i 885 .J Fecundation of Animals. 2I 
In 1875 F. van Beneden found that the observations 
which Butschli had made on Nematoids hold good also as 
regards mammals. He observed, further, that the two 
nuclear elements possess different properties. The one 
makes its appearance near the surface of the ovum at the 
moment when the other already occupies the centre of the 
yolk. Both germs, as they approximate to each other, in- 
crease in size. Even after the union of both elements in 
the centre of the yolk, the nucleus which has been formed 
at the circumference possesses attributes which serve to 
identify it with the essential constituent of the spermatozoa. 
Hence the pronuclei may be distinguished as a male pro- 
nucleus derived from the periphery of the ovum, and a female 
formed in the yolk. 
Soon afterwards there appeared the important investiga- 
tions of O. Hertwig, in which it is proved that the sperm- 
nucleus is evolved from the spermatozoon, and the germ- 
nucleus from the yolk, which then unite in the well-known 
manner. Fol subsequently observed how the spermatozoon 
penetrates into the yolk. This faCt— the penetration of the 
spermatozoon and its share in the formation of the male 
pronucleus — has been verified by all succeeding investigators. 
Observers differed among themselves only in the manner in 
which this penetration occurs, and on the part which it 
plays in the formation of the male pronucleus. Broadly 
speaking, the views of Hertwig and Fol— that the sperma- 
tozoon finds entrance into the yolk, that a portion of it 
becomes the sperm-nucleus which combines with the ovum- 
nucleus to form the segmentation-nucleus — were universally 
acknowledged. On the minor details, such as the formation 
of the fibrillar spindle and the stellar bodies or polar vesicles, 
their signification and destiny, the manner of the penetration 
of the spermatozoon and its part in the formation of the 
sperm-nucleus, there still prevailed differences of opinion. 
This posture of affairs induced Beneden to resume the in- 
vestigation of the process of fecundation. For his observa- 
tions he selected Ascaris megalocephala , the most suitable 
subject at once on account of its size and the simplicity of 
its organisation. 
In his monograph detailing the results of his investiga- 
tions, after a preliminary account of the present state of the 
question and of the methods employed, he discusses the 
changes which the ovum undergoes during the first period 
of ripeness. He then describes the entrance of the sperma- 
tozoon into the ovum, or the conjunction of the sexual 
products ; he considers the second period of the maturity of 
