POLYSPERMY II9 



the middle. Although it has been known for some years that the 

 cytoplasm of an egg alters in various ways immediately after fer- 

 tilization, it seems probable that both mechanisms, a self-pro- 

 pagating cortical change as postulated by Allen (1954), and intra- 

 cytoplasmic diffusion, occur. Whether one, both, or neither are 

 uniquely responsible for the block to polyspermy is a question 

 which still remains to be answered. Kacser (1955) has recently 

 published an interesting and detailed examination of this question. 

 The experiments of Amoroso & Parkes (1947) lend support to 

 the view that some substance in the sperm head may be concerned 

 with the establishment of the block to polyspermy. They found a 

 higher than normal incidence of polyspermy in the eggs of rabbits 

 inseminated with x-irradiated spermatozoa (>2,500 r). The 

 irradiation presumably inactivated some 'block-catalyst' in the 

 sperm head, or modified some substance which normally diffuses 

 out of the sperm head at fertilization and initiates the block to 

 polyspermy. These experiments could be repeated with advantage, 

 on a larger scale. 



Reversal of Fertilization. I referred earlier to all-or-none re- 

 actions. Biologists have often hoped that the reactions of living 

 matter would conform to this principle, but as time goes on, it is 

 found that fewer and fewer do. Fertilization used to be thought of 

 as an irreversible reaction. Once activated, the egg could not be 

 reactivated. Tyler & Schultz (1932) were the first to cast doubts on 

 this concept, when they found that fertilization could be inhibited 

 and reversed in the eggs of Urechis caiipo by treatment of the 

 fertilized eggs with acid sea water. Reversal in this species, which 

 is characterized by the egg reassuming its unfertilized appearance 

 (Plate II) in spite of containing a spermatozoon, could only be 

 achieved if the eggs were exposed to acid sea water within three 

 minutes of fertilization. When such eggs are re-inseminated, a 

 second spermatozoon penetrates and a normal block to polyspermy 

 is estabhshed. More recently, Allen (1953), using the eggs of 

 Spisula solidissima, reversed fertilization during the first four to 

 five minutes after fertilization, by putting the eggs into calcium- 

 free sea water, sea water acidified to pH 5, or sea water containing 

 o-3-o*5% ether. The most interesting experiments on this subject 

 are those of Sugiyama (195 1), using sea-urchin eggs, in particular 

 those of Hemicentrotus pulcherrimiis. Refertilization was achieved 

 by subjecting fertilized eggs to calcium- and magnesium-free sea 

 I 



