CATERPILLAR AND THE MOTH
fore, are differentiated in the embryo into two sets of cells, one set of which will form the special organs of the larva, while those of the other will remain dormant during the larval life to form the adult organs when the larval cells have completed their functions. The cells of the second set carry the hereditary influences that will cause them to develop into the original, or ancestral, form of the species; the cells of the first set produce the temporary larval form, which may retain certain primitive characters from the embryonic stage, but which does not represent an ancestral form in the evolution of the species. An extreme case of anything is always more easily understood when we can trace it back to something simple, or link it up with something familiar. The metamorphosis of insects appears to be one of the great mvsteries of nature, but reduced to its simplest terres it becomes only an exaggerated case of a temporary growth in certain groups of cells to form something of use to the young, which disappears by resorption when the occasion for its use is past. Innumerable simple cases of this kind might be cited from insects; but there is a familiar case of well- developed metamorphosis even in our own growth, namely, the temporary development of the milk teeth and their later substitution by the adult teeth. If a similar process of double growth from the somatic cells had been carried to other organs, we ourselves should have a meta- morphosis entirely comparable with that of insects. TIaE Mo'rla For three weeks or a little longer the processes of re- construction go on within the pupa of the tent caterpillar, and then the creature that was a caterpillar breaks through its coverings and appears in the form and costume of a moth (Fig. I59 J). The pupal shell splits open on the forward part of the back (E) to allow the moth to emerge, but the latter then only finds itself face to face with the wall of the cocoon. It has left behind its cutting instru-
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INSECTS
