122 University of California Publications in Zoology ° (VoL. 9 
thin transparent distended wall of that part of the oviduct. In 
the unsuccessful cases discharged follicles apparently were 
present in the ovary, but there were no eggs in a cluster in the 
oviduct. It is probable that they were isolated and naked and 
hence escaped observation. The search (for scattered eggs) is 
further made difficult by the secretions in the oviduct, the 
presence of follicle cells, some blood corpuscles and fat droplets, 
and also by the rapid drying of the minute oviducts. 
Out of thirteen rats used for obtaining live eggs, ten, killed 
on the average 18.7 hours after parturition, furnished eggs in 
the oviduct. The other three, in which none could be found, 
averaged 20.2 hours. Failure can probably be explained here 
as a result of conditions as in the case of mice, although fixed 
material has not been studied. There is thus a comparatively 
very short period during which eggs are procurable. If the 
animal is killed too soon, the ova will not have had time to leave 
the ovary; if too late, the eggs will have passed on through the 
oviduct and escape even careful scrutiny. 
In regard to ovulation in rats, it will be seen from the data 
just given that it must oceur on the average less than eighteen 
hours after parturition. 
In the case of both mice and rats the method of procuring 
eggs was as follows: the animal when killed (mice by breaking 
the neck, rats by cutting the spinal cord between the cranium 
and atlas) was quickly opened, one ovary and oviduct removed to 
the stage of the binocular in the constant-temperature box and 
placed on a clean, sterile cover-glass of the proper size. When 
the fold of the oviduct containing the eggs was found, it was 
moved to the middle of the cover-glass, cut, and the cluster of 
eggs allowed to flow out. The cluster is jelly-like and highly 
translucent. The eggs being protected by the follicle cells, the 
slight drying serves to make them adhere to the cover. A drop 
of Ringer’s fluid, or in a few cases, blood serum, was placed 
over the eggs, and the cover-glass inverted over the cell of the 
slide. Up to this point the manipulations required from five to 
fifteen minutes. When the slide was connected by the glass 
tubes with the waste and reagent bottles, the chamber was easily 
filled with the desired fluid. With care nearly all air could be 
