ERTE i a mnie ASNO 
THE METAMORPHOSIS OF FLIES. 609 
has ended, and now begins the period of development of the 
same. It lasts from the fifth day to the time of exclusion of the 
fly, and can, as has already been shown above, be divided into 
two subdivisions, of which the first reaches to the end of the sev- 
enth day. 
First to be noticed is the formation of the pupa-case, which, how- 
ever, was in existence at the end of the first period, but lies as the 
cuticula right on the cellular skin (zellenrinde) by which it was 
ensheathed. It now rises up and a space filled with clear liquid 
Separates it from the upper surface of the body. The cellular teg- 
ument (zellenrinde) of the appendages is thickened, partly by the 
increase of the cells present, which seem to receive their plastic 
material by endosmose, but partly through a free formation of new 
cells by a self division of the nuclei. The whole cavity of the 
limbs seem to be compactly filled with nuclei which are uniformly 
from the outer to the innermost transformed into cells. 
By the fifth day the last tarsal joint is divided into two lobes, 
and show the first position of the claws. On the sixth the sutures 
are more distinct, the pulvilli are formed, and on the seventh day 
the external form of the limb is completed. The hypodermis di- 
Vides into two layers whose deeper portion is disposed on the up- 
per surface of the skin and form the hairs and bristles. Inside 
the limbs only the position of the nerves and tracheæ of the pupa 
18 established, the muscles arise afterwards. 
In like manner the wings are formed, their veins arise, the hairs 
appear ; they attain their definitive form and are folded together. 
The halteres grow out, and instead of a single hollow, stunted 
Projection, they are completely formed, and hairy, though still 
Colorless. The antennæ also reach the same grade of perfection, 
and like the appendages, the segments themselves now assume 
their definite form. The four abdominal segments are formed out 
s = eight larval segments which originally formed the abdomen 
e fly. 
While the external form of the body rapidly advances in this 
r to its final perfection, corresponding but slower changes 
“re discovered in the viscera. ‘The fatty tissues continually dis- 
and as often the cavity of the body is filled more com- 
Pactly with nucleolated cells, and fat molecules. The newly formed 
ind “sophagus thickens at the end towards the proventriculus, and 
the first beginnings of the sucking stomach is indicated at 
AMER, NATURALIST, VOL. VIL 
