PRODUCTION OF SPORANGIA IN DIATOMACEjE. 287 



another filament (as is the case with the filamentous Desmidiacece, 

 § 206, and usually but not invariably with the Zygnemacece, 

 § 251), conjugate with each other ; and this may take-place even 

 before they have been completely separated by self- division. Thus 

 in Melosira (§ 227) and its allies, the endochrome of particular 

 frustules, after separating as if for the formation of a pair of new 

 cells, moves-back from the extremities towards the centre, rapidly 

 increasing in quantity and aggregating into a Sporangial mass 

 (Fig. 123, 2, a, £>, c) ; and around this a new envelope is developed, 

 which may or may not resemble that of the ordinary frustules, but 

 which remains in continuity with them, giving rise to a strange 

 inequality in the size of the different parts of the filaments (Figs. 

 131, 132). 



220. Of the subsequent history of the Sporangial frustule, much 

 remains to be learned ; and it is probably not the same in all cases. It 

 has been already shown that the sporangial frustule, even where it 

 precisely resembles its parent in form and marking, greatly exceeds 

 it in size ; and this excess seems to render it improbable that it 

 should reproduce the race by ordinary self -division. Appearances 

 have been seen which make it probable that the contents of each 

 sporangial frustule break-up into a brood of Gronidia, and that it is 

 from these that the new generation originates. These gonidia, if 

 each be surrounded (as in many other cases) by a distinct cyst, may 

 remain undeveloped for a considerable period ; and they must aug- 

 ment considerably in size, before they attain the dimensions of the 

 parent frustule. — It is in this stage of the process that the modi- 

 fying influence of external agencies is most Likely to exert its effects ; 

 and it may be easily conceived that (as in higher Plants and 

 Animals) this influence may give rise to various diversities among 

 the respective individuals of the same brood ; which diversities (as we 

 have seen) will be transmitted to all the repetitions of each, that are 

 produced by the self -dividing process. Hence a very considerable 

 latitude is to be allowed to the limits of Species, when the different 

 forms of Diatomacese are compared ; and here, as in many other 

 cases, a most important question arises as to what are those limits, 

 — a question which can only be answered by such a careful study of 

 the entire life-history of every single type, as may advantageously 

 occupy the attention of many a Microscopist who is at present de- 

 voting himself to the mere detection of differences and to the 

 multiplication of reputed species. * * 



221. Most of the Diatoms which are not fixed by a stipes possess 

 some power of Spontaneous Movement ; and this is especially seen 

 in those whose frustules are of a long narrow form, such as that of 



* See on this subject a valuable paper by Prof. "W. Smith 'On the 

 Determination of Species in the Diatomacea;,' in the "Quart. Journ. of 

 Microsc. Science," Vol. iii. (1855), p. 130 ; a Memoir by Prof. W. Gregory 

 ' On shape of Outline as a specific character of Diatomacea,' in " Trans, 

 of Microsc. Soc," 2nd Series, Vol. iii. (1855), p. 10; and the Author's 

 Presidential Address in the same volume, pp. 44-50. 



