The first is a conditional specification resulting from of an inductive interaction between the animal micromeres
and a centralized 3M macromere.
The first mesenchyme cells to ingress are called primary mesenchyme and are derived from the small micromeres
that arise at the vegetal pole of the embryo during an unequal fourth cleavage.
Three additional quartets of micromeres
are formed (2a-2d, 3a-3d, and 4a-4d; Fig.
collaris, and embryos lack obvious signs of cleavage arrest in the cells that would be the primary trochoblasts in a canonical spiralian (the granddaughters of the l[q.sup.2] cells, which themselves are the vegetal daughters of the first quartet of micromeres
formed at third cleavage).
Fate maps of the first quartet micromeres
in the gastropod Ilyanassa obsolete.
The recent identification of HesC as the Repressor of Micromeres
may be typical of the impact the sea urchin genome has for studies that are not directly concerned with the genome.
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
. This non-identity among macromeres may not affect which of the macromeres later becomes the D macromere, but it may result in some subtle but unknown phenotypic difference among the resulting larvae.
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., 1997).
Recently, MAP kinase (MAPK) signaling cascades in the gastropod Ilyanassa obsoleta have been linked to cell fate specification within the D macromere lineage and the micromeres
of the A, B, and C quadrants (Lambert and Nagy, 2001).
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
. At gastrulation, the macromeres and their descendants are internalized, where they form the larval midgut (Kume and Dan, 1968; Anderson, 1973).
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.
Apical ciliary structures in veliger larvae of apogastropods have been difficult to reconcile with the condition in larvae of patellogastropods and of many other molluscan classes, where a long apical tuft of nonmotile cilia is produced by some or all of the apical rosette micromeres
of the embryo (see Raven, 1966; van Dongen and Geilenkirchen, 1974; Verdonk and van den Biggelaar, 1983; Dohmen, 1992; Dictus and Damen, 1997).