96 METAZOA IN GENERAL 



person skilled in one field exchanges the products of his labor for the 

 products of the labor of another who is more proficient in some other field. 

 This speciahzation in the work of the individual and the exchange of the 

 results of that work develop in proportion as civihzation advances. In 

 the most highly civilized society the individual may spend his whole time 

 doing simply one thing, as the making of a single part for a complex 

 machine, most of the articles which he uses being secured by the purchase 

 of the products of the labor of others. In the study of animals repre- 

 senting various groups from the lowest to the highest a similar reduction 

 in the degree of independence and increase of the interdependence which 

 accompanies specialization may be observed. 



Development 

 of soma . 



Germ 

 cell 



First 9eneration Second ojenerati on Third generation 



Fig. 37. — Diagram to illustrate Weismanii's conception of the continuity of the germ 

 plasm and the development of the somatoplasm anew in each generation. Germ cells are 

 black, somatic cells white. 



118. Somatic and Germ Cells.— The earliest type of cells to become 

 differentiated from the rest are the sex cells, or germ cells. It has been 

 seen (Sec. 113) that they are set aside even among some of the colonial 

 Protozoa. All the cells in the body other than sex cells are termed 

 somatic cells. When a metazoan reproduces sexually, either the egg cell 

 or the zygote which is to participate in the formation of the new indi- 

 vidual separates from the body of the parent, and by differentiation the 

 whole organization characteristic of the particular species of animal is 

 developed. With the death of such an individual all the somatic cells 

 perish. If we call the protoplasm of the somatic cells somatoplasm and 

 that of the sex cells which transmit hereditary characters germ plasm, 

 it may be said that the thread of life continues from generation to 

 generation through the germ plasm alone, the somatoplasm being formed 

 anew in each generation from the germ plasm. Only in the case of 

 asexual reproduction in the Protozoa does the whole animal live on in 

 the bodies of its descendants — that is potential immortahty. 



The genetic continuity of the germ plasm was emphasized in the work 

 of Weismann, whose conception is illustrated in the accompanying 

 diagrain (Fig. 37). 



