NEW TYPES OF CHROMOSOME DISTRIBUTION. 159 



Diabrotica, where they are associated with an odd chromosome 

 instead of a pair of idiochromosomes, Stevens ('08) makes the 

 following suggestion : "It may be possible, as was surmised 

 earlier in the study, that (i) there will prove to be two distinct 

 types (varieties or species), in each of the present species, one 

 having the large unpaired heterochromosome only; the other 

 having an equal pair of chromosomes as in Haltlca, and that (2) 

 the irregularities in time of division and the consequent peculiar- 

 ities in number and distribution of the supernumeraries in Dia- 

 brotica are to be attributed to hybridism." 



The differential chromosomes in Gclastocoris and in the 

 Reduviiclje seem to have undergone a somewhat different history 

 from that of the other types. It is clear that the single large 

 idiochromosome of Diplocodus is represented in FitcJiia by two 

 chromosomes, in Prionidus and Sinea by three and in Gelastocoris 

 by four chromosomes. It is very probable that the condition 

 seen in Diplocodus is the most primitive one, and the essential 

 question is how the other types can have been derived from this. 

 Taking as an example Fitchia, where the female number is 28 

 and the male 27, if a pair of idiochromosomes was the original 

 condition in the species, the original number of chromosomes 

 must have been 26 in both male and female. If, in the male, the 

 original large idiochromosome should break up into two parts, 

 two classes of spermatozoa would be found, one containing 13 

 chromosomes and the other 14. All the eggs would have 13 

 chromosomes. If an egg were fertilized by a 14-chromosome 

 class of spermatozoa, the result would be a female with 27 chro- 

 mosomes. In maturation, this female would produce two kinds 

 of eggs, one with 14 chromosomes and the other with 13. The 

 14-chromosome class fertilized by a spermatozoon with 13 chro- 

 mosomes would produce either a male or female with 27 chromo- 

 somes. From these individuals, then, would arise both eggs and 

 spermatozoa with 13 and 14 chromosomes. As the 14-chromo- 

 some class of spermatozoa would be female-producing, females 

 with 28 chromosomes would be produced upon fertilization of 

 an egg with 14 chromosomes by a spermatozoon with 14 chromo- 

 somes. Further, an egg with 14 chromosomes, fertilized by a 

 spermatozoon with 13 chromosomes would produce a male with 



