Metamorphosis in Animals: The Transformations of the Individual and the Tуpе
The transformations undergone by certain animals in the course of their es have provided human expression with some of its oldest images. From the immemorial the metamorphosis of the caterpillar into a butterfly has served as a metaphor for intimations of higher being. And the same creature, the quiescent pupa, the nympha or chrysalis, has provided a strict hieratic image of contemplation, of serene expectation of things to come, of the pomise of resurrection.
It is scarcely possible to witness the transformations of a dragonfly with experiencing an assault of inner images pointing in the same direction as the meditations in which Jan Swammerdam for the first time reverently described the metamorphosis of the May flies as a “copy of human life.”
The first part of our discussion will deal with cyclic changes of this kind, aleading back to the same starting point, the egg.
The word “metamorphosis” with which we designate these changes of m is employed by biologists in several senses. Those of us who have a erary background are perhaps acquainted with the concept of metamorphosis employed by Goethe and still used by comparative morphology:
the divergent forms successively embodying a fundamental type or bluet. Goethe’s discussion of the metamorphosis of plants treats of this kind of transformation; all theories of evolution deal with such metamorphoses, and accordingly seek to determine how one fundamental tyрe develops from another.
Both possibilities of transformation have been discussed in so many wariants at our Eranos Conference that a biologist may be justified, in his contribution, in considering them both from one standpoint.
Consequently, the second part of our discussion will deal with the transformation of the type. If this lecture comes at the end of our Conference, it is not because I claim the last word. It is an end, because if the study of natural phenomena takes a complete view of its subject matter, it must lead us back to the beginning.
Thus it stands at the end because our conferences are sustained by the spirit of encounter and correlation among all fields of knowledge.
I
We turn first of all to the strange transformation that certain animals undergo in the course of their lives; in these species an individual appears successively in a number of forms.
The first form, and sometimes several of
the early forms, may deviate so radically from the mature form as to obscure their membership in the same species or even genus: the early form “masks” the mature type and is consequently called a “larva.”
To follow the life development of the different forms of larvae is one of the main problems of zoology and it has not yet been fully solved. No one can say how many larvae are still considered by biologists as distinct species. Insects and crabs, worms, snails, mollusks, and echinoderms are the groups in which metamorphoses are most frequent.
But among the vertebrates frogs and salamanders provide examples with which every child is familiar. In this kind of metamorphosis the question of an animal’s change of form is clearly brought home to us, and with it the question of the creature that undergoes the change and of the extent of its inner and outer transformation.
For our discussion of the problem we choose the example of the butterfly-perhaps its familiarity will help to give us a stronger impression of the strangeness of the operations it effects. The study of the metamorphosis
of insects and of the butterfly in particular has given us significant insights into the workings of these processes. And the research carried on in the last few years is so illuminating that there is every reason to favor this sector of animal life as a basis for a general exposition of the problem of metamorphosis.
A first question facing scientists was, in a general sense, clarified in the years from 1926 to 1940. It is this: at what stage of development can we ascertain the presence of the forms that will later mature? By interventions in the development of eggs, we are able to determine the time at which essential traits in the development of the various organs are fixated, in other words, when their development is determined, The irradiation of eggs with ultraviolet light offers the possibility of particularly minute observations: it produces localized injuries which tell us something about the place and time of appearance of the earliest rudiments of organs.
At certain definite moments-the sensitive phases-in the earliest embryonic development we are able to produce injuries which do not at all disturb the formation of the larva but do appreciably affect the metamorphosis to the mature form. In flies, where many organs (proboscis, eyes, legs, wings) become visible only in the last act of development, it has been possible to show that these organs are all prefigured in the earliest germ development, in a form which cannot yet be distinguished under the microscope but which is nevertheless wholly or in large part established.
The time in which this fixation occurs varies from one group of insects to another. In insects with complete metamorphosis, i.e., with larva, pupa, and mature form (= imago), the egg already contains the rudiments of all the organs that distinguish the three stages, and in addition all the equipment for the processes leading from one stage to the next.
These experiments lead biologists to ask how
the structure and development of these organs are prefigured in the submicroscopic structure of the germ. But here we shall merely mention the existence of this question, without discussing it, because it is one of the great riddles which biologists, chemists, virologists, and physicists are all trying to solve.
In any event, three radically different forms are prefigured in the egg of the butterfly. In the caterpillar stage certain organs function fully, but certain others that will develop in the imago are no more than little groups of cells that we call imaginal disks.
Experiment reveals a complex mechanism by which the modifications of the various forms and the transformation from one to another are strictly regulated. But at the same time the study of the first processes of development shows us that the infinitesimal quantity of living substances constituting the egg creates these patterns and guides and regulates the events which will ultimately result in the minutely ordered sequence of the animal’s future development.
The discovery of this mechanism does not reduce the egg to a mosaic of patterns, functioning together like parts of a machine; rather, it leads us to view the mechanism of growth as a system “developing itself,” a process embodying the whole specific nature of the living creature.
This autonomous growth teaches us to look with suspicion on all comparisons between the organism and a machine: such comparisons have only the most limited validity, and in employing them we must remain fully aware that every living creature is very much more than a machine.
None of the episodes of metamorphosis mysteriously prefigured in the seed is so striking as the sudden transformation of the caterpillar into a pupa or as the last act: the emergence and maturation of the butterfly.
The common molts whereby the caterpillars grow from a few
millimeters at the time they leave the egg to finger length or more-these castings off of skin are also transformations; they too require some of the processes that characterize the final metamorphosis.
We shall have to touch on them in our discussion, but we shall concentrate on the last act of the caterpillar’s life, the actual transformation. ~ Adolf Portmann, Metamorphosis in Animals, Page 297-300
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