HISTORY OF THE FERTILIZATION PROBLEM 25 



a short time, and is lost as an immediate consequence 

 of the fertilization reaction, (a) That the fertilizable 

 condition arises suddenly has been shown especially 

 by the work of Delage on the starfish egg and of Wilson 

 on the egg of Cerebratulus. Their experiments on 

 merogony showed that parts of the full-grown ovum 

 taken prior to the rupture of the germinal vesicle are 

 incapable of fertilization; but, soon after the rupture of 

 the germinal vesicle, parts, whether nucleated or not, 

 readily fertilize. Hertwig's observations (1877) also 

 showed a complete failure of the fertilization reaction 

 in primary ovocytes of the sea urchin before rupture 

 of the germinal vesicle, even when spermatozoa pene- 

 trated. I have observed the same thing in Chaetop- 

 terus. (b) Eggs of Platynereis lose their capacity for 

 fertilization almost immediately after coming into sea- 

 water, even though spermatozoa may penetrate (Just); 

 eggs of the frog become unfertilizable after half an hour 

 in water (Spallanzani) ; eggs of the wall-eyed pike com- 

 pletely lose their fertilizability after ten minutes in 

 water (Reighard) . Usually fertilization capacity begins 

 to fall off in one or two hours after eggs are laid in most 

 marine animals, though in some, as in the sea urchin, 

 it may persist much longer. 



2. Irreversibility. Once fertilized, eggs do not ferti- 

 lize again, nor do parts of such eggs that are freed of 

 the fertilization membrane. It should therefore be 

 impossible to superimpose parthenogenesis and ferti- 

 lization; and the studies of C. R. Moore show this to be 

 the case. Apparent superposition appears in all cases 

 to be due to incomplete reactions, which cease and 

 may be subsequently resumed. The fertilization reaction 



