go BOTANICAL GAZETTE [AUGUST 
the life of the individual, while in others it undergoes modification 
during the course of development. In the axial metabolic gradient 
of the chief axis the region of highest rate of reaction becomes the 
apical region or head of the individual, and in other axes the develop- 
ment of organs shows a definite relation to the direction of the 
- gradient. 
In the final analysis such a gradient is not self-determined by 
some sort of organization, but arises as the result of the differential 
action of factors external to the protoplasm, cell, or cell mass acted 
upon. If, for example, an undifferentiated cell or cell mass is 
stimulated at some point by the action of a factor external to it, 
the resulting increase in metabolic activity is not limited to the 
region immediately affected, but a wave of change spreads or is 
transmitted over or through the protoplasm with decreasing energy, 
intensity, or physiological effectiveness, until, if the mass be large 
enough, it becomes inappreciable at a greater or.less distance from _ 
the point of origin. It may be likened to a wave in water or in air 
spreading from the point of disturbance. : 
This is the simplest form of metabolic gradient, but while it 
produces temporary changes in the protoplasm, they are usually 
evanescent or the lasting effect is inappreciable. If, however, the 
stimulation be sufficiently often repeated or sufficiently long con- 
tinued at the same point, more or less persistent changes in the 
protoplasm may occur, which appear as differences in its capacity 
to react. In such a case we say that the irritability of the proto- | 
plasm is altered. Since such changes are in general proportional 
to the energy or intensity of the transmitted change which produces 
them, they must differ at different distances from the point of 
action of the external factor, and the result is what we may call an 
irritability gradient, a gradient in the rapidity, capacity, or intensity 
of reaction of the protoplasm. Such a gradient is, I believe, the 
simplest form of persistent organic or physiological axis. In the 
chief gradient or axis, which is that first established, the region 
of highest metabolic rate or highest irritability becomes the apical 
region, and in the more highly differentiated animals the head, or 
more specifically the cephalic part of the central nervous system, 
develops from it. 
