PS re HE. 



21 



sections thiougli the anterior end of an 

 old larva showing the position of the 

 thoracic discs. The dorsal prothoracic 



Fig. I 

 lelathoi 



B, dorsal frontal s 

 pole ; Pr, Mes and Met, pro-, meso-. 

 ginal discs ; b, body wall. 



pair arise during the larval period and 

 are not embryonic organs as are all the 

 others (Van Rees noticed the same 

 fact about the corresponding muscidian 

 discs). They are invaginations of the 

 larval hypodermis and have external 

 openings which do not close, and their 

 inner surface is lined with a delicate 

 cuticula which is a continuation of the 

 external cuticula of the larva. These 

 discs are rudimentary organs and do 

 not develop into any pupal or imaginal 

 organs. In structure the meso- and 

 metathoracic discs stand exactly mid- 

 way between the same discs in Corethra 

 and in Musca. In Corethra* all tiie 

 thoracic discs are of larval oritrin, aiis- 



Fig. 2. Z>, imaginal disc 

 J Corethra; H, hypoder- 



' Weismann, Corethi 



Fig. 3. D, imaginal disc 

 in Melophagus ; H, hy- 

 podermis ; C, cuticula ; P , 

 parapodial membrane. 



1. c. p. 7»- 



ing as they do after the last larval moult, 

 and each one is a double fold of the 

 hypodermis, of which it remains a part 

 as Fig. 2 shows. In Melophagus, on 

 tlie other hand, each of these discs 

 arises in the embryo, as is also the case 

 in Musca ; it is also a double fold ofthe 

 hypodermis but becomes constricted oft' 

 from it as is shown in Fig. 3. Van 

 Rees' t has called that portion of the 

 invaginated hypodermis which encloses 

 the disc proper, the .parapodial mem- 

 brane (P. Figs. 3 and 4), and the space 

 it encloses, the parapodial space. In 

 Muscat, now, the disc not only becomes 

 constricted oft' from the hypodermis but 

 suft'ers removal to the center of the larva 

 as is shown diagramatically in Fig. 4, and 

 the parapodial membrane lengthens to 

 form the hollow chord which connects it 



Fig 4, D. imaginal di 

 cuticula; /^.parapodial 



with its old position at the Inpodermis. 

 The fate of the thoracic discs in Melo- 

 phagus is exact! V the same as in Coicthra 

 or in Musca. 



In the cephalic discs, now, we ftiul 

 the conditions similar to those in Musca, 

 but even more complicated. Instead of 

 a single pair of head-discs we find two 

 pairs, one dorsal and one ventral. The 

 dorsal pair corresponds to the muscidian 



tl. c. p. 24. 



{Van Rees. 1. c. p. 23. 



