FROM EGG TO INSECT SNODGRASS 



389 



lire 11, in which for convenience the products of only one germ 

 cell are shown for each generation. As the new individual de- 

 velops, the germ cells wander among the body cells for a while, 

 but at last they become lodged in the reproductive organs (figs. 

 24, 27, GCls). Here they multiply in number and undergo their 

 development into the mature eggs or spermatozoa, according to 

 the sex. 



The mass of body cells, consequently, appears to be but a mere 

 side issue from the germ cells, a specialized group of nonreproduc- 

 tive cells that the germ cells have produced for their own protec- 

 tion. It is most probable that in the evolution of animals all the 

 cells were originally alike, and that the division of labor between 

 them has had the usual consequence; the germ cells, in leaving the 

 vegetative functions to the body cells, have lost most of their capac- 

 ity for functions other than that of reproduction; while the body 

 cells, in specializing on the vegetative functions, have lost the 

 reproductive function. The one-celled animals, however, having 



GCls 



BCls 



Fig. 11. — Diagram sliowing relation of germ cells (QCls) and bo(l.v cells 

 (BCls) in successive generations. A germ cell of A forms the germ cells 

 and bod.v cells of B ; a germ cell of B forms tlie germ cells and bod.v cells of 

 C, etc. The offspring (C) of B derives nothing from the body cells of the 

 t)areiit (B), but both offspring (C) and parent (B) have a common origin 

 in a germ cell of A, the grandparent of C 



never differentiated into the two sets of cells, retain in themselves 

 all the normal functions of a cell. But, in some of the lower many- 

 celled animals, the body cells also have not entirely lost the power 

 of reproduction, since in some species a body cell or a group of 

 them detached from the body may multiply and reproduce a new 

 body. The first-formed body cells of the developing egg, too, are 

 rejjroductive cells in this sense, since it is they that carry on the 

 constructive w^ork delegated to them by the parent germ cell; and 

 all the body cells, in retaining the power of division, retain the 

 power of reproduction. All such forms of reproduction, however, 

 are of the asexual type; the distinctive quality of the true germ 

 cells is their ability to unite in fertilization and produce a new 

 individual combining the qualities of two others, though this is not 

 always a necessary condition with the egg cell. 

 76041—26 26 



