February rSgy.] 



PSl'CHE. 



29 



gin is exactly the same in all. The\' 

 all arise as ectodermal thickenings 

 either in the embryo or in the larva ; 

 and further, the position of eaclj pair 

 whether dorsal or ventral, in each seg- 

 ment whether thoracic or abdominal, 

 is the same as that of every other pair. 

 The thoracic thickenings, it is true, 

 develop further and become folds or 

 pockets of the ectoderm because they 

 must furnish the legs and wings of the 

 imago, while the abdominal discs do not 

 develop into pockets, with the exception 

 of those of the external sexual organs*. 

 Then again the thoracic and abdominal 

 discs are alike in that they all help to 

 form the imaginal hypodermis. 



It seems to me that the ventral thor- 

 acic and abdominal discs at least, are 

 homodynamous organs. There can be 

 no doubt that the ventral discs of the 

 ditlerent thoracic segments are homo- 

 dynamous among themselves, likewise 

 the ventral discs of the different abdom- 

 inal segments among themselves. The 

 ventral thoracic discs, too, are undoubt- 

 edly homologous to the thoracic extrem- 

 ities of the other insects; and I think 

 there can be no doubt that the ventral 

 abdominal discs are homologous to the 

 rudimentary extremities which appear 

 in the embryos of all other insects, hut 

 not in the brachyceran dipters. But 

 the thoracic and the embryonic abdom- 

 inal extremities in other insects are 

 undoubtedlv homodynamous organs, 

 therefore, the ventral thoracic and ab- 

 dominal imaginal discs in the brach\cera 



are also homodvnamous organs, as 

 things whicli are equal to the same 

 thing are equal to each other. In the 

 thorax these organs furnish the legs and 

 the ventral half of the imaginal hypo- 

 dermis, in the abdomen thev furnish 

 only the hypodermis, there being no 

 legs. The two pairs of discs which 

 furnish the external sexual organs are, 

 I think, the ventral discs of the absent 

 ninth and tenth abdominal segments. 



When we consider the dorsal discs 

 we find the matter is much moie diffi- 

 cult. We c.'uniot prove that the dorsal 

 thoracic and the dorsal abdominal discs 

 are homodynamous in the same wav, 

 because the dorsal abdominal surface 

 of the insect has no extremities and no 

 rudiments of any :it anj' time. But I 

 think, although reasoning from analogy 

 is very unsafe in such matters, it is at 

 least very probable that the same homo- 

 dynamy exists on the dorsal side as on 

 the ventral side of the insect. The dor- 

 sal discs have exactly the same appear- 

 ance on the thorax and abdomen as the 

 ventral discs, and the same method of 

 origin, and if these facts go for anything 

 there can be no doubt of the homody- 

 namy. 



If, now, this is really the CMse, what 

 is its significance.' The ^■entral discs, 

 thoracic and abdominal, are homolog- 

 ous to extremities. The dorsal thoracic 

 discs are homologous to wings. If they 

 and the dorsal abdominal discs are 

 homodynamous organs, are the latter 

 homologous to wings, too .'' Such an 

 assumption is of course impossible, 

 but it is not impossible that there 



