REGENERATION IN ANNELIDS
187
the regeneration bud. Sayles (1932,1939,1940,1942) arrived at the same
conclusion by grafting portions of the nervous system in Clymenella
torquata. Nerve fragments made up of three pairs of ganglia taken from
anterior regions of the body were grafted to the skin latero-dorsally at
different levels of the body. Regeneration buds developed in contact with
the nervous system and the skin, and their nature depended on their
localization. Implants in the cephalic territory (up to the tenth segment)
gave rise to cephalized buds ; in the region of segments 10-12 the induced
buds were mixed ; in segment 13 they were of caudal type. These results,
which have been confirmed by Baskin (1933) and Painter (1940), are in
agreement with those of Kawakami (1961) who believed that the regenerative power in a given region of the body depended on the histological structure of the wall, the nervous inductor being neutral.
III. Differentiation of the Regeneration Bud
Blastema differentiation will not be the same in posterior regeneration
buds as in anterior buds.
A. Caudal Regeneration
1. Morphogenesis
During the development of a caudal regeneration bud, there remains
at the hinder end a growth zone characterized by blastocytes of the
embryonic type, likely to go on dividing without differentiating. In
Polychaetae, posterior regeneration always begins by reconstruction of
the pygidial segment, bearing cirri. Very rapidly, in the blind connexion
of the ectoderm over the endoderm, mesodermal cells differentiate into
muscles lined with the peritoneal layer while the cirri develop from the
ectoderm. Fig. 23 shows this stage in Nereis diversicolor. The growth
zone (b) is located immediately ahead of this pygidial segment in polychaetes. In Oligochaetae, where there is no segment bearing cirri, the
growth zone is subterminal. This embryonic zone is also characterized
by the connexion between the nervous system and the skin. Ectodermal
blastocytes of the growth zone give rise to elements of the ectoderm and
of the nervous system.
In this same terminal region the alimentary canal also participates
in pygidial growth phenomena, although its epithelium does not
necessarily contain undifferentiated cells of the neoblastic type. Some
authors, for example Abeloos (1950a) working on Magalia perarmata
and Kawakami (1961) on Eisenia, have obtained caudal regeneration
in the absence of the digestive tract; however, anomalies frequently
occur and it does seem that its presence is indispensable for the elaboration of a normal pygidial region (Crowell, 1937).
187
the regeneration bud. Sayles (1932,1939,1940,1942) arrived at the same
conclusion by grafting portions of the nervous system in Clymenella
torquata. Nerve fragments made up of three pairs of ganglia taken from
anterior regions of the body were grafted to the skin latero-dorsally at
different levels of the body. Regeneration buds developed in contact with
the nervous system and the skin, and their nature depended on their
localization. Implants in the cephalic territory (up to the tenth segment)
gave rise to cephalized buds ; in the region of segments 10-12 the induced
buds were mixed ; in segment 13 they were of caudal type. These results,
which have been confirmed by Baskin (1933) and Painter (1940), are in
agreement with those of Kawakami (1961) who believed that the regenerative power in a given region of the body depended on the histological structure of the wall, the nervous inductor being neutral.
III. Differentiation of the Regeneration Bud
Blastema differentiation will not be the same in posterior regeneration
buds as in anterior buds.
A. Caudal Regeneration
1. Morphogenesis
During the development of a caudal regeneration bud, there remains
at the hinder end a growth zone characterized by blastocytes of the
embryonic type, likely to go on dividing without differentiating. In
Polychaetae, posterior regeneration always begins by reconstruction of
the pygidial segment, bearing cirri. Very rapidly, in the blind connexion
of the ectoderm over the endoderm, mesodermal cells differentiate into
muscles lined with the peritoneal layer while the cirri develop from the
ectoderm. Fig. 23 shows this stage in Nereis diversicolor. The growth
zone (b) is located immediately ahead of this pygidial segment in polychaetes. In Oligochaetae, where there is no segment bearing cirri, the
growth zone is subterminal. This embryonic zone is also characterized
by the connexion between the nervous system and the skin. Ectodermal
blastocytes of the growth zone give rise to elements of the ectoderm and
of the nervous system.
In this same terminal region the alimentary canal also participates
in pygidial growth phenomena, although its epithelium does not
necessarily contain undifferentiated cells of the neoblastic type. Some
authors, for example Abeloos (1950a) working on Magalia perarmata
and Kawakami (1961) on Eisenia, have obtained caudal regeneration
in the absence of the digestive tract; however, anomalies frequently
occur and it does seem that its presence is indispensable for the elaboration of a normal pygidial region (Crowell, 1937).