246 TEE POPULAR SCIENCE MONTELY 



was present at the time of the operation (Spemann). In this case, 

 then, the eye appears in an abnormal position. 



Correlative Differentiation. — "We have cited a series of cases that 

 illustrate two apparently contradictory principles known as the principles 

 of correlative differentiation and of self-differentiation. The part that 

 these play in embryonic development should be analyzed. The data 

 of correlative differentiation may be placed in two categories, one of 

 behavior and one of metabolic relations. Considering these separately : 



Behavior. — Any case of behavior involves a stimulus, and a re- 

 sponse ; these imply irritability and reaction capacity. To take a simple 

 case, for instance, the contraction of a muscle, the stimulus may be 

 of a variety of kinds, nervous, chemical, electrical, thermic, mechanical ; 

 in any case the response is contraction. The nature of the response 

 is given in the system and is limited by its reaction capacity. The 

 muscle cell does not contract for one kind of stimulus and secrete in 

 response to another. 



This principle is elementary in physiology and psychology and it 

 must apply also in the physiology of development. It appears to me 

 that it has not been sufficiently borne in mind by students of the sub- 

 ject. Herbst, for instance, divides developmental stimuli into directive, 

 trophic and formative. The first kind of stimulus determines the 

 direction of growth or migration, and so plays an important part in 

 ■development, a really great part illustrated in two of the cases cited, 

 viz., the mode of branching of nerves, and the direction of migration 

 •of wandering cells. Trophic stimuli are those that affect the rate or 

 amount of growth without altering its specific character. 



The conception of formative stimuli implies, if it has any meaning 

 whatever, that the nature of a developmental process is determined 

 by the nature of a stimulus. A case often cited is as follows : the two 

 most fundamental parts of the eye, lens and retina, develop from two 

 entirely distinct primordia, the retina from the embryonic brain and the 

 lens from the epidermis. The retina first grows out from the wall 

 of the brain and reaches the epidermis to which it becomes fused. The 

 latter then produces a lens. Now it was shown for some amphibia, 

 that, if the retina fails to reach the epidermis, no lens forms; there- 

 fore it was argued that the production of the lens is due to a formative 

 stimulus exercised by the retina on the epidermis. But in some other 

 cases the lens forms even if the retina be absent ; which does not prove 

 that it arises without stimulus, only that this specific stimulus is not 

 needed. And the fact that transplanted optic vesicles stimulate lens 

 formation in strange localities from the epidermis merely shows that 

 this form of reaction of embryonic epidermis is widespread at this 

 stage of development. 



The instance is valuable as proving that stimuli are important in 



