MEIOTIC PHASE IN MAIISUPIALS 185 



valuable than the thin ones usually used by cytologists. Most 

 of the work has been done with sections of 15-20 jj. in thickness, 

 mounted between two coverslips instead of in the usual way 

 between a slide and a coverslip. This allows any nucleus to 

 be examined from both sides. For convenience of examination 

 the lower coverslip is temporarily attached to a microscope 

 slide by a drop of immersion oil. 



A. Macropus. 



Number of Chromosomes . — This animal is remarkable 

 among mammals for its small number of chromosomes. The 

 chromosome formula is of the type first clearly established for 

 mammals in the case of Didelphys (Painter, 1922), the 

 males being of the XY type. The diploid chromosome number 

 in the male is 10+XY, or twelve, and in the meiotic division 

 there are five autosome bivalents and the XY bivalent. This 

 very small number allows of great certainty in counting the 

 chromosomes, Macropus being obviously far more favourable 

 for this purpose than any other known mammalian genus. 



Although counts of the meiotic chromosomes leave no doubt 

 that the number is as just stated, it is only rarely that there 

 are twelve separate chromosomes in the spermatogonial mitoses. 

 As a rule there are quite indubitably only eleven (PL 12, fig. 2), 

 of which one, usually occupying the centre of the ring, is very 

 minute. This is the Y-chromosome. In a small percentage 

 of cases, however, there are equally plainly twelve (PL 12, 

 fig. 3), the extra one being smaller than any of the others 

 except Y. This is obviously the X-chromosome, for the 

 meiotic phase shows that X is much smaller than the auto- 

 somes. In the 11 -chromosome spermatogonial mitoses X is 

 presumably attached to one of the autosomes, though I have 

 not been able to identify with certainty the chromosome to 

 which it is joined. Owing to its small size it would not add 

 much to the length of one of the longer chromosomes. I have 

 frequently found a constriction near the end of one of the 

 longer chromosomes, but in view of the widespread tendency of 

 chromosomes to develop such constrictions it would be unjustifi- 



O 2 



