s 



THE PHENOMENA OF LIFE 



The Power of Reproduction. The ameba, to return to our former illus- 

 tration, when the growth of its protoplasm has reached a certain point, mani- 

 fests the power of reproduction, by splitting up into (or in some other way 

 producing) two or more parts, each of which is capable of independent 

 existence. The new amebae manifest the same properties as the parent, 

 perform the same functions, grow and reproduce in their turn. This cycle 

 of life is being continually passed through. 



In more complicated structures than the ameba, the life of individual 

 protoplasmic cells is probably very short in comparison with that of the organ- 

 ism they compose; and their constant decay and death necessitate constant 

 reproduction. The manner in which this takes place has long been the sub- 

 ject of investigation. 



The exact manner of reproduction and growth of protoplasm is a matter 

 of great complexity. Those who have already learned the embryological 

 story have the foundations laid for the physiological uses made of this mate- 



Cell membrane. 



Cell reticulum. - ^ 



Membrane of nucleus. 



Achromatic substance of 



nucleus. 

 Chromatic substance of 



nucleus. 



FIG. 8. Cell with its Reticulum Disposed Radially; from the intestinal epithelium of a 



worm. (Carnoy.) 



rial. Certain of the essential facts are set forth in Chapter II a little later, in 

 so far as the reproduction of the cellular unit is concerned. The reproduction 

 of the animal organism as a whole is a still more complicated story and is 

 presented in the last chapter of this volume. 



THE DIFFERENTIATION OF PROTOPLASM 



The Morphological Unit. Protoplasm was formerly thought to be homo- 

 geneous. It is found, however, that every mass of living protoplasm has one 

 or more special structures imbedded in the mass, the nuclei. In most tissues 

 of the mammalian body each mass of protoplasm is more or less distinctly 

 subdivided into elemental divisions, corresponding to the number of nuclei, 

 the cells. The cells present in the mass, therefore, represent the morphologi- 

 cal units. The arrangements of these units as to size and space relations 

 constitute the form and mass characteristics of the tissues on the one hand, 

 and of the organs on the other. Where there is no distinct marking off of 



