PRATT: IMAGINAL DISCS. 
259 
The Origin of the Imaginal Discs. 
1. The Cephalic Discs: — a. The Early Development of these 
Discs. — Soon after the stage of development represented by Figures 
11-13 (PL 2), but before the lateral mesodermal bands have 
extended to the dorsal side of the embryo and joined each other in 
the mid-dorsal line, and before the endoderm has extended around 
from the dorsal to the ventral side of the embryo (Fig. 15, en’drm ), 
those imaginal discs which are destined to form the imaginal head 
have made their appearance. Three crescentic thickenings of the 
ectoderm appear in the vicinity of and partly encircling the stomo- 
deurn, a median one in front of the stomodeum, and two paired ones 
behind it (PI. 3 , Fig. 17). The median thickening {else. ce. m.) is 
destined to form the ventral cephalic disc, which during the meta¬ 
morphosis develops into' the ventral portions of the imaginal head 
together with the mouth-parts ; the paired thickenings {else, ce.) 
form the dorsal cephalic discs, which are homologous with the 
cephalic discs of the Muscidae, as described by Weismann, Van 
Rees, and others. These develop during the metamorphosis into 
the dorsal and lateral portions of the imaginal head. The median 
disc has no homologue in the Muscidae. 
There is no period in the development of the embryo of Melopha- 
gus when a distinct head is present, although it seems probable that 
the head-fold, since it forms the forward end of the germinal band, 
represents the head. It never shows, however, the slightest trace 
of segmentation. In Musca, according to Weismann (’ 64 ) the head 
is, at a certain stage of the embryo’s development, quite as distinctly 
segmented as the trunk. As will be seen in Weismann’s Figure 71, 
the embryo is divided into fourteen or fifteen segments, of which 
eleven belong to the body, and the remainder to the head. The 
anterior portion of the germinal band does not as in Melopliagus, 
extend onto the dorsal side of the egg, the stomodeum being sub¬ 
terminal. As the development of the embryonic fly advances, the 
cephalic portion gradually becomes reduced in size relatively, and 
its segmentation becomes less distinct. When the young larva is 
finally born its head has been reduced to a mere rudiment at the 
forward end of the body, without a trace of segmentation, and is 
smaller than any of the body segments. Consequently, of the 
twelve segments composing the fly larva the first alone represents 
the head. 
