CLASSIFICATION 



great variation in this respect. After a minimum size is reached, however, the 

 size is restored by the formation of 

 " Auxospores." 



It has been recently discovered 

 that in some marine Diatoms (Cos- 

 cinodiscus, Cheetoceras) there may 

 be formed, by successive division of 

 the protoplast, several (8-16) rounded 

 protoplasts (Fig. 63, B), each of 

 which then secretes a new pair of 

 valves, while still^vithin the old pair. 

 As the valves in these small indi- 

 viduals are very slightly silicifled, it 

 is not improbable that they are capa- 

 ble of increase in size, unlike the 

 strongly silicified valves of most Dia- 

 toms. This formation of several young 

 Diatoms, by repeated divisions of the 

 protoplast of the mother-cell, is very 

 much like that in some of the simpler 

 Peridinese, e.g. Pyrocystis. 



Auxospores. The formation of 

 auxospores may be either non -sexual 

 or sexual. The simplest case, such 

 as that of Melosira (Fig. 60, D), con- 

 sists in the separation of the valves,* 

 so that the protoplast is set free, and 

 increases very much in size, finally 

 developing a continuous membrane 

 about itself. Within this there is first formed a single valve, like one of the 

 original ones, and soon after a second one fitting into it, thus forming a new 

 Diatom of the maximum size of the species. 



FIG. 63. Isthmia nervosa, showing cell- 

 division. Each new individual has 

 formed one new valve inside one of the 

 old ones (X200). B, Coscinodiscus 

 concinnus, showing internal cell-division 

 (X400). (B, after MURRAY.) 



FIG. 64. A, B, auxospore formation in Cocconeis placentula. (After KARSTEN.) 

 C, auxospore formation in Epithemia sp. (After PFITZER.) In Cocconeis a single 

 spore is formed by the fusion of the undivided protoplasts : in Epithemia the proto- 

 plast divides into two parts, each of which conjugates with the corresponding one 

 of the other conjugating cell. 



In other cases, e.g. Cocconeis (Fig. 64), the naked protoplast escapes from 

 two cells, which are generally enclosed in a gelatinous envelope, and the two fuse 



