INTRODUCTORY 7 



joined together to form a ball, some of these colonies producing 

 ova and some spermatozoa ; the latter differ from the ova in 

 being smaller and free-swimming. In all the higher animals 

 the differentiation of the sex cells or gametes is very marked. 

 The spermatozoon is a very small microscopic organism (in man 

 it is -05 millimetre long or about 5^^ of an inch) ; in most species 

 it is formed of a compact head which is the nucleus of the cell 

 and a long slender vibratile tail. The spermatozoon propels itself 

 forward by the movements of the tail. There is generally also 



a 



/ 



9 



Fig. 3. — Ditfeient forms of spermatozoa from different species of 

 animals, as follows : — 



a^ Bat ; h and f, frog ; r/, finch ; e, ram ; f and ^, boar \ h, jelly-fish ; 

 ?', monkey ; /.-, round worm ; /, crab. (After Verworn.) (From 

 Schafer's Essentials of Histology.) 



a more or less cylindrical middle piece between the head and the 

 tail. The spermatozoon contains very little extra-nuclear proto- 

 plasm and therefore very little reserve food material. It is 

 essentially adapted for locomotion, and it is by means of its active 

 swimming movements that it comes into contact with the ovum 

 which it fertilises. Ova or eggs, on the other hand, are inert 

 bodies and contain food material, generally in the form of yolk 

 and sometimes in very great quantity. They are almost invari- 

 ably spherical. They vary in size according to the kind of animal, 

 those of a mammal being microscopic, although much larger than 

 the spermatozoa (the human ovum has a diameter of -2 millimetre 



