CRITICISMS AND CONCLUSIONS. 45 



VIII. CRITICISMS AND CONCLUSIONS. 

 A. MATERIAL. 



This work differs from that of previous investigators in that it has 

 been done on mice of very mixed ancestry. It is therefore open to the 

 possible criticism that the material is unlike that on which other papers 

 have been based. It may be maintained, however, that there is no 

 essential difference in material for the following reasons: first, the fact 

 that the white mice of our stock, whether of colored ancestry or not, 

 breed true, leads one to believe that, in the light of recent work on hered- 

 ity of coat color, they are as pure as other white mice; secondly, there 

 is no dissimilarity in the maturation processes of eggs from mice of dif- 

 ferent coat character; thirdly, there is no real difference in important 

 points between Mr. Kirkham's preparations and our own. 



Sobotta suggests in his paper published in 1907 that some of the 

 differences between his results and those of Gerlach (1906) may be due, 

 in part at least, to the fact that he used eggs set free at an ovulation 3 

 weeks after parturition, whereas Gerlach employed ova obtained during 

 the first 3 days after parturition. Since Sobotta is the only one who has 

 made use of eggs derived from an ovulation later than the first one after 

 the birth of young, his explanation must apply to all other investigations, 

 including the present one. There seems, however, to be no a priori 

 reason for supposing a difference between the maturation processes of 

 eggs maturing and ready for fertilization at different periods after par- 

 turition; moreover, the dissimilarities in the results of the several in- 

 vestigators can be accounted for to a large extent on other grounds, as 

 will appear in the course of the remaining pages. 



Considerable significance attaches to the amount of material studied 

 by other investigators. Tafani, Gerlach, and Kirkham do not state the 

 number of eggs which they observed, but the number was probably 

 small. Lams et Doorme based their paper on only 90 ova. Sobotta in 

 his large work (1895) used 1402 sound eggs; but of this number only 298 

 (compared with our 877, table 2, Stages I-X), at the most, were of 

 such age as to show stages in the formation or division of spindles, or 

 the number of chromosomes, or the abstriction of polar cells. All the 

 rest (1104) were either in stages showing the pronuclei or still older. 



B. METHODS. 



It is probable that the value of our results would have been en- 

 hanced had another set of eggs preserved by other methods been com- 

 pared at each step with those which have served as the basis for the 

 present paper. These have all been carefully studied and in part are 

 described and figured here. However, it should be said that other fix- 

 ing fluids were tried, and that, in cases where the preservation was good 

 enough to give reliable pictures, the eggs showed conditions similar to 

 those obtained with the special preserving fluid described at page 12. 



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