MARSUPIAL SPERMATOGENESIS 209 



In many cases, on the other hand, the sex chromosomes lag 

 behind on the spindle. This is shown in fig. 15 of an anaphase. 

 In this figure it will be noted that the X- and the Y-chromosome 

 are attached to the same spindle-fibre. 



The first division is therefore the reductional division, and 

 gives rise to two daughter secondary spermatocytes which are 

 dimorphic, one containing the X-chromosome and the other 

 containing the Y-chromosome. This dimorphism of the 

 secondary spermatocytes is shown in fig. 16 (PI. 1 6). That these 

 are two daughter spermatocytes is shown by the remains of the 

 spindle-fibres connecting the two cells. In one of the cells can 

 be seen a rather large, deeply stained body in the nucleus which 

 I take to be the X-chromosome. In the other cell nucleus 

 there is a much smaller body not so deeply stained, which 

 probably represents the Y-chromosome. 



Second Meiotic Division. 



Before the onset of the prophase of the second meiotic 

 division, the secondary spermatocyte undergoes a prolonged 

 resting stage and increases greatly in size. From the resting 

 stage with its faintly staining network, the onset of the prophase 

 is shown by the recovery of the staining power of the chromatin 

 in patches. From this the deeply stained, irregular threads of 

 the prophase are formed (PI. 16, fig. 17). Eight through the 

 meiotic stages the X-chromosome has retained its staining 

 capacity and does not thread out, except in the early prophase 

 of the first meiotic division. During the prophase of the second 

 division the X-chromosome remains compact and does not 

 thread out. The division follows on as before, but this time the 

 sex chromosomes divide, one half going to each pole of the cell. 

 The second division is therefore equational. No further reduc- 

 tion in the number of chromosomes takes place during this 

 division. 



Fig. 18 shows an anaphase of the second division with the 

 X-chromosome divided and lagging behind on the spindle. 

 Fig. 19 shows a late anaphase of the same division showing the 

 presence of the Y-chromosome at both poles of the cell. 



