26 



PSrCHE. 



February iSg;. 



before it reaches its final form.* The 

 forward ami of the animal does not 

 change materially, now. from the con- 

 dition we find represented in Fig. 8 

 during the remainder of the embryonic 

 and the entire larval period. Early in 

 the pupal period, however, the dorsal 

 and ventral discs unite to form the head 

 vesicle, as explained in a previous 

 portion of this papi;r, which becomes 

 the imaginal head. 



So much for the origin and embryonic 

 history of the cephalic imaginal discs. 

 The thoracic discs (with the exception of 

 the dorsal prothoracic which arise in 

 the larva) do not make their appearance 

 until the time of the involution of the 

 head. This is late in the embryonic life 

 and at a period when its organization 

 and the formation of the head discs are 

 completed. The thoracic discs arise, 

 like the cephalic, as ectodermic thicken- 

 ings. A dorso-ventral section to one 

 side of the median line of the same em- 

 bryo of which Fig. S represents a sagit- 

 tal section, is shown in Fig. 9. The 



Fig. 9. Dorso-ventral section of anterior end of embryo ; 

 Pro^ MeSt and Met, ventral pro-, meso-, and metathoracic 

 discs ; Mes. D, dorsal mesotboracic disc ; //, bead-disc ; /, 

 intestine ; Z>, dorsal side ; K, ventral side. 



three ventral ectodermal thickenings 

 are the beginnings of the ventral pro- 

 nieso- and metathoracic imaginal discs 

 on one side of the embryo. The dorsal 

 thickening is the beginning of the dorsal 

 mesotboracic disc. The dorsal metatho- 

 racic disc does not appear in this section. 

 All these discs begin very soon to invag- 

 iiiate, the ventral prothoracic begin- 

 ning first, then the me.sothoracic, and 

 finally the metathoracic taking its turn. 

 The invagination begins in each disc at 

 its posterior border as is shown in the 

 ventral metathoracic disc in Fig. 10; 

 then the anterior border sinks in as is 

 shown in the mesotboracic disc in Fig. 

 10 ; finally the entire disc sinks beneath 

 the surface as is shown in the prothor- 

 acic disc in Fig. 10; then the ecto- 



Fig. 10. 1'ro, Mes, and Met, 

 ventral pro-, meso-, and ineta- 

 tboracic discs, showing begin- 

 ning of invagination. 



derm closes over it as is shown in 

 Fig. II, at which stage it is com- 

 parable to the thoracic discs in Coretlira 

 as represented in Fig. 2. Tlie 

 disc now becomes separated from 

 the ectoderm, the thickened middle 

 portion sinks in and forms the disc 



