200 W. E. AGAR 



spermatogonia! metaphase plates probably, however, are 

 X and Y. Early pachytene nuclei show two compact bodies 

 which unite into one, presumably the sex bivalent. 



The second reduction of the chromosome number to one- 

 quarter of the diploid total in the second meiotic division, 

 which has been described for several species of birds and 

 mammals, does not take place either in Ma crop us or 

 Petauroides. 



Chromomeres are very prominent in Petauroides in 

 the zygotene and diplotene stages. 



Probably in Macropus, and more convincingly in 

 Petauroides, the cytological conditions to permit of 

 ' crossing over ' are present in the male. 



The plasmosome which appears in the pachytene stage is 

 probably formed from the plastin or linin basis of the con- 

 tracting sex chromosomes. 



Eeferences to Literature. 



Agar, W. E. (1913).—' Quart. Journ. Mcr. Sci.', 58. 

 AUen, E. (1913).—' Journ. Morph.', 31. 



(1915).—' Anat. Record ', 10. 



Castle, W. E. (1921).— ' Science ', 54. 



Edwards, C. E. (1910).— ' Arch. f. Zellfor.schuug ', 5. 



Frolowa, S. (1912).— Ibid., 9. 



Jordan, H. E. (1911).— Ibid., 7. 



McClung, C. E. (1905).— 'Biological Bulletin', 9. 



Painter, T. S. (1922).—' Journ. Exp. Zool.', 35. 



f^akamura, T. (1920).— ' Journ. Coll. Science Tokyo', 39. 



Wenrich, D, H. (1916).— ' Bull. Mus. Comp. Zool. Harvard', 60. 



Wilson, E. B. (1911).— ' Journ. Morph.', 22. 



(1912).— ' Journ. Exp. Zool.', 13. 



