60 ON THE AGAMIC REPRODUCTION AND MORPHOLOGY OF APHIS 
be produced. When the producing or protozooid possesses no sexual 
organs, I think Professor Owen’s term of “ metagenesis” might well 
be applied to the kind of agamogenesis ; but where the protozooid 
possesses sexual organs, and its buds have all the histological 
characters of ova, then the process may fairly enough be termed 
parthenogenesis. 
Finally, the produced zooid may be incapable of development 
into an independent organism, unless it conjugate with another zooid. 
In this case we have sexual reproduction, or gamogenests. 
The natural character of this classification of the various modes of 
development is manifest when it is thrown into a tabular form :— 
Metamorphosis. 
Gemmation without fission, 
Metagenesis. 
Growth. 
Continuous 
Development. / Agamogenesis. ’ 
Parthenogenesis. 
Discontinuous. 
(Gemmation with 
fission). | Gamogenesis. 
Whatever hypothesis we may entertain with respect to the nature 
of these processes, and however we may think fit to conceive the 
nature of the “individual,” I think it must be admitted, that all the 
phenomena of development in the animal kingdom (and I would 
venture to add in the vegetable kingdom also) fall under one or other 
of these heads. 
Furthermore, all these modes of development pass into one an- 
other. Growth and metamorphosis are combined in all animals. 
Gemmation, so long as the gemma continues attached, is but a 
peculiar kind of growth and metamorphosis. From the fixed bud 
to the separate one, we have all gradations; and fission is little 
more than a peculiar mode of budding. 
Free gemmation is “metagenesis” when the bud is not developed 
within the homologues of the sexual reproductive organs ; it becomes 
“ parthenogenesis” when the bud is developed within such organs ; 
finally, when the free bud requires conjugation with another free bud 
for its development, we have gamogenesis, or sexual reproduction: 
but cases such as those of Daphnia and Apzs show that the histo- 
logical element, which is at one time agamogenetic, may at another 
be gamogenetic. 
Time was when the difficulty of the physiologist lay in under- 
standing reproduction without the sexual process. At the present 
