THE MORPHOLOGY OF FERTILIZATION 47 



II. EXTERNAL AND •INTERNAL FERTILIZATION 



The devices for insuring the meeting of the sexual 

 elements are numerous and varied. In general the 

 conditions must be such as to give scope for movement 

 of the sex cells toward one another. Among the Metazoa, 

 to which our account is limited, the ovum is incapable 

 of movement as a whole. The spermatozoon is motile, 

 and its activities can be maintained only in an aqueous 

 medium of suitable composition (see chap, iv); sea- 

 water is such a medium for most marine animals, and 

 the simplest conditions of union of the germ cells are 

 found in those marine animals that cast their reproductive 

 products into the sea-water, there to meet. Such are 

 most echinoderms, many annelids, tunicates, lamel- 

 libranchs, and bony fishes. These forms are the most 

 favorable for study of fertilization, for the ova, as well 

 as the spermatozoa, are produced in large quantity, and 

 the conditions and time of their union may be arbitrarily 

 determined. For these reasons many of the most 

 thorough studies in both the morphology and the physi- 

 ology of fertilization have been made on such marine 

 animals. But external fertilization is not confined to 

 marine animals; it is also found in fresh- water fishes 

 and in anurous amphibia. 



Many marine and fresh- water animals have, how- 

 ever, acquired methods of internal fertilization, the 

 ovum being fertilized within the body of the female; 

 and the same is true naturally of all terrestrial animals. 

 This involves organs of copulation, more or less complex 

 secondary sexual characters, and special forms of mating 

 behavior. In such cases the meeting of the germ cells 

 is more certainly assured, and they are produced in 



