INTRODUCTION. Xlll 



In Melosira nummuloides, M. Borrerii, and M. subflexilis, the 

 second valve of the conjugating frustule is rarely found united 

 to the mucus surrounding the sporangium, the conjugation 

 taking place only in the last frustule of the filament ; but in 

 Melosira varians and Orthosira orickalcea, conjugation taking 

 place throughout the entire filament, both valves are usually 

 found adherent to the sporangium or its surrounding mucus. 



IV. From a single frustule, as in the last method, two spo- 

 rangia are produced in the process of conjugation : this takes 

 place in Achnanthes and Bhabdonema. 



However varied these methods may appear on a superficial 

 view of the conjugating process, they will not, on a further con- 

 sideration of the nature of these organisms, be found to involve 

 any physiological diversities. 



We have already remarked, that, " during the healthy life of 

 the Diatom, self-division is being continually repeated (Introd. 

 vol. i. p. xxv) ; while the frustule is performing the simple 

 functions of its life and growth, its contents are therefore being 

 continually submitted to the separative process which precedes 

 the self-dividing act. The functions of life and growth are not 

 suspended during the act of conjugation, and in consequence 

 self-division may take place at any stage of the process which 

 accompanies the formation of the reproductive body ; or the 

 latter process may intrude upon, or arrest any step in the 

 progress of self-division. 



In the first mode of conjugation, as occurring in Epithemia, 

 &c, self-division may be regarded as in the earliest stage of its 



Amphora is represented as the result of the conjugation of two frustules ; 

 these genera must therefore be referred to my first class. The conjugation of 

 Cocconeis is described as similar to the others, but two of the figures given, 

 viz. figs. 5 & 6, harmonize the process with that noticed by myself in the 

 same genus. 



The circumstance dwelt upon by Mr. Carter as having an important bearing 

 upon the rationale of the process, viz. that one of the conjugating frustules is 

 invariably smaller than the other, is altogether at variance with my experience, 

 and is totally irreconcilable with the process as it occurs in the genera men- 

 tioned under my third and fourth classes. I am therefore disposed to believe 

 that the difference in size noticed by Mr. Carter was a mere accidental diver- 

 sity, and of no essential signification. 



