190 W. E. AGAR 



illustrated in fig. 17. In Heidenhain preparations both X and 

 Y are at this stage sHghtly paler than the autosomes. 



Two kinds of secondary spermatocytes are therefore pro- 

 duced in the well-known manner, one with the X-chromosome 

 and the other with the Y. There is a complete, and apparently 

 prolonged, resting stage between the two divisions. The 

 difference between the two kinds of secondary spermatocytes 

 is conspicuous in the young nuclei, one member of each pair 

 containing a dense chromatic body (presumably X) which is 

 lacking in the sister nucleus or represented by a very much 

 smaller speck (PL 13, fig. 18). A group of fully resting secondary 

 spermatocytes, presenting the same dimorphism, is shown in 

 fig. 19. 



Fig. 20 shows a prophase for the second division in a 

 secondary spermatocyte containing the larger chromatic body 

 (X) which is seen attached to one of the chromosomes. 



The expected two types of second division are easily found. 

 An early, and rather irregular anaphase with the Y-chromosome 

 is shown in fig. 21. Here Y has just divided. A later anaphase 

 of the other type of second division is illustrated in fig. 22. 

 Here we have apparently only five chromosomes present in 

 each group ; this must clearly be interpreted as a division in 

 which X is present and fused, as usual, with an autosome. 



It is noticeable that there is no trace in Macropu^ (nor 

 in Petauroides) of the second pairing of chromosomes to 

 give a quarter of the diploid number which has so often been 

 described for the second division in birds and mammals. 



•Jordan (1911) described such a second pairing for Didel- 

 phys, but Painter (1922) found that it did not occur in his 

 material. 



B. Petauroides Volans. 



Number of Chromosomes . — The determination of the 

 number of chromosomes in this species presents more difficulty 

 than in the case of Macro pus, owing to their greater 

 number. The number countable in the spermatogonia! 

 mitoses is generally twenty-two, forming typically a ring of 

 twenty, with two smaller, slightly unequal ones, in the centre. 



