CHROMOSOMES AND MITOCHONDRIA 259 



somes (Fig. 64, Z>). The nuclei of all the mature 

 eggs exhibit six chromosomes. When fertilized 

 the spermatozoon nucleus can be recognized, since 

 it lies near the end away from the polar bodies. On 

 the average one-half of the eggs are fertilized by 

 spermatozoa containing five chromosomes and one- 

 half by spermatozoa containing six. The results 

 are as follows : A zygote resulting from the fusion 

 of an egg with six chromosomes and a spermatozoon 

 with six chromosomes possesses twelve chromosomes 

 and develops into a female (Fig. 64, IT) ; and a 

 zygote formed by an egg with six chromosomes and 

 a spermatozoon with five chromosomes contains 

 eleven chromosomes, and hence gives rise to a male 

 (Pig. 64, G). The events during the maturation 

 processes in such a case are similar to those in the 

 bug Protenor, as illustrated in Fig. 65. 



Type II. One X-chromosome and one Y-chro- 

 mosome. In the bug, Lygceus bicrucis, and a num- 

 ber of other species the number of chromosomes in 

 both male and female is the same, but two sex-chro- 

 mosomes of different sizes are present in the male. 

 As shown in Fig. 66, the eggs are all alike, contain- 

 ing six ordinary and one X-chromosome. The sper- 

 matozoa are of two sorts : one-half with the larger, 

 or X-chromosome, the other one-half with the smaller, 

 called by Wilson the Y-chromosome. The zygotes, 

 consequently, produce males if one X-chromosome 

 and one Y-chromosome are present, and females 

 if two X-chromosomes occur. 



Type III. Two chromosomes of equal size 



