194 



ELEMENTS OF BIOLOGY 



acter is a very much reduced number of eggs as compared with the 

 number shed by females whose eggs are externally fertilized. While 

 fertilization is internal in the highest animals, it cannot be said 



SPERMATOZOA 



NEMATODE 



RAT 



ANNELID 



BIRD 



HUMAN 



EGGS 



STARFISH 



FROG 



MOLLUSC HUMAN 



Fig. 133. — Some types of animal germ cells. All of the spermatozoa and the 

 human ovum have been greatly enlarged. (From preserved materials and various 

 authors.) 



that this character is an evidence of advance; for fertilization is 

 also internal in many lower forms, Arthropoda, some Annelida, 

 Nemathelminthes, and even in Platyhelminthes. As regards the 



