DEVELOPMENT OF THE ALBINO RAT 259 
The stage of pronuclei was observed in over 100 ova of the 
white rat by Sobotta and Burekhard. According to these ob- 
servers, the two pronuclei show in the earlier stages of their 
development, large chromatin-like nucleoli, the number of 
which varies. Some little time later, one or several such chroma- 
toid nucleolar bodies with irregularly formed chromatin masses 
arranged on the linin network are to be observed. At a still 
later time, the chromatin becomes distributed over the linin 
network, throughout the nuclear space, giving the appearance 
of a fine chromatin network. One of the pronuclei is, as a rule, 
somewhat smaller than the other. This is regarded as the male 
Cc D 
Fig. 1 Tubal ova, albino rat. X 200. A, rat No. 110, 24 hours, 15 min., 
ovum in pronuclear stage, larger nucleus female pronucleus; B, and C, rat No. 
59, 2 days, 2-cell stages, thin oolemma showing in C, only partially seen in B; 
D, rat No. 62, 2 days, 22 hours, 3-cell stage, the nucleus of the unsegmented 
blastomere in the monaster phase, only one of the other two cells showing in the 
figure. 
pronucleus, since near it the ‘sperm centrum’ was now and 
then observed. The pronuclei lie in about the center of the 
ovum. The pronuclear stages of my own material, observed in 
34 ova, obtained 24 hours after the beginning of insemination— 
thus at the end of the first day of development—all present essen- 
tially the same stage of metamorphosis. As may be seen in A 
of figure 1, the nuclei are distinctly membraned, and are of 
relatively large size. The ovum here sketched measures in the 
stained preparation 70 u by 62 py, and is, therefore, of slightly oval 
form. Sobotta and Burekhard give 55 uw to 60 uw as the size of 
the tubal ova, and 60 uw to 65 u as the size of the ovarian ova in 
