70 GENEEAL HISTOEY OF THE INFUSOEIA. 



facts established in Fitcus by experiment of artificial separation or union of 



the sexes, to the (Edogoma, Vaucheria, Sjphceroplea and Volvox. Pringsheim's 

 declaration, that physiological questions of such a kind as the necessity of the 

 action of the fecundating matter in generation can only be certainly decided 

 by the observation of morphological processes, will not be adopted. Expe- 

 riment has long ago proved the existence of sexes in the Phanerogamia, 

 before the penetration of the pollen-tube into the ovule, and its relation to 

 the germinal vesicle, had been made out, — obseiTations which that theory 

 really no longer required for the establishment of its main question. And 

 if, among so many confirmatory experiments, a few negative results present 

 themselves, in what branch of human knowledge do we not meet with similar 

 phenomena ? The general niles of evidence hold good in such cases. 



" The same analogies, then, which lead ils to recognize a fecundation in 

 the penetration of the spermatic body of (Edogonium into the mother- cell of 

 the spore, in the mixtm^e of that body with the contracted contents of the 

 mother-cell of the spore (with Pringsheim's ' fecundation-globule '), must 

 necessarily lead us to regard conjugation as a fecundation. It is distinguished 

 from the process in (Edogonium only by the fact that the portions of cell- 

 contents which become blended into one cell are of equal size, and that there 

 is not one of them provided with apparatus by means of which, like the 

 spermatic body of (Edogonium by its cilia, it is moved onward until it reaches 

 the cell to be fecundated, — both points, evidently, of no essential importance. 

 " The sporangial frustules difi'er in general from the parent forms not merely 

 in size, but also in the number of striae or of other markings, and to some 

 slight degree in outline. Such variation, M. Thuret contends, proves the phe- 

 nomenon of conjugation to be, not a true mode of reproduction, but only ' a 

 second mode of multiplication of frustules, very curious and very abnormal.' 



*' In the immature condition, we are infoiTaed by Mr. Thwaites, it happens 

 that the sporangia in many species resemble in general characters the mature 

 frustules of another species or even of an allied genus. Thus the sporangia 

 of Gomj)lionema mimttissimum (XI. 17) and of G. dichotomum have a close 

 resemblance to the frustules of Cocconema. On the other hand, in some genera, 

 as in Cocconema, the sporangia take on at once the exact characters of the 

 ordinary frustules, from which they differ only in their exceeding that of the 

 majority of the latter in dimensions. 



^' When a sporangium in a transitional condition is like the frustule of another 

 genus, we are assisted in distinguishing its true natiure and affinity, oftentimes, 

 by the" persistence of the mucus diffused around it ; or by continued observa- 

 tion we may witness its assumption ultimately of its true specific characters, 

 including the development of its pedicle or stalk, where the possession of 

 such an organ is a characteristic (as in Gomjyhonema).^' 



The above fact suggests it as very probable that transitional forms have 

 been described as particular species, or located in wrong genera. Thus Mr. 

 Thwaites thinks that Xiitzing's Epithemia vertagus is no other than the 

 sporangium of Eunotia turgid a, and also that the enlarged frustules of the 

 Melosirece, which that same writer had conjectiu-ally regarded as reproductive 

 bodies, are in. fact the sporangial product of conjugation, and give rise to 

 a chain of frustules larger than those from which they had themselves 

 originated. 



The subsequent history of' the sporangial frustules on being matured is 

 not satisfactorily made out. Prof. Smith has the following on the question 

 (J". M. S. 1855, p. 131) : — " The ordinary Diatomaceous fmstule seems to owe 

 its production to the protoplasmic contents of the sporangial frustule formed 

 by the process of conjugation. These sporangia, like the seeds of higher 



