BLASTOGENESIS AND MORPHOGENESIS
175
regeneration of the head develops, formed exclusively of labeled neoblasts which have migrated from the graft.
If the mesenchymal neoblasts of planarians are polyvalent, they are
not and cannot be totivalent, as can be determined by an analysis of the
stages of regeneration.
The cutting of the animal is followed by cicatrization. The wound
closes; the mass of internal tissues which has just been exposed forms
a "plug" where, among the intact cells, there are mingling destroyed or
traumatized and necrotic cells. The ectodermal cells spread out over
this cicatrix, becoming ameboid and dedifferentiating. They form a very
thin epithelium, a sort of veil which separates the internal tissues from
the external environment [Mates, 1925; Brien, 1944;
(Hydroides);
Cresp, 1964 (serpulids) ]. Cicatrization completed, the neoblasts are concentrated in the blastema of regeneration. Only then does organogenesis
begin. These three stages are observed also in the regeneration of the
urodele limb.
Thus the blastema is formed under the reconstituted ectoblast. Consequently, the ontogenic and organogenic activities of the histoblasts, the
actions of the neoblasts can be manifested only after cicatrization and in
the midst of the older tissues. These tissues are necessarily involved and
contribute to the regenerative activity, not without showing morphallaxis,
as has just been stated. The neoblasts are not able to regain ontogenic
totipotency. There is, perhaps, only one exception to this rule, that of
the mesenchymal amebocytes of freshwater sponges when they are associated to form gemmules or statoblasts, having undergone previously a
true deutoplasmogenesis.
The budding of hydras helps to make clear the significance of this
statement. It is known that the ectoendodermal wall of the polyp possesses neoblasts or interstitial cells, particularly at the base of the
epitheliomuscular cells of the ectoderm. They are polyvalent. At the
point of budding, the interstitial cells are numerous, active, and proliferating, but they are included in the swelling made by the ectoendodermal wall of the polyp (Brien and Reniers-Decoen, 1949, 1950, 1951).
From this swelling they migrate into the endoderm to form there, among
other things, the glandular cells. They furnish to the ectoderm the
histological elements necessary for growth and especially for forming
the four kinds of nematocysts. Their fate, thus, is to participate in the
construction of the polyp. But if their role is polyvalent, is it truly totivalent? Not at all, for the process of regeneration and the formation of a
175
regeneration of the head develops, formed exclusively of labeled neoblasts which have migrated from the graft.
If the mesenchymal neoblasts of planarians are polyvalent, they are
not and cannot be totivalent, as can be determined by an analysis of the
stages of regeneration.
The cutting of the animal is followed by cicatrization. The wound
closes; the mass of internal tissues which has just been exposed forms
a "plug" where, among the intact cells, there are mingling destroyed or
traumatized and necrotic cells. The ectodermal cells spread out over
this cicatrix, becoming ameboid and dedifferentiating. They form a very
thin epithelium, a sort of veil which separates the internal tissues from
the external environment [Mates, 1925; Brien, 1944;
(Hydroides);
Cresp, 1964 (serpulids) ]. Cicatrization completed, the neoblasts are concentrated in the blastema of regeneration. Only then does organogenesis
begin. These three stages are observed also in the regeneration of the
urodele limb.
Thus the blastema is formed under the reconstituted ectoblast. Consequently, the ontogenic and organogenic activities of the histoblasts, the
actions of the neoblasts can be manifested only after cicatrization and in
the midst of the older tissues. These tissues are necessarily involved and
contribute to the regenerative activity, not without showing morphallaxis,
as has just been stated. The neoblasts are not able to regain ontogenic
totipotency. There is, perhaps, only one exception to this rule, that of
the mesenchymal amebocytes of freshwater sponges when they are associated to form gemmules or statoblasts, having undergone previously a
true deutoplasmogenesis.
The budding of hydras helps to make clear the significance of this
statement. It is known that the ectoendodermal wall of the polyp possesses neoblasts or interstitial cells, particularly at the base of the
epitheliomuscular cells of the ectoderm. They are polyvalent. At the
point of budding, the interstitial cells are numerous, active, and proliferating, but they are included in the swelling made by the ectoendodermal wall of the polyp (Brien and Reniers-Decoen, 1949, 1950, 1951).
From this swelling they migrate into the endoderm to form there, among
other things, the glandular cells. They furnish to the ectoderm the
histological elements necessary for growth and especially for forming
the four kinds of nematocysts. Their fate, thus, is to participate in the
construction of the polyp. But if their role is polyvalent, is it truly totivalent? Not at all, for the process of regeneration and the formation of a