THE MORPHOLOGY OF FERTILIZATION 59 



though the former view appears to be more probable. 

 In Ascaris megalocephala a very thick and resistant 

 fertilization membrane is formed as an immediate 

 result of fertilization (Fig. 6). 



These changes have no doubt some common physi- 

 ological basis. Their special features may, however, 



FIG. 6. Entrance of the spermatozoon and formation of the fertili- 

 zation membrane in Ascaris megalocephala: a, The entire spermatozoon 

 within the egg; central germinal vesicle with tetrads; the egg is mem- 

 braneless. b, The spermatozoon has reached the center of the egg and 

 its cytoplasmic parts are disintegrating. First maturation spindle near 

 the surface. A thick fertilization membrane has been formed, separated 

 from the egg by a narrow perivitelline space. 



also be adaptive in other senses. Thus the thick resist- 

 ant fertilization membrane of Ascaris protects against 

 the digestive juices of the host, the horse, and the rotation 

 of the frog's egg rendered possible by the perivitelline 

 space, equalizes internal strain due to differences in 

 specific gravity of parts of the egg, and is quite essential 

 to normal development. 



