44 



PROTOPLASM AND THE CELL 



Reproductionjand Growth. The fourth and last of the classes of 

 functions of protoplasm deals with the means by which the races of 

 animals are continued, as the three other classes discuss some of the 

 protoplasmic functions as means of continuance of the individual organ- 

 ism. Chapter XIII is devoted to the physiology of mammalian and 

 especially of human reproduction. In this connection, therefore, we 

 shall confine ourselves to the underlying principles of cell-division and 

 of cell-growth, and of what might almost be called protoplasmic repro- 

 duction. 



FIG. 18 



Diagram of amitotic cell-division, showing an Ameba polypodia becoming two. 

 (F. E. Schulze.) 



AMITOSIS. Amitosis is direct cell-division or direct nuclear division. 

 This mode of division has been seen to occur in leukocytes and in epithe- 

 lial cells, especially those of arthropods. It occurs also in the protista, 

 and in other forms. The process of amitosis is, so far as the micro- 

 scope shows, very simple; none of the complex, minute organs (spindles, 

 radiations, asters, chromosomes, etc.), soon to be described, are appar- 

 ently employed in this process. In amitosis the nucleus simply flows 

 apart, and this is followed by a similar active division of the cytoplasm, 

 the whole cell thus being made two. First, the nucleus elongates, and then 

 by active protoplasmic streaming one-half separates gradually from the 

 other in the cytoplasm. Meanwhile, the latter has begun the same pro- 

 cess and slowly becomes constricted between the separated nuclei. This 

 constriction becomes gradually more slender and lengthened, until 



