258 CASTRACANE, ON DIATOMACEjE. 



from which it apj:>ears to me that it may be concluded and 

 positively admitted beyond all doubt that in the Diatomacea 

 reproduction takes place by means of germs emitted from the 

 sporangia and sporangial frustules. And in the first place it 

 should be remarked that, whilst the existence of sporangial 

 frustules, very easily distinguishable by their unusual size, 

 can be recognised, we may at the same time note their paucity 

 in proportion to the ordinary frustules — a circumstance that 

 (if I am not wrong) appears to indicate their partial and 

 transitory scope for the elaboration of the reproductive germs. 

 Besides which Rabenhorst, in his work on the ' Freshwater 

 Diatoms,' noticed in 1853 a Melosira with sporangial frus- 

 tules, from one of which, from a lateral aperture, he witnessed 

 the escape of the germs, an occurrence of which he gives 

 a figure in pi. x. In the Sixth Volume of the ' Quart. 

 Journ. Mic. Sci.' it is stated that, at the meeting of the 

 Dublin Natural History Society on the 7th of May, 1858, 

 the excellent microscopist Mr. O'Meara read an account of 

 a circumstance which he had for the first time observed some 

 days before in a recent gathering containing Pleurosigma 

 Spencerii. In these diatoms the endochrome, instead of the 

 usual colour, was of a beautiful green, with scattered granules 

 of a bluish green. These individuals were seen to move with 

 sudden starts to the lower part of the vessel, until first one or 

 two, then others, and at last seven or eight individuals, at some 

 distance from the diatoms, were seen to be furnished at the 

 extremity with vibratile cilia moving with great activity. 

 On the following day the appearance of the frustules was 

 changed, inasmuch as but few granules were visible, and the 

 colour of the endochrome had become olive green, whilst, in- 

 stead of being disposed across the cell, it appeared collected 

 in narrow bands along the two sides of the valves. 



These two observations of Rabenhorst and of O'Meara 

 conclusively prove the formation of the germs of the Dia- 

 tomaccas in the sporangial frustules, and their exit from 

 the interior of the cell. Moreover, other instances have been 

 noticed in which numerous minute diatoms have been ob- 

 served within a cyst, a circumstance which was recorded by 

 Mr. Smith in April, 185£, in a gathering of Cocconema cistula, 

 in which instance he remarked the perfect resemblance 

 between the included frustules and the surrounding ones, 

 amongst which some of the most minute, both of those con- 

 tained in the cysts and the rest, presented every gradation in 

 dimension up to those of the adult form and in the state of 

 conjugation. Similar cysts were observed in October, 1851, 

 by Mr. Christopher Johnson, in a gathering of Synedra 



