In the non-yolky type, exemplified by Ophiothrix, MacBride (1907) states that there is a free-swimming blastula with ciliated cells. These form a cell-layer one-deep and surround the large, central blastocoel. From the vegetal pole mesenchyme cells are budded off (see Fig. 13).

Figs. 13–15.—Diagiams illustrating changes produced in the blastula by increasing yolk mass. Fig. 13, Ophiothrix, a non-yolky type (after Mac-Bride, 1907); Fig. 14, Ophiura, a moderately yolked type (after Grave, 1916); Fig. 15, Kirk's ophiuroid, a heavily yolked type (Fell, 1941).

In the moderately yolky type Ophiura, as described by Grave (1916), shows a thick-walled blastula with somewhat reduced blastocoel. The disposition of the nuclei shows that there is a tendency to form a blastula wall more than one cell deep—and in the later gastrula stage this tendency receives its full expression according to the earlier paper of Grave (1900) where he refers to the wall of the gastrula being more than one-cell deep. From the vegetal pole the mesenchyme in a reduced form bulges into the blastocoel, reducing its cross-section to a major segment of a circle.

Finally, in the heavily-yolked type we see in Kirk's ophiuroid the tendencies already weakly expressed in Ophiura reaching their fullest extent. The wall of the blastula is several cells thick. The blastocoel is reduced to a small cavity crescentic in vertical section owing to the bulging upward of the macromeres—obviously homologous with the mesenchyme producing cells of the other two forms.