610 THE METAMORPHOSIS OF FLIES. 
this period. The chyle-stomach gradually changes in length, its 
walls are transparent and clear, and it is strikingly demarked from 
the dark small intestine rendered so by the walls filled with fat 
corpuscles. With this begins the period of decay, and it reaches 
on the seventh day its complete development. 
Already during the course of seven days usually begins the 
second subdivision of the second period which is characterized by 
the relative position and development of all the organs of the im- 
ago. On the seventh day we find in the cavity of the thorax the 
first trace of the muscles of the wing. Series of cells of the greatest 
fineness pass in determinate directions through the liquid masses 
of fat, and up to the fourteenth day increase in thickness, until 
finally they lie close together to the lateral spaces of the thorax, 
and only leave in the median line a slight space for the free pas- 
sage of the stomach. Their structure is, then, usually definitive, 
it is a sarcolemmous sheath filled with contracted fibres which lie 
together in fascicles, and are kept separate from one another by nu- 
cleated columns. Meanwhile out of the fragments of the old 
intestinal canal appears the new, and shortly after this is 
accomplished there is a union of the small intestine and rectum, 
and by the tenth day the rectal pouch is placed in relation with 
the four rectal apille. At the same time a new plexus of mus- 
cles begins to form on the upper side of the entire intestinal tract. 
Still the most important steps in the formation of the principal 
organs of sense of the fly, the compound eyes, fall into this last 
section of its deyelopmental history. The ocular disks, which 
originated out of the hinder division of the brain-appendage, ' 
still connected with the bulb at the beginning of the second period 
by means of a slender nerve. The bulb gradually extends itself 
so that it covers the whole interior of the eye-disk, and only be 
comes separated from it by a thin layer of fat, which has already 
arisen between the two parts. The bulb shows radiating streaks, 
which are indications of the nervous threads passing through it. 
Only out of the eye-disks will the true eyes be formed, i. €., the 
compartments with the dioptric apparatus, and the perceptive ner 
vous elements. On the twelth day, however, the disks and also 
each compartment leading out of it, have the very small diameter 
of 0-051 ™™., which is gradually at the close of the pupa state ®®- 
arged five times, while at the same time the cellular elements 
lying behind each corneons facet, forms for each chamber a Cry% 
