THE MATURATION OF THE MOUSE EGG. 26 1 



First Polar Body (Figs. 3 and 5).- -The study of many 

 preparations reveals the following facts : None of the eggs in the 

 Fallopian tube have failed to develop at least to the formation of 

 the second polar spindle, and all the 

 ovarian eggs which by their size, 

 slightly denser protoplasm and large 

 follicles appear to be nearly ripe, have 

 already extruded the first polar body. 

 The conclusion arrived at is, that ap- 

 parently every egg which is capable 

 of further development forms a first 

 polar body within the ovary. This 

 agrees with the observations of Ru- 



FIG. 2. Diagram of chromo- 



baschkin ('05) upon the guinea-pig somes in first po ] ar sp i n dle. Note 



egg, and those of Van der Stricht great variation in size. Four 

 ('Ol) upon the egg of a bat, Vesper- more chromatin masses in adjacent 



sections. 



it go noctwa. 



This point established, it is next necessary to explain the dis- 

 appearance of the first polar body in the majority of eggs seen in 

 the Fallopian tube. The zona pellucida may persist in the mouse 

 egg, undiminished, through the early cleavage stages, but in the 

 majority of instances during the process of ovulation the first 

 polar body is either forced through a weakened part of the zona, 

 or frees itself by amoeboid movements, and comes to lie outside 

 the zona, as described and figured by Van der Stricht ('04). 



The first polar body is usually oval -in form, and is character- 

 ized, as found by Van der Stricht ('04) in the egg of K noctnla, 

 by often possessing a little maturation spindle of its own, and in 

 other instances having its chromosomes scattered. In some of 

 these cases which possess a spindle, the first polar body would 

 probably have divided mitotically, as observed by Sobotta ('95) 

 in the mouse egg, and once by Rubaschkin ('05) in the egg of 

 the guinea-pig. The polar bodies vary somewhat in size, and in 

 one series of ovarian eggs there have been found first polar bodies 

 of about four times the average volume. The number of chromo- 

 somes in the first polar body is twelve (dyads). 



Second Polar Spindle (Fig. 3). Immediately after the forma- 

 tion of the first polar body, the twelve dyads remaining in the 



