28 CELLS. 



as an essential element of the cell, inasmuch as the more the sub- 

 ject is investigated the more frequently is this structure found. 

 It is also known by the name of attraction-particle. Radiating 

 from it as a center are fine fibers, which together with it constitute 

 the centrosphere (Fig. 2). Usually there are two of these spheres 

 in a cell ; especially is this the case when the cell is about to 

 divide, and they are connected by fibers forming an achromatic 

 or central spindle. 



Division of Cells. Cells divide and then multiply in two 

 ways : 1. By direct division ; 2. By indirect division. 



Direct Division of Cells. This may be either by gemmation or 

 by fission. In the former a portion of the nucleus and proto- 

 plasm forms a bud-like projection from the parent cell, from which 

 it subsequently separates. The bud develops into a cell similar in 

 all respects to that from which it had its origin. In fission the 

 original nucleus divides into two, and then the protoplasm divides 

 in such manner that each half shall possess its own nucleus, and 

 two new cells are thus produced. Direct division is, however, not 

 the method by which cells, as a rule, reproduce their kind ; indeed, 

 it is regarded as very infrequent. 



Indirect Division, Karyokinesis, Karyomitosis, Mitosis (Figs. 2-17). 

 It is to this method of division that we must look for the com- 

 prehension of the processes by which the tissues produce and re- 

 produce themselves. It has been studied in them all epithelial, 

 connective, muscular, and nervous. While in direct division the 

 nucleus divides into two equal halves, in karyokinesis the changes 

 which take place in the nucleus are complicated, and it is only 

 after a long series that new cells are produced. 



The statement is made by some authors that the division of 

 a cell is preceded by the division of its attraction-sphere, and that 

 the division of the nucleus follows ; indeed, some regard the change 

 taking place in the attraction-sphere as determining or causing the 

 division of the nucleus ; but inasmuch as instances have been 

 observed in which the nuclear changes preceded, they are evidently 

 not under all circumstances dependent upon the influence of the 

 attracti on -sphere . 



The changes which take place in the process of karyokinesis 

 may be concisely described as follows : Prior to the beginning of 

 the process the cell consists of protoplasm containing a nucleus, 

 with one or more contained nucleoli, and enclosed by the nuclear 

 membrane, and a centrosome and attraction-sphere. A close ex- 

 amination of the chromoplasm of the nucleus shows it to be made 

 up of some fibers which form loops at the ends or poles of the 

 nucleus, and are the primary loops, while others less prominent 

 and which help to give to the chromoplasm its reticular or net- 

 work form are secondary fibers. 



When indirect division begins the first change usually, though 



