104
R.
J.
GOSS
communicated to it. It is in this dual perspective, of cause versus effect,
that the problem of morphogenesis may profitably be regarded.
Though morphogenetic influences are abundantly obvious during
embryogenesis, they also persist throughout the adult life of the
organism. Usually held in abeyance once the mature form of an animal
has been realized, the still effective impact of form-determining factors
becomes evident not only in those tissues which must constantly grow
in order to maintain their integrity, but also under the more revolutionary instances of metamorphosis and injury repair. Postembryonic
development reveals, therefore, the belated operation of morphogenetic
influences and indeed provides what is in many ways a very convenient
situation for the experimental investigation of morphogenesis. Especially
useful in such studies are the various instances of appendage regeneration in the lower vertebrates, for here are exhibited all of the basic
mechanisms of morphogenesis which are also encountered in embryonic
development. Yet unlike many embryos, regenerating adult structures
can be obtained in any season at will merely by amputation. Moreover,
the development of regenerates usually occurs on a size scale more
amenable to experimental operations than that of embryos. Finally,
there is an important difference between the adult regenerate and the
developing embryo. In the latter, structures are formed de novo with the
result that the experimental investigator is confronted with the problem
of discovering the site of origin of structures before proceeding to the
more important questions of determining what and how materials are
involved in morphogenesis. However, regeneration, by definition, presupposes the reconstitution of only a portion of a structure. Since part
of an appendage from which the regenerate can be produced must
remain intact, this kind of development, instead of being de novo,
proceeds from the stump of the original structure.
Here, then, the developmental biologist can attempt to analyse a
morphogenetic system with the advantage of knowing that the eventual
form of a regenerate is dictated by the older tissues from which it
originates. This determining influence of the stump on the newly
developing system is apparent from the spatial relations between the
two. The structures which are regenerated from amputated appendages
are (with a few interesting exceptions) exact replicas of the parts they
replace. Quantitatively, there is usually formed precisely the amount
of material which was lost. Moreover, the axes of the regenerates
coincide with those of the stumps. Indeed, the tissues of the regenerate
develop as continuations of those of the stump, thus giving rise to an
anatomically and functionally integrated structure. It is this spatial and
apparently causal relationship between the old and the new that makes
the regenerating appendage such a valuable system for experimental
R.
J.
GOSS
communicated to it. It is in this dual perspective, of cause versus effect,
that the problem of morphogenesis may profitably be regarded.
Though morphogenetic influences are abundantly obvious during
embryogenesis, they also persist throughout the adult life of the
organism. Usually held in abeyance once the mature form of an animal
has been realized, the still effective impact of form-determining factors
becomes evident not only in those tissues which must constantly grow
in order to maintain their integrity, but also under the more revolutionary instances of metamorphosis and injury repair. Postembryonic
development reveals, therefore, the belated operation of morphogenetic
influences and indeed provides what is in many ways a very convenient
situation for the experimental investigation of morphogenesis. Especially
useful in such studies are the various instances of appendage regeneration in the lower vertebrates, for here are exhibited all of the basic
mechanisms of morphogenesis which are also encountered in embryonic
development. Yet unlike many embryos, regenerating adult structures
can be obtained in any season at will merely by amputation. Moreover,
the development of regenerates usually occurs on a size scale more
amenable to experimental operations than that of embryos. Finally,
there is an important difference between the adult regenerate and the
developing embryo. In the latter, structures are formed de novo with the
result that the experimental investigator is confronted with the problem
of discovering the site of origin of structures before proceeding to the
more important questions of determining what and how materials are
involved in morphogenesis. However, regeneration, by definition, presupposes the reconstitution of only a portion of a structure. Since part
of an appendage from which the regenerate can be produced must
remain intact, this kind of development, instead of being de novo,
proceeds from the stump of the original structure.
Here, then, the developmental biologist can attempt to analyse a
morphogenetic system with the advantage of knowing that the eventual
form of a regenerate is dictated by the older tissues from which it
originates. This determining influence of the stump on the newly
developing system is apparent from the spatial relations between the
two. The structures which are regenerated from amputated appendages
are (with a few interesting exceptions) exact replicas of the parts they
replace. Quantitatively, there is usually formed precisely the amount
of material which was lost. Moreover, the axes of the regenerates
coincide with those of the stumps. Indeed, the tissues of the regenerate
develop as continuations of those of the stump, thus giving rise to an
anatomically and functionally integrated structure. It is this spatial and
apparently causal relationship between the old and the new that makes
the regenerating appendage such a valuable system for experimental