MATURATION AND FECUNDATION OF THE OVUM. 117 
stage. The interior of the tail is formed by a prolongation of 
the cortical layer of the protoplasm of the head ; in this internal 
protoplasm at the campanuiiform stage the refringent body 
begins to develop ; it is at first linear. At the lower end of 
the refringent body is a plate of homogeneous substance still 
more refringent, called the limiting plate. Finally, by the 
increase in thickness of the refringent body the shape of the 
spermatozoon becomes conoid. It is at this stage that the 
membrane becomes distinctly visible. The head of the sper- 
matozoon can throw out amoeboid processes. The head 
uncovered by membrane corresponds to the plug of impregna- 
tion in the ovum, while the external protoplasm of the tail 
corresponds to the cortical layer of the vitellus. The sperma- 
tozoon can effect fertilization either at the pyriform, campanu- 
iiform, or conoid stage (see Plate X, fig. 3). 
II. — Penetration of the Spermatozoon or 
Copulation of the Sexual Elements. 
The penetration of a spermatozoon into the vitellus does not 
in itself, as is now well known, constitute fecundation. In 
many ova, as in the present case, a great many processes take 
place after the entrance of the spermatozoon, before the for- 
mation of the segmentation nucleus. In other cases the 
period between the two events is shorter, the polar globules 
being ejected before the spermatozoon enters. Van Beneden, 
therefore, distinguishes the two events as copulation of the 
sexual elements, and actual fecundation. 
Notwithstanding the attention which has been paid to the 
phenomena of fecundation recently, these researches of Van 
Beneden are the first which have traced the history of the 
spermatozoon in a Nematode ovum. Biitschli 1 and Auerbach 1 
were the last investigators who concerned themselves with the 
reproduction of Nematodes, and they obtained no information 
as to the fate of the spermatozoon after its contact with the 
ovum. So recently as 1883, A. Schneider, 1 who specially 
1 For the earlier literature, vide Balfour, ‘Comp. Emb.,’ vol. i. 
