MEIOTIC PHASE IN MARSUPIALS 191 



These are presumably X and Y, the latter being much larger 

 than the corresponding element in Macropus, and being, 

 indeed, but slightly smaller than X. I have, however, found 

 some spermatogonial mitoses with apparently only twenty-one 

 chromosomes, and yet containing this pair in the centre. 

 I am therefore in doubt., from the spermatogonial mitoses, 

 whether the number is 20 + XY or 20 + X. I have some quite 

 unequivocal counts of polar views of the first meiotic metaphase, 

 and some of these show eleven and some twelve separate 

 elements. When twelve are present, one is always distinctly 

 smaller than any of the others. Presumably, when the number 

 is eleven, there are ten autosome bivalents and the XY bivalent. 

 When the number is twelve, X and Y have dissociated. Side 

 views of the meiotic metaphase (of which I have never found 

 one that could be counted) show that one of the bivalents 

 (■? XY) commonly dissociates much in advance of tlie others. 

 The conditions in the early pachytene nuclei also point to the 

 presence of two sex chromosomes. It appears, therefore, to 

 be fairly well established that the formula for the male 

 Petauroides is 20 + XY, or twenty-two in all. This corre- 

 sponds with Painter's enumeration for Didelj)hys. It 

 will also be noticed that the number of autosomes is double 

 that of Macropus. None of my ovarian material proved 

 suitable for chromosome counting. 



The Meiotic Phase.^ — The spermatogonial nuclei of this 

 animal differ from those of Macropus, in that the place of 

 the fine reticulum of the latter genus is taken by a number of 

 irregular blocks of chromatin, united by anastomoses. In the 

 young spermatogonia these chromatic bodies are in approxi- 

 mately the diploid number, and from a study of the spermato- 

 gonial pro- and telophases it appears probable that these 

 blocks are of the nature of ' prochromosomes ', being the 

 undiffused remains of the telophase chromosomes, and passing 

 directly into the chromosomes of the following prophase. 

 When the spermatogonia pass into a more profoundly resting 

 stage the number of these bodies becomes more difficult to 

 determine, owing to their becoming broken up and their 



