IX.
DEVELOPMENT OF THE TELEOSTEAN EGG
397
medullary plate. The material of the 'crescent' is used up in the yolk
which acts as a base, and the germ does not receive a sufficient amount of
'organizing plasm'.
So far, the evidence in favour of the ideas of Oppenheimer and Tung
forms a fairly coherent whole, though still bearing some contradictions.
Waddington (1956) has already stressed the point that some of Tung's
experiments were interpreted in a way which can be criticized, and it
must be acknowledged that this conception still has a very hypothetical
basis. The 'crescent' has not yet been characterized cytologically. The
affiliation suggested between the plasm of Vakaet and the 'crescent'
has only been inferred and awaits confirmation or modification. A priori,
there is no certitude that it corresponds exactly to the localization of a
visible material: in Amphibia, the zone of the grey crescent does not
cover the entire future marginal zone.
On the other hand, how this hypothetical material may reach the
blastoderm needs to be explained. The base of the blastoderm is in
direct communication with the periblast in the early stages; later on,
the syncytium 'buds' off blastomeres that add themselves to the embryonic disc. Without a microscopic study it is impossible to know at
what stage this budding ceases in Carassius; in Salmo this addition of
syncytial cytoplasm seems to disappear around the blástula stage. But
then how can one explain that in later stages the syncytium can still
impose an orientation on the germ? Are diffusing organizing substances
involved? Investigations are now being carried out on Salmo to try and
answer these questions.
C. The Covering of the Ovum
In the ripe egg, the yolk is surrounded by a deep layer of cytoplasm,
which is mobilized by bipolar differentiation, and by the cell envelope
itself, or cortex (pellicle). The terminology of the egg coverings is
rather confused.
In the eggs of Echinoids, the term cortex can be applied both to extraovular and to egg structures (Chambers, 1938) or it may only mean the
egg structures (Monné, 1948).
In the eggs of Amphibia, Holtfreter (1943a) distinguished three superposed layers: coat (outside), membrane (plasmolemma) and plasmagel
(inside). The coat would be extra-ovular so that membrane + plasmagel
together would correspond to the cortex in Monné's sense. This question
has been taken up again by Dollander (1951, 1954, 1956) who places the
coat inside the plasmolemma.
The structural study of the egg coverings in Teleosts is still too rudimentary to enable one to subdivide the covering into constituents. As
DEVELOPMENT OF THE TELEOSTEAN EGG
397
medullary plate. The material of the 'crescent' is used up in the yolk
which acts as a base, and the germ does not receive a sufficient amount of
'organizing plasm'.
So far, the evidence in favour of the ideas of Oppenheimer and Tung
forms a fairly coherent whole, though still bearing some contradictions.
Waddington (1956) has already stressed the point that some of Tung's
experiments were interpreted in a way which can be criticized, and it
must be acknowledged that this conception still has a very hypothetical
basis. The 'crescent' has not yet been characterized cytologically. The
affiliation suggested between the plasm of Vakaet and the 'crescent'
has only been inferred and awaits confirmation or modification. A priori,
there is no certitude that it corresponds exactly to the localization of a
visible material: in Amphibia, the zone of the grey crescent does not
cover the entire future marginal zone.
On the other hand, how this hypothetical material may reach the
blastoderm needs to be explained. The base of the blastoderm is in
direct communication with the periblast in the early stages; later on,
the syncytium 'buds' off blastomeres that add themselves to the embryonic disc. Without a microscopic study it is impossible to know at
what stage this budding ceases in Carassius; in Salmo this addition of
syncytial cytoplasm seems to disappear around the blástula stage. But
then how can one explain that in later stages the syncytium can still
impose an orientation on the germ? Are diffusing organizing substances
involved? Investigations are now being carried out on Salmo to try and
answer these questions.
C. The Covering of the Ovum
In the ripe egg, the yolk is surrounded by a deep layer of cytoplasm,
which is mobilized by bipolar differentiation, and by the cell envelope
itself, or cortex (pellicle). The terminology of the egg coverings is
rather confused.
In the eggs of Echinoids, the term cortex can be applied both to extraovular and to egg structures (Chambers, 1938) or it may only mean the
egg structures (Monné, 1948).
In the eggs of Amphibia, Holtfreter (1943a) distinguished three superposed layers: coat (outside), membrane (plasmolemma) and plasmagel
(inside). The coat would be extra-ovular so that membrane + plasmagel
together would correspond to the cortex in Monné's sense. This question
has been taken up again by Dollander (1951, 1954, 1956) who places the
coat inside the plasmolemma.
The structural study of the egg coverings in Teleosts is still too rudimentary to enable one to subdivide the covering into constituents. As