4 CARL G. HARTMAN 
thereto transitional stages not secured before. Among the latter 
are litters Nos. 194’, 344, 349, 356, 175’, 339, and 347, which 
show the process of entoderm formation in an unbroken series. 
Since this phase of the problem had to be entirely rewritten, 
and since I now have new material on the early stages, besides 
a series of photographs of the living egg in all stages, it has 
seemed desirable and profitable to give a complete account of 
the development of the opossum from the beginning. ‘This has 
been done in the present paper; but the reader is referred to the 
writer’s former publication for certain details. 
The original notes and the preparations upon which this work 
is based, together with alcoholic specimens, will be deposited in 
the archives of The Wistar Institute, where they will be easy 
of access, and anyone who wishes to do so may examine the 
material and test the validity of the conclusions at which I have 
arrived in this paper. 
b. Historical 
In my former publication I reviewed in some detail the work 
of Selenka (’87) on the opossum and that of Hill (10) on Das- 
yurus. Mention was also made of Caldwell’s discovery (’87) of 
the shell membrane enveloping the marsupial egg (Phasco- 
larctus), and of a short paper by Professor Minot (’11) on the 
bilaminar blastocyst of the opossum. Simultaneous with the 
publication of my article, a paper by Spurgeon and Brooks 
appeared, giving a description of two litters of opossum eggs 
in cleavage (2 to 8 cells). I wish at this point to recur briefly 
only to the work of Selenka, leaving the other articles to be 
discussed under appropriate headings in the body of the paper. 
Salenka’s work on the cleavage and blastocyst formation is 
based on 26 eggs secured from two females. One animal yielded 
one 2-celled, one 20-celled, and nine unfertilized eggs, all badly 
shrunken. I suspect that the ‘2-celled’ and the ‘20-celled’ eggs 
are probably specimens in different stages of fragmentation. 
The other animal furnished two unfertilized eggs, one4-celled and 
one 8-celled egg, two blastocysts of 42 and 68 cells, respectively, 
two slightly older blastocysts with a mass of entodermal cells, 
