REGENERATION IN ANNELIDS
185
to allow complete differentiation of the regeneration bud. If the nerve
chain was not removed until 4 days after amputation, regeneration was
normal; cerebral ganglia were found within the bud but their origin
was not ascertained.
From these two series of experiments which complement and confirm
each other, it can be concluded that, in Eisenia foetida, the presence of
growing nerve fibres is absolutely essential for the development of the
ectomesodermal blastema of regeneration.
The interaction of the nervous system and scar tissue has also been
used to advantage by Kawakami (1961) in order to study cephalic
regenerative power in the neutral zone which, in Eisenia foetida, is
located between the sixteenth and thirty-fifth segments. As mentioned
earlier, anterior regeneration does not normally occur in this region.
Kawakami made a large ventral wound by cutting into the body wall
and the nervous system along segments 20-28. During healing, he transected the worm at segment 24 and, in a certain number of cases, he observed reconstruction of a small head provided with a brain and stomodaeum. The author concluded from these experiments that regeneration
is not impossible in the neutral zone. Its absence is not due to a lack of
regenerative induction by the nervous system, but rather to the
structure of the body wäll which, at this point, does not have the
ability to react to a simple transection by differentiating sufficiently to
promote the development of a blastema. If the ventral epidermis is
previously 'embryonized', it acquires this power and, in contact with
the nerve fibres, regeneration takes place, but it is limited; it does not
occur in all cases and the cephalic regenerated end is never quite
normal.
2. Inductive Power
The inductive action of the nervous system can also be evidenced by
the formation of cephalic or caudal heterotopic buds. Morgan (1902) in
his early experiments, demonstrated the induction of a small cephalic
bud where the posteriorly transected nerve chain was in contact with
the epidermis, at points V and 'a" on Fig. 14. Apart from a few fortuitous observations that we have made (Herlant-Meewis and Deligne,
unpublished), there is, to the best of our knowledge, no recent work in
this field. However, it is impossible to overlook this important aspect
of the problem so we shall mention a few of the more suggestive experiments which have thrown light on this question.
Morgan's results have been reproduced by Avel (1930, 1932, 1947).
By bending the nerve chain backwards and inserting it through a buttonhole slit in the skin, he obtained, at this point, a small supernumerary
head, which was mobile and typical but lacked endodermal organs if
185
to allow complete differentiation of the regeneration bud. If the nerve
chain was not removed until 4 days after amputation, regeneration was
normal; cerebral ganglia were found within the bud but their origin
was not ascertained.
From these two series of experiments which complement and confirm
each other, it can be concluded that, in Eisenia foetida, the presence of
growing nerve fibres is absolutely essential for the development of the
ectomesodermal blastema of regeneration.
The interaction of the nervous system and scar tissue has also been
used to advantage by Kawakami (1961) in order to study cephalic
regenerative power in the neutral zone which, in Eisenia foetida, is
located between the sixteenth and thirty-fifth segments. As mentioned
earlier, anterior regeneration does not normally occur in this region.
Kawakami made a large ventral wound by cutting into the body wall
and the nervous system along segments 20-28. During healing, he transected the worm at segment 24 and, in a certain number of cases, he observed reconstruction of a small head provided with a brain and stomodaeum. The author concluded from these experiments that regeneration
is not impossible in the neutral zone. Its absence is not due to a lack of
regenerative induction by the nervous system, but rather to the
structure of the body wäll which, at this point, does not have the
ability to react to a simple transection by differentiating sufficiently to
promote the development of a blastema. If the ventral epidermis is
previously 'embryonized', it acquires this power and, in contact with
the nerve fibres, regeneration takes place, but it is limited; it does not
occur in all cases and the cephalic regenerated end is never quite
normal.
2. Inductive Power
The inductive action of the nervous system can also be evidenced by
the formation of cephalic or caudal heterotopic buds. Morgan (1902) in
his early experiments, demonstrated the induction of a small cephalic
bud where the posteriorly transected nerve chain was in contact with
the epidermis, at points V and 'a" on Fig. 14. Apart from a few fortuitous observations that we have made (Herlant-Meewis and Deligne,
unpublished), there is, to the best of our knowledge, no recent work in
this field. However, it is impossible to overlook this important aspect
of the problem so we shall mention a few of the more suggestive experiments which have thrown light on this question.
Morgan's results have been reproduced by Avel (1930, 1932, 1947).
By bending the nerve chain backwards and inserting it through a buttonhole slit in the skin, he obtained, at this point, a small supernumerary
head, which was mobile and typical but lacked endodermal organs if