In the three animals studied the total number of chromosomes in the male is as follows :
Phascolarctus 16 (14 autosomes + XY).
Sarcophilus 14 (12 autosomes + XY).
Dasyurus 14 (12 autosomes + XY).
In the female the number of chromosomes is as follows :
Phascolarctus 16 (14 autosomes + XX).
Sarcophilus 14 (12 autosomes + XX).
In all animals dealt with in this paper the Y-chromosome is very minute in size compared with the other chromosomes; also the X-chromosome is much smaller than any of the autosomes.
Chromomeres are conspicuous during syndesis, early pachytene, and early diplotene stages.
The early pachytene stage is followed by a late pachytene stage in which the threads become diffuse and lose their capacity for taking up the stain.
Except in the early meiotic prophase the sex chromosome remains compact and deeply stained and does not thread out like the autosomes.
In all the above animals the first meiotic division is reductional, separating the X- and the Y-chromosomes, and the second division is equational, in each cell the sex chromosome dividing. The spermatozoa are therefore of two kinds, one containing an X-chromosome and the other containing a Y-chromosome.
No further reduction in the number of chromosomes takes place during the second meiotic division.
The Y-chromosome could not be identified during the meiotic phase until the metaphase of the first meiotic division. At this stage in Phascolarctus the sex chromosomes are separate and do not form a bivalent.
The archoplasm seems to exert some influence on the chromatin threads at synizesis and during the early pachytene stage. In the former case the contraction takes place to that side of the nucleus at which the archoplasmic mass is situated; in the latter the chromosomes are in the form of thick loops with the ends of the chromosomes pointing towards the archoplasmic mass.
In Phascolarctus the Sertoli cells are very large and possess peculiar rod-like bodies, the origin and function of which was not arrived at. The result of experiments seem to show that the rods are not affected by the action of digestive fluids.
- Copyright © 1923 by the Company of Biologists Ltd.