666 THE METAMORPHOSIS OF FLIES. 
the rise of indifferent cells in the mass to be sloughed off, much 
as happens in the formation of the blastoderm in the egg. This 
renewal of the structure is seen in a measure in the internal or- 
gans. We can distinguish four modes of development, according 
to which the parts of the fly originate. Either certain parts of the 
larvæ become persistent, under modification, or the larval organs 
become a foundation for the parts of a fly, but are thrown off, cell 
by cell, ere they assume a definite character. The third and last 
kind is when an entirely new development of parts takes place, 
the beginning of which either dates at the embryonic or larval 
period or at the close of the pupa state. 
Let us consider this single mode of development more closely 
as regards the direct transfer of the larval parts alone from the 
hypodermis of the eight hinder larval segments, which are after- 
wards developed into the abdomen of the fly. The second mode 
occurs in the alimentary canal, the Malpighian vessels, with the 
dorsal vessel and the central part of the nervous system. The 
same process is observed in all these organs, that of their removal, 
cell by cell, with a succeeding new development. I might term it 
a histolysis. The histological elements of the organ—simple as 
well as compound—suffer a fatty degeneration; there remains & 
residuary mass filled mostly with fat molecules. In the nervous 
centres and Malpighian vessels the nuclei of the cells become pèr- 
sistent, and perhaps give rise to the development of new histo- 
logical elements; whether this persistent mass deports itself like 
the alimentary canal, or whether after its destruction it retains the 
nuclei, must remain undetermined, nevertheless this same mass, 
which had composed the former organs, serves to build up anew the 
new ones. The products resulting from this destruction of organs 
are not wholly dispersed, but remain together and so receive ih 
form of organs in their totality, even if no single histological 
element remain. 
We must here leave undetermined how the cells which are to 
form the new organs originate, though this cannot be doubtful as 
regards the last two modes of development of the parts of os 
imago. They undergo a completely new development, i. e. of al 
those parts which are not generally present in the larva, yo are 
not in their complete state of functional activity. In this last 
category belongs the thorax and head of the fly and their append- 
ages, also those parts of the imago which arise from the disks; 
