CELL DIVISION AND GAMETOGENESIS 233 



inequality consists of a difference in number of the X chromo- 

 somes, but in other cases it may consist in differences in size of 

 one or more of the sex chromosomes. An example of such a 

 condition occurs in the fruit fly Drosophila melanog aster , the 

 zygotic number of which is 8 in both sexes (Fig. 143). In the 

 female these 8 chromosomes are found to be made up of 3 pairs 

 of autosomes plus 2 X chromosomes, and in the male of 3 pairs of 

 autosomes plus one X and one Y chromosome, which is larger 

 than X and is also hook-shaped. The difference in the chromo- 

 somes of the male and the female then becomes the difference 

 between X and Y, which is a difference in form and size. At the 

 end of the maturation divisions of the oocytes, each ootid receives 

 3 autosomes and one X chromosome. In the male two kinds of 



s A * B 



ix a 



Fig. 143. — The diploid series of chromosomes of Drosophila melanogaster. A, 



female; B, male. (After Morgan.) 



spermatids are produced, one receiving 3 autosomes plus one 

 X chromosome and the other 3 autosomes plus one Y chromo- 

 some; resulting in the production of two kinds of sperm. An 

 egg (3 + X) fertilized by. a sperm carrying an X chromosome 

 (3 -f- X) develops into a female (6 + 2X) ; fertilized by a 

 sperm carrying a Y chromosome (3 + Y), it becomes a male 

 (6 + X + Y). A similar condition prevails in man where the 

 zygotic number is 48. In the female these 48 chromosomes are 

 made up of 23 pairs of autosomes plus two X chromosomes, and 

 in the male of 23 pairs of autosomes plus one X and one Y 

 chromosome. X and Y combinations sometimes are found in 

 reversed order so far as the sex is concerned, as in birds where the 

 female has the XY combination and the male the XX. 



Sex Determination. — The term zygotic sex refers to the sex, 

 male or female, established at fertilization, presumably as a 

 result of certain chromosome combinations. The definitive sex is 

 the sex that finally develops. Under normal conditions it would 



