268 Bacillariece 



the multiplication in the filamentous genus Melosira, Miiller 1 

 has drawn up a definite law of division. He has shown that 

 the multiplication of the cells takes place in such a manner 

 as to prevent as much as possible the division of the smallest 

 daughter-cells. This law, although indicating the prevailing 

 conditions of multiplication in Melosira, is not true of all 

 Diatoms. 



On the greatest diminution of size having been reached for 

 any one species, the maximum size of the species is regained by 

 the formation of an auxospore, and there are five methods of 

 reproduction by auxospores. 



(1) The protoplasm of a cell of the smallest size (sometimes 

 termed a ' microfrustule ') swells up and forces apart the halves 

 of the frustule, escaping to the exterior enveloped in a cellulose 

 membrane. This is the auxospore, the wall of which rapidly 

 becomes silicified and assumes the markings characteristic of the 

 species, but the form of the cell is usually very different from that 

 of the original frustule, and often most irregular. This large, 

 newly-formed cell of irregular appearance almost immediately 

 undergoes division, the individuals of each succeeding generation 

 rapidly regaining their characteristic form and elegance. Miquel 2 , 

 who has made a special study of the manner in which the maximum 

 form of a Diatom is re-established, based upon experimental 

 cultures of a number of species, states that such re-establishment 

 of the maximum size is habitually brought about by the formation 

 of this simple type of auxospore. It is merely the rejuvenescence 

 of a single cell accompanied by an increase in size. (Fig. 125 C 

 and D.) 



(2) Two auxospores may be produced by the division of the 

 contents of a single frustule. Each of the two portions of the 

 cell-contents emerges from the cell and develops as in the first 

 method. This method has only been observed in Rhabdonema 

 arcuatum (Ag.) Kiitz. and Achnanthes longipes C. Ag. 



(3) An auxospore may be formed by the conjugation of the 

 contents of two frustules. The two Diatoms become enveloped in 

 a common mucous covering, and the cell-contents emerge and fuse 

 to form a single body, which then develops into an auxospore. 



1 Miiller, ' Die Zellhaut und die Gesetze der Zelltheilungsfolge von Melosira 

 arenaria Moore.' Berlin, 1883. 



2 Miquel in Annal. de Micrographie, iv, 1892. 



