22 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 da} r after 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, 



