HISTORY OF DIATOMS. 2 1 



usually accomplished in the simplest manner ; the protoplasm of each cell of 

 smallest size swells, forces open the frustule, and escapes to the exterior 

 enveloped in a membrane of cellulose, the presence of which may be revealed 

 at any stage by means of reagents. Frequently the cell thus formed presents 

 but a remote likeness to the diatom which has given birth to it ; its envelope 

 silicifies very rapidly and becomes ornamented with the designs which 

 characterise the genus and species ; and then, by subsequently dividing, the 

 frustules of maximum size are rectified and regain their regularity and elegance. 



" The phenomenon of the re-establishment of the maximum form does not 

 appear to be preceded by any act of fecundation ; nor is it usually the effect 

 of conjugation. If certain diatoms do produce spores, auxospores or sporanges, 

 this is not the case universally, for the establishment of their maximum 

 form is usually effected without their undergoing this special modification of 

 vegetable growth. But after all the microscope does not enable us easily 

 to distinguish this glutinous or gelatinous matter in the midst of which, 

 according to certain observers, diatoms germinate. 



" The method of cultivation which I have just described gives observers 

 the opportunity of obtaining diatoms in their re-established form at any time, 

 and in as large a quantity as they desire, and of witnessing under the micro- 

 scope the different phases which precede this re-establishment." 



Thanks to the kindness of Dr. Miquel, who has been good enough to 

 send me a 13th cultivation of Nitzschia palea, I have been able to examine 

 the auxospores described, and to follow in all their phases the curious 

 phenomenon pointed out by him. 



We shall now examine the various methods of reproduction which have 

 been described by different authors. They are four in number : 



r. The reproduction takes place in a single frustule. The diatom secretes 

 a mass of gelatinous matter with which it surrounds itself, the valves are 

 pushed apart, the cellular contents assume a globular form and are hardened 

 into a sporange, which itself gives birth to an auxospore. 



This auxospore is a body of variable form, and is enclosed in a silicious 

 envelope ; it continues to increase and ultimately bursts the sporange and 

 becomes free. Shortly afterwards fresh frustules, which differ slightly from 

 ordinary frustules (particularly in their size), are observed to come into 

 existence in the interior of this auxospore. These frustules, which are called 

 sporangial, in their turn reproduce the primitive frustule by division. 



As regards this method of reproduction it is evident that authors have 

 have either inaccurately observed or described their observations. What 

 has been described as a method of reproduction is nothing more or less 

 than the return to the primitive form as observed and described by Dr. Miquel. 



