DEVELOPMENT OF THE OPOSSUM PA 
To prevent collapse of the blastocysts, which my photographs 
on plates 1 and 2 show to be perfect spheres, it is important to 
pass from one medium to another (Bouin’s to alcohol, water to 
alcohol, aleohol to xylol, and especially xylol to paraffin) by slow 
gradations. I use 5 per cent differences, accurately made up in 
stock solutions. The eggs are placed in small vials and each 
higher percentage is added gradually. Vesicles from about 1.5 
mm. on up may easily be cut in half equatorially with fine 
scissors; but such hemispheres, if of approximately equal size, 
should not be placed in the same vial, because they are likely 
to telescope in such a way that they are hard to separate without 
injury to them. 
The material was imbeded in paraffin and most of the newer 
material was sectioned by Huber’s water-on-the knife method, 
which gives incomparably better results than ribboning the 
series with the rotary microtome. 
6. Unfertilized eggs. Since, unfortunately, the vast majority 
of the eggs removed from animals in captivity are unfertilized, 
it will not be amiss to mention them here. Such eggs remain 
in the uterus for many days, undergoing progressive degeneration 
before being discharged or absorbed. For the first two or three 
days they are not easily distinguished from normal uterine eggs. 
The first sign of degeneration is the breaking up of the ovum 
into masses of various shapes and sizes (fig. 5, pl. 11) and the 
ovum may flatten out into the shape of a crescent (fig. 6, pl. 11). 
Gradually, too, the eggs increase in opacity (fig. 7, pl. 11) and 
become covered with white concretions (fig. 8, pl. 1; figs. 8 
to 10, pl. 11), so that they are only too prominent when one 
opens the uterus hoping to find embryos. The eggs shown in 
figure 9, plate 11, accompanied foetuses near term; hence these 
eggs are at least nine days old. 
7. The illustrations. The drawings (plates 12 to 22) were 
made on Ross stipple board No. 2 and reduced to one-sixth the 
original size. Korn’s lithocrayon No. 1 (a_paraffin-carbon 
pencil) gave the best results, since to be reproduced by the 
line process the dots must be absolutely black, a result not 
easily attained with a graphite pencil. 
