50 MICROSCOPICAL OBJECTS FOUND 



tween the Coscinodisci, &c., and the Diatomaceae. 

 Ehrenberg considers that the former do not 

 occur in chains, though they exhibit the peculiar 

 phenomenon of self-division, so common in the 

 lower tribes of both plants and animals ; a double 

 septum being formed in the interior of the sili- 

 ceous cell or frustule, which encloses the soft 

 colouring matter, so that each half thus becomes 

 an independent organism. The general defi- 

 nition of this curious mode of increase, as given 

 by Ehrenberg, is that " Two individual bodies 

 originate from one individual body, each of 

 which two, possesses and actually is the half 

 of the other, which half perfects itself to its 

 separate and closed individuality. This comple- 

 tion is effected by an internal activity allied to 

 regeneration called into activity, hy the mere 

 tension of the parts."'* The former part of this 

 definition appears to be in accordance with the 

 observed facts, and, as far as the soft structures 

 are concerned, the latter also ; but that it can 

 apply to those, whose hard, fragile and unyielding 

 siliceous cases present anything but a plastic 

 material, capable of tension, is very question- 

 able. It appears more probable that the soft 



* Taylor's Scientific Memors. Vol. iii. Art. 10. 



