This is a hoplonemertean (ribbon worm) embryo. I collected several of such embryos from plankton obtained off of a dock in Charleston, OR on February 8, 2012. In general, nemertean eggs cleave circumferentially, but in some species with large yolky eggs, the cleavage furrow cuts in from one side. This is referred to as “unilateral cleavage”. The embryo shown here is doing just that - it is in the process of dividing from one to two cells, and the cleavage furrow is deeper on one side than on the other. Many hoplonemerteans encase their eggs in some sort of envelope, or chorion. The embryo shown here has an inner chorion (which is tight around the embryo) and a larger outer chorion.
The second picture shows a uniformly ciliated multicellular embryo after one day of development. The
embryo rotates inside the egg chorion at this stage. It also has an apical tuft
of longer cilia (seen at about 5 o’clock). A clear vesicle inside the chorion
at about 2 o’clock is likely a polar body (a product of meiosis). The larvae
that hatched from these eggs (after 4 days of development) had a transitory
larval epidermis which they shed when slightly compressed under a coverslip.
The third picture shows two ciliated cells of transitory larval epidermis left behind by a larva that is just about to exit the frame (at upper left). Other hoplonemertean larvae are known to have similar transient epidermal covering; in some it is reabsorbed, in others it is shed (Maslakova
& von Döhren 2009; Hiebert et al. 2010).
The bottom photograph is of a
week-old larva. It is uniformly ciliated and possesses a prominent apical tuft,
a caudal cirrus, three pairs of eyespots, cerebral ganglia (large clear areas),
gut, and a proboscis. It is essentially a planktonic juvenile. This type of
development is referred to as “direct” because the planktonic stage (if present
at all) possesses a body plan essentially similar to that of the adult (unlike
the nemertean pilidium larva).
Hiebert LS, Gavelis G, von Dassow G and Maslakova SA. 2010. Five invaginations and shedding of the larval epidermis during development of the hoplonemertean Pantinonemertes californiensis
(Nemertea: Hoplonemertea). Journal of Natural History. 44: 2331–2347.
Maslakova SA, von Döhren J. 2009. Larval development with transitory epidermis in
Paranemertes peregrina and other hoplonemerteans. Biological Bulletin. 216:273–292.