Summary
1.
Prophase pairing during spermatogenesis inCimex exhibits reduced body repulsions leading to anomalous types of bivalents at metaphase.
2.
Cimex rotundatus possesses 14 pairs of autosomes and 3 sex chromosomes (X × Y) which are constant in number and regular in their segregation at second anaphase.C. lectularius has 13 pairs of autosomes and a number of univalent chromosomes in addition to the Y. All of these behave like sex chromosomes and the presence of at least two appears to be essential, otherwise their number varies. Their segregation at second anaphase is irregular but not completely random.
3.
The number of univalent elements varies both in different individuals and in different nuclei of the same individual.
4.
This variation in different individuals is found to have limiting values of 4 and 16; while there is a variation of ±3 in second metaphase nuclei of each individual.
5.
Reasons are given for supposing that the numbers of univalents in the female are similar to those of the male and that progressive increase in number due to their combination is limited by the loss of some univalents at first anaphyse.
6.
The distinct size and frequent secondary pairing of two univalents with the Y indicates that two X chromosomes, essential for determination of sex, are present in all nuclei as inC. rotundatus. The other univalents, while behaving like X chromosomes, are thought to be genetically inert and allied to the X chromosomes.