W. B. KirkJiam — Maturation of the Egg of the White Mouse. 73 



When the egg of the mouse has grown to full size, the chromatin 

 is scattered through the germinal vesicle, and from this stage to the 

 prophases of the first maturation spindle no observations have been 

 made on any mammalian egg ; hence it is not known whether there 

 is a pairing of paternal and maternal chromosomes, such as occurs 

 in the invertebrates. 



First Polar Spindle (Pis. I-Il, fig^. 1-4, Text-fig. 4). — The observa- 

 tion of the prophases of the first maturation spindle is confined to 

 the egg of the mouse, a few such stages having been seen by the 

 writer (PI. I, fig. l). A small number of cases showed faint traces 

 of the nuclear membrane, but more often this had entirely disap- 



Figiire 2. — Camera drawing of a living egg soon after its discharge into the 

 Fallopian tube ; showing the two layers of the zona pellucida. The position 

 of the second polar spindle within the egg is shown by the clearer area in 

 the cytoplasm near the polar body. This egg has retained the first polar 

 body, and a space filled with fluid appears around it. x ;j30. 



peared. In this connection it is important to note that no asters 

 such as are seen in eggs of invertebrates are visible at this time. 

 The chromosomes at this stage vary greatly in size and shape, and 

 they number between 12 and 24, which is probably due to preco- 

 cious division. As shown in the figure, the germinal vesicle at the 

 end of the spireme is eccentrically placed. 



At what time the asters and centrioles make their appearance in 

 the mammalian egg is not known, and their definite origin has not 

 become well established even in the eggs of invertebrates. There 

 is, therefore, a considerable gap between PI. I, fig. 1, where there is 

 no indication of any karyokinetic figure, and PI. I, fig. 2, where a 



