62 GENERAL HISTORY OF THE INFUSORIA. 



matm-e, become, like them, transversely striated upon the surface (XI. 8). 

 Around the whole structure a considerable quantity of mucus has, during 

 this time, been developed, by which the empty frastiiles are held attached to 

 the sporangia (XI. 5-8).'* 



The variations in the process are aUuded to in the follo^'ing extracts from 

 the same eminent observer's papers : — " In different genera, slight variations 

 are met with in the method of conjugation : thus, in some species of Gom- 

 phonema the sporangia lie in a direction parallel to the empty frustules, 

 instead of across them, as described in Eunotia turgida. Again, there are 

 examples (in Gomjphonema minutissimmn and Fragilaria pectinalis) where, 

 instead of the conjugated frustules separating into two halves, only a slit 

 appears at one end, to serve for the escape of the endochrome. Instead also 

 of the pail' of conjugated frastules producing between them two sporangia, 

 they may develope but a single one, as happens in Fmgilaria pectinalis. In 

 this species, too, the sporangium, at first cylindrical, soon assumes a flattened, 

 somewhat quadrangular form, and in many cases undergoes fissiparous divi- 

 sion before it has put on the exact appearance of the frustule of a Fragilaria. 



"The Melosirece (GaUionellce, Ehr.) and the Biddulphice,^^ Mr. Thwaites 

 remarks, " would seem, in their development of sporangia, to offer an excep- 

 tion to most Diatomeae ; for in those genera no evident conjugation has been 

 seen. However, something analogous to it must take place ; for, excepting 

 the mixture of endochromes of two cells, the phenomena are of precisely 

 similar character. Thus, instead of the conjugation of two fi-ustules (XV. 

 29, a, h, c, d, 32, 33), a change takes place in the endochrome of a single 

 frustule, — that is, a disturbance of its pre\ious arrangement, a moving 

 towards the centre of the frustule, and a rapid increase in its quantity: 

 subsequently to this it becomes a sporangium ; and out of this are developed 

 sporangial frustules, as in the other Diatomeae. In a single cell, therefore, 

 a process physiologically precisely similar to that occurring between two 

 conjugating cells takes place ; and it is not difficult to believe, taking into 

 view the secondary character of ceU-membrane, that the two kinds of endo- 

 chrome may be developed at the opposite ends of one frustule, as easily as in 

 two contiguous frustules, and give rise to the same phenomena as ordinary 

 conjugation." 



Fui*ther, in his notes on Schizonema suhcohcerens, Mr. Thwaites writes, — 

 " The sporangia of this species are produced by the conjugation of a pair of 

 frustules outside the filaments ; but sporangial frustules are frequently found 

 in a filament intermixed with ordinaiy frustules, from which they differ only 

 in size." - 



Dr. Giiffith and !Mr. Carter, moreover, have portrayed peculiarities in 

 the conjugating process, which Prof. Smith can neither explain nor confii'm, 

 and is equally unable to reduce under either of the leading variations he has 

 defined. The first-named natui-alist stated that in the conjugation of a 

 species of Navicula (amphirhynchus ?) a silicious sheath enveloped the spo- 

 rangial frustule, indestructible by heat and nitric acid. " It is," he writes, 

 " colourless, elongate, rounded at the ends, and furnished with coarse trans- 

 vei-se striae or depressions, through which the line of fracture runs when the 

 object is crushed." This account seems to Prof. Smith erroneous ; and he 

 suggests that this sheath " may probably have been an appearance resulting 

 from the condensation and corrugation of the mucus developed around the 

 reproductive body." This conclusion Dr. Griffiths declares untenable, since 

 no kind of mucus wiU resist the action of a red heat and nitric acid. The 

 specimen examined was, besides, not an isolated one, but hundreds such were 

 present" (A. N. H. xvi. 92). 



