1912] Long: Living Eggs of Rats and Mice 107 
for the present work has been supplied by the Zoological Labora- 
tory of the University of California. i 
Living eggs have been studied and _ artificially seminated 
under the microscope, having been carried as far as the formation 
of the second polar cell. This piece of work bears only on one 
aspect of the larger problem we have undertaken, and although 
the results are at present necessarily incomplete, they are set 
forth with the hope that especially the apparatus devised for the 
problem may be of value to investigators in related subjects. 
The animals used in these experiments were the ordinary 
white rats and mice, the ancestry of which was mixed, partly 
white and partly colored individuals. All were normal and in 
good condition. 
APPARATUS 
In addition to new apparatus presently to be described, some 
improvements have been made in the appliances used in the 
earlier investigation (Long and Mark, 1911). 
CAGES 
The cages are now made entirely of metal (pl. 13, fig. 1). 
They are constructed of one piece of wire netting, one-fourth 
inch mesh, cut into the proper shape, bent over a wooden form, 
soldered at the angles, and reenforced by a frame of heavy wire. 
In the middle is soldered a partition of the same wire netting, 
in which there is a hole connecting the two chambers. In the 
bottom of each chamber is slipped a drawer-like tray of gal- 
vanized iron, which is partly filled with sawdust or shavings 
and forms the floor of the cage. A sloping lid is hinged at its 
upper edge as shown in the figure. 
BIRTH-RECORDING APPARATUS 
Advantage was taken of the method of timing the birth of 
litters which was described in the preceding paper (Long and 
Mark, 1911, pp. 7-10). The apparatus is now made on the same 
principle, but is somewhat simplified, and is fitted with electrical 
contacts and magnets for marking on the chronograph drum. 
Figures A-C and plates 13 to 15 illustrate the construction and 
operation. 
