Seg 
of the Ovum. 
479 
bit, 8, 9, 10, Kupffer on rodents, 29, Selenka on rodents, 
44, 45, 46, and oppossums, 47, van Beneden and Julin on 
bats, 13, Heape on moles, 23, Tafani on white mice, 48. 
The ovum, when discharged from the ovary, is surrounded 
by the corona radiata, which is lost when impregnation takes 
place. Segmentation begins when the ovum is one-half to 
two-thirds of the way through the oviduct. The ovum 
spends about 70 hours in the oviduct in the rabbit, and about 
eight days in the dog. The first cleavage plane passes 
through the axis of the ovum which is marked by the polar 
globules. When first formed, the two segmentation spheres 
are oval and entirely separated from one another, but subse- 
flatten against one another and become 
emarkable phenomenon of which we possess 
I whatever. The second cleavage plane is also 
quently the 
appressed — a 
no explanati( 
meridional. 
The 
nd then 
xt divides into eight 
segments, of which four are larger than the rest. 
The succeeding cleavages have never been followed accu- 
rately, but from Heape's observations on the mole, 28, 166, 
we know that the divisions progress with great irregularity, 
and is probable that the commonly assumed regularity of 
mammalian segmentation does not exist in nature. After a 
time (in the rabbit 
about 70 hours) there 
is reached the stage 
termed Metagastrula 
by van Beneden, 10, 
153-160, in accordance 
with his view of the 
homologies of this 
stage. The metagas- 
trula consists of a 
single layer of cubo- 
idal hyaline cells lying 
close against the zona 
pellucida 
the space 
layer cont 
vithii 
thij 
