78 PROBLEMS OF FERTILIZATION 



I. Physiological polyspermy. — Physiological poly- 

 spermy occurs in vertebrates possessing eggs of large 

 size devoid of a strong membrane and micropyle, in 

 which penetration of the spermatozoon may occur at 

 any spot within a large area. We may conceive in 

 such cases that the protective mechanism against 

 penetration of supernumerary spermatozoa, which be- 

 gins to form at the point of penetration and spreads, 

 does not extend itself with sufficient rapidity to protect 

 the entire fertilizable surface from other spermatozoa. 

 The large eggs of sharks, of some amphibia, of reptiles, 

 and of birds are thus polyspermic. Polyspermy occurs 

 in the eggs of several classes of insects which possess 

 several micropyles (Henking, 1891). Among animals 

 possessing small eggs it occurs apparently only in Bry- 

 ozoa, in which the spermatozoa are united in bundles 

 (Bonnevie, 1907). 



In all cases of normal polyspermy only one of the 

 sperm nuclei formed from the entering sperm heads 

 unites with the egg nucleus, and the supernumerary 

 sperm nuclei are disposed of in certain ways. Thus 

 the fertilization in such cases is finally monospermic. 

 In the fertilization of the pigeon, for instance, from 

 about twelve to twenty-five spermatozoa enter the ger- 

 minal disk as soon as the ovum is released from the ovary 

 (Harper, 1904). The second maturation division occurs 

 after this, and during this time the sperm heads accu- 

 mulate in a ring of protoplasm surrounding the matura- 

 tion spindle at some distance from it (Fig. 12). After 

 completion of this division and formation of the egg 

 nucleus one of the sperm nuclei moves centrally and 

 unites in ' the usual way with the egg nucleus, while 



