164
E . N . W I L L M E R
Mechanocytes make only limited though rather strong contacts with
each other. They are in fact very dependent on these contacts and such
contacts inhibit their subsequent movements (see p. 188), but they are
not usually accompanied by the formation of desmosomes.
Amoebocytes, in which there is far less polarity, seem to prefer their
own company and tend to space the mselves out evenly in a culture so
that they lie as far away from each other as possible. Under certain
rather miscellaneous conditions like low 0 2 , or the presence of various
foreign bodies, amoebocytes may overcome this mutual aversion and
then they fuse together completely, with the formation of multinucleate
giant cells (see pp. 40, 646). There are indications that the development
of pseudopodia, and the mutual repulsion of cells may depend on the
cells being negatively charged, while the cohesion between cells becomes
greater the smaller is this charge as determined by electrophoretic studies
(Abercrombie and Ambrose, 1962).
The three classes of cells must therefore have very different surfaces,
but singularly little is known about the nature of these differences.
Clearly the essential surface properties must be a fairly constant feature
of the cell, for when cells of one type divide, the daughter cells tend
to have properties identical with those of the parents, e.g. mechanocytes produce mechanocytes; and as mentioned earlier, when two
classes of cells, e.g. mechanocytes and amoebocytes, or epithelial cells
and amoebocytes, are explanted together into the same medium they
tend to remain distinct and each to "breed true". Thus the surface
properties are at least partly dependent on intrinsic factors within the
cell, but whether these intrinsic factors are nuclear and gene-controlled
or whether they depend on the inherent and elaborate constitution of
the cell surface itself cannot yet be decided. It is certainly possible to
imagine the cell surface as being so elaborately constituted as to have
some powers of self-duplication and extension with the same pattern
as itself. Indeed the properties of the surface of the fertilized ovum have
been shown in some species to have considerable power of determining patterns of differentiation in the embryo (Curtis, 1960; Raven,
1961).
V I I . T H E P R O B L E M OF I N T E R C O N V E R S I O N
While it is fairly generally agreed that cells, when they emerge from
tissues in the manner described, can often be readily classed into one
of the main types mentioned, at other times such classification may
be difficult, and isolated cells are always much more difficult to identify
E . N . W I L L M E R
Mechanocytes make only limited though rather strong contacts with
each other. They are in fact very dependent on these contacts and such
contacts inhibit their subsequent movements (see p. 188), but they are
not usually accompanied by the formation of desmosomes.
Amoebocytes, in which there is far less polarity, seem to prefer their
own company and tend to space the mselves out evenly in a culture so
that they lie as far away from each other as possible. Under certain
rather miscellaneous conditions like low 0 2 , or the presence of various
foreign bodies, amoebocytes may overcome this mutual aversion and
then they fuse together completely, with the formation of multinucleate
giant cells (see pp. 40, 646). There are indications that the development
of pseudopodia, and the mutual repulsion of cells may depend on the
cells being negatively charged, while the cohesion between cells becomes
greater the smaller is this charge as determined by electrophoretic studies
(Abercrombie and Ambrose, 1962).
The three classes of cells must therefore have very different surfaces,
but singularly little is known about the nature of these differences.
Clearly the essential surface properties must be a fairly constant feature
of the cell, for when cells of one type divide, the daughter cells tend
to have properties identical with those of the parents, e.g. mechanocytes produce mechanocytes; and as mentioned earlier, when two
classes of cells, e.g. mechanocytes and amoebocytes, or epithelial cells
and amoebocytes, are explanted together into the same medium they
tend to remain distinct and each to "breed true". Thus the surface
properties are at least partly dependent on intrinsic factors within the
cell, but whether these intrinsic factors are nuclear and gene-controlled
or whether they depend on the inherent and elaborate constitution of
the cell surface itself cannot yet be decided. It is certainly possible to
imagine the cell surface as being so elaborately constituted as to have
some powers of self-duplication and extension with the same pattern
as itself. Indeed the properties of the surface of the fertilized ovum have
been shown in some species to have considerable power of determining patterns of differentiation in the embryo (Curtis, 1960; Raven,
1961).
V I I . T H E P R O B L E M OF I N T E R C O N V E R S I O N
While it is fairly generally agreed that cells, when they emerge from
tissues in the manner described, can often be readily classed into one
of the main types mentioned, at other times such classification may
be difficult, and isolated cells are always much more difficult to identify