The Behavior of Cells i8i 



fit together so as to leave no spaces between them. 

 This trait not only tends to keep the cells in a com- 

 pact mass, but it has other important functions as 

 we shall see later on. 



The cells of the embryonic tissue called mesen- 

 chyme have long been noted for their 'powers of mi- 

 gration. These cells are usually irregular and 

 changeable in outline, and are able to creep about 

 much like an Amoeba. Usually they form masses 

 lying between other cell layers. Their origin is 

 varied, and they frequently move to a considerable 

 distance from their source. An excellent illustra- 

 tion of their behavior is afforded in the early de- 

 velopment of the sea urchin. In the region of the 

 hollow blastula which is being pushed in to form 

 the primitive gastrula cavity there is given off into 

 the interior of the embryo a number of amoeboid 

 cells. These wander away from their point of or- 

 igin and take up positions on opposite sides of the 

 archenteron, where they form two groups. Why 

 do the mesenchyme cells take up their position in 

 these particular places? Driesch found that by vig- 

 orously shaking the young embryos the mesenchyme 

 cells became loosened and scattered about irregu- 

 larly in the cleavage cavity. After the embryos had 

 been left for some time, however, it was found that 

 these mesenchyme cells were back again in their nor- 

 mal position as if they knew what was their ap- 

 pointed function and took up their station accord- 

 ingly. We may conjecture that it was some chemo- 



