XIV INTRODUCTION. 



progress, which merely involves the separation of the endo- 

 chrome of the parent frustules into two portions, but does not 

 include such a differentiation of these portions as renders them 

 capable of the conjugative act : the endochrome capable of con- 

 jugating with these segregated portions must be sought for in 

 other frustules ; hence the process in these genera involves the 

 presence of two parent frustules, and results in the production 

 of two sporangia. 



In the second mode, met with in Himantidium, the progress 

 of separation is arrested at a still earlier stage ; no differentiation 

 has taken place, and conjugation intervening, necessitates the 

 union of the entire contents of two parent frustules to form a 

 single sporangium. 



In the third mode, the progress of the separation of the endo- 

 chrome in the parent frustule must be considered as so far 

 advanced that complete differentiation has taken place. In 

 every respect but the formation of new valves, self-division has 

 been completed ; the incomplete frustules are therefore pre- 

 pared for conjugation, which, intervening at this stage, leads the 

 observer to believe that but one frustule has been concerned in 

 the production of the single sporangium. This we see in Melo- 

 sira and the other genera mentioned under this class. 



And lastly, self-division occurring during the progress of con- 

 jugation, the endochrome becomes segregated in the very act of 

 intermingling, and a single frustule, whose contents have been 

 already differentiated, gives rise to two sporangia, as in Ach- 

 nanthes and Rhabdonema. 



Nor is the self-dividing disposition in all cases permanently 

 arrested by the complete formation of the sporangium. Having 

 assumed the form of the parent frustules, with a great increase 

 in size (the enlargement in dimensions being in some cases due 

 to the accumulation of the contents of the two conjugating frus- 

 tules, and in others, to a rapid assimilation of nutritive material 

 from the surrounding medium), the sporangial frustule imme- 

 diately submits to self- division, and by the repetition of this 

 act developes a series of frustules equal in size to the original 

 product of the conjugating process. This is notably the case in 

 the filamentous species, as may be easily seen in Melosira, 



