ANIMALIZATION AND VEGETALIZATION
153
of the pluteus. If they act during the second phase, the animalizing and
vegetalizing agents essentially exercise non-specific inhibitor effects on
development. On the other hand, when they act during the stage of
embryonic determination, these agents interfere with the equilibrium
between the ectodermal and entomesodermal tendencies, and animalizing
or vegetalizing effects can be obtained. The maximum sensibility to the
vegetalizing effects of lithium (Hörstadius, 1936a; Bäckström and
Gustafson, 1953) and the animalizing effects of iodosobenzoic acid
(Bäckström, 1953) and zinc ions (Lallier, 1959b) is located at the
beginning of the determination period, during the first stages of
segmentation. The prolongation of the action of the chemical agent
throughout the period of determination contributes to the strength of
their effects on embryonic determination.
3. A Combination of Operative and Chemical Methods
Hörstadius (1936b) used such a combination to show that animal
blastomeres, previously treated with lithium, exert a vegetalizing
influence comparable with the influence of the macromeres. This
observation is important, because it shows that the effects of blastomeres vegetalized by chemical methods are indistinguishable from those
exerted by the normal vegetative blastomeres, the micromeres. The
mixed method is also used to detect the possible animalizing or vegetalizing effects of various chemical agents on isolated animal or vegetal
halves. These halves are more sensitive to the animalizing or vegetalizing
influences of chemical agents than are whole eggs.
C. Heuristic and Practical Value of Operative and Chemical Methods
Operative methods have allowed us to follow the differentiation of the
blastomeres, either isolated or grouped, and to study their interactions
during the progress of morphogenesis. The extension of research on a
biochemical level has set new practical problems. Biochemical techniques
are rarely sensitive enough to analyse the biochemical properties of
fragments of eggs. Moreover it is practically impossible to obtain large
quantities of these fragments because of the difficulties inherent in their
preparation. The utilization of chemical methods which, when whole
eggs are treated, can produce large numbers of vegetalized or animalized
embryos, palliates these difficulties. Most of the biochemical research
done to date concerns vegetalization. It is the possession of very
effective vegetalizing agents, such as lithium, that enables the biochemical aspects of vegetalization to be studied. On the other hand, the
first animalizing agents used produced such inconstant effects that a
systematic study of animalization was not possible. The recent discovery
of very active animalizing agents now permits the biochemical study of
153
of the pluteus. If they act during the second phase, the animalizing and
vegetalizing agents essentially exercise non-specific inhibitor effects on
development. On the other hand, when they act during the stage of
embryonic determination, these agents interfere with the equilibrium
between the ectodermal and entomesodermal tendencies, and animalizing
or vegetalizing effects can be obtained. The maximum sensibility to the
vegetalizing effects of lithium (Hörstadius, 1936a; Bäckström and
Gustafson, 1953) and the animalizing effects of iodosobenzoic acid
(Bäckström, 1953) and zinc ions (Lallier, 1959b) is located at the
beginning of the determination period, during the first stages of
segmentation. The prolongation of the action of the chemical agent
throughout the period of determination contributes to the strength of
their effects on embryonic determination.
3. A Combination of Operative and Chemical Methods
Hörstadius (1936b) used such a combination to show that animal
blastomeres, previously treated with lithium, exert a vegetalizing
influence comparable with the influence of the macromeres. This
observation is important, because it shows that the effects of blastomeres vegetalized by chemical methods are indistinguishable from those
exerted by the normal vegetative blastomeres, the micromeres. The
mixed method is also used to detect the possible animalizing or vegetalizing effects of various chemical agents on isolated animal or vegetal
halves. These halves are more sensitive to the animalizing or vegetalizing
influences of chemical agents than are whole eggs.
C. Heuristic and Practical Value of Operative and Chemical Methods
Operative methods have allowed us to follow the differentiation of the
blastomeres, either isolated or grouped, and to study their interactions
during the progress of morphogenesis. The extension of research on a
biochemical level has set new practical problems. Biochemical techniques
are rarely sensitive enough to analyse the biochemical properties of
fragments of eggs. Moreover it is practically impossible to obtain large
quantities of these fragments because of the difficulties inherent in their
preparation. The utilization of chemical methods which, when whole
eggs are treated, can produce large numbers of vegetalized or animalized
embryos, palliates these difficulties. Most of the biochemical research
done to date concerns vegetalization. It is the possession of very
effective vegetalizing agents, such as lithium, that enables the biochemical aspects of vegetalization to be studied. On the other hand, the
first animalizing agents used produced such inconstant effects that a
systematic study of animalization was not possible. The recent discovery
of very active animalizing agents now permits the biochemical study of