DEVELOPMENT OF THE ALBINO RAT 15 



The stage of pronuclei was observed in over 100 ova of the 

 white rat by Sobotta and Burckhard. 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 si ill 

 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 



$UM 



^ 



Fig. 1 Tubal ova, albino rat. X 200. A, rat Xo. 110, 24 hours, 15 min., 

 ovum in pronuclear stage, larger nucleus female pronucleus; B, and (', 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 p. by 62 /i, and is, therefore, of slightly oval 

 form. Sobotta and Burckhard give 55 m to 60 m as the size of 

 the tubal ova, and 60 m to 65 n as the size of the ovarian ova in 



