METABOLIC GRADIENTS OF VERTEBRATE EMBRYOS. 33 



taneously, but that they die at different time intervals. In other 

 words, the parts of the organism exhibit a differential suscepti- 

 bility towards the toxic substance. This differential susceptibility 

 bears a direct relation to the organization and axiation of the 

 organism. In the simplest cases this relation is : that the anterior 

 or apical end is the most susceptible and dies and disintegrates 

 first, and that the susceptibility decreases in a graded manner 

 from the apical end along the antero-posterior or apico-basal axis. 

 A death or disintegration gradient (so-called because the time of 

 death is recognized by the disintegration of the part affected) is 

 thus observable. This simple gradient has been designated by us 

 the primary gradient. In more complex forms secondary gradi- 

 ents often appear after the early stages of development and may 

 persist throughout life; they consist in the appearance of regions 

 of high susceptibility other than the apical or anterior end. In 

 complex animals it is further found that organs and systems may 

 develop gradients of their own which do not necessarily corre- 

 spond to or bear any relation to the more general primary gradient 

 of the organism. As an example of an organ possessing an 

 axiation independent of that of the rest of the body may be 

 mentioned the vertebrate heart. 



The primary gradient is initiated in protoplasm through the 

 action of the external world upon it. Such environmental action 

 is designated a stimulus and the point of stimulation becomes ipso 

 facto the region of high susceptibility from which both the effect 

 of the stimulus and the susceptibility gradually diminish, finally 

 dying out. Under repeated stimulation such gradients, originally 

 temporary, become morphologically fixed in the protoplasm to a 

 more or less permanent degree. Since these gradients arose in 

 response to the external world they are retained most completely 

 and in a least altered condition in the external surface of organ- 

 isms, and its derivative, the nervous system. As the internal 

 organs and systems increase in number and complexity it is not 

 to be expected that they would necessarily develop gradients 

 corresponding to the gradient of the surface structures but rather 

 that, since the gradient is the expression of physiological activity, 

 their gradients will be the expression of their relation to the 



