270 



FERTILIZATION 



Fig. 138. Polyspermy in the European newt, Triton, f After Fankhauser, '48.) (A) 

 Ten minutes after insemination at 23° C. Metaphase of second maturation division; four 

 sperm have entered the egg, one of which is at the vegetal pole of the egg, and another 

 between the two poles of the egg. (B) One hour and 30 minutes; second polar body 

 given off; small egg pronucleus moves toward nearest sperm nucleus. The latter will 

 become the principal sperm nucleus. Observe that accessory sperm nuclei are enlarging 

 and a sperm aster is developed relative to each. (C) Two hours and 30 minutes. Egg 

 and principal sperm pronuclei in contact; maximum development of sperm asters. (D) 

 Three hours. Fusion of egg pronucleus and principal sperm pronucleus. Accessory sperm 

 nucleus nearest to fusion nucleus shows signs of degeneration. Accessory sperm asters 

 remain undivided, while principal sperm aster has formed an amphiaster. (E) Three 

 hours and 30 minutes. Metaphase of first cleavage; all accessory sperm nuclei degener- 

 ating. (F) Four hours. Early telophase of first cleavage; remnant of accessory nuclei 

 being pushed out of animal pole region by amphiastei and spindle of first cleavage division. 



fertilizin molecule plus the sperm then have the ability to unite with an egg 

 receptor, and the union of the fertilizin-sperm complex with the egg receptor, 

 releases the activating principle within the egg, which spreads "with extreme 

 rapidity" around the egg cortex. The activating principle activates the egg as 

 a whole, setting it in motion toward development. It is thought to work espe- 

 cially upon the cortex of the egg, producing cortical changes, including the 

 formation of a fertilization membrane. Further, it agglutinates or immobilizes 

 all other sperm around the egg. Consequently, polyspermy may be hindered 

 by this agglutination effect and by the fertilization membrane. In regard to 

 polyspermy, Lillie also postulated another substance, antifertilizin, within the 

 egg which unites with the remaining fertilizin molecules in the egg the instant 

 that one sperm has made successful union with a molecule of fertilizin, thus 

 preventing other sperm from entering the egg. 



For discussion, see J. Brachet, '50, p. 143; Dalcq, '28. 



