PRATT: IMAGINAL DISCS. 
245 
larval organs by amoeboid mesoderm cells during histolysis in the 
pupal muscid. These cells, he suggests, are none other than phago¬ 
cytes. Shortly afterward, Van Rees (’84) and Kowalevsky (’85) 
proved the correctness of this position. They showed that the 
process of histolysis consists of the ingestion and digestion of the 
functional larval tissues by phagocytes, and the building up of 
imaginal tissues from imaginal discs. 
Kowalevsky (’87) in another paper took up the investigation of 
the histolysis of the internal organs of Musca where Ganin and 
Viallanes had left it, and gave the first complete account of these 
processes. He showed the exact method by which the muscles, 
digestive tract, and hypodermis of the larva are destroyed by 
phagocytes and the imaginal organs reconstructed from imaginal 
discs. And in the following year Van Rees ( ! 88) published his 
extensive paper on the post-embryonic development of muscids, and 
completed our knowledge of this phenomenon. He showed that 
when the muscidian larva enters upon the pupal stage, histolysis is 
inaugurated by the destruction of the larval muscles, which become 
unfunctional directly after pupation and a natural prey to the 
phagocytes. Soon the thoracic hypodermis and the inner organs 
are attacked, and at the same time the imaginal discs begin to grow 
and widen out, supplying the place of the tissues which are being 
destroyed. The continuity of the hypodermis and of most of the 
internal organs is thus at no time broken, an observation which 
Kowalevsky (’87, p. 585) also made, correcting at the same time the 
statement to the contrary made by Viallanes (’82, p. 221). As these 
processes go on, the two large cephalic imaginal discs, which form 
two irregularly shaped sacs extending as diverticula from the 
dorsal wall of the pharynx back to the brain, begin to move for¬ 
ward, dragging the brain with them. Their anterior ends bend and 
pass ventrally, embracing the pharynx between them. At the same 
time their communications with the pharynx enlarge and their 
lumina fuse more and more completely with the pharyngeal lumen 
until they meet in the median line and form one single median 
opening, which, ever increasing in size, finally extends the entire 
length of the discs. The lumina of the discs and of the pharynx 
thus become completely merged and form together a single con¬ 
tinuous space, and the Avails of the discs and of the pharynx a single 
continuous vesicle. This is the head vesicle or “ Kopfblase,” which 
is destined to become the imaodnal head. This vesicle remains 
