604 THE METAMORPHOSIS OF FLIES. 
phenomena of growth are manifested, and not a deeper reaching 
metamorphosis. As the enlargement of an organ by simple growth 
in the Vertebrates is allied with a new formation of blood vessels, 
so here the origin of a new trachea is accompanied by the speedy 
enlargement of muscles and intestine, and with this very impor- 
tant continual increase of the net-work of tracheæ is combined a — 
expansion and increase in volume, so that after the first moulting, — 
on the anterior end of the body, a new pair of stigmata are formed, 
while the aperture in the hinder one is doubled, and after a m 
ond moulting, a three-fold aperture is made. Accompanying this 
is a certain change in the apparatus of hooks arming the mouth — 
of the larva. All these changes are not of great importance ; they : 
lead to no new feature in the organization of the animal; they aè — 
series of processes which precede the formation of entirely a | 
organs or parts. Transformation in this last sense occurs only in 
those parts of the larva, out of which the parts of the adult insects 
are developed. The genital glands, as well as the outer skin of the : 
- segments bearing the appendages of the fly’s body, are already formed 
in the larva ; indeed they are even formed during the development of 
the embryo. : 
We find ourselves in fact almost going back to the ed : 
theory of Swammerdam, who believed that the larva, pup and bur 7 
terfly were imprisoned from the very first in each other, and ere z 
Iti m 
only this, that the parts of the fly* do not all lie perfec 
within the larva, but exist only as rudiments, 
part of the body of the fly is newly formed, whil 
parts will be produced out of the larva. The hea 
with their appendages are formed within the larva by the 
development of special cell masses. The abdomen, 
arises through a simple change of a number of larval § pa 
The head and thorax arise not as a whole out of by ge : 
mass, but in separate pieces, out of which after pupation g eae 
form is perfected. oe 
The head arises out of two groups of cells which originate “o 
a nervous filament sent off from the supra-æsophageal g groups 
but each segment of the thorax arises out of four separa” 
_ of cells which are partly inserted in the course of @ j aht 
ment, and are in part blended with the peritoneal gkin of SY o 
* This chapter relates wholly to Musca vomitoria. 
