1895.] Embryology. 771 
It appears that only a few single sperms enter the oviducts to meet 
the eggs, since when a sperm was found entering an egg no others could 
be discovered anywhere near. 
When the egg bursts out of a Graafian follicle in the ovary, it is 
accompanied by a large mass of cells of the discus proligerus that may 
continue to surround it till after fertilization. It is probable that some 
of the liquid in the capsule enveloping the ovary and mouth of the 
oviduct passes into the oviduct with the egg, for the egg is found i in a 
part of the tube distended with liquid. 
The egg of the mouse is exceedingly small, only 59 microns in ie 
ter, and is again remarkable amongst Mammalian eggs in having a 
very thin, flexible zona, only 14 microns thick. 
The polar bodies are exceptionally large, as much as 16 microns 
through. One is formed while the egg is still in the ovary, it may 
divide into two, but this was seldom seen. In fact in nine-tenths of 
the eggs observed only one polar body was formed. Without any 
other apparent difference some eggs give rise to two and some to one. 
Since the size and character of the spindle seen in the formation of the 
single polar body is the same as that seen in the second one when two 
are formed, it is inferred that most of the eggs omit the formation of 
the first polar body. In forming the polar body the egg nucleus 
changes into an achromatic spindle, of probably only 12 threads, lying 
tangentially near the surface of the egg and bearing probably 12, at 
the most 14 or 15 rod-shaped chromosomes. There is no sign of radia- 
tions in the protoplasm nor of the existence of a centrosome. This 
spindle then turns into a radial position and the chromosomes divide 
into two groups of each apparently 12 rounded chromosomes that 
move toward the ends of the spindle. One group enters the large polar 
body that is pinched off about it. When there is but one polar body 
(and is the second if there be two) there are marked thickenings of 
the achromatic threads to form conspicuous rounded bodies lying in 
the position of an equatorial plate. 
When the polar body is formed the remaining nucleus of the egg 
forms a dense mass of chromatin about the same size as the male pro- 
nucleus. This is formed from the head of a sperm that enters the egg 
and becomes a spindle-shaped, dense mass lying tangentially near the 
surface. A centrosome is now seen lying near the male pronucleus. 
Both pronuclei enlarge and exhibit remarkably large nucleoli or dense 
spherules of chromatin; there is but one of these in the male while 
there may be several in the female. Finally all differences between 
the two nuclei disappear, they lie side by side and each contains a long, 
much bent strand of chromatin apparently without a free end. 
