160
E . N. W I L L M E R
together as a continuous sheet. Thus, from the point of view of evolution
towards the other cell types it would appear that the epitheliocyte should
be considered as the most primitive form of metazoan cell. It could, of
course, have been that other types of activity, e.g. independent amoeboid activity, were temporarily put into abeyance in the interests of
colony-formation because they certainly occur among the free-living
protozoa. However, in considering the origin of other types of cells in
the metazoa, the types must all have been derived initially from the epitheliocytes. Moreover, among the epitheliocytes, two opposing forms of
activity are likely to be latent if not outwardly expressed.
A stage subsequent to the hollow, but fluid-filled, blastula was in all
probability a blastula in which the fluid in the cavity had a sufficiently
well regulated composition for the cavity to be colonized by cells
leaving the surface epithelium in the way that nowadays cells can be
seen to invade the cavity of the blastula in such organisms as the calcareous sponges or the echinoderms, when these are developing
embryologically. Obviously, such invading cells must be modified
epithelial cells, modified if only because they now no longer make
efficient contact with their next-door neighbours. They could be
derived from either or both of the primary and opposing epithelial
types and thus would be likely to maintain in their new situation something of the original differences between these two types. If this is a
correct hypothesis, it would suggest that the types of cells in unorganized tissue cultures might conceivably revert sooner or later to one of
these four original types, i.e. two types of epitheliocytes and two types
of "mesohyl" or "inner mass" cells derived from the epitheliocytes by
dehiscence or other method of detachment. All epitheliocytes would
be expected to preserve the property of mutual cohesion although it
would not be surprising if it were lost under certain conditions as it may
be during normal development. Although many epithelia in the body
have cells of more than one type, well exemplified by serous and mucous
cells in salivary glands, by ciliated and goblet cells in the trachea, or by
ciliated and "brush-border" cells in the choroid plexus, there is as yet
no evidence that two sorts of epitheliocytes can be distinguished in
cultures of "dedifferentiated" epithelial cells. It is certainly relevant,
however, in this connexion that when epithelia from several sources
are treated, in organ cultures, with large doses of vitamin A some of
the new cells which are produced differentiate into ciliated cells and
others into goblet cells and the relative numbers of each depend in
some cases on the conditions of culture (see p. 661 etseq.). Perhaps a more
thorough investigation of dedifferentiated epithelia may, in the future,
disclose differences corresponding to those which culminate in the
formation of cilia or of mucous globules.
E . N. W I L L M E R
together as a continuous sheet. Thus, from the point of view of evolution
towards the other cell types it would appear that the epitheliocyte should
be considered as the most primitive form of metazoan cell. It could, of
course, have been that other types of activity, e.g. independent amoeboid activity, were temporarily put into abeyance in the interests of
colony-formation because they certainly occur among the free-living
protozoa. However, in considering the origin of other types of cells in
the metazoa, the types must all have been derived initially from the epitheliocytes. Moreover, among the epitheliocytes, two opposing forms of
activity are likely to be latent if not outwardly expressed.
A stage subsequent to the hollow, but fluid-filled, blastula was in all
probability a blastula in which the fluid in the cavity had a sufficiently
well regulated composition for the cavity to be colonized by cells
leaving the surface epithelium in the way that nowadays cells can be
seen to invade the cavity of the blastula in such organisms as the calcareous sponges or the echinoderms, when these are developing
embryologically. Obviously, such invading cells must be modified
epithelial cells, modified if only because they now no longer make
efficient contact with their next-door neighbours. They could be
derived from either or both of the primary and opposing epithelial
types and thus would be likely to maintain in their new situation something of the original differences between these two types. If this is a
correct hypothesis, it would suggest that the types of cells in unorganized tissue cultures might conceivably revert sooner or later to one of
these four original types, i.e. two types of epitheliocytes and two types
of "mesohyl" or "inner mass" cells derived from the epitheliocytes by
dehiscence or other method of detachment. All epitheliocytes would
be expected to preserve the property of mutual cohesion although it
would not be surprising if it were lost under certain conditions as it may
be during normal development. Although many epithelia in the body
have cells of more than one type, well exemplified by serous and mucous
cells in salivary glands, by ciliated and goblet cells in the trachea, or by
ciliated and "brush-border" cells in the choroid plexus, there is as yet
no evidence that two sorts of epitheliocytes can be distinguished in
cultures of "dedifferentiated" epithelial cells. It is certainly relevant,
however, in this connexion that when epithelia from several sources
are treated, in organ cultures, with large doses of vitamin A some of
the new cells which are produced differentiate into ciliated cells and
others into goblet cells and the relative numbers of each depend in
some cases on the conditions of culture (see p. 661 etseq.). Perhaps a more
thorough investigation of dedifferentiated epithelia may, in the future,
disclose differences corresponding to those which culminate in the
formation of cilia or of mucous globules.