Development and Metamorphosis 



49 







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body is filled with a thick fluid in which float bits of degenerating larval 

 tissue. At the same time with this radical histolysis or breaking down of 

 tissue a rapid histogenesis or developing of imaginal parts from certain 

 groups of undifferentiated primitive cells, derived probably mostly from 

 the larval skin-cells, is going on. Thus many of the larval organs and tissues, 

 instead of going over into the corresponding imaginal ones, wholly disinte- 

 grate and disappear, and the imaginal parts are newly and independently 

 derived. In connection with the 

 breaking down of the larval tissues 

 phagocytes or freely moving, tissue- 

 eating, amoeboid blood-cells play an 

 important part, although one not 

 yet fully understood. They are 

 either the causal agents of the 

 histolysis, or are assisting agents in 

 it, the tissue disintegration beginning 

 independently, or — a recent sugges- 

 tion — they are perhaps more truly 

 to be looked on as trophocytes, 

 that is, carriers of food, namely, 

 disintegrating tissue, to the develop- Fig. 8i 

 ing centers of the imaginal parts. 

 Much investigation remains to be 

 done on this interesting subject 

 of histolysis and histogenesis in 

 insects with complete metamor- 

 phosis, but enough has been already accomplished to show the basic and 

 extreme character of the transformation from larva to adult. 



If we ask for the meaning of such unusual and radical changes in the 

 development of insects, we confront at once an important biological prob- 

 lem. Most biologists believe that in a large and general way the develop- 

 ment of animals is a swift and condensed recapitulation of their evolution; 

 meaning by development the life-history or ontogeny of an individual, and 

 by evolution the ancestral history or phylogeny of the species. According 

 to this "biogenetic law" the interpretation of the significance of the various 

 stages and characters assumed by an animal in the course of its development 

 from single fertilized egg-cell to the complex many-celled definitive adult 

 stage is simple: These stages correspond to various ancestral ones in the 

 long genealogical history of the species. Every vertebrate, for example, is 

 at some period in its development more like a fish than any other living 

 kind of animal; it has gill-slits in its throat, is tailed, and is indeed a fish- 

 like creature. This is its particular developmental stage, corresponding 



A cross section of the body of the 

 pupa of a honey-bee, show ing the body-cavity 

 filled with disintegrated tissues and phago- 

 cytes, and (at the bottom) a budding pair 

 of legs of the adult, the larva; being 

 wholly legk ss. Photomicrograph by George 

 O. Mitchell; greatly magnified.) 



