58 VEGETATIVE REPRODUCTION BY FISSION. 



46. Division of the Nucleus. 



As is known, the division of the nucleus can be effected, in 

 the higher plants as well as in animals, in two ways. The simpler 

 of these, generally known as segmentation, or as direct or ami- 

 totic division of the nucleus, occurs, in the higher plants, only in 

 such cells as have ceased to subdivide, and whose multiplication 

 has therefore ceased. The nucleus elongates, becomes constricted 

 at an intermediate point, and finally falls apart in two halves. 

 More complicated, however, is the other process known as in- 

 direct or mitotic division of the nucleus. In this case the final 

 separation of the nucleus into two portions is preceded by a far- 

 reaching transformation of the substance of the nucleus of the 

 mother-cell, an operation entitled karyokinesis ; and it is in this 

 manner alone that the subdivision of the nucleus occurs in the 

 segmentation cells of higher plants and animals. Fundamental 

 researches into this process have been made by FLEMMING (I.), and 

 in the narrower domain of the Thallophytes the investigations of 

 SCHMITZ (I.) merit attention. 



In so far as the Schizomycetes are concerned, BUTSCHLI (I. and 

 II.) made various observations, from which Zacharias concluded 

 that the division of the central body of fission bacteria is amitotic 

 or direct. WAHRLICH (I.), FRENZEL (I.), and SJOBRING (I.) studied 

 the matter more closely, and, according to them, the chromatin 

 granules of the central body (35) are first dissipated, and then 

 the latter stretches and subdivides. Concurrently, the new septum 

 interposes itself between the two moieties of the cell and then 

 splits up into two lamellae, thus effecting the separation of the 

 two cells. 



As will be remarked, the Scliizomycetes assume an exceptional 

 position as regards the behaviour of their nucleus during cellular 

 subdivision, the operation being in this case alone direct, whereas 

 in all other plants karyokinesis occurs. 



47. The Rate of Reproduction 



is, naturally, influenced by external circumstances, especially by 

 the method of nutrition and the temperature. It also varies 

 under the same external conditions in the different species. The 

 time required for the formation of one bacterial cell from another 

 (a new generation) is known as the period of generation. This 

 period was determined by BREFELD (I.) and by PRAZMOWSKI (I.) 

 as twenty minutes at 35; thirty minutes at 30; forty-five 

 minutes at 25; ninety minutes at i8f, and four to five hours 

 at 12.5 C., for the hay bacillus (Bacillus subtilis). Similarly 

 rapid is Koch's Vibrio cholerce asiaticce, the period of generation 

 for which under favourable conditions is only twenty minutes, 



