162
E . N. W I L L M E R
to the half-way conditions between those to which the animal-pole
cells are adapted and those enjoyed by the vegetal-pole cells. These
half-way conditions could be expected to approximate to those which
prevail in the cavity of the blastula by virtue of the opposing actions of
the two classes of cells. Thus the cells in the blastula which would be
most likely to penetrate into the central cavity with ease would be the
intermediate cells which are already most nearly attuned to the conditions within. In other words, during the evolutionary process, the
mesenchymal invasion would be most successful if carried out by the
cells in the intermediate zone, and it may be that this is the main
determining factor in the location of mesenchymal cells in the various
embryos. It also suggests that the mesenchymal cells might rather
easily be able to swing over from one form of activity to the other
should the conditions favour such a change.
All epithelial cells, since they form part of a covering layer separating
two media, must inevitably be polarized cells and in situ have surfaces
in contact with three essentially different media, namely the external
fluid, the internal fluid and the neighbouring epithelial cells. This
arrangement is certainly responsible for some at least of the morphological characters of an epithelial cell, and such characters are likely to
be lost when the cells become disarranged and more isolated in vitro,
or when one or more of these media are changed.
V I .
S O M E P H Y S I C O - C H E M I C A L C H A R A C T E R I S T I C S OF T H E
C E L L T Y P E S
The second approach to the problem of the limited number of types
of cells in unorganized growth in vitro lies in the nature of the differences
between the cells. A particular example may illustrate this. How does a
mechanocyte differ from an amoebocyte? and what causes the difference?
The mechanocyte is, in general, a much more polarized cell than the
amoebocyte. It tends to move in a specific direction with its leading
edge showing many short pseudopodia and "ruffles", and with the
posterior end drawn out into trailing, often filiform processes. Pinocytosis is probably confined to the leading end where the ruffles occur.
The general surface of the mechanocyte appears to be much more
stable than that of the amoebocyte. The amoebocyte may extend any
part of its surface in any direction, in lobose pseudopodia, branching
processes or as lamelliform membranes which may even surround the
whole cell. These membranes may show undulations, and suddenly
break up into more or less radiating processes. Amoebocytes throw out
microvilli and often have extensive ruffles. Pinocytosis, though as we
E . N. W I L L M E R
to the half-way conditions between those to which the animal-pole
cells are adapted and those enjoyed by the vegetal-pole cells. These
half-way conditions could be expected to approximate to those which
prevail in the cavity of the blastula by virtue of the opposing actions of
the two classes of cells. Thus the cells in the blastula which would be
most likely to penetrate into the central cavity with ease would be the
intermediate cells which are already most nearly attuned to the conditions within. In other words, during the evolutionary process, the
mesenchymal invasion would be most successful if carried out by the
cells in the intermediate zone, and it may be that this is the main
determining factor in the location of mesenchymal cells in the various
embryos. It also suggests that the mesenchymal cells might rather
easily be able to swing over from one form of activity to the other
should the conditions favour such a change.
All epithelial cells, since they form part of a covering layer separating
two media, must inevitably be polarized cells and in situ have surfaces
in contact with three essentially different media, namely the external
fluid, the internal fluid and the neighbouring epithelial cells. This
arrangement is certainly responsible for some at least of the morphological characters of an epithelial cell, and such characters are likely to
be lost when the cells become disarranged and more isolated in vitro,
or when one or more of these media are changed.
V I .
S O M E P H Y S I C O - C H E M I C A L C H A R A C T E R I S T I C S OF T H E
C E L L T Y P E S
The second approach to the problem of the limited number of types
of cells in unorganized growth in vitro lies in the nature of the differences
between the cells. A particular example may illustrate this. How does a
mechanocyte differ from an amoebocyte? and what causes the difference?
The mechanocyte is, in general, a much more polarized cell than the
amoebocyte. It tends to move in a specific direction with its leading
edge showing many short pseudopodia and "ruffles", and with the
posterior end drawn out into trailing, often filiform processes. Pinocytosis is probably confined to the leading end where the ruffles occur.
The general surface of the mechanocyte appears to be much more
stable than that of the amoebocyte. The amoebocyte may extend any
part of its surface in any direction, in lobose pseudopodia, branching
processes or as lamelliform membranes which may even surround the
whole cell. These membranes may show undulations, and suddenly
break up into more or less radiating processes. Amoebocytes throw out
microvilli and often have extensive ruffles. Pinocytosis, though as we