The first is a conditional specification resulting from of an inductive interaction between the animal micromeres
and a centralized 3M macromere.
Euechinoids embryos from which micromeres
have been excised still produce a larval skeleton, but the skeleton is derived from the veg2 cells (Horstadius, 1973).
Three additional quartets of micromeres
are formed (2a-2d, 3a-3d, and 4a-4d; Fig.
It is as much diverged from the spiralian stereotype as the nemertean pilidium, with which Owenia's development indeed shares some traits, including enlarged first quartet micromeres
, gastrulation by invagination into a large blas-tocoel, and the formation of a convoluted primary ciliated band from numerous proliferative cells.
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
The geometry of early cleavage is modified in this species such that the micromeres
and the early embryonic pattern of msp 130 expression is absent and an unusually large number of mesenchyme cells enter the blastocoel, where they begin directly forming the adult skeleton.
Just before sixth cleavage MAPK expression is restricted to the six central micromeres
destined to form the shell gland primordia at the dorsal-most tip of the embryo.
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
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).