166 
PHYSIOLOGY: C. M. CHILD 
in many cases low temperature also give the same death gradient and 
by means of the Tashiro biometer a gradient in C02-production in the 
same direction can also be demonstrated. 
In much lower concentrations of the cyanides and narcotics which 
permit some degree of acclimation the susceptibility gradients of the 
organisms are reversed, because in these low concentrations the regions 
of high metaboKc rate undergo more rapid and more complete acclima- 
tion to the reagent, while the regions of lower rate are less capable of 
accKmation and sooner or later die. 
While the metabolic gradient along the longitudinal axis is usually 
the most clearly defined, similar gradients exist in relation to other 
axes or planes of symmetry. In at least most bilaterally symmetrical 
forms metabolic rate decreases from the median region laterally toward 
each side and in the invertebrates ventral regions have primarily a higher 
rate than dorsal, while in the earlier developmental stages of the verte- 
brates the metaboHc rate decreases from the dorsal toward the ventral 
region. This difference between invertebrates and vertebrates is of 
interest in relation to the position of the postcephalic parts of the cen- 
tral nervous system, which in the former are usually ventral, in the latter 
dorsal. 
When we examine the course of development in nature we find that 
it confirms these experimental results. In general, wherever we can dis- 
tinguish an axis or a plane of symmetry we can distinguish a gradient 
or gradients in rate or sequence of development along the axis or in defi- 
nite relation to the plane. This gradient corresponds in direction to 
the metabolic gradient and in all cases where a central nervous system 
appears, it is the first organ to become morphologically distinguishable, 
it arises from the region of highest metabolic rate in the primary gradi- 
ents, and its cephahc part represents the region of highest rate in the 
whole organism. 
There is also evidence that such metabolic gradients exist in plants, 
as well as in animals. Most plants are multiaxial forms, but as regards 
single axes, there are various indications of more or less definite metabo- 
lic gradients along these axes. We find, for instance, that the highest 
rate of growth occurs in the apical regions of each axis and that a more 
or less definite growth gradient extends from this region along the axis. 
In general a relation of dominance and subordination exists between 
regions of higher and those of lower metaboKc rate. If a region of high 
metabolic rate is once established in any way in an undifferentiated cell 
or cell mass a more or less definite gradient in rate extending to a greater 
