266 G. CARL HUBER 
spindle, and the first segmentation division would appear to 
fall to a period ranging from the beginning to near the middle 
of the second dayafter the beginning of insemination, probably 
about 30 to 32 hours after insemination. In the mouse, in which 
these stages have been very completely and carefully investi- 
gated by Sobotta, the conjugation of the pronuclei and the 
first segmentation spindle formation falls to the end of the first 
day after copulation. These phenomena appear to be passed 
through rather quickly in the mouse ovum, covering a period of 
only about one and a half to two hours. 
The 2-cell stage with resting nuclei extends through a relatively 
long period. In the mouse it extends through nearly an entire 
day, as shown by Sobotta, who found 2-cell stages present through 
a period ranging from 25 hours to 48 hours after copulation. 
Melissinos often observed the 2-cell stage with resting nuclei in 
both mice and rats in material gathered 24 hours after copulation 
and to 44 hours thereafter. It is to be regretted that this ob- 
server does not differentiate more specifically between ova of 
mice and rats in his description. As a rule it is impossible to 
determine except by inference to which of the two varieties of 
ova his account refers. It may be assumed that the statements 
made apply equally well to the ova of either the mouse or the 
rat. 
In my own material, the 2-cell stage was observed during a 
period extending from 1 day, 18 hours to 2 days, 22 hours after 
the beginning of insemination, thus for a period extending over 
more than 24 hours. In the albino rat, the first: two blastomeres 
are equivalent cells of essentially the same size and structure, 
as may be seen from B and C, of figure 1, drawn respectively of 
ova found in the right and left oviducts of rat No. 59, killed two 
days after the beginning of insemination, and regarded as repre- 
sentative ova. The two cells of each ovum are not spherical, 
but of slightly oval form, with relatively large, distinctly mem- 
branated nuclei, with fine chromatin granules scattered on the 
linin network and a number of relatively large chromatoid nucleoli. 
The cytoplasm presents a granular appearance, the granules 
being evenly distributed throughout the cell. In my own 
material, I seldom find the two cells lying in the same plane, 
