PRATT: IMAGINAL DISCS. 
243 
Agassiz (’51) somewhat later. Leuckart (’57) mentions the tho¬ 
racic and cephalic imaginal discs of Melophagus, hut without describ¬ 
ing them or knowing their ultimate fate. 
It was in the years 1863 to 1866 that Weismann (’63, ’64, ’66) 
laid the foundations of our present knowledge of imaginal discs in 
a series of investigations on the development of Musca and other 
Dipt era, both brachycerous and nematocerous. He (’63, p. 229) found 
that the dipterous larva, which is apodous, and in the case of the 
Bracliycera, acephalic, contains within its thorax six pairs of disc-like 
bodies. In Simulia, a nematocerous dipter, which is the first form he 
studied, they are situated near the animal’s integument, although 
apparently not in connection with it. There are three pairs ven- 
trally located, each pair belonging to one of the thoracic segments, 
and three pairs dorsally located, these being similarly distributed 
among the thoracic segments. All of these bodies he found in close 
relation to nerves or tracheae or both. He further found that they 
remain functionless during the life of the larva, although increas¬ 
ing greatly in size with the growth of the larva, and that during the 
metamorphosis they develop into certain organs of the imago. 
He called them, consequently, imaginal discs. He found that the 
three pairs of ventral discs develop into the imaginal legs, the 
dorsal metathoracic pair into the balancers, the dorsal mesothoracic 
pair into the wings, and the dorsal prothoracic pair into the anterior 
pupal spiracles, when these are present. 
In Corethra (Weismann, ’66), also a nematocerous dipter, he 
found similar conditions. 
In Musca he (’63, ’64) found the conditions very much more 
complicated. The six pairs of discs just mentioned he found pres¬ 
ent; but, instead of being located near the integument, they were 
sunk into the centre of the animal’s body. An additional pair of 
discs was also present in the forward portion of the thorax, directly 
in front of and closely applied to the brain-ganglia ; these lie found 
were destined to develop into the imaginal head. Weismann also 
found that only a small portion of the larval body passes directly 
into the imaginal body, the greater part of it undergoing disintegra¬ 
tion, so that the tissues entirely lose their identity, and afterwards 
the imaginal body is built up anew from the imaginal discs. To 
this process, the entire significance of which, however, was not 
understood until later, he gave the name “ histolysis .” 
These early papers of Weismann have furnished the starting- 
