TV. B. Kirkham — Maturation of the Ecjg of the White Mouse. 67 



stated that the eggs leave the ovary during the winter (Decem- 

 ber to February) and are at once fertilized, but onh^ begin to seg- 

 ment in March or April. These workers further found that each 

 Q^& commonly formed three polar bodies, and they saw both pro- 

 nuclei and a few early cleavage stages. Van Beneden ('99) has 

 since continued his study of the bat's ^^^^ and has noted all the 

 important cleavage stages. 



Van Beneden ('80) described some late stages in the development 

 of the rabbit's Q^^., and also made the important observations that 

 the female pronucleus comes from the germinal vesicle and that the 

 chromatin of the polar bodies has the same origin. 



Rein ('83), working with the eggs of rabbits and guinea-pigs, 

 observed in the latter the pronuclei in different stages of develop- 

 ment and conjugation, as van Beneden ('75) had previously described 

 for the rabbit. He also discovered that the formation of both polar 

 bodies takes place within the ovary, and mistakenly considered that 

 the female pronucleus, as well, was formed there. 



Flemming ('85) found in the rabbit some ovarian eggs which had 

 extruded the first polar body, — an observation confirmed the same 

 year by Bellonci. 



Selenka ('86) described some eggs of the opossum, ranging from 

 two- to eighteen-celled stages. They were found in the upper part 

 of the uterus. 



Heape ('86) worked on the Qg& of the European mole. He was 

 unable to follow the formation of the polar bodies, but observed 

 the two pronuclei, two- and four-celled stages, and the later cleavages 

 up to the formation of the blastula. 



Keibel ('88) has described a two-celled stage of the European 

 hedgehog's Q^Q,. 



The investigations which have yielded the most conclusive and 

 extensive results, however, have been conducted upon the ^^^ of the 

 mouse, — an qq^^ more favorable for detailed cytological study than 

 that of almost any other mammal. The names connected with this 

 work are those of Bellonci ('85), Tafani ('88, '89), Sobotta ('93, 

 '94, '95, '99), and Gerlach (:o6). 



Bellonci states that, in the mouse, the first polar spindle is similar 

 to that found in the eggs of invertebrates, and that it is formed 

 from the germinal vesicle. He also saw some ovarian eggs accom- 

 panied by the first polar body, the latter possessing a distinct mem- 

 brane and lying under the zona. 



Tafani observed both the first and second maturation spindles, 

 found first polar bodies associated with degenerating ovarian eggs, 



