II. THE EMBRYOLOGY
OF ASCIDIANS
79
1. Morphological Modifications of the Egg
When in Contact with Foreign Sperm
Observing accurately the Ascidian eggs within the 2-3 minutes
immediately after fertilization one is struck by the spectacular modifications which occur in the shape of the egg. It passes from a spherical
form to a piriform, then to an elliptical, and, finally, after many other
ameboid changes to a spherical form again; these modifications occur
very suddenly, and are evanescent, lasting only one minute. The egg
does not change its shape again until the emission of the first polocyte.
These changes cannot be easily observed in eggs which have not been
liberated from their membranes, in fact they have escaped the notice
of many authors. Probably simultaneously with these modifications,
the peculiar migration of the pigmented plasms occurs, as was described
by Conklin (1905) in Styela.
Postponing for a while a discussion of the problem of the mechanism
by which these modifications are produced, let us consider if and how
the egg changes when it meets a foreign sperm. The results, of course,
vary with the crosses and batches of eggs. Sometimes, certain eggs do
not modify their shape (or, if they do, only some 12 to 14 hours after the
addition of the sperm), but generally they respond to the foreign sperm,
although abnormally. In fact modifications very often occur violently,
with elimination of parts of the cytoplasm, and polyspermy is also
frequent. These facts allow one to draw some conclusions about the
events of fertilization. One can affirm that the barrier to foreign
fertilization is not exclusively constituted by the egg membranes. The
cortex is certainly involved, otherwise all the eggs would be penetrated by
foreign sperms, and at the same time. It seems, however, that the cortex
must somehow be 'modified' in order to permit the entry of the sperm.
2. Sperm-agglutination in Heterospecific Hybridizations
The alleged modification might be the consequence of an interaction
between the antifertilizins of the foreign sperm and the fertilizins of the
egg. In fact, another interesting occurrence in the fertilization of the
Ascidian eggs with heterospecific sperm is sperm-agglutination. This
phenomenon is not absent in normal fertilization, but it occurs quite
differently. According to Minganti (1951b) the eggs of Ciona, if allowed
to stay in sea-water for a while, release fertilizins; these, like the
fertilizins of the sea-urchin eggs, activate the movement of the sperm,
agglutinate the sperm, and, possibly, attract the sperm. In heterospecific
fertilizations these phenomena are not abolished, but occur in an abnormal way. The agglutination at times does not take place, while at
other times it is most spectacular.
OF ASCIDIANS
79
1. Morphological Modifications of the Egg
When in Contact with Foreign Sperm
Observing accurately the Ascidian eggs within the 2-3 minutes
immediately after fertilization one is struck by the spectacular modifications which occur in the shape of the egg. It passes from a spherical
form to a piriform, then to an elliptical, and, finally, after many other
ameboid changes to a spherical form again; these modifications occur
very suddenly, and are evanescent, lasting only one minute. The egg
does not change its shape again until the emission of the first polocyte.
These changes cannot be easily observed in eggs which have not been
liberated from their membranes, in fact they have escaped the notice
of many authors. Probably simultaneously with these modifications,
the peculiar migration of the pigmented plasms occurs, as was described
by Conklin (1905) in Styela.
Postponing for a while a discussion of the problem of the mechanism
by which these modifications are produced, let us consider if and how
the egg changes when it meets a foreign sperm. The results, of course,
vary with the crosses and batches of eggs. Sometimes, certain eggs do
not modify their shape (or, if they do, only some 12 to 14 hours after the
addition of the sperm), but generally they respond to the foreign sperm,
although abnormally. In fact modifications very often occur violently,
with elimination of parts of the cytoplasm, and polyspermy is also
frequent. These facts allow one to draw some conclusions about the
events of fertilization. One can affirm that the barrier to foreign
fertilization is not exclusively constituted by the egg membranes. The
cortex is certainly involved, otherwise all the eggs would be penetrated by
foreign sperms, and at the same time. It seems, however, that the cortex
must somehow be 'modified' in order to permit the entry of the sperm.
2. Sperm-agglutination in Heterospecific Hybridizations
The alleged modification might be the consequence of an interaction
between the antifertilizins of the foreign sperm and the fertilizins of the
egg. In fact, another interesting occurrence in the fertilization of the
Ascidian eggs with heterospecific sperm is sperm-agglutination. This
phenomenon is not absent in normal fertilization, but it occurs quite
differently. According to Minganti (1951b) the eggs of Ciona, if allowed
to stay in sea-water for a while, release fertilizins; these, like the
fertilizins of the sea-urchin eggs, activate the movement of the sperm,
agglutinate the sperm, and, possibly, attract the sperm. In heterospecific
fertilizations these phenomena are not abolished, but occur in an abnormal way. The agglutination at times does not take place, while at
other times it is most spectacular.