PHYSIOLOGY: C. M. CHILD 
165 
ordering factor, or they have ignored the whole problem. Nevertheless, 
orderly behavior in development is one of the most fundamental and 
significant characteristics of the organism, for upon it depend the form 
and structure of the adult and the possibility of definite and coordinate 
fimction of parts. 
During some fifteen years of experimental investigation of certain 
aspects of the problems of reproduction and development, I have been 
able to establish certain facts which throw some light on the problem 
of the mechanism which underlies the orderly character of development 
and at the same time point very definitely to certain conclusions concern- 
ing the nature of the so-called polarity and S3niimetry of the organism. 
Some of the more important of these facts and the conclusions drawn 
from them are briefly stated in the following paragraphs. 
In all axiate forms (forms possessing one or more axes with reference 
to which an order is perceptible) which I have examined, including sev- 
eral species of ciliate infusoria. Hydra, and several hydroids, various 
flatworms, the eggs and developmental stages of several anneHds, of 
starfish and sea urchin and of several fishes, and amphibia, there exists, 
at least during the earher stages of development and in many cases 
throughout life, a gradient in rate of the metaboKc processes or of cer- 
tain fimdamental metabolic reactions, which is coincident in direction 
with the longitudinal or chief axis of the body.^' ^' The region 
of highest rate in this gradient always becomes the apical region of the 
organism, i.e., the region which precedes in locomotion, or which is 
farthest removed from the region of attachment or insertion in the case 
of sessile forms, or is the chief region of growth or of reaction to external 
conditions according to the nature of the organism. Moreover, in those 
organisms in which a definite head appears, this head, or primarily the 
cephalic nerve ganglion which is its fundamental part and the first to 
appear in development, always arises from this region of highest meta- 
bolic rate. 
This metaboKc gradient can be distinguished in various ways ; for ex- 
ample, the susceptibiHty of different regions of the body to various chemi- 
cal agents such as the cyanides and at least many other narcotics in con- 
centrations which kill in the course of a few hours, varies in general with 
metabolic rate.^ Consequently when organisms and particularly the 
simpler forms and earlier stages of development, are killed by such sub- 
stances a metabolic gradient appears as a death or susceptibility gradi- 
ent, the regions of highest rate being most susceptible and dying first. 
Various other conditions such as lack of oxygen, high temperature, and 
