144 JOSEPH KRAFKA, JR. 



Weismann (1864) was the first to show the relation of the imag- 

 inal disks to adult structure. The development of these disks can 

 be readily followed in Drosophila either by direct dissection or by 

 serial section. 



The primordia of the imaginal disks of the compound eye arise 

 as invaginations of the so-called pharynx, with which they retain 

 their connection by a delicate membrane. They can be found as 

 late as the two-day larva , anterior to and independent of the brain 

 or supra-esophageal ganglia ( Fig. i ) . 



The posterior migration of these disks results in their fusion 

 with the anterior aspects of these ganglia. This connection is of 

 a membranous character and the disks and brain may be readily 

 removed together in dissection. This condition is met within the 

 three-day larva (Fig. 2). As determined by temperature experi- 

 ments on the bar-eyed mutant, this is the critical period in facet 

 formation. 



That the rudiments of the ommatidia are fixed at this time is 

 shown by the peculiar arrangement of cells in the imaginal disks 

 of the four-day larva. Four terminal cells and six basal cells, 

 arranged around a deeply staining axis, form a cylindrical unit 

 that is repeated over the entire inner surface of the imaginal disk 

 (Figs. 3 and 4). 



Pupation occurs on the fifth day of larval life. The imaginal 

 disks take their position as a part of the body wall of the pupa. 



On the sixth day the various parts of the dioptric apparatus are 

 clearly recognizable as such. The rudimentary ommatidia are 

 spherical in shape, with four terminal pseudocone cells and six 

 retinuke cells arranged around a rhabdome. The ommatidia are 

 supported by a double row of pyramidal cells, one basal and the 

 other peripheral. The peripheral cells underlie the short optic 

 bristles (Figs. 5 and 6). 



Material slightly more advanced shows an elongation of the 

 retinulse cells and the rhabdome. The outer surface of the four 

 pseudocone cells is convex and a very thin cuticula has formed. 

 This separates from the ommatidia in the process of fixation in 

 the same manner as the corneal facets (Fig. 7). 



By continued elongation the rhabdome and the retinulse cells 



