260 G. CARL HUBER 
the white rat. Kirkham and Burr give the diameter of the 
living unsegmented egg of the rat as of 0.079 mm. As may be 
seen from A and B, of figure 2, the tubal ova, even when free in 
the oviduct, are not of necessity spherical in shape, but often 
slightly compressed, as may be clearly seen in four models of 
tubal ova in the pronuclear stage, reconstructed at a magnifica- 
tion of 1000 diameters, in my possession. Depending on the 
plane of section, the diameter of a tubal ovum may thus vary 
to the extent of 5 u to 8 uw. The two nuclei in the preparation 
shown in A of figure 1, measure, the larger one, regarded as the 
female pronucleus, 23 » by 16 uw, the smaller 17 u by 15 yw. Essen- 
tially all of the chromatin is distributed over the linin network 
in fine granules, the larger nucleus presenting one large, faintly- 
Fig. 2. Models, made after the Born method, of two tubal ova of the albino 
rat in the pronuclear stage. X 200. A,rat No. 106, 24 hours; B, rat No. 110, 
24 hours, 15 min. Reconstructions made at a magnification of 1000 diameters, 
figure reduced in reproduction. 
staining chromatoid nucleolus. The ooplasm is finely granular, 
distributed so as to give the section a slightly mottled appear- 
ance. When compared with figures given by Sobotta and 
Burekhard (figs. 21 to 24, plates 9-10) showing pronuclear 
stages of the ova of the rat, my own seem to fall in about the 
middle of this series, thus some little time after their formation, 
but not immediately preceding the stage of segmentation spindle 
formation. In the albino rat, and perhaps in other mammals, the 
pronuclear stage, in its various phases of nuclear metamorphosis, 
must constitute a stage covering a relatively long period. If it 
is assumed that semination occurs about 10 to 12 hours after the 
beginning of insemination, such assumption being justified 
by the observations of Sobotta and Burckhard, according to 
whom the life of the spermatozoa in the genital tract of the 
white rat is only about 10 hours, and if it is recalled that in 
