THE IMAGINAL DISCS. 73 



These exist in the larva as encapsulated groups of embryonal 

 cells, and vary greatly in form ; for the several discs not only 

 differ from each other, but exhibit considerable variations at 

 different stages of development. Many are capable of easy 

 demonstration in the adult or even in the young larva ; whilst 

 others do not become apparent until the nymph is partially 

 developed. As all the discs preserve an embryonic character, 

 it is probable they are all present in the young larva, or even in 

 the embryo, as distinct groups of cells, but only the larger ones 

 have been actually demonstrated at the earlier periods of de- 

 velopment ; and it is only in the later stages that the smaller 

 discs can be safely recognised. 



History. — The larger discs in the larva of the MuscidEe were 

 first seen by Swammerdam [4], who, from their relation with 

 the nerve-centres, mistook them for ganglia. Lachat and 

 Audouin [31] termed them ' Plaques ' ; Leon Dufour [32], 

 * Corps ganglionoides ' ; and Leuckart [20] and Scheiber [21] 

 described them as ganglia. Weismann [2] discovered their 

 true nature, and named them ' Imaginal Schciben,' of which 

 the English equivalent, Imaginal discs, is generally accepted. 

 Ganin [34], in a paper in Russian, added much to our know- 

 ledge ; but, unfortunately, Ganin's work is only accessible to 

 me by short translations of certain portions of it [27, 34]. 



Morphology. — The view of the nature of the discs which I 

 have adopted (p. 2i), although supported by the researches of 

 Dewitz [35], Kunckel d'HercuIais [25], and others, is at vari- 

 ance with the views of Weismann [2] and Ganin [25, 27, 34]. 

 Both these distinguished naturalists examined the structure of 

 the thoracic discs with great care, at the earliest stage of de- 

 velopment in which they are recognisable, and concluded that 

 they originate from the nerve-sheaths and the peritoneal tissue 

 of the tracheae to which they are attached. 



It must be confessed that in the earlier stages of develop- 

 ment, appearances favour the views of Weismann and Ganin ; 

 on the other hand, the hypothesis which derives epiblastic 

 structures from the mesoblastic connective tissues, and severs 

 the development of the Muscid^e from all other insects, is one 



