: 
: 
1883.] Growth and Development. 729 
It reproduces by gemmation and continues larval through many 
generations. Only in autumn, when the conditions of nature 
grow unfavorable to its larval nutrition, do the long-checked 
energies of development assert themselves, and the final progress 
to maturity take place. 
Marked instances of the same kind as those here considered 
appear in other fields of life, and notably in the Hydrozoa. Here 
as in insects we have species which progress directly to the 
mature or Medusa stage, while in others there are long periods of 
restraint in the larval stage, and of differentiation and reproduc- 
tion of the larval form. In some cases the advancement to the 
Medusa stage is checked to such an extent that the free-swim- 
ming state is not entered, and occasionally only an aborted rep- 
resentative of the Medusa, or mature Hydrozoan, appears. 
In all cases one thing is evident; the development of repro- 
ductive organs seldom occurs in the larval form, and is always 
the last stage in the attainment of maturity. Though the larva 
represents a former mature animal possessing reproductive organs, 
it now fails to gain them, and such reproduction as it displays is 
always by gemmation. It is a nutritive not a reproductive organ- 
ism. The production of the reproductive organs is the final phase 
of individual life. It is the signal that the animal has attained the 
apex of its individual life, and is about to continue its existence in 
the person of its offspring. Did these organs appear in the larva 
they would indicate a retrogression, since sexual offspring would 
be produced, and the final life stage fail to appear. Thus larval re- 
tardation effects a lengthening of the individual life, and in some 
insects constitutes the whole of the nutritive stage. In these 
cases no nutriment is taken in the imago state, and only sexual 
reproduction attended to. 
The marked production of adventitious organs in Echinoderm 
larvae leads to another thought. The modern theory is, that all 
animals in their progression from the germ to maturity pass through 
form phases indicating every ancestral type. But it Would be use- 
less to seek for detailed indications of the ancestral forms in the 
embryo, since probably only the core of these forms is repro- 
duced. The general, deep-lying, essential features of structure 
are displayed, but not the special superficial organs. Only when, 
-asin insect and echinoderm larva, development is retarded, do 
7 One case in which it does occur is that of the Amblystoma, above given. 
