DEVELOPMENT OF THE EXTERNAL FORM OF THE IMAGO. 



377 



or tubular portion of the " brain appendages " we find the frontal 

 disc (ss), from which the antennal rudiment soon grows out in just 

 the same way as do the limb-rudiments from the base of the imaginal 

 disc that gives origin to them. 



Originally (Fig. 185 ^4) the "brain appendages" lie somewhat far 

 back, in the thorax of the larva, so that they connect the posterior 

 part of the wall of the pharynx with the most anterior segment 

 of the brain. Later, however, after the pupal stage has been entered 

 upon, they, together with the central nervous system, shift further 

 forward (Fig. 185 B), so that their anterior extremities, which are 



Fig. 186.— Diagram illustrating the transformations that take place in the pupa of Mvsca 

 before it hatches (adapted from Kowalevsky and Van Rees). The wing-rudiments are not 

 drawn, as, optic disc ; at, antennal rudiment ; b 1 , b-, b 3 , rudiments of the three thoracic 

 limbs ; bg, ventral chain of ganglia ; g, brain ; k, cephalic vesicle (formed by the union of 

 the pharynx with the brain appendage) ; oe, oesophagus ; r, rudiment of proboscis ; ss, 

 frontal disc ; /, II, III, the three thoracic segments. 



now bent ventrally, embrace the pharynx laterally (if we have 

 rightly understood the descriptions of Weismann and Van Eees). 

 At the same time the communication between the "brain-appendages" 

 and the pharynx (Fig. 185 B, o) becomes wider and wider, and soon 

 extends in the form of lateral oesophageal slits along the whole 

 length of the brain-appendages. By this means, the lumina of 

 the brain-appendages and of the pharynx flow together so completely 

 that the two soon represent only one single vesicle, the cephalic 

 vesicle (Fig. 186, k). The walls of the cephalic vesicle are nothing 



