THE IMAGINAL DISCS. 75 



appear to depend on the period at which the discs are first 

 developed. 



' In the abdominal region of the fly the discs first appear at 

 the end of the larval period, so that their evolution follows 

 immediately; thus they remain superficial. In the thorax of 

 Corethra — Weismann — and of Volucella — Kiinckel — the discs 

 are formed at the commencement of larval life, and become 

 invaginated, only retaining their union with the integument by 

 a [hollow] pedicel of variable length. In the thorax of the fly 

 they are formed much earlier; they already exist in the embryo 

 in the &gg. It is perhaps on this account that there is no ap- 

 parent connection with the integument ; but further researches 

 on this point are needed. 



' It is, perhaps, important to observe that this peculiar method 

 of development is not confined to the Insecta. The researches 

 of Barrois have established the fact .hat in Pilidium, amongst 

 the Nemertids, the phenomena observed are similar, since in 

 these worms the larval exoderm is shed and is replaced by one 

 developed from germs analogous to imaginal discs ; and the 

 analogy is rendered the more striking, as in the different species 

 of Nemertid the same differences are met with as in insects 

 exhibiting a higher or lower degree of metamorphosis ' [27, 

 p. 224]. 



My own researches show that in the adult larva of the Blow- 

 fly there is precisely the same connection between the epiblast 

 of the disc and the hypoderm as that which has been observed 

 in the ant (Dewitz [35] ), and in Corethra [33] and Volucella [26] 

 (Fig. 15, 2). Moreover, in the maxillary discs and those of the 

 anterior spiracle the sac is still a wide open ampulla, and the inva- 

 gination of the frontal lobes has been already referred to (p. 42). 

 I attribute the failure on the part of Weismann and Ganin to 

 trace the connection as due to the difficulties which attend the 

 investigation in young larvee. 



In the advanced types of disc, such as the leg discs of the 

 fly (Fig. 15), the invaginated hypoderm exhibits a distinct 

 differentiation into two parts, a disc sac, s, the provisional 

 capsule of Ganin, and the epiblast of the disc. The latter 



