External Factors that Infiuence 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.’ 
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. 
