MEIOTIC PHASE IN MARSUPIALS 189 



rough estimate of the relative volumes of the total chromatin 

 content at these two stages made by means of plasticene 

 models showed that the volume of the chromatin is more 

 than twice as great in the later as in the earlier stage. 



The XY bivalent is visible as before, attached to an auto- 

 some. It is now, however, seen to be attached to one limb 

 onl}^ of the bivalent. 



Figs. 14-16 (PI. 13) represent metaphases of the first meiotic 

 division, to show the relations of the sex chromosome. In 

 fig. 14 they form a compact body attached to one end of one 

 of the autosome bivalents. In fig. 1 5 they are similarly attached, 

 but somewhat drawn out towards the equator of the spindle. 

 At this stage no distinction between X and Y is visible, but 

 as the metaphase progresses the two components begin to 

 separate, as shown in figs. 16 a and b. The minute Y is very 

 characteristically pulled out along the spindle-fibre at this 

 stage. 



Fifty metaphase I's were examined especially in respect to 

 the mode of attachment of XY. 



In fourteen cases it was attached as in fig. 14. 



In twenty-five cases it was attached as in fig. 15. 



In eleven cases it was apparently free, forming an independent 

 bivalent. In many of these cases, however, it was probably 

 attached as in the manner of fig. 15, but by a longer and finer 

 thread. There is also little room for doubt that an attachment 

 as in fig. 14 indicates an early metaphase, and that later the 

 relations shown in fig. 15 are always assumed. 



I have not been able to determine whether the autosome 

 to which XY is attached is always the same one, but it appears 

 probable that it is (note the distinctive shape of this chromo- 

 some in the two groups figured in fig. 13, and in figs. 14 and 15). 

 The bivalent in question is, however, certainly always one of 

 the larger ones. 



The first meiotic division is the differential division for X 

 and Y (PL 13, figs. 16, 17). During anaphase Y becomes still 

 further pulled out along the line of the spindle-fibre, and 

 presents in the late anaphase the characteristic appearance 



