CHROMOSOMES AND MITOCHONDRIA 261 
(Fig. 67). In the bug, Oncopeltus fasciatus, the 
number of chromosomes (16) in both male and 
female is the same, but they are of equal size in 
both sexes. It is probable, however, that one of 
those of the male represents an X-chromosome and 
the other a Y-chromosome as in Type II, although 
they are not visibly different. 
Type IV. One X-chromosome attached to an 
ordinary chromosome. There are a number of 
cases on record in which the X-chromosome is 
attached to an ordinary chromosome as in Ascaris 
megalocephala. Probably on this account the sex- 
chromosome was overlooked in these species for 
many years. The resulting zygotes, as Fig. 68 
shows, are comparable to those of Type I (Fig. 65). 
Type V. Spermatozoa alike, but eggs of two sorts. 
In a few animals it has been found that the eggs are 
dimorphic and the spermatozoa all alike, as repre- 
sented in Fig. 69.1 
Numerous variations have been discovered in 
the number and size of the X- and Y-chromosomes ; 
some of these are illustrated in Fig. 70. When more 
than one X-chromosome is present they act as a unit, 
and two sorts of zygotes are produced asin other cases. 
Chromosome cycles of more than ordinary interest 
have been described in the honeybee, in phyloxerans 
and aphids and in certain hermaphrodites. It 
has long been known that the female honeybees 
(queens and workers) develop from fertilized eggs 
1 The recent contributions of Tennent and Baltzer make the occurrence 
of this type seem very doubtful. 
