22 



As the principal studies on this subject I may mention those 

 of Landois 1871, Ganin 1876, Pancritius 1884, Dewitz 1887, 

 VAN Bemmelen 1889, Bugnion 1892, Gonin 1894, Mayer 1896, 

 Mercer 1900, Bauer 1904, v. Voss 1912. 



Gonin's work especially is often cited. He examined in the 

 first place Piens brasslcae and says (1894) of the origin of the 

 so-called disque imaginal that it is better to use the term repli 

 imaginal. 



I wish to point out the concurrence between him and Lijonet. 

 „The repli imaginal originates from evagination of the hypoder- 

 mis, preliminarily invaginated. The part which the tracheae and 

 the nerves play in this formation is a secondary one. The tracheae 

 are neither the cause of the duplication nor of the extension of 

 the walls of the wing. The rudiments of the wings are developed 

 from the first larval age but do not participate in the larval 

 moults, their surface does not produce a cuticle till towards the 

 end of the last stage". 



Mercer who I think, has been one of the latest investi- 

 gators of these organs, has found in Pieris also that the rudiments 

 of the wing grow. 



In a few words his results may be summarized as: instar /, 

 over against a trachea a thickening of the hypodermis is found, 

 in the middle of which lies a cavity, and round the trachea 

 some detached cells (= lymphocytes ?). Instar 7/, a chitinous plug 

 penetrates into the cavity ; Instar 7/7, the wingbud arrives in 

 the body cavity, but remains connected with the hypodermis by 

 the „peripodal membrane" (v. Rees). The trachea becomes larger 

 and grows into the two layers of the hypodermis. Instar 7F, 

 the tracheoli enclosed in the wing-bud reach the edge. Instar F, 

 the tracheoli grow further, the „ wing-rudiment" reaches the leg 

 and becomes folded. Pupa, the larval tracheoli disappear and are 

 replaced by a quite different pupal system. 



Bauer (1904) adds to this that the form of the wing-folds is 

 only governed by mechanical forces. If the thick larval cuticle 

 is an obstacle, the fold becomes invaginated, but directly evagi- 



