DEVELOPMENT OF THE ALBINO RAT 13 



first segmentation spindle are by inference derived from the 

 sperm centrosome. The data here given, as concerns the matu- 

 ration and fertilization phenomena pertaining to the albino 

 rat, unless otherwise credited, have been drawn from the account 

 of Sobotta and Burckhard, whose account is accompanied by 

 excellent figures. 



Long has studied in living ova of mice and rats the phenomena 

 of maturation and fertilization. Tubal ova were placed in Ring- 

 er's solution on an especially constructed slide and spermatozoa 

 introduced. It was possible to seminate the ova of rats with 

 rat spermatozoa and to observe the formation of the second 

 polar body. The formation of the second polar body, "usually 

 near the first polar cell, may begin within five minutes to two or 

 more hours after the spermatozoa are introduced. The con- 

 striction may be finished three-fourths of an hour later." "The 

 first appearance is an elevation clearer than the rest of the cell. 

 The swelling becomes higher, and at one side of the elevation 

 there appears a depression which is the beginning of the constric- 

 tion which presently encircles the whole swelling and cuts it 

 off from the egg." Nothing could be said as to the changes which 

 the chromatin undergoes after the spermatozoa have penetrated 

 the egg. The eggs remained alive and apparently normal for 

 about twelve hours, after which they began to degenerate. 



PRONUCLEAR STAGE 



As has been stated, my own observations on the develop- 

 ment of the albino rat (Mus norvegicus albinus) begin with the 

 pronuclear stage. The material at hand for this stage is listed 

 in table 1, page 258. 



Thus there are present in the series 34 ova showing a pro- 

 nuclear stage and 9 ova showing the second maturation spindle 

 in the monaster phase. The latter may be dismissed with the 

 brief statement that they represent unfertilized ova. In rat 

 No. 108, with 7 ova in the stage of the second maturation spindle, 

 killed 24 hours after the observed copulation, there was found 

 no trace of spermatozoa in the oviduct. Two reasons may be 

 offered for the non-appearance of fertilization in this case: 



