194
H E N R I E T T E HERLANT-MEEWIS
thoracic ones also affects the gut, which, at this level, takes on a
pharynx-like structure. According to Abeloos, the morphallaxis which
affects the ectoderm or the endoderm would be induced by the intense
rearrangements to which the mesoderm and more particularly the
circulatory system are subjected. Such alterations in the circulatory
system have previously been noted by von Haffner (1928) in Lumbriculus
variegatus. After amputation of eighteen anterior segments, eight of
them regenerated, and in the ten adjacent old segments the circulatory
system underwent changes to comply with that normally appearing in
segments 9-18.
IV. Importance of the Nervous System in Morphogenesis
If the nervous system is necessary as a chemical mediator during the
make-up and growth of the blastema, the part it plays does not stop
there; it acts later as an organizer during morphogenesis. In all cases
where the nervous system is lacking in the regeneration bud, the differentiation of the other organs is incomplete. However, a small nervous
rudiment appearing independently in a regeneration bud is sufficient for
the cephalization mechanism to be triggered off.
I t is during asexual reproduction that the influence of the nervous
system on morphogenesis can be particularly well observed. In limicolous oligochaetes fissiparity leads to the formation of chains of zooids
of widely differing ages (Herlant-Meewis, 1958). Stolte (1955) distinguished two phases in the development of these zooids during experimental regeneration. Firstly, the arrangement of the regeneration
material where the nervous system acted by embryonizing the different
layers, and secondly the morphogenetic phase during which the brain
appears first and acts as an organizer.
Observations on the Aeolosomatidae (Herlant-Meewis, 1954) lend
support to this point of view. In this group, asexual multiplication is
very active and the number of individuals making up a chain is particularly high : these zooids can appear by strobilization, pygidial budding
or bud fractionation. Furthermore, the group is characterized by the
localization of the nervous system within the ventral ectoderm (Fig.
24A,N).
In the growing pygidium, all tissues are in an undifferentiated state ;
there is no distinct nerve chain. The very first evidence of pygidial bud
formation is the appearance of an autonomous nervous rudiment (NJ
in the ventral ectoderm ; the rudiment is isolated from the differentiated
nervous system in the anterior regions (N) by a completely undifferentiated zone (P). The appearance of a second nervous rudiment (N 2 )
behind the first one leads to a second bud formation. Arrows 2 and 3
H E N R I E T T E HERLANT-MEEWIS
thoracic ones also affects the gut, which, at this level, takes on a
pharynx-like structure. According to Abeloos, the morphallaxis which
affects the ectoderm or the endoderm would be induced by the intense
rearrangements to which the mesoderm and more particularly the
circulatory system are subjected. Such alterations in the circulatory
system have previously been noted by von Haffner (1928) in Lumbriculus
variegatus. After amputation of eighteen anterior segments, eight of
them regenerated, and in the ten adjacent old segments the circulatory
system underwent changes to comply with that normally appearing in
segments 9-18.
IV. Importance of the Nervous System in Morphogenesis
If the nervous system is necessary as a chemical mediator during the
make-up and growth of the blastema, the part it plays does not stop
there; it acts later as an organizer during morphogenesis. In all cases
where the nervous system is lacking in the regeneration bud, the differentiation of the other organs is incomplete. However, a small nervous
rudiment appearing independently in a regeneration bud is sufficient for
the cephalization mechanism to be triggered off.
I t is during asexual reproduction that the influence of the nervous
system on morphogenesis can be particularly well observed. In limicolous oligochaetes fissiparity leads to the formation of chains of zooids
of widely differing ages (Herlant-Meewis, 1958). Stolte (1955) distinguished two phases in the development of these zooids during experimental regeneration. Firstly, the arrangement of the regeneration
material where the nervous system acted by embryonizing the different
layers, and secondly the morphogenetic phase during which the brain
appears first and acts as an organizer.
Observations on the Aeolosomatidae (Herlant-Meewis, 1954) lend
support to this point of view. In this group, asexual multiplication is
very active and the number of individuals making up a chain is particularly high : these zooids can appear by strobilization, pygidial budding
or bud fractionation. Furthermore, the group is characterized by the
localization of the nervous system within the ventral ectoderm (Fig.
24A,N).
In the growing pygidium, all tissues are in an undifferentiated state ;
there is no distinct nerve chain. The very first evidence of pygidial bud
formation is the appearance of an autonomous nervous rudiment (NJ
in the ventral ectoderm ; the rudiment is isolated from the differentiated
nervous system in the anterior regions (N) by a completely undifferentiated zone (P). The appearance of a second nervous rudiment (N 2 )
behind the first one leads to a second bud formation. Arrows 2 and 3