260 W. WALDEYER. 
The action of the spermatozoon on the egg-cell undoubtedly 
did not become clear immediately after the knowledge of its en- 
trance. Some regarded the action as obscurely ‘“ dynamic,” 
others as “ chemical,” but not to be determined more clearly. 
The knowledge that the penetrating spermatozoon becomes a 
nuclear structure, which has to enter into close relation with 
the nucleus of the egg-cell in order to complete fertilization, 
forms the last advance in our knowledge in this direction. 
We have above discussed the researches on this point in 
detail, and we have seen that with this advance the names of 
O. Hertwig and E. van Beneden must be especially connected. 
Both have indeed described the relation to the egg-nucleus 
differently, and therefore explain in different ways the mode of 
fertilization, so far as this can be understood at all from the 
facts at present known. O. Hertwig’s view can be described 
as the “ fusion theory,” that of van Beneden as the “ nuclear- 
replacement theory.”” In fact, O. Hertwig and his followers 
see as the essential act of fertilization, the material fusion of 
the male and female nuclear structures to form a single nucleus, 
which becomes the nucleus of the egg-cell, fertilized and more 
perfectly capable of development. 
E. van Beneden lays no stress on fusion, since in fact 
he proved in Ascaris megalocephala that in far the 
greater number of cases no fusion occurs. He states that 
fertilization is complete at the moment when a new 
nucleus (pronucleus) arises from the remnant of the germinal 
vesicle, and from the nuclear constituents of the head of the sper- 
matozoon. A fusion of these two structures is unnecessary, 
since in so very many cases it does not occur in Ascaris. Van 
Beneden is evidently quite right in declaring that fusion is not 
necessary if it does not happen in any given animal. But E. 
van Beneden goes still further in the theoretical discussion of 
fertilization, since he deals with the formation of the directive 
corpuscles. With the formation of these the original nucleus 
of the egg-cell loses a portion of its substance ; the nucleus of 
the sperm mother-cell undergoes a similar loss, so that the 
nuclear portion of the complete spermatozoon is likewise 
