282 



THE MICROSCOPE AND ITS REVELATIONS. 



the form and markings of the parent-frustules, but grow to a very much 

 larger size, the sporangial masses having obviously a power of self-increase 

 up to the time when their envelopes are consolidated. It seems to be in 

 this way that the normal size is recovered, after the progressive diminu- 

 tion which is incident to repeated binary multiplication ( 278). Of 

 the subsequent history of the ' zygospores' much remains to be learned; 

 and it may not be the same in all cases. Appearances have been seen 

 which make it almost certain that the contents of each zygospore break- 

 up into a brood of gonidia, and that it is from these that the new genera- 

 tion 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 augment considerably in size before they obtain 

 the dimensions of the parent frustule. It is in this stage of the process 

 that the modifying 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 pro- 

 cess. Hence a very con- 

 siderable latitude is to be 

 allowed to the limits of 

 Species, when the differ- 

 ent forms of Diatomaceas 

 are compared; and here, 

 as in many other cases, 

 a most important question 

 arises to what are those 



Self -Conjugation (?) of Melosira Italica (Aulacoseira cren- limits a Question which 



ata, Thwaites): 1, simple filament; 2, filament developing -, nnwprprJ hv 



.auxospores; a, 6, c, successive stages in the formation of Can Olliy D6 ailSWeiea Dy 



auxospores; 3, auxospore-frustules, in successive stages, a, 6, gucn a care f u l study of the 



c. of multiplication. ,. , . , . , J 



entire nie-history ot every 



single type, as may advantageously occupy the attention of many a Mi- 

 'Croscopist 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 



281. 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 (Figs. 177, 178), in which a strange inequality 

 presents itself in the diameters of the different cells of the same filaments, 

 the larger ones being usually in various stages of binary subdivision, by 

 which they multiply themselves longitudinally. According to the obser- 

 vations of Mr. Thwaites (loc. cit.}, these also are the products of a kind 



1 See on this subject a valuable Paper by Prof. W. Smith ' On the Determina- 

 tion of Species in the Diatomacece, ' in the " Quart. Journ. of Microsc. Science," 



' - 



a Memoir Prof. W. Gregory ' On Shape of Outline as a spe- 

 Trans, of Microsc. Soc.," 3d Series, Vol. iii. 



Vol. iii. (1855), p. 130 



cific character of Diatomacece,' in 



<1855), p. 10; and the Author's Presidential Address in the same volume, pp 



44-50. 



