When the ectodermal micromeres
divide faster than the macromeres, then 3D induction and mesentoblast formation occur at a more advanced cell stage.
In early development there is an apparent lack of a hyaline layer, irregularly arranged blastomeres, variable numbers of micromeres
, and an absence of primary mesenchyme (Schroeder, 1981).
Third cleavage leads to the formation of the first quartet of micromeres
located at the animal pole (1a-1d).
Feeding veligers acquire a metatroch and food groove, which in Crepidula fornicata originate from progeny of a group of 2nd tier micromeres
(Hejnol et al.
A missing link in the sea urchin embryo gene regulatory network: hesC and the double-negative specification of micromeres
However, unequal distribution of red cytoplasm within the four macromeres resulting from second cleavage is not consistent with the notion that the initial four macromeres of equally cleaving gastropod eggs are identical prior to later induction of the dorsoventral axis by micromeres
Spicules of the sea urchin embryo are synthesized by primary mesenchyme cells (PMCs), which are descendants of the micromeres
of the 16-cell-stage embryo (Gustafson and Wolpert, 1967; Okazaki, 1975; Ettensohn et al.
Eight species (Chaenactis glabriuscula, Cryptantha micromeres
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.
latifrons embryos formed a vegetal quartet of micromeres
[ILLUSTRATION FOR FIGURE 1E OMITTED], and the larvae developed an elaborate endoskeleton [ILLUSTRATION FOR FIGURE 1I OMITTED].
Injection of fluorescent lineage tracers (DiI) into identified blastomeres of Mnemiopsis embryos showed that lithocytes originate from oral micromeres
of the E lineage and continue to produce new lithocytes in the epithelial floor throughout adult life (Martindale and Henry, 1999).
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