METAMORPHOSIS, a Greek word meaning change of shape, is applied in zoology. (See INVERTEBRATE EMBRYOLOGY.) All animals begin their existence as undifferentiated cells and at tain their characteristic features by growth which entails changes of shape. If we were to magnify the model of a baby to the size of a man we should discover how striking are the changes in the pro portions of the human figure as manhood is attained and yet these changes are not regarded as metamorphosis. The best definition of metamorphosis is "a conspicuous change in shape and mode of life in an animal occurring in a comparatively short time without any increase in size or even with a decrease in size." In the com mon English frog, the tadpole stage lasts from two to four months, during which the animal increases very much in size but retains the same general appearance and proportions. The rudi ments of the hind limbs gradually increase in length whilst the f ore-limbs break through the skin of the breast region. The creature then ceases to feed and begins to crawl out of the water, though it remains within reach and takes to it if alarmed. In about four days the tail is reduced to a vestige and finally disappears and the young frog takes up land life. The froglet is smaller than the completely grown tadpole and requires four years to attain sexual ripeness and full size.
In the development of the butterfly, the caterpillar, like the tadpole, requires about two months to grow to full size. Then in a day or so it constructs its cocoon and changes into a chrysalis or pupa. The pupal stage may last as long as the entire growth of the caterpillar, or longer, for a whole winter may be passed in this state, but the emergence of the perfect insect from the pupal skin only occupies an hour or two. The adult life may last only about three weeks.
The term metamorphosis is not applied to embryos, only to larvae. Since the change in shape is accomplished without increase in size, it always involves the casting off or absorption of certain larval tissues. In the case of the tadpole's tail, wandering cells (phagocytes) carried by the blood-stream attack and devour the contained muscles, nerve-cord and notochord, the skin is thrown into internal folds and these folds in turn are attacked by phagocytes and so the whole skin area shrinks in size, till the tail is reduced to a stump.
Metamorphosis is found only in those life-histories where the larva and the adult have very different habits. Among verte brates it only occurs in Cyclostomes in certain groups of fish and in Amphibia (frogs, toads and newts). Below the level of verte brates, however, it is found in every great division of the animal kingdom. The newt is especially interesting. This creature differs from a frog in having the fore- and hind-limbs of approximately equal size and in retaining the tail throughout life. The larva differs from the tadpole in retaining during the whole of its aquatic existence the external feather-like gills which the tadpole at first possesses but later casts off when they have been covered by the growth of the gill cover. This cover in the tadpole hides the fore-limbs which are only revealed a short time before the little frog leaves the water, but in the newt larva this cover is rudimentary and the fore-limbs appear in development first, to be followed later by the hind-limbs so that in the older larvae all four limbs are obvious and fully formed. Now in certain species of newt such as the Mexican axolotl the habit has been acquired of spending the whole life in water. In normal circum stances the larvae of these newts never metamorphose but de velop their genital organs and eventually lay eggs. For reasons discussed below it is possible to induce metamorphosis in these newts by feeding them with thyroid gland. No more startling phenomenon can be witnessed than this. In a week or two in the writer's laboratory a large fat axolotl lost its gills, closed up its gill slits, lost the blade-like fin on its tail, darkened in colour, shrunk considerably in size and emerged from the water as a rather small black newt.