THE I M AGIN A L DISCS. 



83 



Kiinckel d'Herculais [25] figured and described three pairs of 

 appendicular discs in the resting larva of Volucella — a pair of 

 mandibular, a pair of maxillary, and a pair of labial discs. The 

 two latter correspond with those of the Blow-fly larva, but I 

 have been unable to find any traces of mandibular discs. 

 Weismann knew nothing of the appendicular head discs. 



The Thoracic Discs are arranged in two groups. Four neural 

 discs attached to the second and third pairs of nerves lie 

 beneath the neuroblast and the great cephalic discs. These 

 are the inferior, pro- and mesothoracic discs. Four pairs are 

 closely related to the great tracheal trunks, the upper pro-, 

 meso- and metathoracic, and the inferior metathoracic discs 

 (Fig. 16). 



Fig. 15. — Leg Disc= : /, the prothoracic leg disc from the adult larva; 2, longi- 

 tudinal section of the mesothoracic leg disc of the same ; j, transverse section of 

 the same ; a, neural stalk ; b, neck of sac ; c, nerve after traversing mesoblast of 

 the disc ; in, mesoblast ; s, sac of disc ; h, hypoderm ; tr, trachece. 



The inferior thoracic discs may be called for brevity leg 

 discs. The prothoracic pair are enclosed in a single sac, and 

 are connected with the hypodermis by two distinct necks 

 (Fig. 15, I h), and with the neuroblast by a pair of nerves. 



The mesothoracic leg discs are not united ; they lie below 

 and a little behind the prothoracic discs. 



The concentric structure which the leg discs exhibit in optical 

 section is due to the arrangement of the epiblast (Fig. 15). 

 The central papilla is the rudiment of the last tarsal joint, and 



6—2 



