728 Growth and Development. [July, 
lying organs are in the true line of development. When develop. 
ment is resumed only these internal organs are retained as part 
of the mature animal, and the secondary larval organs are thrown 
off, or absorbed as nutriment by the new body. To so great an 
extent has this secondary development proceeded, that in some 
cases the discarded organs retain their power of swimming and 
devouring food, though with no means of digesting it. The 
energy of further development resides only in the core of the 
strange creature which has surrounded itself with a temporary 
shell of swimming, food catching and masticating organs. The 
tissues of the mature form are molded out of those of the larva, 
and its useless series of temporary organs discarded. 
It would appear, then, that if any animal during its embryonal 
development enters, at any stage of this process, conditions favor- 
able to the persistence of that stage, its further development 1$ 
temporarily checked. The energy of outer influences resists the 
action of internal energies. Nutrition opposes development. The 
vigor of the organism is devoted to growth, and its energies of 
change are restrained. Many animals pass on to maturity with- 
out a halt. Others make several halts in the path of develop- 
ment, in each of which nutrition checks unfoldment. For devel- 
opment to be resumed, nutrition must be checked. The insect 
larva must cease to eat ere it can resume its life progress. It also 
seeks some shelter to secure it from danger during this pror 
this being probably an instinct arising from natural selection. 
And now proceeds a series of organic changes, arising perhaps 
from the exercise of chemical affinities inherent in the tissues, by 
which the molecules of these tissues are rearranged and ae 
forms of tissue produced, the nutriment stored in the form ofis 
sue during the larval period serving as material for the new-form- 
ing tissues. all 
It is quite possible that in the embryonal development of 
animals there are periods of active nutrition in which gre d 
replaces unfoldment, and periods of quiescence and cessal 
nutrition in which the chemistry of evolution resumes 15 gee 
acting on the products of nutrition and molding them mee 
forms. In some cases these changes rapidly succeed each saa 
In others the period of larval restraint grows abnormally sr 
In such a case as that of the Aphis, the larva is so well se K 
with food that its further development is completely chet 
