DEVELOPMENT OF THE OPOSSUM 97 
nuclear division, the nuclei being in the resting stage; the other 
two are in late anaphase. 
However, two of my own specimens, both of which contain 
short spindles in each blastomere, cast doubt upon Hill’s view 
as stated above, with which I had agreed before I came into 
possession of the eggs (from No. 306). For in one of these eggs 
the spindles are exactly parallel; in the other they deviate 36° 
from the parallel. 
It is, therefore, apparent that, unless we assume a rapid 
shifting of the blastomeres during the early phases of the second 
cleavage, the matter must for the present remain in doubt. 
It is interesting to note that D. aurita has two breeding seasons 
a year, whereas D. virginiana has but one. 
Professor Hill states it as his belief that twelve is the reduced 
number of chromosomes in the opossum, and with this I fully 
agree. 
LITERATURE CITED 
CaLpweLL, W. H. 1887 The embryology of Monotremata and Marsupalia, 
Part I. Phil. Trans. Soy. Soc., vol. 178 B. 
HarTMAN, Cart G. 1916 Studies in the development of the opossum, Didel- 
phys virginiana L., Parts I and II. Journ. of Morph., vol. 27. 
Hiuz, J. P. 1910 The early development of the Marsupalia, with special refer- 
ence to the native cat (Dasyurus viverrinus). Quart. Jour. Mier. 
Sci., vol. 56. 
Mrnot, CHarLes R, 1911 Note on the blastodermie vesicle of the opossum. 
Anat. Rec., vol. 5. 
PaTTERSON, J. P. 1913 . Polyembryonic development in Tatusia novemcincta. 
Journ. of Morph., vol. 24. 
Sevenka, E. 1887 Studien in der Entwicklungs geschichte der Thiere, Band 
4, Das Opossum (Didelphys virginiana). Wiesbaden. 
SPURGEON AND Brooks 1916 The implantation andearly segmentation of the 
ovum of Didelphys virginiana. Anat. Rec., vol. 10. 
Van DER Stricut 1909 Lastructure de l’ceuf des Mammiféres (Chauve-souris, 
Vesperugo noctula): Troisiéme Partie., Mem. de |’Acad. roy. de Bel- 
gique, IIe ser., t. 2. 
