THE PHYSIOLOGY OF FERTILIZATION 131 



It is usual to regard penetration of the spermatozoon 

 as synonymous with fertilization, but we may have 

 fertilization well begun after mere attachment of the 

 spermatozoon and without any penetration, as in 

 Nereis; on the other hand we may have penetration 

 without any fertilization, if the egg is not in the proper 

 condition. A complete fertilization reaction involves 

 penetration, 'which thus furnishes one of the problems 

 of fertilization; but it is not fertilization in itself. 



H. THE FERTILIZABLE CONDITION OF THE GAMETES 



The fertilization reactions are possible only during a 

 definite limited time in the life of the gametes. This 

 has usually been attributed to rapid aging after the 

 fertilizable condition is once attained, but a careful 

 examination of the loss of fertilizing power may enable 

 us to define this problem more exactly. 



i. The spermatozoon. Spermatozoa attain fertilizing 

 power after the completion of histogenesis and the 

 attainment of full motility. Unripe spermatozoa will 

 not fertilize. After ripening, spermatozoa retain ferti- 

 lizing power for variable periods depending on the 

 species and on the conditions. Within the gonads or 

 in the seminal ducts spermatozoa are motionless and 

 may remain in good condition apparently indefinitely 

 until ejaculated. In the case of animals with internal 

 fertilization the life of spermatozoa within the genital 

 tract of the female may also be considerable. In birds 

 (fowl) spermatozoa may retain power for two or three 

 weeks. In most mammals the period is shorter; but 

 in bats, for instance, copulation occurs in the fall, and 

 ovulation and fertilization are delayed until the spring. 



