I 



80 11 '. H. KirkhcDn — Maturation of the Egg of the White Mouse. 



or less solid mass (PI. Nil. tiLC. 15), and would have finally formed 

 a resting nucleus (PI. VJII, tigs. 16-17, Text-fig. 10). 



This second polar body is characterized by being generally spheri- 

 cal in form, with an average diameter of about ,007 to .012"'™, and 

 W\ containing 12 univalent chromosomes, which are usually seen 

 cither lumped together or as a resting nucleus. It is this polar 

 body, furthermore, which is commonly found associated with the 

 early cleavage stages (PI. VIII, figs. 16-17). 



The important question concerning the fate of the first polar bodk- 

 in cases where it has disap])eared (PI. VI, fig 12 ; Text-fig. 1) will 

 now be considered. That this disappearance is not peculiar to the 

 j)reparations used by the writer is established by the fact that eggs 



Figure 10. — Egg in the Fallopian tube soon after fertilization, with the second 

 jjolar body, the chromatin of which is forming a resting nucleus. The first 

 polar body has disajDpeared. At the right within the egg is seen the female 

 pronucleus ; at the left, the sperm head. The zona pelliicida has been 

 dissolved by the reagents used. (Somewhat diagrammatic.) x 675. 



which ])ossessed only a (second) polar spindle, no ])olar bodj' accom- 

 ]»anying them, were seen in the Fallopian tube by Tafani ('89), 

 while Sobotta ('95) saw similar eggs, but mistook the spindle they 

 contained for that of the first polar mitosis. Van der Stricht (:04), 

 furthermore, has described and figured an egg of F! noctula with 

 both polar bodies lying outside the zona. Finally, the writer has 

 seen several series of living eggs before fertilization, in one of which 

 only one egg in six possessed a first polar bod}^, while in another 

 series three out of the five eggs possessed this polar body. 



The zona in the mou.se may persist undiminished through the 

 early cleavage stages of the egg, but in the guinea-pig Rubaschkin 

 (:05) has found that at the time of ovulation the zona is soft, and 



