THE PHENOMENA OF LIFE. 7 



for the same purpose of getting new matter for growth and nutrition, 

 take food into their stomachs. But in both these cases the materials are 

 much altered before they are finally assimilated by the structures they 

 are destined to nourish. 



(4. ) The growth of all living things has a definite limit, and the law 

 which governs this limitation of increase in size is so invariable that we 

 should be as much astonished to find an individual plant or animal with- 

 out limit as to growth as without limit to life. 



4. Reproductive powers. The amoeba, to return to our former illus- 

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

 manifests the power of reproduction, by splitting up into (or in some 

 other way producing) two or more parts, each of which is capable of in- 

 dependent existence. The new amoebae manifest the same properties as 

 their 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 amoeba, the life of indi- 



FIG. 4. Diagram of an ovum (a) undergoing segmentation. In (6) it has divided into two ; in 

 (c) into four ; and in (d> the process has ended in the production of the so-called " mulberry mass." 

 (Frey.) 



vidual protoplasmic cells is probably very short in comparison with that 

 of the organism they compose : and their constant decay and death 

 necessitate constant reproduction. 



The mode in which this takes place has long been the subject of 

 great controversy. 



It is now very generally believed that every cell is descended from 

 some pre-existing (mother-) cell. This derivation of cells from cells 

 takes place by (1) gemmation, or (2) fission or division. 



(1) Gemmation. This method has not been observed in the human 

 body or the higher animals, and therefore requires but a passing notice. 

 It consists essentially in the budding off and separating of a portion of 

 the parent cell. 



(2) Fission or Division. As examples of reproduction by fission, we 

 may select the ovum, the blood cell, and cartilage cells. 



In the frog's ovum (in which the process can be most readily ob- 

 served) after fertilization has taken place, there is first some amoeboid 

 movement, the oscillation gradually increasing until a permanent dimple 

 appears, which gradually extends into a furrow running completely 

 round the spherical ovum, and deepening until the entire yelk-mass is 

 divided into two hemispheres of protoplasm each containing a nucleus 

 (Fig. 4, #). This process being repeated by the formation of a second 

 furrow at right angles to the first, we have four cells produced (c) : this 



