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Table 1 Comparison of inversion in V. globator and V. carteri.

From: There is more than one way to turn a spherical cellular monolayer inside out: type B embryo inversion in Volvox globator

Characteristic V. carteri V. globator
Type of inversion A B
Duration of inversion Approximately 45 min [20, 22] Approximately 50 min
Sequence of processes The phialopore opens;
the anterior hemisphere inverts;
the posterior hemisphere contracts;
the posterior hemisphere inverts;
the phialopore closes [20, 22, 26]
The posterior hemisphere contracts;
the posterior hemisphere inverts;
the phialopore opens;
the anterior hemisphere inverts;
the phialopore closes
[25, 36, 37]
Site of the initiation of inversion Point-shaped initiation site at the anterior pole of the spheroid [22] Ring-shaped initiation site at the equator of the spheroid
Direction of cell shape changes From the anterior pole to the posterior pole [22] Initially, from the equator to the posterior pole; then, from the equator to the anterior pole
Diameter of embryo Immediately before inversion: approximately 65 μm [22]
immediately after inversion: approximately 60 μm [22]
Immediately before inversion: approximately 80 μm
Immediately after inversion: approximately 60 μm
Relative size (volume) of the cells of the embryo during inversion Reproductive cells are roughly 100 to 200 times larger than somatic cells [22] Reproductive cells are roughly one to five times larger than somatic cells
Cell shape immediately before inversion All cells are spindle shaped [20, 22] All cells are teardrop shaped
Cell shape in bend region during inversion Flask-shaped cells
(radially symmetrical at the chloroplast ends) [20, 22, 24]
Paddle-shaped cells (wedge shaped at the chloroplast ends)
Cell shape of cells that have passed the bend region Column-shaped cells [20, 22] Pencil-shaped cells (the column shape does not appear until inversion is completed)
Order of cell shape changes All cells undergo the same consecutive cell shape changes: spindle → flask → column [20, 22, 24] Posterior and anterior hemispheres invert with different cell shapes changes;
cells in the posterior hemisphere: teardrop → spindle → paddle (except for the cells close to the posterior pole) → pencil → column;
cells in the anterior hemisphere: teardrop → disc → pencil → column
Shape of nuclei throughout inversion Minor changes [20] Major changes
Cytoplasmic bridges Only between cells of the embryo [20, 22, 24, 29] Between cells of the embryo and between embryonic and parent cells
Phialopore Swastika-shaped opening; four lips of the cell monolayer curl outward and backward [20, 22] Circular opening; while the opening widens, the cells and cytoplasmic bridges around the phialopore become passively stretched
Denting Only before inversion [20, 26, 39] Before and after inversion
Flagella Only flagellar stubs approximately 2 μm in length that do not grow during inversion; flagella grow asymmetrically after inversion [76] Flagella grow symmetrically throughout inversion