250 LECTURES ON BIOLOGY 



body is not only divided into body-cells and germ-cells, but the 

 body-cells again have become differentiated and changed, accord- 

 ing to the functions which they perform, into muscle-, nerve-, 

 blood-, intestinal, kidney-, connective tissue-, and bone-cells. 

 Indeed, the degree of division of labour is a measure of the 

 degree of development of an organism : the higher the organism 

 the more completely does the individual cell give up its indepen- 

 dence and individuality and become a serving link of the whole 

 without any self-purposes. In many cases the cell must even die 

 in order to fulfil its task in the organism. The epidermis cells of 

 man, at first living cells full of plasm, become harder and harder 

 and finally die, thus forming the cover under which the con- 

 nective tissue and other delicate organs are protected against 

 injuries and other noxious external influences. The bone- 

 forming cells become osseous, and as dead bone-corpuscles help 

 to build up the skeleton. 



Is it not now clear why the Volvox body and the body of 

 all higher multicellular organisms must die, and why only the 

 germ-cells are able to reproduce a species ? Having through 

 division of labour undergone a one-sided transformation, the body- 

 cells are only able to perform certain definite functions that have 

 been given to them. But as the parts of every engine deteriorate 

 by wear and tear and finally disintegrate, so life uses up the 

 body-cells, and, being deprived of the faculty to regenerate by 

 reproduction, they are doomed to early decay. The continuity 

 of life rests now only on the germ-cells. With the production 

 of germ-cells the most important mission of each organism is 

 fulfilled, for its death no longer imperils the preservation of the 

 species, and thus we see that death and reproduction frequently 

 synchronize. 



