4. C E L L T Y P E S
161
If, during development, each sort of epitheliocyte produced its own
type of mesenchymal cell, the mechanocytes could well represent one
of these types and the amoebocytes the other. This hypothesis therefore
groups cells into (a) the cells of the primary surface (the epitheliocytes)
and (b) the cells of the "inner mass", and, while the former may be
more or less versatile cells in that they are normally in contact with
a variable external medium, the cells of the mesenchyme or inner mass
are more likely to be dependent upon the epitheliocytes for providing
a suitable milieu. It is significant in this connexion that mechanocytes
are much more fastidious about their diet than are most epitheliocytes,
and they have been shown to be assisted in their growth by amoebocytes,
whose phagocytic properties no doubt liberate these cells to some extent
from such dietary restriction. It need hardly be mentioned that the
"primary surface" of the organism, of course, includes the alimentary
canal and all its diverticula and also the lining of the urino-genital
system.
It is interesting to notice that, in the body, epithelial cells seem to
show two tendencies in their micro-structure. There is, for example,
some correlation between the presence of cilia, microvilli, laminar
endoplasmic reticulum, laminar Golgi body and desmosomes on the
one hand and infolded cell surfaces, extensively vacuolated endoplasmic
reticulum, Golgi body with numerous terminal vesicles and with cells
locking together in a jig-saw pattern, instead of with desmosomes, on
the other.
In most blastulae, the two types of cells, present as extremes at the
animal and vegetal poles, are not as clearly defined as they are, for
example, in the blastulae of some of the calcareous sponges and there is
usually a definite gradient of cells merging slowly and continuously
from animal-pole cells to vegetal-pole cells. If this was the case in the
hypothetical primitive metazoan which gave rise to mechanocytes and
amoebocytes in the mesohyl, then presumably there should be among the
mesenchyme or mesohyl cells all gradations from "strong" mechanocytes
through "weak" mechanocytes and "weak" amoebocytes to "strong"
amoebocytes. Since, however, the original concept involved the opposing
characters of the two classes of cells, any change-over between the two
types must at some point involve a change of orientation, a change of sign
as it were. Presumably those cells which were not very "strongly" committed either way might be the more easily able to reverse their activity.
Those more heavily committed one way or the other would only be
able to change their form and character with greater difficulty, or not
at all. This is clearly important in any consideration of problems
concerned with the interchangeability of cell types. Moreover the cells
in the middle of the gradient are presumably those most nearly adapted
a
161
If, during development, each sort of epitheliocyte produced its own
type of mesenchymal cell, the mechanocytes could well represent one
of these types and the amoebocytes the other. This hypothesis therefore
groups cells into (a) the cells of the primary surface (the epitheliocytes)
and (b) the cells of the "inner mass", and, while the former may be
more or less versatile cells in that they are normally in contact with
a variable external medium, the cells of the mesenchyme or inner mass
are more likely to be dependent upon the epitheliocytes for providing
a suitable milieu. It is significant in this connexion that mechanocytes
are much more fastidious about their diet than are most epitheliocytes,
and they have been shown to be assisted in their growth by amoebocytes,
whose phagocytic properties no doubt liberate these cells to some extent
from such dietary restriction. It need hardly be mentioned that the
"primary surface" of the organism, of course, includes the alimentary
canal and all its diverticula and also the lining of the urino-genital
system.
It is interesting to notice that, in the body, epithelial cells seem to
show two tendencies in their micro-structure. There is, for example,
some correlation between the presence of cilia, microvilli, laminar
endoplasmic reticulum, laminar Golgi body and desmosomes on the
one hand and infolded cell surfaces, extensively vacuolated endoplasmic
reticulum, Golgi body with numerous terminal vesicles and with cells
locking together in a jig-saw pattern, instead of with desmosomes, on
the other.
In most blastulae, the two types of cells, present as extremes at the
animal and vegetal poles, are not as clearly defined as they are, for
example, in the blastulae of some of the calcareous sponges and there is
usually a definite gradient of cells merging slowly and continuously
from animal-pole cells to vegetal-pole cells. If this was the case in the
hypothetical primitive metazoan which gave rise to mechanocytes and
amoebocytes in the mesohyl, then presumably there should be among the
mesenchyme or mesohyl cells all gradations from "strong" mechanocytes
through "weak" mechanocytes and "weak" amoebocytes to "strong"
amoebocytes. Since, however, the original concept involved the opposing
characters of the two classes of cells, any change-over between the two
types must at some point involve a change of orientation, a change of sign
as it were. Presumably those cells which were not very "strongly" committed either way might be the more easily able to reverse their activity.
Those more heavily committed one way or the other would only be
able to change their form and character with greater difficulty, or not
at all. This is clearly important in any consideration of problems
concerned with the interchangeability of cell types. Moreover the cells
in the middle of the gradient are presumably those most nearly adapted
a