216 A. W. GREENWOOD 



also the X-chromosome is much smaller than any of the auto- 

 somes. 



Chromomeres are conspicuous during syndesis, early pachy- 

 tene, and early diplotene stages. 



The early pachytene stage is followed by a late pachytene 

 stage in which the threads become diffuse and lose their capacity 

 for taking up the stain. 



Except in the early merotic prophase the sex chromosome 

 remains compact and deeply stained and does not thread out 

 like the autosomes. 



In all the above animals the first meiotic division is reduc- 

 tional, separating the X- and the Y-chromosomes, and the 

 second division is equational, in each cell the sex chromosome 

 dividing. The spermatozoa are therefore of two kinds, one 

 containing an X-chromosome and the other containing a 

 Y-chromosome. 



No further reduction in the number of chromosomes takes 

 place during the second meiotic division. 



The Y-chromosome could not be identified during the meiotic 

 phase until the metaphase of the first meiotic division. At 

 this stage in Phascolarctus the sex chromosomes are 

 separate and do not form a bivalent. 



The archoplasm seems to exert some influence on the 

 chromatin threads at synizesis and during the early pachytene 

 stage. In the former case the contraction takes place to that 

 side of the nucleus at which the archoplasmic mass is situated ; 

 in the latter the chromosomes are in the form of thick loops 

 with the ends of the chromosomes pointing towards the archo- 

 plasmic mass. 



In Phascolarctus the Sertoli cells are very large and 

 possess peculiar rod-like bodies, the origin and function of 

 which was not arrived at. The result of experiments seem to 

 show that the rods are not afTected by the action of digestive 

 fluids. 



