PRATT: IMAGINAL DISCS. 
249 
concluded that they must arise in the embryo. Concerning, however, 
the exact manner in which they take their origin, he was not able 
to make any positive statements, but was of the opinion that they 
first appear within the body-cavity as proliferations of the epithelial 
coverings of tracheae and of the neurilemma of nerves. In Core- 
thra, on the other hand, he observed that the discs arise as folds 
of the larval hypodermis, and he calls attention to the fundamen¬ 
tally different method of origin of imaginal discs in these represent¬ 
atives of the two great divisions of the Diptera. 
Landois ('71) , investigating the development of wings in the larva 
of Lepidoptera, follows Weismann in his conclusions as to their ori¬ 
gin in the embryo. Ganin (’76) also supports Weismann as to the 
embryonic origin of imaginal discs in all the different groups of 
insects he studied. Ktinckel d’Herculais (’75) was the first one who 
discredited Weismann’s conclusions. He found each disc in the 
muscidian larva to be connected with the hypodermis by means of 
a cord, and he rightly concluded that it resulted from an invagina¬ 
tion and that the discs are therefore of ectodermic origin. Weis¬ 
mann (’64, p. 139) also, it is true, saw these connecting cords, but 
failed to interpret them correctly. Dewitz (’78, ’81) after a study 
of lepidopterous larva came to the same conclusion concerning the 
origin of the imaginal discs as Ktinckel d’Herculais, namely, that 
they are ectodermal invaginations. Pancritius (’84), also studying 
the Lepidoptera, reached the same conclusion. Balfour, in his text¬ 
book (’80) declared that, notwithstanding the authority of Weis¬ 
mann to the contrary, the cephalic and thoracic imaginal discs of 
Musca must be derivatives of the ectoderm, as they are in Corethra. 
Ivowalevsky ('86), after a study of the embryo of Musca, declared 
himself unable to determine the method of origin of the imaginal 
discs. He arrived at the negative result, however, that they do not 
arise as growths from the epithelium of tracheae, but that tracheae 
and nerves unite with them while they are still young. 
Van Rees (’88) in his studies of the muscidian larva, demonstrated 
the existence of a fine lumen, a continuation of the peripodal space, 
in the cord connecting the disc with the hypodermis, and showed 
that both lumen and peripodal space are lined with a fine cuti- 
cula. He asserts that this discovery is anatomical proof that the 
cord, the peripodal membrane, and the disc itself have all been 
parts of a single invagination of the embryonic ectoderm in exactly 
the same way as the imaginal discs in Corethra arise as invagina- 
