1912] Long: Living Eggs of Rats and Mice 125 
egg. The swelling becomes higher and at one side of the eleva- 
tion there appears a depression which is the beginning of the 
constriction which presently encircles the whole swelling and 
cuts it off from the egg. The polar cell is at first clear, as in. 
the mouse, and later becomes granular like the egg cytoplasm. 
There is at one side of the polar cell on the surface of the egg a 
temporary elevation, for which at present I have no explanation. 
While the second polar cell is at first spherical, it becomes 
irregular and distinctly amoeboid, undergoing, perhaps amitot- 
ically, division into unequal parts. It seems to degenerate, grow- 
ing smaller and more granular. 
At present nothing can be said as to the changes which the 
chromatin undergoes after the spermatozoon has penetrated the 
egg. 
The egg itself becomes more nearly spherical after the second 
polar cell is abstricted. 
The eggs remain alive and apparently normal for about 
twelve hours, when they begin to degenerate, the outlines grow- 
ing ragged, the granular contents becoming dull and contracted 
toward the center, leaving a clear peripheral zone. The causes 
are not at present determined. In some cases bacteria play a 
part. 
No reason can at present be assigned for the failure of mouse 
eggs to produce a second polar cell; nor can the precise con- 
ditions under which it is formed in rat eggs be set down. There- 
fore a further discussion at present is not desirable. 
Zoological Laboratory, University of California. 
Transmitted July 28, 1911. 
