118 
J. T. CUNNINGHAM. 
studied Ascaris megalocephala, denies the existence of a 
male pronucleus; he states that the spermatozoon is entirely 
absorbed by the vitellus, and that the two pronuclei are formed 
by the division of the germinal vesicle. 
In the immense majority of cases only one spermatozoon 
penetrates into the vitellus. Having examined preparations 
which Prof. Van Beneden kindly sent me, I am convinced of 
the perfect accuracy of what he says on this point. 
In the earliest stages a spermatozoon is seen fixed by its 
head to the plug of impregnation ; in the next stage half 
immersed in the vitellus, in the next completely immersed. 
Thei’e is no uncertainty as to the identification of the sperma- 
tozoon ; its size enables it to be seen with a low power, its 
refringent body distinguishes it from all the elements of the 
vitellus. Before entering an ovum a spermatozoon fixes car- 
mine only in its nucleus, the moment it attaches itself to the 
plug of impregnation all its protoplasm is affected by the 
staining solution. Thus it is possible, without the evidence 
afforded by focussing, to distinguish immediately between a 
spermatozoon simply attached accidentally to the surface of the 
ovum and one actually in the interior of the vitellus. 
In one or two instances two spermatozoa were seen within the 
vitellus ; but the question whether both of them form male pro- 
nuclei which unite with the female pronucleus was not decided. 
The author points out that it has not yet been proved for all 
animals that fertilization is effected normally by a single sper- 
matozoon. In Echinoderms, Fol and Hertwig have shown 
that only a single spermatozoon enters the ovum. In Ascaris 
megalocephala and Petromyzon , 1 according to Calberla, the 
micropyle is closed by the spermatozoon itself. Kupffer and 
Benecke , 2 on the other hand, describe the entrance of several 
spermatozoa into the ovum of Petromyzon ; and there are 
numerous published researches to support either side of the 
controversy. But it has never been proved in the ovum of 
any animal that the formation of more than one male pronucleus 
1 Calberla, ‘ Z. f. w. Z., 5 Bd. xxx. 
1 ‘ Vorgang der Befruchtung im Ei von Petromyzon,’ Konigsberg, 1878. 
