1912 | Long: Living Eggs of Rats and Mice 123 
driven out, for the eggs at first withstand a violent current 
without becoming detached. 
Spermatozoa are easily introduced into the chamber after 
mixing them with a few drops of Ringer’s fluid. Unfortunately 
for clear observation the eggs, especially those of the rat, are 
greatly obscured by the follicle cells, and thus far the penetra- 
tion of the spermatozoon into the egg has not been observed. 
The immediate effect of the spermatozoa is not quite the same 
in all cases. Mouse spermatozoa are exceedingly active in the 
salt solution. In one experiment they had within two minutes 
thrashed the follicle cells loose, and bared the eggs, which they 
then kept in rapid rotation. Usually the follicle cells are 
loosened less violently, becoming scattered within twenty to 
forty-five minutes. It is then possible to wash out the excess of 
spermatozoa and the follicle cells, leaving the eggs quite free in 
the chamber of the slide. By judiciously pinching the rubber 
tubing leading to the waste bottle the eggs may be moved about 
and freed from debris. While most of the spermatozoa when 
first introduced became attached to follicle cells and lashed the 
water violently, a number penetrated between the follicle cells 
and moved about with a slower sinuous motion, strikingly like 
that of a free-living nematode. The latter sort of motion is more 
effective, for the spermatozoa exhibiting it progress much more 
rapidly with the same amount of activity. Presumably this is 
the way they move up the uterus and oviduct. 
At first scarcely anything can be observed of the zona pel- 
lucida. However, after the eggs are freed of the follicle cells 
in Ringer’s solution, the zona is clearly visible as a highly 
transparent, thick envelope. In blood serum the zona is invisible. 
It is not closely applied to the egg, from which it is sometimes 
separated by a considerable space, especially in the region of the 
first polar cell. Sometimes the first polar cell seems hardly to 
touch the egg, again it lies in a slight depression in the surface 
of the egg. 
The cytoplasm of the eggs of both mice and rats is beauti- 
fully transparent, so clear that it is possible to see distinctly 
granules at the lower pole. In both there is a granular in- 
gredient, though it is different in the two ova. In eggs of mice 
