662 THE METAMORPHOSIS OF FLIES. 
derm larva at no time perform their function; they continue to 
develop more perfectly; they grow more complicated in their 
structure ; they throw off their parts singly; they only hold in 
reserve their histological structure in order to become physiolog- 
ically capable of performing their functions. 
In the Muscidz, on the contrary, each organ of the larva does 
not become entirely lost, though bordering upon a histolysis, i. e. 
becoming functionally incapable, their histological elements dis- 
solve themselves into a blastema, from which a new histological 
element must arise. The only difference from the total destruc- 
tion, such as befalls the muscles, the fat bodies, etc., is this, that 
the destruction of tissues here becomes a continuous process, and 
the new organs are built up out of the same material which com- 
posed the old ones. This obtains in the intestine, the nervous 
system and the dorsal vessel. But a surprising analogy to the 
development of the Echinoderm occurs in the formation of the 
imaginal disks. As the body of the Echinoderm selects at several 
points around the alimentary canal of the larva indifferent cell 
masses, and then all unite and consolidate into a single mass, 8° 
arise at different places within the body of the fly larva—heré 
still in genetic relation with the organs of the larva — masses of 
indifferent cells, which become differentiated in the course of the 
development of the different parts of the imago, and become trans 
formed into a common whole. It cannot be considered as aN 
essential deviation, that in the Pluteus larva these cell masses ar? 
formed during the life of the larva, while in the muscid larva they 
are formed before that, in the egg; and this deviation occurs to 
such a slight extent, as we have seen above, that a pair of the 
formative disks, those out of which the upper half of the p1% 
thorax is formed, here makes an exception, and is only developed 
shortly before the pupation. -Had we considered the formation o 
the cell masses of the Echinoderm larva as buds, then for 4 still 
stronger reason is the formation of the imaginal disks of the Mus- 
cidæ a budding process. They are outgrowths of the tegai 
tary membrane of the nerves and tracheæ of tissues, which, ; 
they are not histologically, are yet physiologically equivalents to 
the complete fibrous tissues of the vertebrates. Both tissues i 
essentially derived from an amorphous, fundamental sib 
which seems to have the capabilities which the more recent studie 
in histology ascribe to the peculiar restorative quality of the com 
