Macropus ualabatus has twelve chromosomes, namely 10 + XY in the male and 10 + XX in the female.
In Petauroides the number is almost certainly twenty-two, the male being of the formula 20 + XY. No female counts were obtained for this animal.
In the male Macropus Xis generally attached to one of the autosomes in spermatogonial mitoses. Y, which is exceedingly minute, is free. During the pachytene stage, while the autosomes are still elongated, X and Y condense into a bivalent. In the first meiotic division this bivalent is attached to an autosome.
As a result of the first meiotic division the usual two classes of secondary spermatocytes are formed one with X and the other with Y. In the second meiotic division, those with X show only five separate chromosomes, showing that X, as usual, is fused with an autosome. The other class of second divisions shows five autosomes and the minute Y.
In the female Macropus the sex chromosomes were never found free from the autosomes in the ovarian follicle cells, which therefore show only ten separate chromosomes.
In Petauroides the sex chromosomes cannot be distinguished with certainty from the autosomes. An unequal pair of small chromosomes usually situated in the centre of the spermatogonial metaphase plates probably, however, are X and Y. Early pachytene nuclei show two compact bodies which unite into one, presumably the sex bivalent.
The second reduction of the chromosome number to onequarter of the diploid total in the second meiotic division, which has been described for several species of birds and mammals, does not take place either in Macropus or Petauroides.
Chromomeres are very prominent in Petauroides in the zygotene and diplotene stages.
Probably in Macropus, and more convincingly in Petauroides, the cytological conditions to permit of ‘crossing over’ are present in the male.
The plasmosome which appears in the pachytene stage is probably formed from the plastin or linin basis of the contracting sex chromosomes.