IX. DEVELOPMENT OF THE TELEOSTEAN EGG
405
have not quite finished rolling up are put in contact by the small zone of
deep blastomeres that are still bare, the surface of contact enlarges little
by little, the two explants partly unroll and each becomes a hemisphere
covered by the two joined enveloping layers.
If the blastoderm is detached from the yolk, the syncytial ring contracts (see Section III, D); if, before the ring closes up completely the
blastoderm is put back in its place when it has not completely rolled up,
it adheres, and, as in the preceding case, the explant unrolls and opens
the syncytial ring; and the whole has once more the aspect of a normal
eggTo conclude, the form of the germ in the intact egg is the result of a
balance between the forces of adhesion of the deep blastomeres with one
another and with the syncytium; a result also of the contractility of the
peripheral syncytium and of the tension of the perivitelline cortex. The
variations in intensity of one or other of these components are responsible for the changes in the shape of the blastoderm during development.
The third proposition is demonstrated by the fact that an explant of
deep blastomeres has difficulty in acquiring a regular shape and does not
become spherical. On the other hand, the enveloping layer, when
isolated, rolls up very quickly into a sphere (Devillers et al., 1957b).
The study of the cortical properties during healing might appear to
be of limited interest, since such an occurrence does not arise in normal
development. Actually, under normal conditions, the same properties
come into play during gastrulation, especially in epiboly. From the
beginning of premorphogenesis, the enveloping layer thus potentially
contains the elements that provide the motive forces of epiboly; as for
the motive forces of invagination, their sources have not yet been disclosed.
D. The Vitelline Syncytium
This was discovered by Lereboullet in 1854. It can be considered as
an adaptive embryonic structure, related to the particular distribution
of material which is characteristic of the telolecithic egg.
In Salmo, the syncytium acquires its individuality in relation to the
blastodisc at the morula stage (Kopsch, 1911). One can distinguish, by
the distribution of cytoplasm and nuclei, a central portion, displaced towards the future ventral zone, a thin annular zone poor in nuclei and a
peripheral ridge without nuclei which is in continuity with the periblast
(Fig. 3).*
If the blastodisc of Salmo is lifted from the syncytium, by the action
* In other cases, the syncytium is formed around the edge of the germ and then
extends towards the centre (Crenilabrus, Ctenolabrus, Fundulus).
405
have not quite finished rolling up are put in contact by the small zone of
deep blastomeres that are still bare, the surface of contact enlarges little
by little, the two explants partly unroll and each becomes a hemisphere
covered by the two joined enveloping layers.
If the blastoderm is detached from the yolk, the syncytial ring contracts (see Section III, D); if, before the ring closes up completely the
blastoderm is put back in its place when it has not completely rolled up,
it adheres, and, as in the preceding case, the explant unrolls and opens
the syncytial ring; and the whole has once more the aspect of a normal
eggTo conclude, the form of the germ in the intact egg is the result of a
balance between the forces of adhesion of the deep blastomeres with one
another and with the syncytium; a result also of the contractility of the
peripheral syncytium and of the tension of the perivitelline cortex. The
variations in intensity of one or other of these components are responsible for the changes in the shape of the blastoderm during development.
The third proposition is demonstrated by the fact that an explant of
deep blastomeres has difficulty in acquiring a regular shape and does not
become spherical. On the other hand, the enveloping layer, when
isolated, rolls up very quickly into a sphere (Devillers et al., 1957b).
The study of the cortical properties during healing might appear to
be of limited interest, since such an occurrence does not arise in normal
development. Actually, under normal conditions, the same properties
come into play during gastrulation, especially in epiboly. From the
beginning of premorphogenesis, the enveloping layer thus potentially
contains the elements that provide the motive forces of epiboly; as for
the motive forces of invagination, their sources have not yet been disclosed.
D. The Vitelline Syncytium
This was discovered by Lereboullet in 1854. It can be considered as
an adaptive embryonic structure, related to the particular distribution
of material which is characteristic of the telolecithic egg.
In Salmo, the syncytium acquires its individuality in relation to the
blastodisc at the morula stage (Kopsch, 1911). One can distinguish, by
the distribution of cytoplasm and nuclei, a central portion, displaced towards the future ventral zone, a thin annular zone poor in nuclei and a
peripheral ridge without nuclei which is in continuity with the periblast
(Fig. 3).*
If the blastodisc of Salmo is lifted from the syncytium, by the action
* In other cases, the syncytium is formed around the edge of the germ and then
extends towards the centre (Crenilabrus, Ctenolabrus, Fundulus).