208 A. W. GREENWOOD 



(PL 15, fig. 9). One of the plasmosomes (Px) remains in contact 

 with the sex chromosome, the other (Px') appears to be formed 

 from a division of this plasmosome. 



The early pachytene stage is followed by a late pachytene 

 stage in which the chromosomes show a marked diminution in 

 staining capacity. They become diffuse and furry in appear- 

 ance, the X-chromosome or possible XY bivalent alone remain- 

 ing as a deeply stained body. The onset of the diplotene stage 

 is marked by the recovery of the staining power of the chromo- 

 somes. The chromosomes begin to split ^o that two long, 

 thin threads are formed. The chromomeres can be distinctly 

 seen on these threads occurring in pairs (PI. 15, fig. 10). 



The thin threads now begin to contract, but never entirely 

 separate from one another, remaining in contact at two or more 

 points (PL 15, fig. 11). At this stage the nucleus, which has been 

 increasing in size from the onset of the meiotic prophase, has 

 now attained its maximum volume. Follo^\ang this stage the 

 nuclear membrane breaks down and the chromosomes lie free 

 in the cytoplasm. 



In a few cases during the diplotene stage I have seen the 

 X-chromosome apparently attached to the end of one of the 

 autosomes, but this condition appears to be exceptional. In 

 most cases it lies free. It is in the metaphase of the first meiotic 

 division that the Y-chromosome can first be identified with 

 certainty. Figs. 12 and 13 are of metaphase plates. The first 

 is a cell just after the nuclear membrane has disappeared. The 

 seven large bivalents and the separate X- and Y-chromosomes 

 can be seen attached to one another by threads. Fig. 13 is 

 a rather later view. In Phascolarctus the X- and the 

 Y-chromosome do not usually form a bivalent. In division 

 figures they are always found separate. In the meiotic prophase 

 they may possibly be in the form of a bivalent, but, as men- 

 tioned before, this point could not be determined. 



Fig. 14 shows a metaphase side-view. In this it will be seen 

 that the X- and Y-chromosomes are on opposite sides of the 

 mass of chromosomes at the equator of the cell, and are travel- 

 ling to opposite poles of the cell ahead of the other chromosomes. 



