1877.] Recent Literature. 429 
ward tegument, can be explained, I think, as follows: The fact that 
imaginal discs, formations homologous to ambulatory legs, are situated 
in the cavity of the larval body, in connection with the tracher and 
nerves, must undoubtedly be understood as a consequence of the com- 
pound process of the displacement of the imaginal dise from the surface 
towards the inner cavity along the tracheal tube or nerve. The larve 
of Corethra, Miastor, Chironomus, have the imaginal discs more on the 
surface of the skin than those of the ant (Myrmica). In the former 
larve, these formations are walled in by a comparatively less developed 
fold of the skin. In the larve of Myrmica, this deep fold is transformed 
into a well-developed bag, which, together with the leg of the imago, 
developing within it, is placed during a certain time within the cavity of 
the larval body, below its muscular, subcutaneous stratum. In these 
insects, after the leg is stretched outside, the bag enclosing it is atrophied, 
and has no ulterior meaning. If we represent to ourselves that the 
outside aperture, leading into the provisional bag, with the incipient leg 
of the ant, is closed, we obtain all the homological parts of the disc 
of an ant as compared to the disc of Muscide in the corresponding 
stage of development. - That is, the part of the disc of the Muscide 
which I described as its outside tegument, becomes the homologue of the 
closed fold of the skin in the disc of the ant; the provisional cavity of 
the disc of Muscide, between its outer and inner tegument, is homol- 
ogous to the cavity of the bag in the disc of the ant; the inner tegu- 
ment of the disc of the Muscide and the thickening of the anterior 
half of the disc of the ant represent the beginnings of the leg of 
the imago, and are homologous formations. ‘The phase of develop- 
ment of the imaginal disc of the ant, before it begins to project exter- 
nally, when the extremity consists only of three provisional segments, 
and the corresponding phase in the dise of Muscide, entirely con- 
cealed within the cavity of the body, are remarkably alike anatomically, 
if we do not pay attention to the external opening in the disc of the 
ant. It seems very probable that, when the post-embryonal develop- 
ment of different insects is better investigated, embryological facts will 
be found, which will favor the view, explained above, of the imaginal 
dise of Muscide being comparable to the disc of the ant (Myrmica). 
I mean to say, that an intermediate stage of the imaginal disc will be 
found, during which it occupies in the full-grown larva a position sim- 
ilar to its position in the larve of Muscide, and has at the same time 
its outer integument and provisional cavity similar to those of the ant. 
`“ The paired abdominal thickenings which appear on the sides of the 
abdominal segments in the larvæ of Muscide before the formation of the 
Permanent abdominal segments of the imago are, I think, morphologic- 
ally homologous with the thoracic imaginal discs, and may be called ab- 
dominal imaginal discs. : 
“The facts communicated by me concerning the formation of the ali- 
