600 MICROSCOPIC FOEMS OF VEGETABLE LIFE THALLOPHYTES 



that are produced by the process of binary division. Hence a very 

 considerable latitude is to be allowed to the limits of species, when the 

 different forms of Diatomacece 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 advan- 

 tageously occupy the attention of many a microscopist who is at 

 present devoting himself to the resolution of the markings on 

 diatom- valves, and to the multiplication of reputed species by the 

 detection of minute differences. 1 



This formation of what are termed auxospores as serving to 



augment the size of the 

 cells which are to give 

 origin to a new genera- 

 tion takes place on a 

 very different plan in 

 some of those filamentous 

 types, such as Melosira 

 (fig. 444, A, B), in which 

 a strange inequality 

 presents itself in the 

 diameters of the differ- 

 ent cells of the same 

 filament, the larger ones 

 being usually in various 

 -stages of binary sub- 

 division, by which they 

 multiply themselves 

 longitudinally. Accord- 

 ing to the observations 

 of Mr. Thwaites (loc. 

 FIG. 446. Conjugation of Epithemia turgida : A, c it.), these also are the 

 front view of single frustule ; B, side view of the v ^ f ^ j f 



same; C, two frustules with their concave surfaces Products Ol a kind of 

 in close apposition ; D, front view of one of the conjugation between the 

 wing the separation of its valves E, adjacent cells of the or- 

 dinary diameter, taking 

 place before the comple- 

 tion of their separation. He describes the endochrome of particular 

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

 as moving back from the extremities towards the centre, rapidly 

 increasing in quantity and aggregating into a zygospore (fig. 447, 

 No. 2, a, 5, c) : 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 ; and this zygospore soon undergoes binary 



1 See on this subject a valuable paper by Prof. W. Smith ' On the Determination 

 of Species in the Diatomacece ',' 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 Diatomacece,' 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 ; ' On 

 Navicula crassinervis, Frustulia saxonica, and N. rhomboides, as Test-objects,' by 

 W. H. Dallinger, Monthly Micro. Journ. 1876, vol. xvii. p. 1 ; also an Additional 

 note on the identity of these, by the same Author, ibid. p. 173. 



frustules, si 



F, side and front views after the formation of the 



zygospores. 



