THE FERTILIZATION-PROCESS 



that, widespread though the phenomenon is, it would reveal 

 no common factor. This point of view is often maintained 

 for all animal biology, some workers going so far as to say- 

 that every animal and every egg is a law unto itself. How- 

 ever, such a point of view does not properly envisage a 

 phenomenon as widespread as fertilization. It is our pur- 

 pose to look beyond differences, to seek a common factor. 

 This can not be related to a particular stage in maturation, 

 as our classification shows. It thus becomes necessary 

 to describe briefly the process of fertilization as it occurs 

 in a representative of each of the classes in order to learn 

 if any feature is common to eggs of all classes. Said other- 

 wise, having examined the structural make-up of the 

 co-partners in the moment when they come together in the 

 act of fertilization, we address ourselves to an exposition 

 of the events that follow this coming together. At this 

 point one general word concerning the way in which egg 

 and spermatozoon meet may not be amiss. 



Eggs of animals, in which the sexes are separate, may be 

 laid before the spermatozoa reach them. In such cases 

 the eggs are shed into the sea — or fresh water — with or 

 without copulation between male and female. In other 

 cases, eggs are reached within the female's body by sperma- 

 tozoa deposited within her genital tract; here copulation 

 between the sexes is the rule. Eggs in these cases may be 

 deposited at once to undergo development outside the ani- 

 mal's body. Or they may remain within the female where 

 they undergo development completely or in part. The fore- 

 going is true also of normally hermaphroditic animals — i.e., 

 those in which the sexes are united. It should be added 

 that among these, eggs and spermatozoa produced by one 

 individual may unite, so-called self-fertilization; sometimes 

 this is brought about by the presence of some structure 

 that prevents the access of spermatozoa from another indi- 

 vidual to the eggs. In many hermaphroditic animals self- 



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