150 STRUCTURE AND ENDOWMENTS OF ANIMAL TISSUES. 



ration should be performed by the reunion of the products of two dis- 

 tinct orders of cells ; the coalescence of which produces a germ, that is 

 the starting-point of a new organism ; thus differing from the act of 

 Reproduction by gemmation or budding, which essentially consists in an 

 extension or out-growth from the original organism, by the subdivision 

 of cells in the manner already described ( 212). Among the lowest 

 Cellular Plants, in which every cell is apparently similar to the rest, 

 this operation is effected by the "conjugation" of any pair (as it would 

 seem) of the cells which have been produced by multiplication from the 

 original germ. But in all the higher organisms, both Vegetable and 

 Animal, we find that certain cells are set apart for this purpose ; and 

 that there is an obvious distinction between the " germ-cells," from 

 within which the germ ultimately makes its appearance, and the " sperm- 

 cells," which communicate to them a fertilizing influence. Still in the 

 lower tribes, both of Plants and Animals, we find that " sperm-cells" 

 and " germ-cells" are developed in the midst of the ordinary tissues of 

 the body; and it is only as we ascend the scale, and find the principle 

 of division of labour carried out in other ways, that we meet, as in Man, 

 with particular organs set apart for their evolution, and find these organs 

 appropriate respectively to distinct individuals. 



241. The spermatic cells of Man are developed within the tubuli of 

 the Testicle ; where they appear to hold exactly the same relation to 

 the membranous walls of those tubuli, as do the secreting cells to the 

 tubes and follicles of the proper Glands, being, in fact, the representa- 

 tives of their epithelial cells (Fig. 30, a). Each of these developes in its 

 interior a variable number of secondary cells, or " vesicles of evolution ;" 

 and within every one of these is produced a single thread-like body, 

 dilated at one extremity, and possessed of a remarkable self-moving 

 power, which is termed a Spermatozoa. Sometimes the vesicles of evolu- 

 tion remain inclosed within the parent-cell, until their spermatozoa have 

 been completely developed, and have been set free by their rupture (6) ; 



Fig. 30. 



Formation of Spermatozoa within seminal cells ; a, the original nucleated cell ; b, the same enlarged, with 

 the formation of the Spermatozoa in progress; c, the Spermatozoa nearly complete, but still enclosed within 

 the cell. 



and thus, when they have all performed their office, the parent-cell con- 

 tains nothing but a bundle of spermatozoa (c), whose dispersion takes 

 place as soon as its cell-wall gives way. From the very peculiar motion 

 they possess, the Spermatozoa were long regarded a.s distinct and 



