CHAPTER 9 



POLYSPERMY 



After fertilization the haploid egg nucleus normally fuses with a 

 single haploid sperm nucleus : in the rare event of it uniting with 

 more than one sperm nucleus, development is almost invariably 

 abnormal. Two mechanisms exist to prevent polyandrous syn- 

 gamy of egg and sperm nuclei. One of these, Type I Inhibition, 

 prevents all but one spermatozoon from entering the egg. In the 

 second mechanism. Type II Inhibition, several spermatozoa 

 enter the egg but only one sperm nucleus unites with the egg 

 nucleus. 



Type II Inhibition of Polyspermy. The eggs of selachians 

 (Riickert, 1899), urodeles (Jordan, 1893), reptiles (Oppel, 1892), 

 Polyzoa (Bryozoa) (Bonnevie, 1907), birds (Blount, 1909; Hamil- 

 ton, 1952), some molluscs (Bretschneider, 1948), and many insects 

 (Richards & Miller, 1937) are normally polyspermic in the sense 

 that several spermatozoa enter the egg at fertilization. Only one 

 of these fuses with the female nucleus, the remainder degenerating 

 near the surface of the egg, though abortive divisions of the super- 

 numerary sperm nuclei sometimes occur. Fankhauser (1925-1948) 

 has made a most interesting study of polyspermy in the eggs 

 of Triturus helveticus (Razoumowsky) and of Diemictylus viri- 

 descens (Rafinesque), in which several spermatozoa usually enter 

 the egg after insemination. In spite of polyspermy, cleavage is 

 normal and monospermic, provided less than ten spermatozoa 

 enter the egg, although for three hours after fertilization the egg 

 appears to be typically and pathologically polyspermic. At the 

 time when the sperm nucleus (the one which happens to be nearest 

 the egg nucleus after maturation) fuses with the egg nucleus, the 

 supernumerary sperm nuclei, which may have proceeded as far as 

 prophase or even to the release of chromosomes (particularly in 

 Diemictylus viridescens), begin to degenerate, those nearest the fusion 

 nucleus degenerating first. If, after fertilization, an egg is ligatured 

 so that one half contains the egg nucleus and sperm nuclei, the 

 half in question develops normally. The other half, which con- 

 tains sperm nuclei but no egg nucleus, cleaves as frequently as a 



H 103 



