248 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
double folds of the hypodermis, but become constricted off from 
it, as shown in^Figure D. In Musca the discs are also of embryonic 
origin. They also become constricted off from the hypodermis; 
but, instead of remaining where they have originated, they suffer 
removal toward the centre of the larva, as is shown in Figure E, 
and the peripodal membrane, as Van Rees calls the outer wall of 
the disc, lengthens to form the hollow cord which connects it with 
its old position at the hypodermis. 
In the cephalic discs the conditions are similar to those in Musca, 
but even more complicated. Instead of a single pair of head-discs 
there are two pairs present, one dorsal and one ventral. The dorsal 
pair corresponds to the muscidian head-discs-in every respect; they 
are destined to form the dorsal and lateral portion of the imaginal 
head together with the compound eyes. The ventral head-discs 
have no counterpart in Musca. In the embryo they arise as a 
single median thickening, from which paired diverticula develop. 
From the bottom of each of these diverticula, which in the larva 
project from the ventral pharyngeal Avail, there extends into its 
lumen a long projection. The diverticula fuse in the median line 
during the latter portion of the larval period, and the wall thus 
formed betAveen them gradually disappears, so that in the full-grown 
larva the ventral discs appear as a single ventral diverticulum of the 
pharynx, at the bottom of which a pair of long projections extends 
tOAvards the Avide opening. The fate of these discs is to form the 
ventral portion of the head, the paired projections being the funda¬ 
ments of the proboscis. The formation of the head-vesicle during 
the metamorphosis proceeds in a way similar to that in Musca. 
The ventral disc fuses early at its lateral edges with the dorsal pair; 
the communications betAveen both ventral and dorsal discs and the 
pharynx become rapidly larger (in the old larva they have already 
become very Avide), and soon the discs and the pharynx form 
together a single vesicle, the head-vesicle. 
2. Imaginal Discs in the Embryo. — As already stated, the em¬ 
bryonic history of imaginal discs has not been studied in the higher 
insects. The folloAA T ing are the speculations of authors as to their 
origan. 
O 
In Corethra, a nematocerous dipteron, Weismann (’66) found that 
the imaginal discs do not make their appearance until after the last 
larval moult. In the Bracliycera, on the other hand, he found the 
cephalic and thoracic discs present in the youngest larvae, and he 
