SPERMATOZOA. 



173 



the "pin-shaped form" which occurs in Man and other 

 Mammals (Fig. 17). 



FIG. 17. Seed-cells or sperm-cells from the semen of various Mammals. 

 The broad side of the flattened, pear-shaped nucleus portion of the sperm- 

 cell (the so-called " head of the sperm-animalcule") is represented in the 

 drawings marked I; the narrow side in those marked II : k, kernel of the 

 sperm-cell; m, central portion (protoplasm); s, active tail-like process 

 (whip) ; N, four human sperm.cells ; A, two sperm-cells of the ape ; K, of 

 the rabbit; H, of the common mouse ; 6', of the dog ; S, of the pig. 



In 1677, when the Dutch naturalist, Leeuwenhoek, first 

 discovered these filamentous and very active tiny bodies 

 in the human semen, they were generally supposed to be 

 distinct, independent animalcules, resembling Infusoria, and 

 they were at once named " seminal animalcules." As we 

 have already observed, they played an important part 

 in the erroneous theory of preformation which was then 

 prevalent, according to which the whole of the developed 

 organism with all its parts exists preformed, though very 

 small and as yet unexpanded, in each seminal animalcule. 

 (See p. 36.) These animalcules had only to penetrate 



