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 as in 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. 



