558 



SEXUAL GENERATION : SPERM-CELLS. 



preceding in exactly the same manner as the flowering and 

 fruiting of Plants differ from their extension and propagation 

 by leaf-buds. In all save the very lowest tribes of Animals, 

 we meet at particular seasons with two peculiar sets of cells, 

 termed sperm-cells and germ-cells ; these are sometimes borne 

 by the same individuals, which then correspond as regards 

 their reproductive apparatus with the generality of Flowering 

 Plants ; but they are more commonly separated, as in dioecious 

 Plants (VEGET. PHYSIOL., 409) ; the individual bearing the 

 "sperm-cells" being then designated as the male, and the 

 individual bearing the "germ-cells" as the female. 



731. The " sperm-cells " very closely resemble those con- 

 tained within the antheridia of Cryptogamia (VEGET. PHYSIOL. 

 399 ; BOTANY, 737, 776). When mature, each cell is 



found to contain one or more spirally 

 coiled filaments (fig. 301), which, 

 when set free by the bursting of 

 the cell, have an active spontaneous 

 movement lasting for some time like 

 ciliary action. These filaments were 

 formerly regarded as true Animal- 

 cules ; but since other examples of 

 independent movement have been 

 discovered in what are certainly 

 nothing else than detached parts of 

 the organism, and more especially 

 since moving filaments of a precisely 

 analogous character have been dis- 

 covered in Plants, all idea of their 

 independent animality has been laid 

 aside, and they are now known as 

 spermatozoids. The use of their 

 motor activity is obviously to bring 

 them into contact with the germ-cells, when both have been 

 set free from the interior of the bodies within which they 

 were formed. When the sperm-cells are developed in a 

 special or distinct organ, as happens in all save the lowest 

 types of Animal structure, this organ usually more or less 

 resembles the ordinary glands in structure ( 356, 357), and 

 is termed the Testis. 



732. The "germ-cells" are not so clearly distinguished 



Fig. 301. SPERMATOZOIDS : 

 e, immature sperm-cells. 



