Professor Owen on Metamorphosis and Metagenesis. 273 



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 

 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, gene- 

 ralizes 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 aff'orded 

 by the process which the germinal part of every egg under- 

 goes before the embryo begins to be formed. 



A principle, answering to that of the pollen, which fer- 

 tilizes the seed of plants, is the efficient cause of these 

 changes ; its mode of operating is best seen in the trans- 

 parent 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 two parts. This division is repeated with the 

 same result, until the principle has diffused itself by indefi- 

 nite multiplication through the whole yelk which then con- 

 stitutes the ' germ-mass.' 



The next stage is the formation of the embryo : certain of 

 the minute subdivisions, called * nuclei ' or nucleated cells, 

 combine and coalesce to constitute the tissues of the embryos : 

 they ar& afterwards incapable of generating. If all be so 

 metamorphosed the organism cannot procreate of itself ; but 

 if a part only of the germ-mass be metamorphosed into 

 tissues the unchanged remnant may, if nutrition, heat and 

 other stimuli are present, repeat the same actions as those 

 that formed the first germ-mass, and lay the foundation of 

 future embryos. 



In proportion to the amount of the substance of an or- 



VOL. L. NO. C. — APRIL 1851. 8 



