Page:Insects - Their Ways and Means of Living.djvu/313

 METAMORPHOSIS

groups or islands of cells in the larval tissues. These dormant cell groups are known as imaginal discs, or his- toblasts. (Imaginal is from imago, an image, referring to the adult; histoblast means a tissue bud.) When analvzed closely, the apparent "double" struc- ture of the embryo will be round to be only the result of an exaggeration of the usual processes of growth, ac- companied by an acceleration in certain tissues and a retardation in others. In general, wherever an adult organ is represented by an organ in the larva, even though the latter is greatly retuced, the cells that are to give this organ its adult form do not begin to develop until the larval growth is completed. But if an organ is lacking in the larval stage, the regenerative cells may start to develop at an earlier period--even in the embryo in a few cases. Hence, t/te remodeling q[ a larval orgat in the pupal stage is only a comp/etion of that organ's normal de- velopment, and te production q[ a "ne"' organ is onl tke deferred development of one tkat kas been suppressed during tke larval period. The special organs or forms of organs that the larva bas built up for its own purposes necessarily become useless when the larval life has been completed. Such organs, therefore, must be destroyed if they can not be directly made over into corresponding adult organs. Their tissues consequently undergo a process of dissolu- tion, called histolvsis. It can not be explained at the pres- ent time what causes histolysis, or why it begins at a certain time and in particular tissues, but histolysis is only one of the physiological processes that depend probably on the action of enzymes. In some insects a part of the degenerating tissues of the larva is devoured during the pupal stage by ameboid cells of the blood, known as p]2agocytes. It was once supposed that the phagocytes are the active agents of the destruction of the larval tissues, but this now seems improbable, since histol- ysis takes place whether phagocytes are present or absent.

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INSECTS