MATURATION AND FECUNDATION OF THE OVUM. Ill 
been made known by Rabl , 1 Hatschek , 2 Roux , 3 and 
Rauber . 4 
The eggs at the lower end of the oviduct, in the condition 
in which they receive the spermatozoon, take, when liberated 
on a slide in an indifferent liquid, the form of an ovoid. There 
is little difference between the length of the longer axis of the 
ovoid and the shorter. In the ovum a morphological axis can 
be distinguished with dissimilar poles ; this axis coincides with 
one of the shorter axes of the ovoid. At one of its poles there 
is a slight projection of the substance of the ovum; this pro- 
jection is limited by a circular outline, and it is to the centre 
of the projection that the spermatozoon attaches itself. This 
pole of the axis may therefore be named the pole of impreg- 
nation, while the opposite end is called the neutral pole. 
In order to see well the structure of the ovum it must be 
examined a little before it has reached the lower end of the 
oviduct. The slightly projecting portion may be called the 
polar disc. The portion of the vitellus beneath the disc is 
more sombre than the rest, and is called the central medulla. 
This is enveloped, interiorly, by a layer of the vitellus, a little 
more transparent, which reaches the surface of the ovum in a 
zone all round the polar disc, distinguished as the parapolar 
region. This second layer of the vitellus is the intermediate 
layer. Finally enveloping the intermediate layer in its lower 
portion is a cortical layer of vitellus, the most transparent of 
all, which extends to the surface of the ovum from the neutral 
pole to the limit of the parapolar region. This limit between 
the cortical layer and the parapolar region, is the parapolar 
circle. The germinal vesicle lies in the intermediate layer 
close to the inner surface of the cortical layer. 
When the ovum leaves the rhachis of the ovary it has the 
form of a mallet ; the extremity of the handle of the mallet is 
1 ‘ Jena. Zeit.,’ Bd. x. 
2 ‘ Arb. Zool. Inst., Wien,’ Bd. iii. 
3 ‘ Zeit. der bestimmung der hauptrichtungen des Freschembryos,’ Leipzig 
1883. 
* ‘Zool. Anz.,’ 1883, No. 147. 
