rS THE STRUCTURE AND LIFE 



of Bacteria. For the future much time will have to be devoted to these 

 cultivations, though Dr. Miquel has also studied the return of the reduced 

 frustule to its largest (Sporangial) form which he has been able to bring 

 about artificially by successive special cultivations. 



Dr. Miquel presented to the Academy of Sciences of Paris (24th October, 

 1892) a paper on this subject, which, on account of its importance, is here 

 reproduced in its entirety : 



" It is ascertained that when a single frustule of a living diatom in a free 

 state is propagated in a sterilised and suitably nutrient maceration the 

 daughter cells produced from the mother cell vary greatly in size. In 

 measuring frustules of a similar cultivation it is found that the figures which 

 represent diatoms of the same size, are approximately related to one another 

 as the terms of the binomial expansion (1 + i) n in which n denotes the 

 number of generations which have occurred. 



By taking certain diatoms of medium size from a first cultivation and 

 transferring them to a second maceration, a second cultivation is obtained in 

 which the size of the medium cells differs in length from that of the same 

 diatom from some tenths to several //, according to their species. By 

 continually producing cultivations in this way, the diatoms are made to 

 assume very reduced dimensions, and the phenomena which accompany 

 the re-establishment of the so-called Sporangial form are quickly observed. 

 If the silicious Pheophyceae are associated in chains of joints the filaments, 

 as they become smaller and smaller, should be transferred into fresh 

 sterilised macerations, and in the same way the formation of auxosporial cells 

 is easily promoted. 



" I have myself followed the re-establishment of the maximum form in 

 Melosira and Nitzschia. 



" In Melosira nummuloides the protoplasm of the joint which gives birth 

 to the so-called Sporangial frustule increases in volume, forces back the 

 cylindro-spherical valves which contain it, slowly abandons them, and 

 forms on their exterior an almost spherical mass, the outer covering of which 

 immediately silicifies. This newly-formed cell soon generates by division 

 individuals like itself, and produces a string of joints of a diameter about 

 double that of the cylindro-spherical generating frustule. 



; 'The cells of Melosira varians when they have reached a sufficiently 

 reduced size, also produce a large number of spheres, but these have 

 diameters more than double the size of those of the generating frustules. 



" When these large cells have reached maturity they free themselves from 

 the filaments and are enveloped in a thick membrane, and then the vitality of 

 the diatom is relaxed. To all appearances these spheres constitute the spores 



