280 INVERTEBRATA CHAP. 



proctodaeum in the embryo, is due to the existence of these zones of 

 proliferation, and that by their activity in budding off cells external 

 to themselves they mechanically cause their own invagination. The 

 cells of the mid-gut likewise have a limited life they are all 

 eventually transformed into glandular cells, they exhaust themselves 

 in elaborating secretion and then die and are replaced by young cells 

 budded from the imaginal islands. 



The great changes which occur at metamorphosis are due therefore 

 to the fact that a large number of cells terminate their normal 

 careers simultaneously, and to the fact that a large amount of 

 proliferation from the formative zones occurs at the same time. But 

 it is probably true that in animals of all classes, as they grow, the 

 individual cells forming the tissues wear out and die and are replaced 

 by young cells lying between the bases of the functional cells at 

 least such renewal of tissue is often observed. In some cases, as in 

 Galerucella, the replacement cell may be a rejuvenated portion of the 

 original cell, a portion of the nucleus and of the worn-out cytoplasm 

 being cast off. 



Finally, the fact that Arthropoda are enclosed in. a rigid cuticle 

 which is periodically cast off, must lead to this casting off of worn- 

 out cells and their replacement by new ones being more or less 

 restricted to the period of moulting the life-rhythm of the cell 

 tends, in fact, to be synchronous with the physiological rhythm of 

 the production of new cuticle ; and so it may be assumed we have in 

 the metamorphosis merely an intensification of the change that goes 

 on at every moult. Such critical moults are found in Crustacea also 

 (see p. 198), and indeed they occur outside the limits of the phylum 

 Arthropoda altogether, for we have every right to consider the 

 amniotic invagination of the Echinopluteus larva to be an imaginal 

 disc bike that of Musca, and the destruction of larval tissue which 

 marks the end of their larval life as quite comparable to the histolytic 

 changes which accompany the metamorphosis of Insecta. 



