274 



INTERNAL FACTORS 



IV. 2 



blood-vessels. When the creature is deprived of oxygen the 

 pigment disappears, and it is supposed that under normal con- 

 ditions the pigment cells are chemotactically attracted by the 

 oxygen in the blood (Loeb). 



Two other cases come from Echinoid larvae. Driesch has been 

 able to displace, by shaking, the two groups of primary mesen- 

 chyme cells, those destined to secrete the skeletal spicules of the 



Fia. 164. Diagrams of Driesch's experiment into the responsiveness 

 to stimuli of mesenchyme cells in Echinus. A, Normal arrangement of 

 mesenchyme cells. 2?, The cells deranged by snaking, returning in C to 

 their original position. (From Korschelt and Heider, after Driesch.) 



larva. The cells, displaced towards the animal pole, return to 

 their original position after six hours, and subsequent develop- 

 ment is normal (Fig. 164). Driesch has attributed this move- 

 ment of the cells to a stimulus exerted by the endoderm. 



Again Herbst has found that when the larvae of sea-urchins 

 are placed in solutions of lithium salts or in sea-water deprived 

 of the sulph-ion, or devoid of the carbon-ion, the skeleton of the 



