22 



PSYCHE. 



[Februarj' iSy?. 



head-discs in every respect; lliey nrc 

 destined to form tlie dorsal and lateial 

 portions of the imaginal head together 

 with the compound ej'es. The ventral 

 head-discs have no counterpart in 

 Musca. In the embryo, as we shall see in 

 a moment, they appear as a single median 

 thickening, but in the young larva they 

 have become paired diverticula of the 

 ventral pharyngial wall ; and at the bot- 

 tom of each diverticulum there arises a 

 projection. These diverticula soon fuse 

 in the median line an'd the wall between 

 them gradually disappears. In the old 

 larva, consequently, the ventral discs 

 appear as a single ventral diverticulum 

 of the pharynx at the bottom of which 

 a pair of long projections extend toward 

 the wide opening. The fate of these 

 discs is to form the ventral portion of 

 the head, the paired projections forming 

 the anlage of the proboscis. 



The formation of the head-vesicle pro- 

 ceeds in a way similar to that in Musca. 

 The ventral disc fuses early at its lateral 

 edges with the dorsal pair. The com- 

 munications between both ventral and 

 dorsal discs and the pharynx rapidly 

 widen (in the old larva they have already 

 become very large), and soon tlie 

 discs and pharynx form together a 

 single vesicle, which is the head-vesicle. 



The imaginal discs of the abdomen 

 in Melopliagus are exactly the same as 

 the corresponding ones in Musca. 



1 shall now speak of the origin and 

 early development of imaginal discs. 

 But little has been published on this 

 subject and nothing, so far as I know, 

 on the origin of muscidian discs. 



Weismann* showttl in iS66 that in 

 Corethra, that is, in the neniatocera, the 

 thoracic discs arise as ectodermic infold- 

 ings after the last larval moult, and just 

 before pupation (Fig. 3). This is 

 rathei' surprising, as we should expect 

 these discs, as they represent extremi- 

 ties, to appear in an earlv embryonic 

 period when the extremities usually 

 arise in insects. In Musca, Weismannf 

 ilid in fact find that the imaginal discs 

 arise in the embryo, but their genesis 

 was not observed by him, and, as he 

 found them invariably attached to a tra- 

 chea or a nerve, he made the mistake of 

 supposing that they take their origin in 

 the epithelial coverings of these organs. 

 This method of growth was, however, 

 early discredited by Ktinckel d"Hercu- 

 laist who, in 1875, found the chord con- 

 necting the thoracic discs with the hvpo- 

 dermis, and ]ightly concluded that they 

 have an ectodermic origin. Balfour§ 

 also, in his text book, declared that the 

 thoracic and cephalic discs must be de- 

 rivatives of the ectoderm in Musca as 

 they had been proved to be in Corethra. 

 Dewitzll in 187S confirmed d'Herculais' 

 observation of the chord connecting the 

 disc with the hypodermis, and Van 

 Rees some years later in his paper al- 

 readv quotedlT showed that this chord is 



*\Veisraann. Die Metamorphose v. Corethra etc. p. 7S. 

 tWeisniann. Die iiachembry. Entvv. v.d. Musciden, etc., 



p. 223- 



il. c. p. .46. 



JBalfour. Comparative Embryology, vol. I, p. 425. 



llDewitz. Reitrage z. Keimtnis d. postembry. Glied- 

 massen 1). d. Insecteii. Zeit. f wiss. Zool. Bd. 30. 

 Sup. p. 95. 



ri. c. p. 25 



