The fine structure of Comanthus japonica (Echinodermata: Crinoidea) from zygote through early gastrula

The fine structure of the early embryo of Comanthus has been described by scanning and transmission electron microscopy at approximately 20-min intervals from zygote (20 min) through early gastrula (260 min). In normally developing (and presumably monospermic) embryos, some non-fertilizing sperm were invariably trapped in the perivitelline space; this suggests that there is an effective block to polyspermy at the level of the plasma membrane. No trace of a hyaline layer is encountered in the pervitelline space. At first cleavage, which begins unilaterally at the animal pole, the contractile ring filaments are rather thick (50–150 Å) in comparison to those known for other marine invertebrates. From first cleavage through early gastrula, the lateral surfaces of the blastomeres are broadly adherent, and there is an intercellular material, presumably an adhesive, in the intercellular space. The blastocoel first appears during the four-cell stage. From the eight-cell stage through the start of gastrulation, only one opening, the vegetal pore, connects the blastocoel with the perivitelline space. Gastrulation begins at the 50–100-cell stage, while the vegetal pore is still open, and a clearly defined blastula stage is bypassed. Gastrulation is by a novel process, which I have called holoblastic involution. At gastrulation the eight most vegetal blastomeres, which encircle the vegetal pore, shoot out erect, unbranched filopodia for many microns through the blastocoel. The filopodia adhere to the blastocoelic surfaces of the animal blastomeres and contract, pulling the vegetal blastomeres into the blastocoel. The migrated vegetal blastomeres adhere to one another, forming the entoderm in the vegetal region of the embryo; the remaining blastomeres become the ectoderm. Soon after the completion of cell migration, the entodermal blastomeres appear to cast off their contractile microappendages and adhesive membranes into the blastocoel.