20 PHYSIOLOGY. 



To get a better conception of the direct form of division, we will 

 study one of the infusorians, the typical amoeba., and the changes 

 occurring in it during reproduction. The first intimation of a divi- 

 sion is noted in the spherical nucleus, which becomes elongated, the 

 middle portion of it being indented by a constriction, which gives to 

 the nucleus a dumb-bell shape. The constricted portion becomes 

 gradually narrower and slenderer until the two heads of the ball sepa- 

 rate and each assumes the same shape as its mother spherical. The 

 cell thus contains two distinct nuclei. Following the division of the 

 nucleus is that of the protoplasm by constriction also. The indenta- 

 tion always appears between the two nuclei. Eventually two cells 

 are thus formed, each with a separate nucleus; each daughter-cell is, 

 of course, smaller than its mother, but by the assimilation of the 

 nutrient material surrounding it, it soon grows to the normal, definite 

 size. This process often requires several hours for its completion, 

 the various stages being frequently accomplished in an uncertain 

 manner. 



2. Indirect Cell=di\ision (Mitosis, or Karyokinesis). 



By far the greater number of animal- and plant- cells follow the 

 more complicated and intricate method of indirect, or karyokinetic, 

 form of division. The division of the protoplasm is simple enough, 

 following only the laws of constriction until the mass is completely 

 separated into two parts, by means of a furrow, or fissure. It is the 

 nucleus which undergoes very remarkable and typical changes, very 

 complicated in their nature, but which in plants and animals are con- 

 stant and agree very much in regard to essentials. Thus the indirect 

 method is very nearly, though not quite, universal. 



As a cell prepares for division the most evident and important 

 fact noticed is a change in its nucleus, both physical and chemical. 

 The nucleus becomes somewhat enlarged, and its chromatic nuclear 

 substance, or chromoplasm, so called because it has an affinity for 

 stains, begins to become changed little by little, from the netlike 

 arrangements of its minute granules* and particles, until the substance 

 is arranged in the form of threads loosely rolled up, like a coil or 

 convolution, called the skein or spirem. These consist principally 

 of nuclein, and stain more deeply than the surrounding parts, and 

 are, hence, more easily discerned. It is the presence of these threads 

 that gives to the process the name mitosis. In most cases there is 

 but a single thread, which is coiled or convoluted throughout its 

 entire length; occasionally, there occur several such threads. The 



