192
H E N R I E T T E HERLANT-MEEWIS
To conclude, nervous ganglia can be repaired autonomously from
peripheral elements of ectodermal origin which bear no relation to the
remaining nervous system in situ. I t could well be that, in head regeneration also, the nerve cord is concerned only in its own restoration
and that the ganglia of the peri-oesophageal ring are formed from ectodermal cells. Further investigation is needed to solve this problem.
2. Regenerative Power
I t is well known that anterior regeneration generally occurs within
the so-called 'cephalic territory', is very variable in extent depending on
the species, and limited most often to the reconstruction of some segments corresponding to a 'cephalic region'. This zone has been defined
as the most extended region of the front end of the body likely to be
repaired integrally. I t is a distinct morphogenetic system which can be
reproduced entirely by the posterior zones but is itself incapable of
regeneration. This region is often characterized by the absence of
nephridia and the presence of specialized circulatory and respiratory
systems. In Lumbricidae, it is most often made up of five segments, i.e.
transection between segments 5 and 6 will be followed by an equimeric
reconstruction of five segments ; if amputation is more posterior, again
five segments will be repaired, but in this case the regeneration bud will
be hypomeric.
However, many exceptions have been observed in several Lumbricidae; they have been listed by Gates (1949). A recent study in sedentary
polychaetes has shown that in most cases the cephalic region contains
the head and the prothoracic segment, and sometimes the first thoracic
segment as well. Anterior regeneration is limited to these segments
alone; this has been demonstrated by Cresp (1952) in Hydroides norvegica and by Abeloos (1949, 1950b) in Terebella lapidaria and Magalia
perarmata. Here again, there are many exceptions particularly in species
with high regenerative power and in those that tend to reproduce
asexually. Head regeneration then is not limited to the cephalic territory
but is possible at all levels of the body ; furthermore, it is not necessarily
limited to reconstruction of the cephalic region. This applies in the case
of Salmacina incrustans (Cresp, 1956) where the anterior regeneration
bud includes the head and two setigerous segments and of Polydoraflava
(Abeloos and Thouveny, 1957) where eight segments are reconstructed
whatever the level of transection. The same applies to Clymene oerstedii
(Abeloos, 1952) in which all segments suppressed in the anterior region
of the body are completely reconstructed. In most of the cases considered
here, the authors observed that the ability to regenerate progressively
decreases along a gradient from the cephalic to the caudal region, except in those varieties susceptible to budding where morphogenetic
H E N R I E T T E HERLANT-MEEWIS
To conclude, nervous ganglia can be repaired autonomously from
peripheral elements of ectodermal origin which bear no relation to the
remaining nervous system in situ. I t could well be that, in head regeneration also, the nerve cord is concerned only in its own restoration
and that the ganglia of the peri-oesophageal ring are formed from ectodermal cells. Further investigation is needed to solve this problem.
2. Regenerative Power
I t is well known that anterior regeneration generally occurs within
the so-called 'cephalic territory', is very variable in extent depending on
the species, and limited most often to the reconstruction of some segments corresponding to a 'cephalic region'. This zone has been defined
as the most extended region of the front end of the body likely to be
repaired integrally. I t is a distinct morphogenetic system which can be
reproduced entirely by the posterior zones but is itself incapable of
regeneration. This region is often characterized by the absence of
nephridia and the presence of specialized circulatory and respiratory
systems. In Lumbricidae, it is most often made up of five segments, i.e.
transection between segments 5 and 6 will be followed by an equimeric
reconstruction of five segments ; if amputation is more posterior, again
five segments will be repaired, but in this case the regeneration bud will
be hypomeric.
However, many exceptions have been observed in several Lumbricidae; they have been listed by Gates (1949). A recent study in sedentary
polychaetes has shown that in most cases the cephalic region contains
the head and the prothoracic segment, and sometimes the first thoracic
segment as well. Anterior regeneration is limited to these segments
alone; this has been demonstrated by Cresp (1952) in Hydroides norvegica and by Abeloos (1949, 1950b) in Terebella lapidaria and Magalia
perarmata. Here again, there are many exceptions particularly in species
with high regenerative power and in those that tend to reproduce
asexually. Head regeneration then is not limited to the cephalic territory
but is possible at all levels of the body ; furthermore, it is not necessarily
limited to reconstruction of the cephalic region. This applies in the case
of Salmacina incrustans (Cresp, 1956) where the anterior regeneration
bud includes the head and two setigerous segments and of Polydoraflava
(Abeloos and Thouveny, 1957) where eight segments are reconstructed
whatever the level of transection. The same applies to Clymene oerstedii
(Abeloos, 1952) in which all segments suppressed in the anterior region
of the body are completely reconstructed. In most of the cases considered
here, the authors observed that the ability to regenerate progressively
decreases along a gradient from the cephalic to the caudal region, except in those varieties susceptible to budding where morphogenetic