Although the size and the architecture of the lobe forming region may differ (Dohmen 1983), great similarities appear to exist in the developmental stage at which
macromere 3D starts its organizing role (van Dongen 1976a,b).
fornicata embryos, which themselves specify the 3D
macromere, suggests that these micromeres may be responding to an even earlier, unidentified signal.
At first cleavage, most of the red cytoplasm was briefly extruded as a bulbous polar lobe before rapidly consolidating back into one of the two
macromeres (Fig.
As a result of unequal cleavages from the third through the fifth cleavage cycles, these cells, known as
macromeres, are usually far larger than the remaining embryonic cells, the micromeres.
Lamour Family: Siphonocladaceae Cladophoropsis
macromeres W.R.
4C), with 4 cells of
macromeres at the vegetal pole (lower hemisphere) and 4 cells of micromeres at the animal pole (upper hemisphere).
At the third division, micromeres and
macromeres could be differentiated and egg development progressed to the gastrula with cell cleavage being total, unequal and spiral (stages 6-11) (Fig.
Cleavage is dexiotropic and unequal at the 8-cell stage: the animal cells (micromeres) are larger than the vegetal cells (
macromeres) (Fig.
Embryonic development up to the trocophore stage occurs within the egg membrane and corresponds to the typical development described for gastropod molluscs, with elimination of the two polar bodies after fertilization; segmentation is of the spiral type with the formation of micromeres and
macromeres. The trocophore larva F.
Molluscs, like most other lophotrochozoans, exhibit a highly conserved pattern of spiral cleavage that generates four
macromeres surmounted by four tiers of micromere quartets.
Third cleavage was equatorial (= perpendicular to the animal-vegetal axis), resulting in four micromeres at the animal pole (la-1d); the four
macromeres remained at the vegetal pole (lA-1D) (Fig.
The gut has not achieved its functional form and is still filled with the progeny of the yolky
macromeres. A bilateral pair of tufts of diffuse hooked cilia, termed lateral hooked cilia (Henry, 1986), form during this stage.
