88
G.
REVERBERI
increase in the egg's volume, the primitive cortex must shrink. One
might imagine therefore the cortex as a net of oriented protein fibrils,
which at fertilization would close together, and the granules attached
to them would thus be displaced. The above mentioned observation of
Conklin on the test cells of Styela, is easily explained if one admits that
the test cells are connected physically with the cortex; electronmicroscope observations support this explanation. The hypothesis of
the existence of a semi-oriented structure in the cortex of the unfertilized egg is supported by the results which are obtained by the
treatment of the egg with trypsin (Ortolani, 1957a) and particularly,
by observations with the polarized light microscope (Monroy, 1953).
According to Monroy, the weak birefringence which is observed in the
cortex of the unfertilized egg increases after fertilization at the top of
the lobopodia, where the cortical layer appears thicker.
An extraordinary importance is attributed today to the role of the
egg cortex in morphogenesis. Distinguished embryologists consider the
cortex as being the site of the 'organ-potencies'; Raven (1958) maintains that the ooplasmic segregations themselves are controlled by the
cortex. It is also well known, as a result of the experiments of
Hörstadius et al. (1950), that in the unfertilized sea-urchin egg large
quantities of endoplasm can be aspirated by a micropipette without any
unfavourable consequence on the development.
In the unfertilized egg of Ascidians also, one can remove more or less
endoplasm by a micropipette (unpublished work) without any consequence; neither does the removal of restricted parts of the cortex
interfere with normal morphogenesis. However, the same operations
carried out on the fertilized egg impede segmentation.
3. Endoplasmic Substances and their Displacement by Centrifugation
No embryologist would deny the importance of the visible or invisible endoplasmic substances. The plasmatic segregations occurring in
the Ascidian egg at fertilization are too impressive and one cannot deny
their significance.
With regard to the mitochondria, the meaning of their migration and
segregation (Figs. 62 to 73) for the larval musculature has been illustrated above. The plasmatic migrations, the causes of which were
discussed by Costello (1948), take place, at least for the mitochondria,
in two periods, and are connected with two events. The first, is certainly
connected with the already mentioned emission of fluid hyaloplasm at
the animal pole, as soon as the sperm reaches the cortex; at this time,
mitochondria gather at the vegetal pole. The second, is connected with
the displacement of the sperm nucleus; consequently, the mitochondria
gather in the region of the yellow crescent.
G.
REVERBERI
increase in the egg's volume, the primitive cortex must shrink. One
might imagine therefore the cortex as a net of oriented protein fibrils,
which at fertilization would close together, and the granules attached
to them would thus be displaced. The above mentioned observation of
Conklin on the test cells of Styela, is easily explained if one admits that
the test cells are connected physically with the cortex; electronmicroscope observations support this explanation. The hypothesis of
the existence of a semi-oriented structure in the cortex of the unfertilized egg is supported by the results which are obtained by the
treatment of the egg with trypsin (Ortolani, 1957a) and particularly,
by observations with the polarized light microscope (Monroy, 1953).
According to Monroy, the weak birefringence which is observed in the
cortex of the unfertilized egg increases after fertilization at the top of
the lobopodia, where the cortical layer appears thicker.
An extraordinary importance is attributed today to the role of the
egg cortex in morphogenesis. Distinguished embryologists consider the
cortex as being the site of the 'organ-potencies'; Raven (1958) maintains that the ooplasmic segregations themselves are controlled by the
cortex. It is also well known, as a result of the experiments of
Hörstadius et al. (1950), that in the unfertilized sea-urchin egg large
quantities of endoplasm can be aspirated by a micropipette without any
unfavourable consequence on the development.
In the unfertilized egg of Ascidians also, one can remove more or less
endoplasm by a micropipette (unpublished work) without any consequence; neither does the removal of restricted parts of the cortex
interfere with normal morphogenesis. However, the same operations
carried out on the fertilized egg impede segmentation.
3. Endoplasmic Substances and their Displacement by Centrifugation
No embryologist would deny the importance of the visible or invisible endoplasmic substances. The plasmatic segregations occurring in
the Ascidian egg at fertilization are too impressive and one cannot deny
their significance.
With regard to the mitochondria, the meaning of their migration and
segregation (Figs. 62 to 73) for the larval musculature has been illustrated above. The plasmatic migrations, the causes of which were
discussed by Costello (1948), take place, at least for the mitochondria,
in two periods, and are connected with two events. The first, is certainly
connected with the already mentioned emission of fluid hyaloplasm at
the animal pole, as soon as the sperm reaches the cortex; at this time,
mitochondria gather at the vegetal pole. The second, is connected with
the displacement of the sperm nucleus; consequently, the mitochondria
gather in the region of the yellow crescent.