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 often differ in size and appear- 

 ance, and there is every reason to believe that 

 the spermatozoa of each species are peculiar in 

 certain respects even though we may not be 

 able to distinguish any structural differences 

 under the microscope. The human sperma- 

 tozoa (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 



