Facts and Factors of Development n 



. T.. M H 



FIG. 3. Two HUMAN SPERMATOZOA. A, showing the side of the flattened 

 head; B, its edge; H, head; M, middle-piece; T, tail. (After G. Retzius.) 



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 spermatozoon swims forwards 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 appearance, and there is every reason to 

 believe that the spermatozoa of each species are peculiar in cer- 

 tain respects even though we may not be able to distinguish any 

 structural difference under the microscope. The human sperma- 

 tozoa (Fig. 3) 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 usually broken off 

 and left outside (Fig. 4). About the'time of the entrance of the 

 spermatozoon into the egg the latter divides twice, giving off two 

 minute cells known as polar bodies which lie at the upper or 

 animal pole of the egg. The nucleus in the head of the sperm, 

 after it has entered the egg, begins to absorb material from the 

 egg and to grow in size and at the same time a minute granule, the 

 centrosome, appears, either from the middle-piece or from the 

 head of the sperm, and radiating lines run out from the centro- 

 some into the substance of the egg. The sperm nucleus and cen- 



