DEVELOPMENT OF THE OPOSSUM 95 
the opossum egg is much more Eutherian than Dasyurine. Of 
these perhaps the most striking feature is very early differentia- 
tion of the entoderm. Since in other marsupials, according to 
Hill, the entoderm is formed early (Macropus, Perameles) it 
seems probable that when the other marsupials have been more 
thoroughly studied it will be found that the opossum is more 
typical of the marsupials in general and that Dasyurus represents 
a more primitive, albeit, therefore, an even more interesting 
form. 
ADDENDUM 
Since the completion of my manuscript there has come to 
hand Prof. J. P. Hill’s paper on ‘The Early Development of 
Didelphys aurita,’’ published in the April, 1918, number of the 
Quarterly Journal of Microscopical Science. This work is based 
on eggs secured from six females: one animal furnished unseg- 
mented, unfertilized eggs; another numerous 2-, 3-, and 4-celled 
eggs; from two others, cleavage stages of 4 to 16 cells were 
taken, and from two animals bilaminar blastocysts about 1 mm. 
in diameter. 
It appears from this contribution that the developmental 
stages of the South American opossum, so far as Professor Hill’s 
material goes to show, are closely duplicated by my own speci- 
mens of the local species. The same is true of a number of 
somewhat later stages (primitive streak) which I have secured 
from the small black D. marsupialis occurring in south Texas 
and Mexico. 
In most respects my own work finds full corroboration as well 
as interesting extensions in the careful study made by Professor 
Hill. I wish briefly to refer to several points discussed by our 
able British colleague. 
In his analysis of the 2-, 4-, 8-, and 16-celled stages, he presents 
additional evidence of polarity in the opossum egg; for he finds 
that in a large proportion of such eggs the cells are plainly made 
up of two groups, differing somewhat in size, the smaller cells 
being considered by Hill as constituting the upper or formative 
pole of the egg. He also finds that the majority of eggs in 
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