. _ pendages. The imaginal disks of the two anterior limbs are 
716 : THE METAMORPHOSIS OF FLIES. 
ously originated not as a direct protrusion, but as an invagination, 
also forming no projection on the upper surface of the hypodermis, l 
as in fact seems to be the case in the wings of Lepidoptera. e 
But we will take a nearer glance at the mode in which®he form- 
ation of the appendages and especially those of the thorax takes 
place. Many differences between Musca and Corethra appear, and 
also many resemblances, and indeed in points of great importance, 
so that we must say that the formation of the thoracic appendages 
goes on in the two insects in essentially the same way, and indeed the 
process of formation may be considered as simply local protrusions 
of a broad, flat, basal membrane. This basal membrane is in both 
cases the hypodermis of the thorax, which in Corethra at the time : 
of formation of the appendages at once arises as a whole, while is 
in Musca it exists only in a rudimentary way in the form of many 
pieces separated from one another. Ideally in fact the thorax of 
Corethra consists of twelve such pieces, each one of which is des- 
tined to grow out as an appendage. Only an apparent similarity 
lies in the relation which exists in the two Diptera between the . 
nerves and the newly forming appendages. In Corethra all the 
appendages of the thorax, with the exception of the gills around 
the end of the body are provided with sensible (sensibeln) nerves; 
_ for then with their increasing growth the nerve-stalk will be sur- 
rounded. From the neurilemma of these nerves arises the forma 
tion of the tissues filling the cavity of the appendages ; they form 
a store of cells (zellzucherungen) which are transformed into the 
tracheze, muscles, sinews and nerves of the appendages. Only in 
the origin of the gills, the place of the nerve is supplied by 4 
trachea, which in those organs wanting nerves and muscles cer 
tainly play the same part as the nerve. : : 
he imaginal disks of the Muscide also stand in connection 
with the nerves, but the significance of this fact in regard to the 
development of the appendages is still quite another thing. This 
is due to the fact that not all the disks are attached to nerve-fila- 
ments, as indeed are not all those from which originate similar ap- 
attached to nerves; the hinder spring up from the tracheal gE 
_ without any connection with nerves. The conclusion is inevitable 
that the nerve-stems are here nothing else than points of sust 
ment for the new formations. Certainly it is. evident that the 
nerve passes through the new formation in order to reach the 
