§ II.] Cell-forms. 9 



to escape observation, and becomes visible only under the influ- 

 ence of reagents which give a deep colour to the protoplasm 

 and make it shrink, especially alcoholic solution of iodine. 

 This must not be forgotten in determining the length of cells. 



The successive bipartitions are either all in the same direction, 

 and the transverse walls are therefore parallel ; or more rarely 

 the walls lie in two or three directions in space, so that they 

 successively cut one another, and may actually cross at a right 

 angle. 



II. 

 Cell-forms, cell-unions, and cell-groupings. 



Single Bacterium-cells, the simple structure of which has been 

 considered in the preceding chapter, may appear in very various 

 forms, the variety depending partly on their own shape and on 

 that of their simplest aggregations, partly on whether they are 

 united into larger aggregations or not, and on the peculiar 

 characters of these aggregations. 



I. The shape of the individual cells and their simplest 

 genetic combinations give rise to the distinction into round- 

 celled, and straight or spirally twisted rod-like forms. A billiard- 

 ball, a lead pencil, and a cork-screw, so exactly illustrate these 

 three chief forms, that no one requires for his instruction in 

 this case the costly models which are offered for sale. The 

 figures on subsequent pages, which will be examined more 

 closely in later lectures, will for the present give a sufficiently 

 clear idea of the matter. 



These forms have received a variety of names in the course 

 of the development of our knowledge. The round forms are 

 at present most commonly known as Cocci (Figs. 3, 4), and are 

 spoken of as Micrococci or Macrococci according to their size, 

 or as Diplococci when they still remain united in pairs after a 

 bipartition ; earlier writers called them monads, a name which 

 they applied to a variety of heterogeneous objects. 



