FACTS AND FACTORS OF DEVELOPMENT 15 



sands of times smaller than the egg. In most 

 animals, and in all vertebrates, it is an 

 elongated, thread-like cell with an enlarged 

 head which contains the nucleus, a smaller 

 middle-piece, and a very long and slender tail 

 or flagellum, by the lashing of which the sper- 

 matozoon swims forward in the jerking fashion 

 characteristic of many monads or flagellated 

 protozoa. In different species of animals the 

 spermatozoa differ more or less in size and ap- 

 pearance, and there is every reason to believe 

 that the spermatozoa of each species are pecu- 

 liar in certain respects even though we may not 

 be able to distinguish any structural differ- 

 ences under the microscope. The human sper- 

 matozoa (Fig. 2) closely resemble those of 

 other primates but are still slightly different, 

 and the conclusion is logically inevitable, as we 

 shall see later, that the spermatozoa as well as 

 the ova of each individual differ slightly from 

 those of every other individual. 



2. Fertilization. — If a spermatozoon in its 

 swimming comes into contact with a ripe but 

 unfertilized egg, the head and middle-piece of 

 the sperm sink into the egg while the tail is 



