288
CHARLES
Ε. WILDE
donor and host. Only following subsequent amputation does the donor
nucleus dominate the synthesis of its own type of cap (Hämmerling,
1953).
In the parthenogenetic merogony experiments of Harvey (1936),
non-nucleated fragments of Arbacia punctulata cleave and form a
blástula. The blástula becomes ciliated with regional variations typical
of normal blastulae repeated in the non-nucleated 'embryo'. Here there
has been maintained and fostered, in the contemporary absence of the
nucleus, the new synthesis of cellular organelles of a high degree of
complexity. Normal topographic variation in the synthesis has also
been maintained. Either the merogonic fragment has been previously
cued by metabolic activities initiated in the nucleus at some prior time,
or the cytoplasm with its own synthetic activities can cue the differentiation of cilia. It must be concluded that the metabolism of the cytoplasm can carry out differentiative activities in the absence of the
nucleus, and that this is true for the cytoplasm of prospective pigment
cells. These facts do not necessarily decry the primacy of the nucleus.
The form and identity of pigment cells as between species is obviously
due to nuclear activities. It is often possible to recognize absolutely, cells
migrating and differentiating in a host of differing genotype stemming
from a transplant donated by a different species (Twitty, 1949).
In the final analysis differentiation probably reflects an interplay
between micro-environment, cytoplasm and nucleus. Much of the
material which follows will be referred to such an interplay.
In view of the difficulties of separating nucleus and cytoplasm, it has
proven convenient to design precise in vitro systems where the microenvironment can be altered at will. Two parameters in this work must
be set however. It is essential that the experimenter know precisely, on
a cell for cell level, the presumptive fate of the explanted cells in order
to obviate the criticism that any micro-environmentally altered differentiation is due, not to the alteration itself, but to contamination of
the culture with cells of differing fate. The second parameter lies in
the means of environmental alteration. The compounds added to or
subtracted from the micro-environment should be chosen for some
particular reason. The biochemical aspects of their metabolism should
be reasonably well worked out, and the material should be added in
amounts approximating by one or two orders of magnitude the occurrence of the same or closely-allied compounds in nature. Under these
circumstances exacting experiments can be designed and pertinent
information with regard to cellular differentiation can be obtained.
Failing this, data may become confused.
Fell and Mellanby (1953) reported the first positive results insofar as
cellular differentiation is concerned. In their experiments, Vitamin A
CHARLES
Ε. WILDE
donor and host. Only following subsequent amputation does the donor
nucleus dominate the synthesis of its own type of cap (Hämmerling,
1953).
In the parthenogenetic merogony experiments of Harvey (1936),
non-nucleated fragments of Arbacia punctulata cleave and form a
blástula. The blástula becomes ciliated with regional variations typical
of normal blastulae repeated in the non-nucleated 'embryo'. Here there
has been maintained and fostered, in the contemporary absence of the
nucleus, the new synthesis of cellular organelles of a high degree of
complexity. Normal topographic variation in the synthesis has also
been maintained. Either the merogonic fragment has been previously
cued by metabolic activities initiated in the nucleus at some prior time,
or the cytoplasm with its own synthetic activities can cue the differentiation of cilia. It must be concluded that the metabolism of the cytoplasm can carry out differentiative activities in the absence of the
nucleus, and that this is true for the cytoplasm of prospective pigment
cells. These facts do not necessarily decry the primacy of the nucleus.
The form and identity of pigment cells as between species is obviously
due to nuclear activities. It is often possible to recognize absolutely, cells
migrating and differentiating in a host of differing genotype stemming
from a transplant donated by a different species (Twitty, 1949).
In the final analysis differentiation probably reflects an interplay
between micro-environment, cytoplasm and nucleus. Much of the
material which follows will be referred to such an interplay.
In view of the difficulties of separating nucleus and cytoplasm, it has
proven convenient to design precise in vitro systems where the microenvironment can be altered at will. Two parameters in this work must
be set however. It is essential that the experimenter know precisely, on
a cell for cell level, the presumptive fate of the explanted cells in order
to obviate the criticism that any micro-environmentally altered differentiation is due, not to the alteration itself, but to contamination of
the culture with cells of differing fate. The second parameter lies in
the means of environmental alteration. The compounds added to or
subtracted from the micro-environment should be chosen for some
particular reason. The biochemical aspects of their metabolism should
be reasonably well worked out, and the material should be added in
amounts approximating by one or two orders of magnitude the occurrence of the same or closely-allied compounds in nature. Under these
circumstances exacting experiments can be designed and pertinent
information with regard to cellular differentiation can be obtained.
Failing this, data may become confused.
Fell and Mellanby (1953) reported the first positive results insofar as
cellular differentiation is concerned. In their experiments, Vitamin A