PROGRESS OF MICROSCOPICAL SCIENCE. 
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probably, as tbe expression of the ultimate segmentation of the leg 
which it formerly possessed ; or, in other words, the provisional seg- 
ments of the leg, developing from the imaginal disk, remind us of the 
permanent segments of the larval legs of Lepidoptera, beetles, &c., 
which, in these latter, are used as temporary, provisional, locomotive 
organs. 3. I believe that great morphological importance must be 
attached to the fact that during the development of the imaginal disk 
of the Muscidce, the Hymenoptera. Corethra, and Miastor, the pro- 
visional cavity in the disk, which has no ultimate meaning, appears 
first of all. The scientific meaning of this provisional cavity, as well 
as of its outward tegument, can be explained, I think, as follows : The 
fact that imaginal disks, formations homologous to ambulatory legs, 
are situated in the cavity of the larval body, in connection with the 
tracheae and nerves, must undoubtedly be understood as a consequence 
of the compound process of the displacement of the imaginal disk from 
the surface towards the inner cavity along the tracheal tube or nerve. 
The larvae of Corethra, Miastor, Chironomus, have the imaginal disks 
more on the surface of the skin than those of the ant ( Myrmica ). In 
the former larvae, these formations are walled in by a comparatively 
less developed fold of the skin. In the larvae 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 repre- 
sent 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 disk of an ant as compared to the disk of 
Muscidce in the corresponding stage of development. That is, the part 
of the disk of the Muscidce which I described as its outside tegument, 
becomes the homologue of the closed fold of the skin in the disk of 
the ant ; the provisional cavity of the disk of Muscidce, between its 
outer and inner tegument, is homologous to the cavity of the bag in 
the disk of the ant ; the inner tegument of the disk of the Muscidce and 
the thickening of the anterior half of the disk of the ant represent the 
beginnings of the leg of the imago, and are homologous formations. 
The phase of development of the imaginal disk of the ant, before it 
begins to project externally, when the extremity consists only of three 
provisional segments, and the corresponding phase in the disk of 
Muscidce, entirely concealed within the cavity of the body, are 
remarkably alike anatomically, if we do not pay attention to the 
external opening in the disk of the ant. It seems very probable that, 
when the post-embryonal development of different insects is better 
investigated, embryological facts will be found, which will favour the 
view, explained above, of the imaginal disk of Muscidce being com- 
parable to the disk of tbe ant (Myrmica). I mean to say, that an 
intermediate stage of the imaginal disk will be found, during which it 
occupies in the full-grown larva a position similar to its position in 
the larvm of Muscidce, and has at the same time its outer integument 
and provisional cavity similar to those of the ant.” 
