External Factors that Influence Growth 267 



of the content of the cells, so that the heavier parts sink down- 

 ward and the lighter rise upward. This interpretation finds its 

 chief support in the fact that in plants when a centrifugal force 

 is substituted for gravity the direction of growth is thereby 

 determined ; and also in the fact that in the frog's egg an actual 

 rotation of the protoplasm has been observed. 



Effects of Electricity on Growth 



There is even less to be said on this subject than in the case of 

 gravity. Certain writers have found that a current of electricity 

 running at right angles to the axis of an embryo chick either 

 brings the development to an end or causes abnormal develop- 

 ment. 



Roux placed the unsegmented eggs of the frog between two 

 electrodes and found that the pigment arranged itself around two 

 centers corresponding to the magnetic poles. After segmenta- 

 tion, each cell showed a similar arrangement of its pigment. 

 No influence was found on the direction of the cleavage planes, 

 and normal development took place. The cause of the arrange- 

 ment of the pigment granules is not clear, nor is it understood 

 why, in each cell, a separate center was formed. Whether the 

 current actually passed through the egg or only over its surface 

 is not certain. 1 



Pressure and^ Contact 



That the growth of parts of an animal may be changed by 

 pressure is a familiar experience. Our own epidermis responds 

 to pressure, and even such resisting structures as bones respond 

 most surprisingly to continued pressure, as when, for example, 

 a new socket can be made in the pelvis for the head of the femur, 

 or when the shape of the head of the children of savage races 

 is altered by pressure, or when the ribs of women become 

 deformed as a result of lacing. 



A direct response to contact is best shown in fixed animals. 



1 Rossi repeated this experiment and found abnormal cleavage-and abnormal 

 development. 



