262 FERTILIZATION 



egg pronucleus enlarges and develops the amphiaster, whereas the more dis- 

 tantly located sperm asters fade and disintegrate. This fact suggests that some 

 influence from the egg pronucleus stimulates the further development of the 

 amphiaster in the sperm nearest the egg pronucleus. In experiments on in- 

 semination of egg fragments in the urodele, Triton, Fankhauser ('34) found 

 that the sperm aster in that fragment which did not contain the egg nucleus 

 failed to reach the size of the aster in the fragment containing the egg nucleus. 

 He concludes, p. 204, "The interactions between the sperm complex and the 

 cytoplasm of the egg seem, therefore, to be stimulated in the presence of the 

 egg nucleus." 



On the other hand, the experiments on androgenesis by Whiting ('49) in 

 Habrobracon, and the insemination of the "red halves" of the sea-urchin egg 

 by Harvey ('40) demonstrate that the sperm aster can, without the egg 

 pronucleus, produce the first cleavage amphiaster. However, the presence of 

 a nucleoplasmic substance in both of these cases cannot be ruled out. For 

 example, A. Brachet ('22) and Bataillon ('29), the former working on the 

 sea-urchin egg and the latter on the eggs of two amphibian species, demon- 

 strated that large, normal sperm asters and large vesicular sperm nuclei do 

 not form until after the germinal vesicle breaks down and the egg becomes 

 mature. Premature fertilization results in polyspermy, small sperm nuclei, and 

 small sperm asters. In normal fertilization, therefore, it is very probable that 

 the development of the sperm aster into a normal cleavage amphiaster is 

 dependent: 



( 1 ) upon the egg cytoplasm, and 



(2) upon some factor contributed to the egg cytoplasm by the nuclear sap 

 or from the chromosomes of the female nucleus at the time of the break- 

 down of the germinal vesicle or during the maturation divisions. 



I. Some Related Conditions of Development Associated with the 

 Fertilization Process 



1. Gynogenesis 



The word gynogenesis means "female genesis." Therefore, gynogenesis is 

 the development of the egg governed by the female pronucleus alone. The 

 male gamete may enter the egg but plays no further role (Sharp, '34, p. 406; 

 Wilson, '25, p. 460). In the nematode, Rhabdites aberrans, the egg produces 

 but one polar body, and diploidy is retained. The egg is penetrated by the 

 sperm which takes no part in later development, as it degenerates upon entering 

 the egg. 



In the above instance, it is doubtful whether or not sperm is necessary to 

 activate the egg. However, in the nematode, Rhabdites pellio, the egg is pene- 

 trated by the sperm which plays no further role in development. Nevertheless, 

 in the latter instance, sperm entrance appears to be necessary for egg acti- 



