Vlll INTRODUCTION. 



midiece, ZygnemaceG, and in other Algse, and which had already 

 been noticed and described by various writers under this sig- 

 nificant title. The facts, he observed, are thus recorded by 

 Mr. Thwaites ; and we have copied his figure to give greater 

 distinctness to his description : — " The process of conjugation 

 consists in the union of the endochrome of two approximated 

 fronds, this mixed endochrome developing around itself a proper 

 membrane, and thus becoming converted into the sporangium. 

 In a very early stage of the process, the conjugated frustules 

 have their concave surfaces in nearly close apposition (Plate A. 

 fig. 2. I.); and it may be observed that from each of these 

 surfaces two protuberances arise, which meet two similar ones 

 in the opposite frustule : these protuberances indicate the future 

 channels of communication by which the endochrome of the two 

 frustules becomes united, as well as the spot where is sub- 

 sequently developed the double sporangium, or rather the two 

 sporangia. 



" The mixed endochrome occurs at first as two irregular masses 

 between the connected (conjugating) frustules, but these masses 

 shortly become covered with a smooth cylindrical membrane — 

 the young sporangia (fig. 2. II.), which gradually increase in 

 length, retaining nearly a cylindrical form until they far exceed 

 in dimensions the parent frustules (fig. 2. III.), and at length, 

 when mature, become, like them, transversely striated upon the 

 surface (fig. 2. IV.). Around the whole structure a considerable 

 quantity of mucus has, during this time, been developed, by 

 which the empty frustules are held attached to the sporangia." 



Nothing can be more accurate than this description, which 

 would apply to almost every instance of conjugation in the 

 Diatomaceae. 



A few circumstances omitted by Mr. Thwaites, which become 

 of significance when the process is observed in other genera, 

 will be noticed hereafter ; in the mean time it is interesting to 

 remark, how exactly conformable to the above account are the 

 phenomena attending the conjugation of other species of the 

 genus Epit/iemia, viz. Epithemia Sorecc (Plate A. fig. 9), Epi- 

 i hernia gibba (Plate A. fig. 13), and Epithemia venlricosa (Plate A. 

 fig. 14). 



