THE IMAGINAL DISCS. 77 



the hypodennis. In the later stages it becomes so thin that 

 its original cellular character can no longer be recognised. 



The Mesoblast of the Disc was first discovered by Ganin ; it 

 consists of a stroma of stellate cells, permeated by fusiform 

 cells, tracheae, and nerve-fibres. It frequently covers the whole 

 outer surface of the provisional sac, but is thickest in the 

 hollow of the disc itself, in which a cavity is usually present, 

 which communicates with the body cavity of the larva, and, 

 like the latter, is filled with blood. 



The origin of the mesoblast of the disc, like that of the meso- 

 blast of the embryo, is unknown. Kiinckel d'Herculais [25] 

 supposed it to be developed from peritracheal ceils, or from the 

 leucocytes of the larva, but left these alternatives unsupported by 

 facts. Its continuity with the tissue of the nerve and tracheal 

 sheaths is perhaps in favour of the former view ; but it is equally 

 probable, I think, that it is developed by differentiation from the 

 disc itself. This much is certain, it cannot be demonstrated 

 in the earlier stages of disc development. 



b. The Cephalo-Thoracic Discs. 



The cephalo-thoracic discs may be studied in the larva ; the 

 abdominal discs are more readily demonstrated during the 

 earlier stages of the development of the nymph. 



There are nine pairs of discs concerned in the development 

 of the head and thorax. The great cephalic discs, and the 

 maxillary and labial discs, form the head and proboscis, an 

 upper and a lower pro-, meso- and metathoracic disc on each 

 side unite to form the thorax. 



The Great Cephalic Discs (PI. IV., op d, an d, and pc d, and 

 Fig. 13) are formed, as already indicated, by an invagination 

 of the frontal lobes of the embryo (Fig. 7).* They extend from 

 the posterior extremity of the cephalo-pharynx to the anterior 

 portion of the neuroblast, and are suspended by the cephalo- 

 pharyngeal band and the ring (Fig. 13, cp, and Fig. 14, r). The 

 manner in which they are disposed will be understood by a 



* Weismann [33] has figured a similar condition in Chironomus ; com- 

 pare his Figs. 17 and 25 with my Fig. 7. 



