Spicule Formation by Isolated Micromeres of the Sea Urchin Embryo

Micromeres are isolated at the 16-cell stage from three species of Japanese sea urchins, Hemicentrotus pulcherrimus, Pseudocentrotus depressus, and Anthocidaris crassispina, and are cultured in sea water containing a small amount of horse serum. In all species used, isolated micromeres first divide unequally as they do in vivo. The pattern and number of the subsequent cleavages are also the same as in vivo, although they are not necessarily clear in all cases, since the border of the adjacent cells become invisible at each resting stage in some batches of embryos. After cleavage, passing through the stage when the contours of the individual cell are obscure, decendants of the isolated micromeres form cell aggregates similar to the group of primary mesenchyme cells in a blastula. Within such aggregates, a spicular rudiment appears which develops either into a triradiate spicule as in normal gastrulae or into a rod. The triradiate spicule grows into a three-dimensional skeleton which is very similar to the normal pluteus skeleton, not only in its final shape and size including species-specific characters but in its developmental course and crystallographic nature. The rod, on the other hand, develops into either a one-dimensional or two-dimensional skeleton. These skeletons probably correspond to a part of the complete skeleton.