IX. DEVELOPMENT OF THE TELEOSTEAN EGG
413
vegetal pole taken as fixed point. This movement can co-operate with
the other epibolic forces except in the last stages of the closing of the
blastopore; the pellicle can then be removed without impeding the end
of the process. Trinkaus (1949b) notes that a pellicular wound at the
vitelline pole speeds up the closing of the blastopore. He considers this
as a proof of the non-intervention (non-contraction) of the pellicle,
but I think that the acceleration is the result of a pellicular traction
increased by the addition of the closing movement of the wound acting
in the same direction.
Trinkaus thinks that the pellicle of Fundulus does not contract itself
but disappears by solation with the spreading of the embryonic disc.
By placing charcoal marks on the blastoderm, the periblast and the
pellicle, this author measured the variations in their relative distances
and in the distance between them and the vegetal and animal poles
during epiboly. Apart from technical inaccuracies, the space between
two pellicular marks does not vary, though it should do so if there were
a contraction. In the same way the distance between such marks and
the vegetal pole stays constant as long as they are not reached by the
edge of the blastoderm. The diminution of the interval between the
edge of the blastoderm and the mark is equal to the lengthening of the
radius of the blastoderm; it would be greater if the mark moved back by
contraction. Thus the pellicle plays no part in epiboly, as shown also
by the fact that when the blastoderm is separated from the pellicle it
still achieves its epiboly.
I think that some corrections should be made to Trinkaus's conclusions. On the one hand, the pellicle of Salmo visibly contracts itself
On the other hand, even if this contraction affects at the same time the
whole surface, the displacement of the different points will not be of the
same magnitude at all levels because of the slowing down caused by
friction on the underlying yolk. Retraction in an intact egg has not the
same appearance as in the pellicle emptied of its contents. Lastly, since
a mark placed on the pellicle never passes onto the blastoderm but is
always pushed back, how could it stay fixed to a base which disappears?
Since the elastic tension of the syncytium is inferior to that of the
blastoderm, if the pellicular contraction were the only process involved
in epiboly, then the spreading of the syncytium stripped of its blastoderm should take place at a greater speed. Trinkaus notes that this
speed stays constant, but on the other hand, if the same experiment is
carried out in the last stages of the closure of the blastopore, this speed
then becomes greater—a result which contradicts the first.
Trinkaus has not taken into consideration the fact that the peripheral
syncytial ridge, when bare, contracts under the influence of water (see
413
vegetal pole taken as fixed point. This movement can co-operate with
the other epibolic forces except in the last stages of the closing of the
blastopore; the pellicle can then be removed without impeding the end
of the process. Trinkaus (1949b) notes that a pellicular wound at the
vitelline pole speeds up the closing of the blastopore. He considers this
as a proof of the non-intervention (non-contraction) of the pellicle,
but I think that the acceleration is the result of a pellicular traction
increased by the addition of the closing movement of the wound acting
in the same direction.
Trinkaus thinks that the pellicle of Fundulus does not contract itself
but disappears by solation with the spreading of the embryonic disc.
By placing charcoal marks on the blastoderm, the periblast and the
pellicle, this author measured the variations in their relative distances
and in the distance between them and the vegetal and animal poles
during epiboly. Apart from technical inaccuracies, the space between
two pellicular marks does not vary, though it should do so if there were
a contraction. In the same way the distance between such marks and
the vegetal pole stays constant as long as they are not reached by the
edge of the blastoderm. The diminution of the interval between the
edge of the blastoderm and the mark is equal to the lengthening of the
radius of the blastoderm; it would be greater if the mark moved back by
contraction. Thus the pellicle plays no part in epiboly, as shown also
by the fact that when the blastoderm is separated from the pellicle it
still achieves its epiboly.
I think that some corrections should be made to Trinkaus's conclusions. On the one hand, the pellicle of Salmo visibly contracts itself
On the other hand, even if this contraction affects at the same time the
whole surface, the displacement of the different points will not be of the
same magnitude at all levels because of the slowing down caused by
friction on the underlying yolk. Retraction in an intact egg has not the
same appearance as in the pellicle emptied of its contents. Lastly, since
a mark placed on the pellicle never passes onto the blastoderm but is
always pushed back, how could it stay fixed to a base which disappears?
Since the elastic tension of the syncytium is inferior to that of the
blastoderm, if the pellicular contraction were the only process involved
in epiboly, then the spreading of the syncytium stripped of its blastoderm should take place at a greater speed. Trinkaus notes that this
speed stays constant, but on the other hand, if the same experiment is
carried out in the last stages of the closure of the blastopore, this speed
then becomes greater—a result which contradicts the first.
Trinkaus has not taken into consideration the fact that the peripheral
syncytial ridge, when bare, contracts under the influence of water (see