140 PROBLEMS OF FERTILIZATION 



O. and R. Hertwig (1887) were the first to observe this: 

 under the influence of chloral hydrate spermatozoa pen- 

 etrate unripe sea urchin eggs in large numbers, but the 

 eggs remain unchanged and the sperm heads undergo no 

 change within the egg. Under normal conditions sper- 

 matozoa do not usually penetrate into such unripe eggs. 

 The writer has observed penetration of unripe ovocytes 

 of Chaetopterus without any subsequent reaction of egg 

 or sperm. Wilson (1903) has observed for Cerebratulus, 

 and Delage (1901 a) for the starfish, that enucleated 

 portions of ova, full grown but with intact germinal 

 vesicle, will not give any fertilization reaction, but as 

 soon as the germinal vesicle has broken down similar 

 pieces readily fertilize. 



Fertilization capacity thus arises suddenly in ova. 

 The most natural working hypothesis is that this is 

 due to formation of a definite substance essential for 

 fertilization. This is a conception that we shall exam- 

 ine more fully later on. For purposes of reference we 

 shall call this (for the present) hypothetical substance 

 fertilizin. 



The fertilizable condition of the ovum is not of 

 indefinite duration. Indeed in many cases its duration 

 is exceedingly brief. The most remarkable case of this 

 kind is found in the annelid Platynereis megalops, 

 which has been very beautifully analyzed by E. E. 

 Just (1915). Fertilization is normally internal in this 

 animal and the eggs are laid as soon as fertilized. 

 Artificial insemination may be successfully performed 

 by mixing the ova and spermatozoa dry, i.e., without 

 any contact with sea- water until after insemination; 

 but sea-water may be added 5 seconds later. If, how- 



