W. B. Kirkham— Maturation of the Egg of the White Mouse. 81 



varies considerably in thickness in different eggs. A similar con- 

 dition prevails in the mouse, so that at the time the eggs leave the 

 ovary most of the first })olar bodies are pressed through the zona. 

 In fact, in a living Qg^ possessing a first polar body, the writer has 

 seen this polar body, while the ^gg was being stained and dehydrated 

 on the stage of the microscope, forced through the zona by the con- 

 traction of the latter under the influence of changing osmotic con- 

 ditions. 



Pronuclei. — Immediately after the extrusion of the second polar 

 body, the 12 univalent chromosomes remaining in the agg of the 

 mouse assemble to form the female pronucleus (PI. XIV), and the 

 sperm head increases in size and approaches it. The male and female 

 pronuclei now come to lie close together near the center of the egg, 

 but somewhat nearer the animal than the vegetal pole, where they 

 form the so-called cleavage nucleus. 



Both the male and female pronucleus have been seen in the ^gg 

 of the mouse by Tafani ('89), Sobotta ('95) and Gerlach (:o6) ; in 

 the rabbit's Qgg by Weil ('73\ van Beneden ('75), and Rein ('83) ; 

 in the guinea-pig by Rein ('83) and Rubaschkin (105) ; in the %gg 

 of the bat by van Beneden and Julin ('80) and van Beneden ('99) ; 

 in the mole's Qgg by Heape ('86) ; in the ^gg of Tupaja javanica 

 by Hubrecht ('96), and in the Qgg of Tarsius spectrum by Hubrecht 

 (:02). 



The later fertilization stages as well as the entire process of 

 cleavage and implantation have been described for the egg of the 

 mouse by Sobotta ('93, '94, '95) and Burckhard (:0l). 



The writer welcomes this opportunity again to acknowledge his 

 gratitude and indebtedness to Prof. Wesley R. Coe, whose encourage- 

 ment and critical knowledge have been of the very greatest assistance 

 in carrying on this work. 



Sheffield Scientific School of Yale University. 

 May, 1907. 



Ill, — Summary. 



1. Two polar bodies are apparently formed by every egg which is 

 capable of development, the first polar body appearing within the 

 ovary, the second after the entrance of the spermatozoon into the 



2. At the breaking up of the spireme the number of chromatin 

 masses is between twelve and twenty-four. 



Trans. Conn. Acad., Vol. XIII. 7 August, 1907. 



