18 MORPHOLOGY 



of the excreta (bacterial products such as acids, &c), particularly in pure 

 cultures, arrests development. As standards of comparison it may be 

 mentioned that a complete division of nucleus and cell in the staminal 

 hairs in Tradescantia takes from eighty to one hundred minutes, and that 

 the process may be effected in Amoeba in from ten to twenty minutes. 

 The rate of multiplication among the bacteria is therefore nothing extra- 

 ordinary, particularly when we consider the simple structure of their cells 

 and the absence of a complicated karyokinetic process. 



When a cylindrical bacterium divides into two it is immaterial, as far as 

 the result is concerned, whether the division is longitudinal or transverse ; 

 but, as a matter of fact, we find that it is always transverse, be the cylinder 

 straight or spirally twisted. This is in harmony with the generally observed 

 law in cell-division, that the new membrane is always formed in the 

 most economical manner ; that is to say, in such a way that a minimum of 

 material is required. In cylindrical cells this is manifestly the transversal 

 position. When the cells resulting from such division remain connected, 

 chains and filaments arise, particularly in the case of non-motile species. 

 They occur sometimes even among motile forms, such as the cholera vibrio 

 (Fig. 28,.^), although in this case the moving cells are more easily separated. 

 Inasmuch as transverse fission is the only mode of increase among cylindrical 

 bacteria, it follows that chains or filaments are the only kinds of colony or 

 ' growth-form ' that can occur, save where there is a subsequent shifting of 

 single cells (as in Cladothrix). 



In the monotrichous and lophotrichous bacteria it is always the non- 

 ciliated end of the dividing cell which bears the cilia for the new individual 

 (Fig. 8, d), the inner ends of the two rods never bearing them. In all cases 

 where the cell seems to have cilia at both poles there are in reality two 

 young cells still united by their inner ends. A curious possibility arises 

 out of this fact. At every division it is only one individual that is provided 

 with new cilia, the other cell bearing the old ones, and this process may be 

 repeated time after time. As a result, it may happen that of two bacteria 

 swimming about and not as yet disconnected one has brand-new cilia, and 

 the other a set which has lived through hundreds of generations. 



In peritrichous forms it seems probable that, during the elongation of 

 the cell preparatory to fission, new cilia arise between the old ones along the 

 side of the cell. 



Among the spherical bacteria it is manifestly, as far as the economy of 

 the cell is concerned, a matter of indifference in which direction the plane 

 of fission lies, because every plane that passes through the centre of the cell 

 represents a minimum. A predilection for one particular plane must there* 

 fore be the outward expression of hereditary morphological characters that 

 possess a classificatory value. The closest resemblance to the cylindrical 

 bacteria is shown by those spherical forms in which the planes of fission of 



