Then, the micromeres of the second and third generation divide more or less synchronously, simultaneously with the central 3D macromere
fornicata embryos, which themselves specify the 3D macromere
, suggests that these micromeres may be responding to an even earlier, unidentified signal.
For example, patellogastropods (true limpets) and euthyneuran gastropods (pulmonates and opisthobranchs) with equally cleaving eggs depend on regional segregation of morphogenetic determinants to specify the animal-vegetal axis (4), but an inductive interaction between micromeres and one of the macromeres
later specifies the dorsoventral axis of the embryo (5, 6).
At gastrulation, the macromeres
and their descendants are internalized, where they form the larval midgut (Kume and Dan, 1968; Anderson, 1973).
Lamour Family: Siphonocladaceae Cladophoropsis macromeres
The first two cleavages were equal and the third unequal, producing four micromeres and four macromeres
at the animal and vegetal poles, respectively (Fig.
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.
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
At the 8-cell stage, animal cells (micromeres) are larger than the vegetal macromeres
, similar to all nemerteans but unlike most annelids (Shankland and Savage, 1997).
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