CHROMOSOMES AND MITOCHONDRIA 259 
somes (Fig. 64, D). The nuclei of all the mature 
eggs exhibit six chromosomes. When fertilized 
the spermatozoén 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 spermatozoén 
with six chromosomes possesses twelve chromosomes 
and develops into a female (Fig. 64, H); 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 
(Fig. 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, Lygeus 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 
