192 W. E. AGAR 



fragments scattered, till at last a stage is reached ^vhere the 

 chromatin is finely distributed throughout the nucleus except 

 for three or four masses representing the remains of the larger 

 blocks which have escaped complete fragmentation. Very 

 often, however, the number of the chromatic masses remains 

 at approximately the diploid number throughout the whole 

 interphase from the telophase to prophase. 



The interstitial nuclei of the two genera present a similar 

 difference, those of Ma cr opus being finely reticular, and 

 those of Petauroides containing about the diploid number 

 of irregular chromatic masses. 



The leptotene stage develops from a primary spermatocyte 

 having the same structure as a spermatogonium, i.e. containing 

 a number of massive chromatic bodies (PI. 13, fig. 23). Each 

 of these becomes the centre of a process of thread formation 

 (PI. 13, figs. 24, 25), to produce in the aggregate the leptotene 

 nucleus (PL 13, fig. 26). This process resembles that by which 

 the leptotene stage in certain insects is developed from a 

 nucleus containing the diploid number of chromatic bodies 

 (Wilson, 1912). In Petauroides, however, the evidence 

 that these blocks represent each a single chromosome, though 

 strong, is not complete. 



The massive centres from which the thread-spirming starts 

 persist for a long time, and indeed appear to form the 

 basis of the synizetic knot. Synizesis is more pronounced in 

 Petauroides than in Ma crop us — at least in my material 

 (PI. 13, fig. 27). 



Parasyndesis occurs during the synizetic contraction (PI. 13, 

 figs. 27, 2S), but not with the regularity observable in those 

 animals in which the leptotene nucleus is orientated into a 

 bouquet. In the nucleus shown in fig. 27 it is proceeding over 

 one length of the thread, but appears to be already completed 

 over the rest of the nucleus. That syndesis concerns, not the 

 chromosomes as a whole but their constituent chromomeres, is 

 beautifully shown in Petauroides (PI. 13, figs. 27, 28). In 

 this animal the chromomeres are unusually distinct and large 

 at this, and some other, stages. 



