The Behavior of Cells 



187 



different osmotic pressure. It is essential to the per- 

 formance of many important functions that these 

 membranes be absolutely continuous with no aper- 

 tures at any point. This continuity is insured through 

 the traits of behavior we have mentioned. 



Fig. 7. — A secondary membrane formed by the approximation of 

 originally isolated cells from the ectoderm of Hyla, such as 

 shown in B. Very fine pseudopodia are shown in the free 

 margins. 



One very general and Important feature of bod- 

 ily organization therefore is traceable largely to 

 certain peculiarities of the behavior of cells. In 

 many animals the sex cells arrive at their final posi- 

 tion only after considerable migration. In the 

 sponges they are derived from amoeboid cells and in 

 hydroids they often migrate from one germ layer to 



