176 Cc. M. CHILD 
If we may accept this view of structure in the organism, and all 
the facts are in its favor, then it is actually very similar in its rela- 
tion to the energy current to the morphological characteristics 
of a river system except of course that the latter are mechanically 
produced. The constructed islands and bars, the depositions of 
the river, represent those particles or masses which have, under 
the conditions existing at a given time and place, been left behind 
by the current. Under certain conditions the river may produce 
structure of a certain kind at a certain point in its course, while 
under different conditions this structure may disappear and give 
place to structure of a different kind. 
But the most important fact for present purposes is that in the 
organisms, as in the river, structure, as soon as itappears, begins 
to influence the metabolism, the energy current. From this time 
on the metabolic processes, like the flow of the river, occur in a 
certain structure and here the mutual interactions begin. - Of the 
character of these interactions in organisms we are only beginning 
to obtain some vague conceptions, but that they occur, it is 
impossible to doubt. 
Perhaps a few words will not be out of place concerning the 
bearing of these facts and suggestions upon the theory of ‘forma- 
tive substances’ which has played a considerable réle in embryo- 
logical investigation during the last few years. Most of the sup- 
porters of this theory have attempted to identify the so-called 
formative substances with visible granules or other accumula- 
tions in the cytoplasm, without considering the fact that the 
appearance of these substances in visible structural form indicates 
that they are, at least for the time being, relatively inactive, and 
that they are first of all products or incideats of metabolism 
(Child, ’06b). Of course some or all of these substances might 
reénter metabolism under altered conditions and so play a part 
in determining its character, but the important point is that they 
are indications of a difference in metabolism already existing in 
the different regions where they are formed. We might expect 
that the differences in metabolism, which are certainly more im- 
portant as formative factors than these accumulations of granules, 
would persist in the different regions, even if the granules could 
