164 Venous System of Didelphys Marsupialis (LL) 
abundance of material the writer has been unable to obtain, and, there- 
fore, appreciates that many modifications, as well as additions, to his 
account may possibly be necessary before a complete history of the veins 
is at hand. 
In the following pages an attempt has been made, on the basis of the 
material at hand, to present an account of the development of the post- 
cava from a time soon after its first appearance until the adult stage is 
reached; also, an account of the development of the azygos veins, as 
well as the transformations which the umbilical and omphalomesenteric 
veins undergo during the different stages of development. 
An attempt to breed opossums in captivity proved only partially suc- 
cessful, the failure being due, I am convinced, to the unsuitable condi- 
tions which necessarily prevail in my laboratory. Most of my Didelphys 
material was therefore obtained outside of the laboratory, and, for this 
reason, it is impossible to give the exact age in days and hours of any 
of the embryos or pouch young studied. It has been possible, however, 
by means of Selenka’s, 86 and 87, figures and descriptions to approxi- 
mate their ages in a few cases and to establish the fact that the embryo 
of Dasyurus which Dr. J. P. Hill, of the University of Sydney, kindly 
sent me is, in point of structure, relatively younger than my youngest 
Didelphys embryo. 
According to Selenka, the interval between copulation and birth in 
Didelphys virginiana is about thirteen days (twelve days and twenty 
hours), while that between copulation and the beginning of cleavage is 
five days. In the following list where ages are mentioned the age has 
been reckoned from the beginning of cleavage. 
List OF MATERIAL STUDIED. 
1. One Dasyurus embryo measuring about 6 mm. in length (crown- 
rump measurement). From a comparison of the structure and external 
characters of this embryo with that of Selenka’s, 86 (Fig. 3, Taf. XX VI), 
six days old Didelphys embryo, it is evident that the latter is slightly 
more advanced than the Dasyurus embryo, which, in point of structure, 
corresponds to a Didelphys embryo of about five and one-half days. 
2. Eleven Didelphys embryos averaging 8 mm. in length. From a 
comparison of their measurements these embryos may be a few hours 
older than Selenka’s, 86, (Fig. 3, Taf. XX VI) six days old embryo, al- 
though in their external characters the two appear to be identical. 
2 All measurements were made in this manner. Embryos and pouch young 
were measured by the writer after fixation. 
