INTRODUCTION. XV11 



cannot be explained in accordance with the ascertained phe- 

 nomena which characterize embryonic development in its earliest 

 stages in all the simpler forms of Thallogens and Protophyta. 



Another mode of reproduction, or rather a modification of 

 the process I have detailed, has been briefly described and im- 

 perfectly figured by Rabenhorst (Stisswasser Diatomaceen, p. 3. 

 taf. x. Suppl. P. 18), who represents the contents of the spo- 

 rangium of a Melosira as being converted into active " gonidia," 

 and escaping from the cell in the same manner as the zoospores 

 of Sjoharoplea from the cells of its filament It is possible that 

 such a process may take place, this method of development 

 being far from unfrequent in the lower Algae ; its occurrence in 

 the Diatomacee cannot, however, be received as established 

 without further observation, and a more careful record of the 

 phenomena attending its progress. 



Section IX. 



On the Nature of the Diatomace^e. 



It will be seen from the observations already made, that the 

 opinion of the writer of the present work accords with that of 

 the generality of authors who have treated of the Diatomacese, 

 and leads him to place these forms in the ranks of the vege- 

 table kingdom. Some of the earlier writers upon the sub- 

 ject, misled by superficial and obvious characteristics, placed 

 these organisms among animals, and, influenced by this idea of 

 their nature, persuaded themselves of the presence in the Diato- 

 macee of prehensory, digestive, and locomotive organs. The 

 careful and unbiassed researches of later observers, aided by 

 microscopical appliances of a more trustworthy kind, have satis- 

 factorily demonstrated that the simplicity of function which 

 belongs to a Diatom is wholly irreconcileable with the existence 

 of such organs as a mouth, stomach, or feet ; and the more accu- 

 rate the examination, and the more complete the instrument of 

 observation, the more certain is the persuasion that the Diato- 

 maceous frustule is a single cell, homologous with the same 

 organisms in the Desmidiee and other Protophyta, and with the 



VOL. II. b 



