128 WILLIAM B. KIRKHAM 



OBSERVATIONS 



The yellow-mouse material used in this investigation consists 

 of complete series of sections of ovaries and fallopian tubes 

 from healthy, non-suckling females representing each of 

 the first three days of pregnancy, and like material, with the 

 addition of entire uteri with all contained embryos for each day 

 of pregnancy from the fourth to the twentieth. 



There were also available four sets of preparations from sickly 

 females. As a control there were full sets of similar material 

 from healthy, non-suckling white mice, and serial sections of 

 seventeen degenerating white-mouse embryos, at least sixteen 

 of which came from sickly females; all of these white mice 

 coming from lines which had been subjected to cross-breeding 

 with those of other coat colors, and found to be without the 

 factor for yellow. The details of the yellow-mouse material 

 are set forth in table 1. 



The time of ovulation, with respect to previous parturition, 

 the time of fertilization, rate of cleavage, and time of implanta- 

 tion are the same for all mouse ova, whether whites, homozy- 

 gous yellows, or heterozygous yellows, and the number of cleav- 

 ing eggs, morulas, and blastulas found in individual yellow 

 females falls within the same limits as found in white females, 

 thus disposing as mathematically invalid, of any theory that the 

 missing homozygous yellows are to be accounted for on the basis 

 of the failure of eggs with the yellow factor to undergo matura- 

 tion or fertilization. The two-cell stages examined all appear 

 normal, but by the time the eggs have reached the morula stage 

 in yellow mice certain ones show abnormaUties, thus one of the 

 morulae in set Y 40 has very indistinct cell boundaries. Ab- 

 normal blastulas can be distinguished easily by their shrunken 

 appearance, the cells in such cases being small and crowded, and 

 the blastodermic vesicle smaller then normal (fig. 1). Never- 

 theless, these abnormal individuals were alive at the time of pres- 

 ervation, and it is not until they are due to implant that they 

 begin to plasmolyze and are overwhelmed by phagocytes. 



