REGULATORY PROCESSES IN ORGANISMS Les 
to some extent at all times. Under certain conditions, however, . 
?. €., in the absence of certain other substances, they, or some of 
them, may reénter metabolism to a much larger extent and fur- 
nish energy. It is a familiar fact that in the absence of nutritive 
material from without’ the organism uses up its own substance, 
2.e., the relatively inactive substances which under other conditions 
had accumulated in it. In certain of the lower organisms this 
process may continue until the organism is reduced to a minute 
fraction of its original size. These facts do not, however, conflict 
with the suggestions made above as to the relative inactivity 
of structural substance, but serve rather to confirm the idea that 
the accumulation of these substances, when other nutritive mate- 
rial is present, is due to their relative inactivity. 
Various authors have attempted to distinguish between a mor- 
phological and a functional metabolism, but it is doubtful whether 
such a distinction is valid, except as an expression of the fact that 
substances of different degrees of chemical activity arise in the 
course of metabolism and that certain of the less active substances 
constitute the structural basis of the organism, while others 
undergo chemical transformation and elimination. 
It is of course not merely the nature of the substances them- 
selves, but the existing conditions as well, which determine the 
degree of activity or inactivity. Under certain conditions a cell 
or an organ may accumulate certain substances and so acquire 
a certain characteristic structure, while under altered conditions 
these substances may rapidly disappear and others be accumu- 
lated. Thus, for example, the odcyte, during its growth period, 
accumulates yolk, which under the existing conditions is almost 
wholly inactive chemically and so appears as a structure-building 
substance. But when fertilization occurs the conditions within 
the cell are so altered that the accumulated yolk rapidly reénters 
metabolism and serves as nutritive material. In fact we may say 
that the egg does not produce yolk because it is to develop into 
a new organism, but that it develops as it does because it has 
accumulated yolk. In the periodic changes in the cell connected 
with growth and division there is also abundant evidence for the 
occurrence of changes of this character. 
