INTRODUCTION 



21 



' Apical body or acrosome. 



Nucleus. 



• End-knob. 



• Middle-piece. 



• Envelope of the tail. 



-Axial filament. 



flagellate type (Fig. 4a), consisting of a head, largely 

 made up of chromatin, a middle piece, and a vibratile 

 tail. Spermatozoa are comparatively minute, rang- 

 ing in size from those of Amphioxiis, which are less 

 than 0.02 mm. long, to those of 

 the amphibian, Discoglossus, 

 which reach a length of 2.0 mm. 

 According to Wilson it would 

 take from 400,000 to 500,000 

 sea urchin spermatozoa to equal 

 in volume the egg of the same 

 species. It is not surprising, 

 therefore, to find that the num- 

 ber of spermatozoa produced by 

 a single male may be hundreds 

 of thousands times as great as 

 the number of eggs developed 

 in a female. Eggs are, as a rule, 

 incapable of locomotion, but 

 spermatozoa are active, swim- 

 ming about by means of their 

 tails until they reach the passive 

 eggs which they are to fertilize. 

 Since generally only one sperm- 

 atozoon fuses with an egg, it is 

 obvious that most of them never perform the function 

 for which they are specialized ; but apparently an 

 enormous number are formed to make the ferliHza- 

 tion of the eggs more certain. 



The experiments of Loeb and Bancroft (1912) on 

 spermatozoa have shown that when the living 



. End-piece. 



Fig. -ia. — Diagram of a 

 flagellate spermatozoon. 

 {From WUson, 1900.) 



