MEIOTIC PHASE IN MARSUPIALS 187 



incorporated with the plasmosome, it is clear that the sex 

 chromosomes at this stage are in a diffused condition hke the 

 autosomes. 



The earhest stages of the primary spermatocytes which are 

 distinguishable from the spermatogonia are early leptotene 

 stages — a rather later leptotene nucleus being shown in fig. 6. 

 The large nucleolus is shown by its staining reaction to 

 be a plasmosome, and is still the only compact body in the 

 nucleus. The X-chromosome is therefore at this stage in the 

 leptotene condition like the autosomes. The same presumably 

 applies to the Y-chromosome, though this is too small to permit 

 of definite statement. 



The synizetic contraction, though unmistakable, is not very 

 pronounced (PL 12, fig. 7). 



The process of syndesis is difficult to follow in this animal. 

 It begins about the stage shown in fig. 6, and is completed 

 by the stage illustrated in fig. 7, which is a pachytene nucleus. 

 About the stage of fig. 6 frequent duplicity and parallelism of 

 threads can be observed, from which parasyndesis may be 

 inferred. The direct evidence for this mode of syndesis is, 

 however, certainly not strong in this species, but the indirect 

 evidence is very convincing. Firstly, this mode of syndesis 

 can be observed in Petauroides, and the general course 

 of meiosis is so similar in the two genera that it is incredible 

 that the mode of syndesis should not be the same. Secondly, 

 as we shall see Ijelow, there is no doubt that the components 

 of the definitive bivalents in M a c r o p u s are derived from the 

 pachytene threads by the longitudinal splitting of these, and 

 not by their doubling over. That being so, it follows that the 

 mode of syndesis must have been by longitudinal fusion, unless 

 one of the most fundamental hypotheses of modern cytology — 

 that is to say the individuality of the chromosomes — is false. 



In the early pachytene nuclei (PL 12, fig. 7), as in still earher 

 stages, the sex chromosomes are not visibly different from the 

 other chromosomes. As the pachytene threads begin to 

 contract, however, X soon becomes visible by reason of its 

 much more rapid condensation, so that it soon comes to form 



