300 DESCRIPTION OF \Vi)lijgasirica 



thrown off and disappears ; in othei's, it reuiaius for sonic time 

 linking the frustulcs in pairs, as in Mdosira and Odontclla ; and 

 sometimes it is only partially torn away or absorbed, and unites the 

 fi'ustules successively formed in a zig-zag chain, by portions re- 

 maining attached to their angles, examples of which we find in 

 Biatoma, Isthnia, &c. (Ann. Nat. Hist. 1851 p. 4.) 



" There appears, however," says Mr. Thwaitcs, *' a limit to this 

 mode of propagation of the finistule, except by the intcr-s-ention of 

 another phenomenon — viz., conjugation, or a mixture of endo- 

 chromcs; after which process, fissiparous division proceeds as before." 

 (Ann. Nat. Hist. 1848, p. 161.) 



Moreover, self-division is an act only of muUvplicaUor>i physiologi- 

 cally speaking, for all the fnistules so produced, are but parts of the 

 original individual producing them, — of that one which derived its 

 individuality from a sporangium, the result of conjugation, which 

 is a process of actual propagation. 



The elucidation of this w^onderful phenomenon of conjugation in 

 the Diatomacem, we owe* to Mr. Thwaites ; from whose papers we 

 shall endeavour to give a condensed account of it. 



For the most part, conjugation in the Diatomacea, as in the 

 Desmidiem, consists in the union of the endochrome of two approxi- 

 mated 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, as in 

 JEunotia turgida, have their concave surfaces in nearly close apposi- 

 tion (P. 14, f. 1) and from each of these surfaces two protuberances 

 arise, which meet two similar ones in the opposite frnstule (P. 14, 

 f. 3) ; these protuberances indicate the future channels of com- 

 munication by which the endochrome of the two fnistules becomes 

 united, as wx^ll as the spot where is subsequently developed the 

 double sporangium, or rather the two sporangia. A front view of 

 two frustules at the same period, shows each of these to have 

 divided longitudinally into two halves (P. 14, f. 4.), which, though 

 some distance apart, are still held together by a very delicate mem- 

 brane ; that, however, soon disappears. 



The mixed endochrome occurs, at first, as two irregular masses 

 between the connected frustules, but these masses shortly become 



