/ 4 GEXEEAL HISTOEY OF THE IXFrSOKIA. 



of the economy of nature that the same result should be obtained in the same 

 species in two different ways." 



M. Focke has satisfied himself of the reproduction of some species of Navi- 

 culce (A. iV. H. 1855, 237) by a strange complication of the phenomena of 

 ^' alternation of generation " and conjugation. Navicula hifroiis, for example, 

 forms, he says, by the spontaneous fission of its internal substance, spherical 

 bodies which, hke gemmules, give rise to Surirella microcora. These by. 

 conjugation produce N. sphndida, which gives rise to iV^. hifrons by the same 

 process. This last act of gemmation has been obsei^ed by the author in all 

 its phases. He saw two specimens of N. spJendida, enveloped in a sort of 

 mucosity, open and evacuate the whole of their contents, which serv^ed to form 

 a N. hifrons. The production of the reproductive bodies by the latter was 

 also observed; but their development into Surirella microcora, and the pro- 

 duction of N. splendida by conjugation, rest solely on the inductions of the 

 author. 



These facts require revision and confirmation, but they are, nevertheless, 

 worthy of the attention of observers, and appear to point to phenomena quite 

 as singular as those which have been revealed to us within the last few years 

 by the study of the reproduction of so many of the lower animals. They, in 

 fact, present in a manner the converse of the phenomena exhibited in the 

 ordinary alternation of generation, as several germs or eggs are necessary for 

 the production of the last individual of the cycle. 



Kiitzing has sunnised the existence of another mode of development, viz. 

 by germs or sj^ores prepared from the gonimic contents of the fmstules. This 

 method of j^ropagation was indeed comprehended in Ehrenberg' s doctrine that 

 much of the granular contents were ova ; an hj^iDothesis started rather to bring 

 the stnicture of the Diatomeae in accordance ^ith the generally assumed poiy- 

 gastric organization, than to explain any observed j^henomena, complicated as 

 it also was with other suppositions of fecundating male glands or seminal 

 vesicles and a sexual discharging orifice. 



Eabenhorst {S'lissw a sser- Diatom, p. 3) has followed up Kiitzing's suggestion, 

 and affinns that the frastules of Diatomeae swell up in a vesicular manner 

 and become filled with a greater or less number of cells, which at first have 

 an irregular figiu-e, but subsequently assume a regiilar oval shape. This 

 having happened, the cells move in a current from right to left within the 

 cavity of the parent-ceU, which by-and-by sphts open and emits its progeny, 

 each of which has, at an anterior clear space, two long projecting cilia. Por a 

 very short time these germs enjoy a swarming movement, and afterwards, on 

 becoming stationary, attain with extreme rapidity, or even sui-pass, the size of 

 the parent-cell, which is itself destroyed in the act. This plan of reproduc- 

 tion by the development of a brood of young organisms ^vithin a parent -cell, 

 or, in more technical terms, this formation of active gonidia (microgonidia), 

 prevails in many of the lower ^Ugae, and consequently has no a-priori argu- 

 ment against it. However, as Prof. Smith remarks, " Its occurrence in the 

 Diatomeae cannot be received as estabhshed without fm^ther observation and 

 a more careful record of the phenomena attending its progress " {op. cit. 

 vol. ii. p. x^ii). 



Eabenhorst has illustrated this mode of development in only one species of 

 Melosira, although he puts it foi-ward in a general manner as if tnie of aU 

 the Diatomeae. Indeed it occau\s to us that it is not a special and otherwise 

 unobseiwed process of reproduction, but merely that variety of the act of con- 

 jugation described by Mr. Thwaites in the genus Melosira, in which a change 

 in the endochrome of a single frustule, attended by an increase of contents 

 and a consequent enlargement — such as is intimated in Rabenhorst's account — 



