Development and Metamorphosis 
49 
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¬ 
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 
Fig. 8i.—A cross section of the body of the 
pupa of a honey-bee, showing the body-cavity 
filled with disintegrated tissues and phago¬ 
cytes, and (at the bottom) a budding pair 
of legs of the adult, the larvae being 
wholly legless. Photomicrograph by George 
O. Mitchell; greatly magnified.) 
