II.
THE EMBRYOLOGY OF ASCIDIANS
85
observation that the fortunate results were obtained almost entirely
from two experiments alone (9 larvae from one experiment, 2 larvae
from another) led Minganti to suppose that genetieal conditions might
be at work; the gametes of the specimens of Phallusia and A. malaca
used might have possessed, in other words, genes capable of harmonic
collaboration. With regard to the matroclinous characters in development or in the larvae, it must be noted that such results have already
been observed in the Echinoderms (Hörstadius, 1936; von Ubisch, 1954,
1957). One cannot exclude the possibility that the cytoplasm of the egg
might be 'conditioned' by its nucleus much earlier, perhaps in the
ovary, but in this case, the problem escapes us.
IV. Egg Constituents and Morphogenesis
Morphogenesis is produced by the interaction of the egg constituents.
One would like to know which constituents are indispensable for morphogenesis, how they work and when they receive the 'orders' to enter in
action.
Embryologists are continuously confronted with these problems and
in order to solve them, they have used, or invented, the most acrobatic
techniques; particularly useful have been the techniques of centrifuging
the eggs, or separating, rotating and transplanting the blastomeres.
For the Ascidians also, these techniques have been used: in the third
part of this review, the latest results which have been reached by these
techniques will be presented.
The subject matter will be divided as follows: (A) the 'organ-forming'
potencies of the unsegmented egg; (B) the 'mosaic' character of development; (C) the 'inductors'.
A. The Organ-forming' Potencies of the Unsegmented Egg
1. The Egg Envelopes and their Removal
The Ascidian egg, when shed, floats freely in sea-water; its membranes
are, in fact, wonderfully suited for floating. In Phallusia eggs there are
two membranes: the exterior, equipped with floating cells; and the
interior, very tenuous and transparent, under which lie, spread all over
the surface of the egg, the so-called 'test' cells. The origin of these
membranes is debated; and the function of the 'test' cells is still more
questioned. They seem to be connected somehow with the cortex, and
are endowed with a yellow pigment. The egg membranes are attacked
by trypsin; as they are dissolved, the eggs become 'naked'. The test cells
are not dissolved by trypsin, so that they can be collected by differential
centrifugation and analysed; under ultraviolet radiation they display
THE EMBRYOLOGY OF ASCIDIANS
85
observation that the fortunate results were obtained almost entirely
from two experiments alone (9 larvae from one experiment, 2 larvae
from another) led Minganti to suppose that genetieal conditions might
be at work; the gametes of the specimens of Phallusia and A. malaca
used might have possessed, in other words, genes capable of harmonic
collaboration. With regard to the matroclinous characters in development or in the larvae, it must be noted that such results have already
been observed in the Echinoderms (Hörstadius, 1936; von Ubisch, 1954,
1957). One cannot exclude the possibility that the cytoplasm of the egg
might be 'conditioned' by its nucleus much earlier, perhaps in the
ovary, but in this case, the problem escapes us.
IV. Egg Constituents and Morphogenesis
Morphogenesis is produced by the interaction of the egg constituents.
One would like to know which constituents are indispensable for morphogenesis, how they work and when they receive the 'orders' to enter in
action.
Embryologists are continuously confronted with these problems and
in order to solve them, they have used, or invented, the most acrobatic
techniques; particularly useful have been the techniques of centrifuging
the eggs, or separating, rotating and transplanting the blastomeres.
For the Ascidians also, these techniques have been used: in the third
part of this review, the latest results which have been reached by these
techniques will be presented.
The subject matter will be divided as follows: (A) the 'organ-forming'
potencies of the unsegmented egg; (B) the 'mosaic' character of development; (C) the 'inductors'.
A. The Organ-forming' Potencies of the Unsegmented Egg
1. The Egg Envelopes and their Removal
The Ascidian egg, when shed, floats freely in sea-water; its membranes
are, in fact, wonderfully suited for floating. In Phallusia eggs there are
two membranes: the exterior, equipped with floating cells; and the
interior, very tenuous and transparent, under which lie, spread all over
the surface of the egg, the so-called 'test' cells. The origin of these
membranes is debated; and the function of the 'test' cells is still more
questioned. They seem to be connected somehow with the cortex, and
are endowed with a yellow pigment. The egg membranes are attacked
by trypsin; as they are dissolved, the eggs become 'naked'. The test cells
are not dissolved by trypsin, so that they can be collected by differential
centrifugation and analysed; under ultraviolet radiation they display