vi METAMORPHOSIS 215 



life a certain amount of cell-replacement. Individual cells, or groups of 

 cells, worn out by their activities become moribund, die, and are replaced 

 by substitutes cells which lag behind in their development and retain 

 their juvenile characteristics until their services are needed, when they 

 rapidly complete their development and take the place of their worn-out 

 predecessors. A conspicuous example of this process has already been 

 alluded to on p. 135 that of the yellow cells of Lumbricus, but it is 

 probable that its occurrence, although in less conspicuous form, is a 

 common characteristic of living tissues. 



In the process of metamorphosis we have to do not merely with 

 the concentration of change of bulk and change of form in a particular 

 stage of the life-history but also with a similar concentration of this 

 process of cell-replacement. The reinforcing cells are either scattered or 

 in certain parts of the body form definite, easily recognizable patches, 

 to which the old-fashioned name " imaginal discs " is still commonly 

 given. Apart from the blood, the nervous, and the reproductive systems, 

 the process of cell-replacement is to a great extent held back until the 

 time of metamorphosis, when there sets in a process of wholesale dis- 

 integration of the tissues (histolysis) their cells becoming moribund, 

 dying, and disintegrating, and their remains being devoured by amoebo- 

 cytes. At the same period the replacement cells burst into activity, 

 multiply rapidly and undergo tissue specialization, until by the end of 

 the pupal period they have provided a complete new outfit of tissues 

 and organs, replacing those which have disintegrated and, it may be, 

 differing greatly from them, in correlation with the changed functions 

 which they will have to perform in the new life of the perfect insect. 



It has been established in the case of some insects that the epidermis 

 behaves in a different fashion at metamorphosis. In its case the cells 

 do not die and become replaced by others : the cells of the larva appear 

 to persist in the adult. This is apparently rendered possible by their 

 possessing the power of recovering their juvenile activity by the extrusion 

 from their inner ends of a quantity of chromatin which is at once ingested 

 by amoebocytes. This process of rejuvenation of cells by the elimination 

 of, presumably effete, chromatin is not at all understood : it appears to 

 be of not infrequent occurrence in the animal kingdom, and it is probably 

 the expression of a phenomenon of very deep biological significance. 



Apart from the general effects which have been so far alluded to, the 

 chitinous exoskeleton of the arthropod has definite functions of its own. 



(i) It forms a magnificent protective envelope to the soft living 

 protoplasm of the body, guarding it from mechanical violence, from 



