250 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
tions of the larval hypodermis. Graber (’77-’79), however, thought 
otherwise. In his valuable treatise “Die Insekten,” part 2, p. 563, 
following the results of the first paper of Dewitz (’78), which had 
just been published, he introduces a diagram (Fig. 206, E) repre¬ 
senting the thoracic discs taking their rise as deep invaginations of 
the hypodermis. He does not, however, commit himself to this 
interpretation, but says “ Eine solche Annahme entbehrt aber 
vorlaufig jeglicher Begrtindung, und so dtirfte es doch besser sein, 
das Vorkommen einer endogenen Insekten-Metamorphose einfach 
anzuerkennen als sie mit Gewalt zu einem Vorgang umzudeuteln . 
der mit den bishermen Beobaclitinmen nicht ubereinstimmt.” 
O O 
Shortly after the publication of the above-mentioned paper of Van 
Rees, Graber (’89) published an extensive series of studies on the 
embryonic development of the Muscidae and certain other insects, 
in which the origin of the imaginal discs is touched upon. He 
adheres to the conclusions of his former work just quoted, and 
takes exception to those of Van Rees. Supported by certain obser¬ 
vations on the embryonic development of Lucilia, he asserts that 
the anatomical evidence of that author does not prove the cephalic 
and thoracic discs to be hypodermal invaginations. Graber’s obser¬ 
vations were made on the cephalic discs and were as follows: The 
first traces of these discs which he found were a pair of thick plates 
composed each of a single layer of epithelium situated at the right 
and left of the pharynx of the larva and in connection with its lat¬ 
eral walls (see his Figs. 116, 117, 117*). The origin of these 
plates, therefore, he did not observe, but the fact that when he first 
noticed them they were interjial structures, and the further fact 
that they were not at this early stage in the form of sacs, but of 
plates, led him to think that the sac-form these discs possess in their 
later and larval developmental stages is a secondary adaptation. 
In other words, he believed these discs to arise in the body-cavity 
of the animal as epithelial thickenings which afterward assume a 
sac-fonn. He asserts also.that the possibility cannot be denied that 
the thoracic imaginal discs have a similar origin. In fact, this is 
the interpretation of the matter he himself believes. 
In later years, Verson (’90) and Gonin (’94), after studying Lepi- 
doptera, Bugnion (’91), after studying a hymenopteron, and Wahl 
(’99), a dipteron (Eristalis), have adopted the opinion of Van 
Rees, that imaginal discs are of ectodermal origin. 
My pwn investigations on the embryology of Melophagus will 
