KAROYKINESIS AND ITS RELATION TO FERTILIZATION. 223 
size in spermatozoa is astonishing ; at least, I know of no single case of 
spermatozoa of different animal species being absolutely alike; and I must 
here point out that one might with advantage take into account the form 
of the spermatozoa in questions as to the definition and identification of 
species. 
In normal impregnation within the same species, the reason, according to 
the Hertwigs, why normally only one spermatozoon enters, lies in the rapid 
formation of a sort of yolk-membrane, caused by the irritation of the sperma- 
tozoon which enters first (Echinoderms, Fol, Hert wig) ; butit must be mentioned 
also that such a membrane is not everywhere formed in impregnation, therefore 
some other repelling power is present in the egg-protoplasm. If, before 
fertilization, one allows irritating or enfeebling reagents (e. g. chloral) to act 
on the egg-cell, if one cools the egg, or warms it above the normal tempera- 
ture, or if one applies chloroform, numerous spermatozoa will generally 
enter. In all super-fertilized eggs treated in this way, if, after all, develop- 
ment does take place, it is abnormal. The hypothesis is suggested by 
Fol that multiple-formations (‘‘ Mehrfachbildungen”) are connected with 
polyspermy. Hertwig (‘Lehrbuch der Entwickelungsgeschichte,’ p. 34) 
strongly urges, as a proof of this, the fact that in Lumbricus trapezoides 
studied by Kleinenburg (105), two embryos regularly develop from each 
egg-cell. However, it appears quite premature to lay down such an hypo- 
thesis for a wide range of the animal kingdom, since in many Vertebrates 
(Batrachia, Lamprey, Trout) it is to be regarded as certain, according to the 
very careful and detailed researches of Kupffer (117), that a physiological 
polyspermy exists, i. e. that numerous spermatozoa enter the egg, and their 
substance mixes with that of the egg. In lamprey, for instance, Kupffer ob- 
served the following process: first one spermatozoon with head and tail entered 
the egg; near this spermatozoon others perforated the egg-membrane, but did 
not enter into the yolk; their heads however swelled up into hyaline drop-like 
structures, which were found on the inner side of the egg-membrane, between 
it and the yolk. There now rises up from the egg-protoplasm a plug-like 
process (“cone d’attraction ” of Fol), which glides along the inner surface of 
the membrane and gathers up the hyaline sperm-drops, which then fuse with 
the substance of the plug. Nevertheless, as Kupffer expressly points out, 
there are present in all cases only two pronuclei, which conjugate to form the 
new nucleus of the fertilized egg. H. van Beneden, too, observed in the rabbit, 
that even if numerous spermatozoa were fused with the periphery of the 
yolk, yet always only one male pronucleus was formed, which then fused with 
the second—the female pronucleus—to form the “segmentation nucleus,” 
This “ segmentation nucleus ”—in Hertwig’s sense—can very easily contain, 
it appears to me, constituents of “ numerous ” spermatozoa, but also (in other 
cases) of only “one.” The process of pronuclear formation in Vertebrates is 
not well known. It is, however, quite clear that the entrance of many sper- 
matozoa in Vertebrates does not act so as to disturb the development. In 
