MEIOTIC PHASE IN MARSUPIALS 199 



appearance of being formed out of their substance, though no 

 figures were discovered quite so striking as those illustrated for 

 Petauroides. 



(4) C h r o m a t o i d Bodies . — In Petauroides one or 

 two bodies staining densely with iron haematoxylin appear 

 suddenly in the cytoplasm in the pachytene stage. Their 

 origin and fate have not been determined, but they seem to be 

 distributed capriciously at cell-division. In fig. 37 they have 

 all passed to one daughter cell (that containing the sex chromo- 

 some), but this mode of allocation is not invariable. In 

 Ma crop us chromatoid bodies are either absent or incon- 

 spicuous. 



Summary. 



Macropus ualabatus has twelve chromosomes, namely 

 10 -fXY in the male and 10 -l-XX in the female. 



In Petauroides the number is almost certainly twenty- 

 two, the male being of the formula 20 + XY. No female counts 

 were obtained for this animal. 



In the male Macropus Xis generally attacked to one of 

 the autosomes in spermatogonial mitoses. Y, which is exceed- 

 ingly minute, is free. During the pachytene stage, while the 

 autosomes are still elongated, X and Y condense into a bivalent. 

 In the first meiotic division this bivalent is attached to an 

 autosome. 



As a result of the first meiotic division the usual two classes 

 of secondary spermatocytes are formed one with X and the 

 other with Y. In the second meiotic division, those with 

 X show only five separate chromosomes, showing that X, as 

 usual, is fused with an autosome. The other class of second 

 divisions shows five autosomes and the minute Y. 



In the female Macropus the sex chromosomes were never 

 found free from the autosomes in the ovarian follicle cells, 

 which therefore show only ten separate chromosomes. 



In Petauroides the sex chromosomes cannot be distin- 

 guished with certainty from the autosomes. An unequal pair 

 of small chromosomes usually situated in the centre of the 



NO. 266 p 



