PRATT: IMAGINAL DISCS. 
267 
cerebral nerve-masses (gn. cb.), as is also shown in the parasagittal 
section (PI. 5, Fig. 33). The ganglia allata ( gn . ala.) and their 
fusion to form the median nerve are shown in Figure 43. The 
forward end of the heart (cr.) is also cut. Figure 44 represents a 
section posterior to the dorsal discs and through the hinder part of 
the ganglia allata. Mesoderm does not make its appearance in 
the cephalic discs, nor do nerves or tracheae enter them. 
The process of the involution of the embryonic head of Melopha- 
gus, by which the ventral cephalic disc, the sucking tongue, and 
the median opening of the dorsal cephalic discs, are changed from 
external to internal organs, and the head of the animal telescoped 
into the thorax, is really the final act of a longer operation. The 
head of the embryo, which, in my opinion, is represented by the 
head-fold of the germinal band, begins to disappear when the stomo- 
deum begins its migration towards the anterior end of the embryo. 
The involution of the head has been observed in only one other 
representative of the brachycerous Diptera besides Melophagus, 
viz. in Musca vomitoria by Weismann (’64). Here the disappear¬ 
ance of the head is also a gradual process. The embryonic head 
gradually grows shorter and at the same time loses its segmenta¬ 
tion, its posterior edge moves forward until finally its anterior 
portion invaginates into the mouth, forming the so-called 'larval 
pharynx. The dorsal cephalic discs then appear as appendages of 
the dorsal pharyngeal wall. But Weismann did not observe the 
method by which these structures originate. 
2. The Thoracic Discs .— Six pairs of thoracic imaginal discs 
make their appearance in the embryo of Melophagus. Three pairs 
are dorsal and three pairs ventral, a dorsal and a ventral pair 
belonging to each of the pro-, meso-, and metathoracic segments, 
respectively. The three ventral pairs give rise during metamor¬ 
phosis to the three pairs of imaginal legs; the dorsal metathoracic 
pair to the rudiments of the balancers, and the dorsal meso- and 
prothoracic pairs to no adult structures, they being rudimentary 
structures. All of these discs first appear late in the embryonic life 
of the insect; namely, at about the time of the involution of the head, 
— at a time when the formation of the dorsal cephalic discs is prac¬ 
tically complete, when the back of the embryo is closed, and the 
wall of the intestinal tract is formed. 
The earliest appearance of the thoracic discs figured is that 
shown for the embryo represented in Figure 32 (PI. 5), in which the 
i 
