II. THE EMBRYOLOGY OF
ASCIDIANS
77
cytochemical reactions) was already present in the unfertilized egg,
where it combined with the inhibitor and was thus prevented from
synthesis. We should like to suppose, generally, that most enzymes are
already present everywhere in the cells of the embryo: their further
active synthesis, however, occurring only in the 'proper' cells. But what
is it that awakens an enzyme from its quiescent state, in that cell, at the
appropriate time, and brings about its multiplication?
C. Heterospecific Hybridizations
The study of the interrelations between nucleus and cytoplasm is
fundamental for any embryologist. Every organism is, in fact, the result
of a continuous exchange between the two. The interest in the nucleus
has surpassed that in the cytoplasm, with the result that we have come
to attribute to the nucleus a sort of omnipotence with regard to
development. According to this view, every detail in development and
in morphogenesis is due to the genes, including the fundamental
processes, such as the pattern and velocity of segmentation, gastrulation,
neurulation. Embryologists, however, cannot accept too easily this
affirmation, since they are compelled, by their research, to give importance also to the cytoplasmic constituents. This difficulty was fully
understood by Morgan (1934), who, embryologist and genetician at the
same time, proposed a hypothesis which has inspired much research.
According to Morgan, the nuclei of the first cells derived by segmentation of the egg are identical. However, under the action of cytoplasmic
substances, which are not alike in all blastomeres because of the
Ooplasmic segregation' which occurs during development, these nuclei
would become gradually different. Certain cytoplasmic constituents
would activate only certain genes, and so, little by little, structures and
organs would differentiate. As can be seen, Morgan's theory attributes
a role to the cytoplasm in differentiation, and holds that the nuclei
themselves become gradually differentiated. The same theory, in more
chemical language would sound like this: the enzymes would originate in
the nucleus as the very first products of the genes ('one gene, one
enzyme'); the enzymes would then pass from the nucleus into the
cytoplasm, where, if they found their specific substrate, they would
initiate a chemical reaction. The result of this reaction might be the
proper substrate for another enzyme which in turn would enter into
action, and so on. The substrates of the enzymes are certainly different
from the very beginning, because the cytoplasmic contents are not
alike in all blastomeres, and therefore different chemical reactions will
go on in the different cells. The results of the various chain-reactions
will be different molecular configurations, from which would derive the
basis for morphological structures. In this way, the interplay between
ASCIDIANS
77
cytochemical reactions) was already present in the unfertilized egg,
where it combined with the inhibitor and was thus prevented from
synthesis. We should like to suppose, generally, that most enzymes are
already present everywhere in the cells of the embryo: their further
active synthesis, however, occurring only in the 'proper' cells. But what
is it that awakens an enzyme from its quiescent state, in that cell, at the
appropriate time, and brings about its multiplication?
C. Heterospecific Hybridizations
The study of the interrelations between nucleus and cytoplasm is
fundamental for any embryologist. Every organism is, in fact, the result
of a continuous exchange between the two. The interest in the nucleus
has surpassed that in the cytoplasm, with the result that we have come
to attribute to the nucleus a sort of omnipotence with regard to
development. According to this view, every detail in development and
in morphogenesis is due to the genes, including the fundamental
processes, such as the pattern and velocity of segmentation, gastrulation,
neurulation. Embryologists, however, cannot accept too easily this
affirmation, since they are compelled, by their research, to give importance also to the cytoplasmic constituents. This difficulty was fully
understood by Morgan (1934), who, embryologist and genetician at the
same time, proposed a hypothesis which has inspired much research.
According to Morgan, the nuclei of the first cells derived by segmentation of the egg are identical. However, under the action of cytoplasmic
substances, which are not alike in all blastomeres because of the
Ooplasmic segregation' which occurs during development, these nuclei
would become gradually different. Certain cytoplasmic constituents
would activate only certain genes, and so, little by little, structures and
organs would differentiate. As can be seen, Morgan's theory attributes
a role to the cytoplasm in differentiation, and holds that the nuclei
themselves become gradually differentiated. The same theory, in more
chemical language would sound like this: the enzymes would originate in
the nucleus as the very first products of the genes ('one gene, one
enzyme'); the enzymes would then pass from the nucleus into the
cytoplasm, where, if they found their specific substrate, they would
initiate a chemical reaction. The result of this reaction might be the
proper substrate for another enzyme which in turn would enter into
action, and so on. The substrates of the enzymes are certainly different
from the very beginning, because the cytoplasmic contents are not
alike in all blastomeres, and therefore different chemical reactions will
go on in the different cells. The results of the various chain-reactions
will be different molecular configurations, from which would derive the
basis for morphological structures. In this way, the interplay between