THE METAMORPHOSIS OF FLIES.* II. 
BY DR. AUGUSTUS WEISSMANN. 
WE now attempt to consider morphologically the phenomena of 
the development of the Muscidae, as discussed in the foregoing 
chapters. In the first place, the theories above laid down seem to 
be thoroughly established, 7. e. that we have in the metamorphosis 
of the flies nothing to do with a simple change of skin. Indeed, 
the additional metamorphosis is accompanied by a casting aside 
and new formation of the chitinous framework which underlies 
the body, but this has a very subordinate signification. -The sys- 
tems of organs of the larva disappear simultaneously, it may be 
completely, it may be only histologically; and out of the frag- 
ments the tissues build up a new body. It is a matter of doubt 
Whether to consider the larva and pupa as one and the same indi- 
vidual, or whether we have not in reality an alternation of genera- 
tions. I think that those who speak of a metagenesis in certain 
Echinoderms should regard this also as such, if with V. Carus,t 
We consider metamorphosis as a series of developments in which 
the animal, during a certain stage in its development, is provided 
with provisional organs; but metagenesis as that in which this 
Whole developmental stage itself (Amme) must be considered as a 
Provisional one, so that it readily follows that the system of organs 
of the larva taken collectively is provisional, or in other words, 
that the larva itself is a provisional stage, while the fly must be 
considered as a new individual; such process taking rank as an 
alternation of generations. At all events from the Plateus-form 
Va new organs arise in the sea star, as from the larva in the fly. 
There is the alimentary canal and the water vascular system, which 
last, though in a plainly undeveloped condition, is already par- 
tially developed in the larva. Here we have, except the hypo- 
derm of the abdominal segments of the larva, no parts which pass 
into the pupa without previously undergoing a total revolution. 
The alimentary canal and the water-vascular system of the Echino- 
E eee een 
Forming the closing chapter in “ Die Entwicklung der Dipteren,” 1864. 
tSystem der thierischen Morphologie, Leipzig, 1853, p- (661) 
