MARSUPIAL SPERMATOGENESIS 207 



nucleus which is opposite to that on which the archoplasmic 

 mass is found, and it appears as if this body exerts some 

 influence, if it is not wholly the cause of the synizetic con- 

 traction. 



In the early leptotene stage no sign of a compact X- or Y- 

 chromosome could be seen, so that thej^ are evidently threaded 

 out like the autosomes at this stage. 



Fig. 5 shows a much later stage in syndesis and synizesis. 

 The nucleus is not complete, but shows very clearly the pairing 

 of the chromomeres in homologous threads. The chromomeres 

 exhibit great variability in size. In one of the threads syndesis 

 appears to be nearly complete. In the centre of the nucleus 

 can be seen the compact mass of the synizetic contraction. 



The leptotene stage is followed by the pachytene stage. 

 Fig. 6 shows an early pachytene stage consisting of thick, 

 looped chromosomes Avhich have emerged from the synizetic 

 contraction. These chromosomes are seen to be distinctly 

 double in composition, the presence of the chromomeres showing 

 up as darkly stained bodies in the more hghtly stained thread. 

 The threads are now very thick. The ends of the threads 

 at this stage are directed towards the archoplasmic mass. Later, 

 these threads lie scattered through the nucleus and all trace 

 of duplicity is eventually lost. The X-chromosome makes its 

 appearance in the early pachytene stage. It appears first as 

 a thin, deeply stained thread (PL 15, fig. 7), but contracts down 

 to form a round mass, which is typical of the X-chromosome 

 in later prophase. I have been unable to identify the minute 

 Y-chromosome at this stage. Whether the Y-chromosome fuses 

 with the X-chromosome to form a bivalent could not be 

 determined with certainty owing to the minuteness of the 

 Y-chromosome and the presence in the nucleus at this stage 

 of several deeply staining granules. 



Always in contact with the X-chromosome there is a large 

 pale plasmosome (PI. 15, figs. 7, 8, 9). This varies considerably 

 in shape, and usually contains a number of deeply staining 

 granules. In later pachytene stages two plasmosomes are 

 visible, each containing one or more deeply staining granules 



