DEVELOPMENT OF THE OPOSSUM 19 
The pressure now being removed, the hypertrophied mucosa is 
pulled apart, preferably under the binocular microscope, with 
two pairs of finely pointed forceps, and the lumen exposed. 
The eggs may be picked out from among the delicate folds of 
the mucosa by means of a pipette. But this method is un- 
necessarily tedious; the uterus may instead be simply turned inside 
out in the Ringer’s solution and the eggs picked out from the 
bottom of the dish. To insure finding all of the eggs, a little 
Bouin’s fluid added to the salt solution, after removal of all the 
eggs that can be seen, makes any specimens overlooked promi- 
nently visible. The uterus should also be shaken out in another | 
dish of Ringer’s solution for any eggs that may have been hidden 
in the uterine folds. To keep the solution clear of blood, it is 
well, before opening the organ, to slit all the superficial blood- 
vessels and drain them of blood. I may add that the neck of 
the uterus should be ligated with a ‘lifting’ ligature before it is 
cut from the body, in order to prevent the loss of eggs through 
the os uteri. 
Young eggs in cleavage and small blastocysts are mostly 
found near the caudal end of the uterus, often closely bunched 
together. Hence one cannot speak of ‘implantation’ of the 
opossum egg at any early stage. The ‘uterine cups’ described by 
Spurgeon and Brooks (’16) do not mark implantation surfaces, 
but merely accidental pits produced by pressure into the delicate 
oedematous mucosa. 
If, on opening the animal, pregnancy seems to be advanced, 
in order to remove entire vesicles intact, it is best to slit the 
uterus superficially in many places and to trim off the entire 
musculature before attempting to remove the vesicles, which 
are closely applied to the mucosa, but never fused with it. 
This procedure renders the use of a killing fluid to paralyze the 
musculature entirely superfluous. With a pair of forceps and a 
fine brush an entire litter of delicate vesicles may be removed 
intact. They may be transferred to the fixing fluid in a deep 
mustard spoon or in a shallow, neckless vial. A collapsed 
vesicle may again be dilated in the fixing fluid by injection with 
a fine pipette; in fact it is well to irrigate with the fixing fluid the 
lumen of every vesicle containing a large embryo. 
