12 NOTICES OF THE MRETINGS [Feb. 7, 
a ciliated monad, then to a lobed rotifer, and next to a long-armed 
polype. 
This polype stage of the Medusa had been previously recognised 
in 1788, but without a suspicion of its true nature, by O. F. Miiller, 
who called it Hydra gelatinosa. 
It was next observed, and its habits more fully described, by Sir 
John Dalyell, in 1834, as Hydra tuba: and in 1836 he made known 
its singular metamorphoses into forms which Sars had previously 
described as Scyphistoma and Sirobila; and Dalyell saw the sponta- 
neous division of the latter into a pile or series of small Meduse. 
All the stages of the metagenesis were independently noted by 
Sars who described them in 1841. 
The difficulty of accounting for the presence of Entozoa in the 
interior parts of animal bodies is rapidly disappearing as the know- 
ledge of their course of development advances. 
The principal stages of this development were described in a small 
worm (Menostoma mutabile), parasitic in the air-cells, intestines, and 
peritoneal cavity of many water-fowl. 
The ovum is converted into a ciliated monadiform embryo, which 
escapes from the bird, and swims about freely in the water. A clear 
mass may be discerned in the interior which exhibits independent 
movements. This body is liberated, grows rapidly, and generates in its 
interior a number of independent organisms provided with a cephalic 
speculum and a caudal appendage, referable by their form to the genus 
Cercaria. They are very active and insinuating, could even bore 
through the skin by the sharp needle-like armature of the head, and 
somehow or other do, under the guise of the Cercaria, again get 
access to the interior of the water-fowl ; fall into a state of torpor ; 
become circular flattened pupz ; and are finally metamorphosed into 
monastomes — a sluggish pendant parasite utterly deprived of the 
power of existing in water, or of gaining access, as a monostome, to 
the interior of any animal. 
Steenstrup, who has the merit of having first grouped together 
and pointed out the analogies of the different stages in the animals 
that undergo these successive changes, generalizes the facts under 
the phrase of ‘ Alternate Generation,’ and he calls the procreant 
larve ‘Amme,’ or Nurses, and ‘Gross-amme,’ or Grand-nurses. 
There is no particular objection to these names; but we naturally 
desire to know on what power the metageneses depend. 
Professor Owen thought the key to the power was afforded by the 
process which the germinal part of every egg undergoes before the 
embryo begins to be formed. 
A principle, answering to the pollen, that fertilizes the seed of 
plants, is the efficient cause of these changes: its mode of operating 
is best seen in the “transparent eggs of some minute worms; the 
principle manifests itself as a transparent, highly refractive globule 
