DEVELOPMENT OF THE OPOSSUM 23 
which must also be filled with the solution; for it is absolutely 
essential that there should be no air space for the reflection of 
light between the transparent glass holding the eggs and the 
black glass serving as a background. 
In the photographs of preparations, as well as in the drawings, 
for ease of comparison, magnifications of 50, 200, and 500 have 
been adhered to with few exceptions. In this connection plates 
12 and 13 are especially adapted to serve as a résumé of the 
stages covered in this paper. Comparison of these two plates 
shows that the young eggs shrink greatly on account of their 
delicate albumen layer. 
Altogether the twenty-two plates accompanying this paper 
contain over 750 representations of more than 600 different 
opossum eggs, mostly, of course, in groups on the photographs. 
The drawings and the photomicrographs of preparations number 
some 240 of over 180 different eggs. . 
d. External changes at ovulation in the female opossum 
In common with many other wild animals, the opossum does 
not breed well in captivity. I have worked with hundreds of 
animals kept in cages or in large rooms, isolated or in groups of 
dozens or of a hundred or more; yet the number of observed 
copulations that I have to record is disappointingly small. 
Many births have taken place in the cages, but the cases of 
,pseudopregnancy outnumber the cases of true pregnancy many 
times over. 
In spite of careful personal attention to the habits of the 
captive animals, I was unable during the first two years’ col- 
lecting to determine from outward signs the sexual state of the 
female. In this regard I was at first forced to agree with 
Selenka who says: ‘‘Ohne operative Eingriffe ist ttber die Trach- 
tigkeit eines Weibchens keine Gewissheit zu erlangen, da man 
weder durch Tasten mit dem Finger die weichen Uterushérner 
auffiden kann, noch auch an den Milchdriisen eine Verainderung 
wahrnimmt, bevor nicht die Embryonen nahezu ausgewachsen 
sind.”’ I have since learned, however, that Selenka was wrong 
