MATURATION AND FECUNDATION OF THE OVUM. 119 
is a normal occurrence. Fol has shown that in Asterias two 
male pronuclei are sometimes formed, and give rise to ab- 
normal segmentation and abnormal larvae. 
In prepared eggs of Ascaris megalocephala, in which a 
spermatozoon is attached to the plug of impregnation, a space 
is often seen between the vitelline membrane and the vitellus 
in the region round the micropvle. This lifting of the mem- 
brane never occurs at later stages, or at other parts of the 
vitellus. The author discusses very fully the literature on the 
subject of micropyles, pointing out that in very few cases has 
it been proved that the micropyle, when it exists, forms the 
exclusive means of entrance for the spermatozoon. For the 
bibliography of this subject the reader may refer to Van 
Beneden’s work, and to Balfour, ‘ Comp. Embryology/ vol. i. 
Van Beneden is inclined to believe that every ovum has a 
morphological axis, at one end of which a single spermatozoon 
enters ; but he does not maintain that this has yet been 
proved. 
After the spermatozoon has fixed itself to the plug of im- 
pregnation (the two elements uniting by their homologous 
poles), the latter descends towards the interior of the vitellus, 
taking the spermatozoon with it. The head of the spermatozoon 
executes amoeboid movements, which aid its penetration. When 
the head has passed the micropyle, the membrane covering 
the tail unites with the vitelline membrane so as to close the 
aperture ; and the complete membrane is called the oosper- 
matic envelope. As the spermatozoon advances into the in- 
terior of the vitellus the membrane follows it, so that when it 
is entirely immersed there is a very slight prominence to show 
where it entered. After the spermatozoon has entered, its 
protoplasm becomes less granular ; its nucleus is less refrin- 
gent and less deeply stained. Apparently some of the chro- 
matin of the nucleus diffuses through the protoplasm of the 
head. 
After the spermatozoon is completely immersed it is impos- 
sible to distinguish either plug of impregnation or polar disc. 
The penetration of the spermatozoon is effected, at least in part. 
