234 FERTILIZATION 



egg (probably with the agglutinin factor) ; this reaction presumably agglutinates 

 the sperm to the egg's surface. 



d) An egg-surface, liquefying factor, androgamone III, has been isolated 

 by Runnstrom, et al. ('44), from sea-urchin sperm (Runnstrom, '49, p. 270). 

 A similar "sperm lysin" has been isolated also from mackerel testes. This work 

 suggests that a specific sperm lysin may be involved in the activation processes 

 within the egg cortex. (See theory of fertilization according to J. Loeb at end 

 of chapter.) 



e) Lastly, in certain animal species, substances may be present in the 

 seminal fluid which induce the spawning reaction in the female, while in the 

 egg secretion of certain species, a factor may be present which induces spawn- 

 ing in the male. 



3. Fusion of the Gametes or the Second Stage of the 

 Process of Fertilization 



The actual fusion or union phase of fertilization begins once the sperm has 

 made contact with the egg (fig. 12 IB). From this instant the rest of the fer- 

 tilization story becomes essentially an egg problem. The egg up to the time 

 of sperm contact literally has been waiting, discharging fertilizin substances 

 into the surrounding medium. However, when a sperm has made successful 

 contact with the surface of the egg, the waiting period of the egg is over, its 

 work begins, the fusion of the two gametes ensues, and the drama of a new 

 life is initiated! 



The following events of the fusion process may be listed — events which 

 occur quite synchronously, once the mechanisms involved in egg activation 

 and gametic fusion are set in motion: 



(a) The separation of an egg membrane (fertilization membrane, vitelline 

 membrane, chorion, zona pellucida, etc.) from the egg's surface and 

 the exudation of fluid-like substances from the egg's surface. 



(b) A fertilization cone may be elaborated in some species. 



(c) Changes in the physicochemical activities of the egg. 



(d) The maturation division (or divisions) is completed in most eggs. 



(e) Profound cytoplasmic movements occur in many eggs which bring 

 about various degrees of localization of cytoplasmic substances; these 

 substances orient themselves into a pattern definite for the species. 

 In some species a cytoplasmic pattern composed of future, organ- 

 forming substances is rigidly established and definitely correlated with 

 the first cleavage of the egg (Styela); in others it is less rigid (frog); 

 and in still others it appears gradually during cleavage of the egg 

 (teleost fishes). 



(f ) The sperm nucleus enlarges, and the middle-piece area in most animal 

 species develops a cleavage aster. 



