THE METAMORPHOSIS OF FLIES. 663 
nective tissue of vertebrates, i. e. of building up cells, and of re- 
turning to that from which it originated. 
I find it generally true that the nuclei lying near a recent disk 
become metamorphosed into disk-forming cells; tha‘ they sur- 
round themselves with cells, and multiply like the other disk-cells. 
Such a process may be seen in the exceptionally late appearance 
of the upper prothoracic disks. These disks are already formed 
in the egg out of the embryo cells,—out of the same material as 
the tegumentary membrane with which they develop. They could 
only in an ideal sense be regarded as outgrowths of this tegu- 
mentary membrane; so the upper prothoracic disks-are, in reality, 
nothing but outgrowths. The nuclei of the peritoneal skin form 
cells, and increase in number, constituting the disks. We have here’ 
a bud which scarcely differs from the buds which are concerned 
in the formation of new stigmata in the first moulting of the larva, 
and we should almost err in considering the morphological value 
of this disk to regard it as a true imaginal disk ; it should at least 
not be compared with those of the free Tipulide larve, in which 
they have a by far more complicated structure, while they are con- 
siderably larger, and are indicated contemporaneously with the 
formation of the other disks of the thorax. 
As I cannot agree with that opinion which regards the well- 
known metamorphosis of Echinoderms as a metagenesis, so am I 
Still far from proposing that there is such a metamorphosis in the 
Muscide. We must certainly, with V. Carus and J. Müller, con- 
Sider it in this respect as irrelevant whether the nurse produces 
one or more germs (in a monogenous way); whether the animal 
rowing from the egg develops into a sexual form, or whether it, 
not capable of that, at the end of its development produces germs 
(buds) which build up a sexual animal; or whether the series of 
developmental forms from the embryo up to the sexually ripe 
animal end in one or two individuals. The answer in both cases 
seems to me not doubtful. In the Echinoderms as in the Muscide 
We have to deal with a metamorphosis, not with an alternation 
of generations. Larva and sexually mature animal are one and 
the same individual. In the Echinoderms it seems to me this is 
evident in that the internal organs (intestine and water canal sys- 
tem) are present, and without any interruption of their functions 
_ Pass over from the larval stage to the adult sea star; so that a 
single germ will not from the beginning pass by gradual differen- 
