232 ANIMAL BIOLOGY 



individuality, while in others certain cells are restricted more or 

 less in their functions, so that a physiological division of labor is 

 established which involves the shifting of individuality from the 

 cells to the colony as a whole. This specialization is exhibited 

 chiefly with regard to reproduction and reaches its highest expres- 

 sion among colonial Protozoa in Volvox, where among ten thou- 

 sand or so cells, perhaps a score are specialized for reproduction 

 and the rest are somatic. Usually each of the reproductive cells 

 (germ cells) divides to form a group which is set free as a mini- 

 ature colony; but in certain cases some of the reproductive cells 

 become transformed into male and others into female gametes. 

 After fertilization of the eggs, usually by sperm from another 

 colony, the zygotes develop into new colonies which eventually 

 are liberated from the parent colony. (Figs. 29, 30.) 



As has been previously suggested, the physiological division of 

 labor in the colonial Protozoa, involving, as it does, a segregation 

 of reproductive from somatic structures, affords a logical transi- 

 tion from the unicellular condition to that characteristic of the 

 multicellular forms. These, to all intents and purposes, may be 

 considered highly complex colonies of cells in which specialization, 

 no longer confined merely to demarking germinal and somatic 

 regions, has transformed the latter into a complex of tissues and 

 organs, the body (soma) of the individual, while the germinal tis- 

 sue (germ) is confined to the essential reproductive organs. 



It is customary, therefore, to draw a more or less sharp distinc- 

 tion between the soma and germ — to consider the soma the 

 individual which harbors, as it were, the germ destined to continue 

 the race. This theory of germinal continuity, which is chiefly 

 associated with the name of Weismann, recognizes that the germ 

 contains living material which has come down in unbroken con- 

 tinuity ever since the origin of life and which is destined to persist 

 in some form as long as life itself. On the other hand, the soma may 

 be said to arise anew in each generation as a derivative or offshoot 

 of the germ; and, after playing its part for a while as the vehicle 

 of the germ, to pass the germ on at reproduction, and then die. 

 The germinal continuity concept has altered the attitude of biolo- 

 gists toward certain fundamental questions in heredity and evolu- 

 tion, as will be apparent when these subjects are considered. 

 (Figs. 154, 162, 180, 305.) 



Though Volvox and other colonial forms afford a glimpse of the 



