198 AGAR E. W. 



condition of the plasmosome in M a c r o p u s has already been 

 described. In Petauroides a plasmosome can occasionally 

 be seen in the primary spermatocytes, but usually none can be 

 identified with certainty — probably because, as in M a c r o p u s , 

 this plasmosome is partly chromatic in the resting nucleus, 

 and therefore does not stand out clearly from the other 

 chromatic bodies. In the leptotene, synizetic, and early 

 pachytene nuclei of Petauroides, a plasmosome can 

 sometimes be identified, but is usually concealed among the 

 dense tangle of chromatic threads. In the later pachytene 

 stages, two plasmosomes are plainly visible (PI. 14, fig. 32). 

 One is considerably darker than the other, and has no relation 

 to the sex chromosomes. This one I take to be the plasmosome 

 of the earlier stages. The other plasmosome is in close relation 

 to the sex chromosomes, and indeed appears to be formed out 

 of their substance. It makes its appearance as a large pale 

 body at the time that the sex chromosomes are uniting into 

 a bivalent, and at first is an elongated structure closely attached 

 to the bivalent. In some cases its shape and relations suggest 

 that it is the persistent part of the bivalent from which the 

 chromatin has flowed away into the rounded mass which forms 

 the condensed sex bivalents : this appearance is strengthened 

 by the fact that sometimes rounded granules or drops of 

 chromatin are left embedded in the plasmosome. In other 

 cases it is pear-shaped and is attached by its neck to the 

 bivalent, irresistibly suggesting that it has been squeezed out 

 of the contracting chromosomes like a viscid fluid from a 

 narrow aperture. These appearances are illustrated in figs. 32 

 and 38. In later stages this plasmosome becomes approxi- 

 mately spherical and often becomes detached from the sex 

 bivalent, though always lying close to it. 



For a time the two plasmosomes — one presumably the 

 remains of the pre-leptotene nucleolus, and the other apparently 

 formed out of material (plastin or linin '?) derived from the sex 

 chromosomes — coexist, the former being the first to disappear. 



In M a c r p u s the larger pale plasmosome which appears in 

 close connexion with the uniting sex chromosomes has also the 



