120 BIOLOGY: GENERAL AND MEDICAL 



by the development of the soma or body cells into 

 groups set aside for special functions, such groups be- 

 coming more and more differentiated by the continu- 

 ally increasing complexity. The movement once started 

 advances rapidly after the void between the unspecial- 

 ized unicellular and the definitely specialized multi- 

 cellular forms of life has once been bridged. 



With the increasing complexity of the soma come 

 differentiations which seem to affect it alone, but react 

 upon the germ. Thus, for example, one might doubt 

 that the possession of ornamental appendages could 

 in any way benefit the germ, but analysis of the question 

 will show that it is one of the circumstances by which 

 the good of the host is advanced and so is of impor- 

 tance to the germ. Indeed the evolution of living matter 

 depends in large measure upon the reciprocal relation- 

 ships of the soma, and the germ. 



Continuing to study increasing complexity, we pass 

 from those examples in which the combination of cells 

 appeared to be temporary and loose, any detached 

 member of the colony being able to maintain itself and 

 eventually to establish a new colony, as in Microgromia, 

 Carchesium, and Epistylus, to such as Volvox where the 

 combinations are fixed and permanent and individual 

 members detached from the whole languish and die. 



Among the metazoa we find no fixed combinations of 

 undifferentiated cells forming single organisms. In 

 the very lowest we find definite disposition of the cells 

 a regular arrangement to subserve a definite function. 

 Thus among the sponges and the hydras we find the 

 cells disposed in two chief layers whose functions differ 

 more than their general appearance. The fresh-water 

 hydra consists of two layers of cells, an outer forming 

 the ectoderm or covering layer and an inner forming the 

 entoderm which is digestive and nutritive. The entire 

 animal, its body as well as its tentacles, and any budding 

 offspring that may be attached to it, is composed of 

 these two simple layers. Between them is a narrow in- 



