results when bred. Types Numbers 3 and 8 do not 

 appear in the cultures, probably because they die very early. 

 The original male has red eyes and the original female white 

 eyes. Red eyes (represented by the dots) appear in every fly 

 bearing the X-chromosome of the original male." 



(Diagram by Sharp based on data from Bridges and Morgan.) 



7. SECONDARY SEXUAL CHARACTERS AND HORMONES 



It will be seen from the preceding illustrations that 

 the primary differences between the sexes is in the kind 

 of gametes which they form. The female is an egg- 

 producer, the male a sperm-producer. In many ani- 

 mals especially invertebrates, it is very difficult to dis- 

 tinguish males from females without first examining 

 the gonads, although there is no lack of forms in which 

 one can with ease distinguish the sexes solely by ex- 

 ternal appearances. 



Very often this sexual dimorphism is confined, first, 

 to the genitalia or to accessory apparatus used in 

 copulation, oviposition, or rearing of the young; and 

 second, to extra genital characteristics not associated 

 directly with reproduction, such as color, ornamenta- 

 tion, and the like. Both of these types of sexual 



