172 THE EVOLUTION OF MAN. 



clear fluid, but a dense mass of exceedingly numerous cells, 

 floating about in a comparatively small quantity of fluid. 

 It is not this fluid, but the cells suspended in it, which 

 produce fertilization. In most animals, these sperm-cell* 

 are possessed of two special properties. In the first place, 

 they are extraordinarily small, usually the smallest cells in 

 the organism ; and secondly, they are possessed of a very 

 peculiar quick motion called the spermatozoid movements. 

 The form of the cells is in correlation with this movement. 

 In most animals, as also in many of the lower plants (but 

 not in the higher), each of these cells consists of a very 

 small naked cellular body, enclosing an oblong nucleus, 

 and of a long vibrating filament attached to the body of 

 the cell (Fig. 17). It was a very long time before it was 

 discovered that these structures are simple cells. In former 

 times they were universally regarded as actual animals, 

 and were called sperm-animals (Spermatozoa). It is only 

 through the searching investigations of the past few years 

 that we have acquired positive evidence of the fact that 

 each of these so-called spermatozoa is really a simple cell. 

 It is, therefore, best to call them simply seed-cells or sperm- 

 cells. In Man these possess the same form as in many 

 other Vertebrates, and in the majority of Invertebrates. 

 In many of the lower animals, however, the form of the 

 seed-cells is very different. Thus, for example, in the Cray- 

 fish, they are fixed, round cells, motionless, and furnished 

 with peculiar stiff, bristly processes. So, too, in certain 

 Worms, e.g. the Thread-worms, the sperm-cells possess a 

 very anomalous form. Some of these are amoeboid, re- 

 sembling very small egg-cells. Yet even in most of the 

 lower animals, e.g. the Sponges and the Polyps, they possess 



