244 PROCEEDINGS : BOSTON SOCIETY NATURAL HISTORY. 
point for a large number of investigations upon the imaginal 
discs. He confined his studies to the Diptera; other insect 
groups were soon investigated. Landois (’71) following Herold, 
Agassiz, and most of the older authors, studied the imaginal wing- 
discs to be found beneath the dorsal thoracic integument of the 
caterpillar. Ktinckel d’Herculais (’75) showed that in the larva of 
Volucella, a muscid, the imaginal discs, although situated at a dis¬ 
tance from the integument, are connected with it by a delicate 
chord, the remnant of an invagination. He also discovered two 
pairs of imaginal discs near the hinder end of the body of the larva, 
which develop into the external genital organs. 
Ganin (’76) studied imaginal discs in several groups of insects, — 
namely, the Hymenoptera, Neuroptera, Coleoptera, Lepidoptera, 
and Diptera,—-in the larvae of all of which he found wing-discs, and 
in those which are apodous, leg-discs as well. He added to these 
observations the discovery, in the larvae of brachycerous Diptera, of 
other discs than those described by Weismann. That author 
believed that the hypodermis of the larval abdomen went directly 
with piodifications to form that of the imago. Ganin now showed 
that in, and forming a part of, the hypodermis of each of the eight 
abdominal segments of the muscidian larva, are four discs, two 
dorsal and two ventral, the tissue of which resembles that of the 
thoracic discs, and that they form the starting point for the growth 
of the imaginal hypodermis of the abdomen. Ganin likewise discov¬ 
ered similar discs in the epithelium of the larval mid-gut, whose fate 
it is to form in the same way the imaginal mid-gut; and he also dis¬ 
covered the important fact that each imaginal disc is made up of 
two kinds of embryonic tissue, ectoderm and mesoderm. Further¬ 
more he discovered the amoeboid mesoderm cells which destroy the 
larval organs during histolysis. 
Dewitz (’78) took up the study of imaginal discs for the purpose 
of reviewing the work of Ganin, Landois, and Weismann. A few 
years later Viallanes (’82) studied afresh the post-embryonic devel¬ 
opment of the Muscidae, and laid the basis of our present knowledge 
of the histological details of the process of histolysis in the meta¬ 
morphosis of insects. 
In 1882 and 1888 Metschnikoff published the first of his epoch- 
making studies on the destruction of tissues in certain invertebrates 
by leucocytes, or, as he called them, phagocytes. He discussed 
Ganin’s observations and especially that of the destruction of the 
