CONDITIONS ASSOCIATED WITH FERTILIZATION PROCESS 



263 



MIDDLE FERTILIZATION 



PIECE ^ CONE 



\4,HEAD .M^ 



Fig. 131. Fertilization in the sea urchin, Toxopneustes variegatus. (After Wilson and 

 Mathews, 1895.) (A) Sperm head and middle piece. (B) Fertilization cone (attrac- 

 tion cone; "cone of exudation" of Fol). The fertilization cone forms after the sperm 

 head and middle piece have entered the egg, and persists through (F) when the pronuclei 

 begin to come together. (C-J) Different stages in the fusion of the pronuclei. Observe 

 that the sperm rotates at about 180° and that the sperm aster appears near base of nucleus 

 (D, E). The aster grows rapidly (F, G) as the sperm pronucleus advances toward the 

 female pronucleus, and appears between the two pronuclei in (G). In (H) the aster has 

 divided, and the daughter asters are found at either end of the two fusing pronuclei. 

 In (I, J) the two asters are at either end of the fusion nucleus. (J) Fusion nucleus 

 between the amphiaster of the first cleavage. 



vation. A somewhat similar phenomenon may also occur in other animal 

 species taking part in hybrid crosses, where some or all of the paternal chromo- 

 somes may be eliminated; activation normally occurs in these instances, and 

 development results. Gynogenesis is experimentally produced in amphibia by 

 radiating the sperm before fertilization. Development is carried on by the 

 female pronucleus in the latter instance, although it may produce larvae which 

 ultimately die. Parthenogenesis, natural and artificial, in all its essential fea- 

 tures in a sense may be regarded as gynogenesis. 



2. Androgenesis 



This form of development is experimentally produced by removal of egg 

 pronucleus with a small pipette before nuclear syngamy occurs (Porter, '39) 

 or by treating the egg with x-rays before fertilization (Whiting, '49). The 

 male pronucleus seems incapable of bringing about normal and full develop- 

 ment in amphibia, but in wasps, where the egg pronucleus has been destroyed 

 by radiation, it has been successful (Whiting, '49). 



