44 \l.(.I.T\TI\i; RKPRonrCTlON BY FISSION 



45. Form of the Daughter-Cells. 



i. when observed immediately after their production by subdivision, 

 exhibit a more or le.-s angular outline, since the surface of separation between 

 the mother- and daughter-cell is flat, appearing to the eye as a straight line. 

 This shape, however, very soon undergoes alteration, the cell-membrane being 

 caused to bulge outward by the pressure of the plasma, whereby the plane 

 Mirfaoes are rounded and the cell assumes the customary appearance of the non- 

 facetted coccus. By the same cause the initially plane circular outline of the 

 smaller side of a cylindrical bacillus, newly formed by fission, becomes dome- 

 shaped. 



46. Division of the Nucleus. 



A> 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 amitotic 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 con- 

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

 complicated, however, is the other process known as indirect 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 Thallopliytes 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. WAHKLICH (I.), 

 PitKNZEL (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 

 lamella', thus effecting the separation of the two cells. 



As will be remarked, the Schizonii/cetes 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 con- 

 ditions 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 gene- 

 ration. This period was determined by BKEFELD (I.) and by PKAZMOWSKI (I.) 

 M twenty minutefl at ^5 ; thirty minutes at 30 ; forty-five minutes at 25; 

 ninety minutes at i8J, and four to five hours at 12.5 C., for the hay bacillus 

 (Bacillus subtilis). Similarly rapid is Koch's \'il>ri<> cholera axinticu . the 

 period of generation for which under favourable conditions is only twenty 

 minutes, as ascertained by BUCIINKK, L<>.\<. \i;i>. and KIKIM.IX (I.). If the number 

 of cell> present in a bacterium culture at the commencement of an experiment 



