MORPHOGENESIS IN STENTOR
19
cells with their huge chromosomes, the ciliate macronucleus seems to be
another way of enlarging the nucleus in support of an unusually large
and active cell. In multicellular forms it has always been attractive to
associate cellular differentiation with nuclear differentiation of some
sort. However arising, the nucleus of one type of cell is said to become
irreversibly changed so that the progeny of this cell, even when explanted from the body, would produce only cells of its own type. Such
theories, even as we encounter them today, are always challenged by
the difficulty of explaining why, after development is completed, a
minor part, like the leaf cell of a Begonia or branchial basket of a
tunicate, can nevertheless regenerate the whole with all the normal
diversity of tissues (Harrison, 1933).
In stentors or in other unicellular forms it is obvious that differences
between different regions of the cortex or cytoplasm develop without
corresponding nuclear differentiation simply because all parts of the cell
draw upon the same nucleus. Yet even in Stentor a regional differentiation of the macronucleus corresponding to adjacent cytoplasmic differences in structure was proposed by Weisz (1949b) but this could not
be confirmed (Tartar, 1957b). Direct inheritance of the cortical pattern
and the intimate involvement of this pattern in the formation of holdfast and feeding organelles indicates that in ciliates a large part of the
'information' or basis for epigenesis resides in the semisolid ectoplasm,
with the nucleus merely supplying essential substances.
B. Necessity of the Nucleus for Primordium Formation and Development
That the presence of the macronucleus is necessary for oral regeneration and survival was the outstanding conclusion from all earlier studies
of regeneration in Protozoa. This is really not surprising because so
important a cell organelle as the nucleus certainly should perform indispensable functions.
More remarkable perhaps is that the cell should go on living and
behaving fairly normally for so long a time without its nucleus, though
death eventually ensues. In the unicellular alga, Acetabularia, the cell
can survive for months and even grow and regenerate in the absence of
the nucleus, but this possibility is in part attributable to the persistence
of 'morphogenetic substances' which have been emitted from the
nucleus prior to its removal (Hämmerling, 1953). In enucleated stentors
neither growth nor digestion occur, nor even autodigestion, for recent
studies indicate that enucleated stentors do not decrease in volume as do
their nucleated partners when starved (Tartar, unpublished). There is
also little lag between enucleation and cessation of the elaboration of cell
parts. In the earlier stages of primordium development the anläge can
at most continue one or two stages further, whereupon development
19
cells with their huge chromosomes, the ciliate macronucleus seems to be
another way of enlarging the nucleus in support of an unusually large
and active cell. In multicellular forms it has always been attractive to
associate cellular differentiation with nuclear differentiation of some
sort. However arising, the nucleus of one type of cell is said to become
irreversibly changed so that the progeny of this cell, even when explanted from the body, would produce only cells of its own type. Such
theories, even as we encounter them today, are always challenged by
the difficulty of explaining why, after development is completed, a
minor part, like the leaf cell of a Begonia or branchial basket of a
tunicate, can nevertheless regenerate the whole with all the normal
diversity of tissues (Harrison, 1933).
In stentors or in other unicellular forms it is obvious that differences
between different regions of the cortex or cytoplasm develop without
corresponding nuclear differentiation simply because all parts of the cell
draw upon the same nucleus. Yet even in Stentor a regional differentiation of the macronucleus corresponding to adjacent cytoplasmic differences in structure was proposed by Weisz (1949b) but this could not
be confirmed (Tartar, 1957b). Direct inheritance of the cortical pattern
and the intimate involvement of this pattern in the formation of holdfast and feeding organelles indicates that in ciliates a large part of the
'information' or basis for epigenesis resides in the semisolid ectoplasm,
with the nucleus merely supplying essential substances.
B. Necessity of the Nucleus for Primordium Formation and Development
That the presence of the macronucleus is necessary for oral regeneration and survival was the outstanding conclusion from all earlier studies
of regeneration in Protozoa. This is really not surprising because so
important a cell organelle as the nucleus certainly should perform indispensable functions.
More remarkable perhaps is that the cell should go on living and
behaving fairly normally for so long a time without its nucleus, though
death eventually ensues. In the unicellular alga, Acetabularia, the cell
can survive for months and even grow and regenerate in the absence of
the nucleus, but this possibility is in part attributable to the persistence
of 'morphogenetic substances' which have been emitted from the
nucleus prior to its removal (Hämmerling, 1953). In enucleated stentors
neither growth nor digestion occur, nor even autodigestion, for recent
studies indicate that enucleated stentors do not decrease in volume as do
their nucleated partners when starved (Tartar, unpublished). There is
also little lag between enucleation and cessation of the elaboration of cell
parts. In the earlier stages of primordium development the anläge can
at most continue one or two stages further, whereupon development