It has simply been traced as far as the formation of a diblastula which acquires a ciliated band, and becomes a nearly spherical trochosphere.
A well-marked trochosphere is formed by the development of an equatorial ciliated band; and subsequently, by the disproportionate growth of the lower hemisphere, the trochosphere becomes a veliger.
B, The diblastula has become a trochosphere by the development of the ciliated ring y r (optical section).
C, Side view of the trochosphere with commencing formation of the foot.
D, Further advanced trochosphere (optical section).