INTRODUCTION. XXV 



the ordinary mode of increase in all sucli tissues. Certain it is, 

 that in the great class of unicelbilar Algse, it is the universal 

 mode of growth and multiplication. It, however, presents several 

 pecidiarities in the organisms with which we are now concerned, 

 and is also so readily noted and followed in all its stages in the 

 Diatomacese, that a close observance and clear comprehension 

 of the process in their case may illustrate and guide inquiry in 

 other tribes. 



The first step in the process of self-division in the Diato- 

 maceous frustule, is the fission of the internal cell, probably by 

 the doubling-in of its membranous wall, and consequently the 

 separation of the endochrome or cell-contents ; the central vesicle 

 or cytoblast also dividing into two parts, which remove to a little 

 distance from each other ; these movements being simultaneous 

 with a retrocession of the epidermal valves and the formation of 

 the siliceous connecting-membrane already described. In the 

 centre of the enlarged frustule, in exact apposition to the original 

 valves, and closely applied to them, there are now formed two 

 new valves, covering the surface of the cell-membranes along the 

 line of fission. The divided portions of the endochrome spread 

 themselves along the membrane which is embraced by the new 

 valves, and there result two half-new frustules, bound together 

 by the connecting-membrane, generated during the process we 

 have described. The figures of self-division given throughout the 

 plates will serve to illustrate this description ; and more especially 

 those in PI. VIII. fig. 59^, PI. XV. fig. 126 d, PL XXII. fig. 216 d. 



Diuring the healthy life of the Diatom, the process of self- 

 division is being continually repeated ; the two half-new frus- 

 tules at once proceed to divide again, each into two frustules, 

 and thus the process continues. I have been unable to ascer- 

 tain the time occupied in a single act of self-division ; but sup- 

 posing it to be completed in twenty-four hours, we should have, 

 as the progeny of a single frustule, the amazing number of one 

 thousand millions in a single month : a cu'cumstance which will 

 in some degree explain the sudden, or at least rapid, appearance 

 of vast numbers of these organisms, in localities where they were, 

 but a short time previously, either unrecognized, or only sparingly 

 diffused. 



