62 Royal Institution. 



The first step was made by Siebold, who, in 1839, traced the 

 development of the Medusa aurita from the egg to a stage resembhng 

 a cihated 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 Strobila ; and Dalyell saw the sponta- 

 neous division of the latter into a pile or series of small Medusae. 

 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 {Monostoma 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-hke 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 pupae ; and are finally metamorphosed into 

 Monostomes — a sluggish pendent 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 

 larvae * 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 

 in the centre of the egg ; it then divides ; and each division, attract- 

 ing the vitelline matter of the egg about it, divides that matter into 



