The opening frames show the two daughter cells of the Corella zygote just after first cleavage.  The video ends when the tadpole larva, consisting of a muscular tail and bulbous head, breaks free from the chorion and swims out of the frame.  The events in between – a series of cell divisions, cell shape changes, and cell rearrangements – transform those two cells into a complex animal.  Remarkable, isn't it, what those individually-simple steps add up to: programmed within those two innocuous cells with which this story begins is the information to craft an animal with skin on the outside, guts on the inside, muscles in between, sense organs up front, and nerves connecting them.  It's only a little more than a day after fertilization that the tadpole hatches and, with muscular tail propelling it from the back, swims off in search of an appropriate place to spend the rest of its life. 

Early cell divisions in Corella are synchronous, and the first three cell divisions are roughly equal.  At fourth cleavage, an unequal division by two of the eight cells results in two cells very much smaller than their sisters.  At fifth cleavage, these small cells divide unequally again.  The smallest of this group ultimately finds itself at the tip of the larva’s tail.  The choreography of successive cell divisions is bilaterally symmetric, and as early as the eight-cell stage one can discern the axes of the animal-to-be. (See this video for a better view.)

Gastrulation transforms the blastula into a multi-layered embryo.  Ten cells around the vegetal pole – the future endoderm – change shape from wide-side-out to wide-side-in and thus dive into the middle of the embryo.  (For a better view of the endoderm cells invaginating, see this video.)  More cells, which will form tail muscles, the notochord, and other mesoderm, follow the endoderm inside.

After gastrulation, a patch of cells on the future dorsal side of the ascidian embryo folds over from the edges to form the tube; this tube forms the larval nervous system: sense organs, "brain", and a strand of nerve fibers running down the tail.  This process, called neurulation, is thought to be homologous between ascidians and vertebrates; the dorsal neural tube, along with the notochord, is among the shared derived traits that reflect our proximity to ascidians on the tree of life.  The neural tube runs along the anterior-posterior axis and is enclosed except for a single opening, called the neural pore (which is visible, and labeled, for a brief moment in this video).   

This is a good time to point out something about the movement of the embryo.  The neural pore is visible as the embryo rolls over within the chorion.  But how is it moving?  It isn’t ciliated.  Ascidians certainly have and use cilia, but not on the larval skin.  When it hatches, it will swim using its muscular tail, not cilia, for propulsion.  It is the test cells that are moving it!  Test cells, a singular feature of the hardware associated with ascidian eggs, play an important role in later stages of morphogenesis.  They crawl very actively on the surface of the embryo.  According to R. A. Cloney, who studied test cells comparatively by light and electron microscopy, they secrete "ornaments," with which they decorate the outer cuticle of the tunic.  These secretions serve to make the larva hydrophilic, helping it avoid getting trapped in the surface tension.  In contrast, larvae from which test cells are experimentally removed are hydrophobic, and easily become stuck in the surface of the water.

After neurulation, the embryo begins the dramatic morphogenetic events that will transform it into a tadpole larva.  The tail begins to form as the presumptive notochord cells, which were located in the anterior part of the embryo, move posteriorly, becoming enclosed by two bands of muscle.  Their conjoint movements elongate the tail.  The notochord, a distinguishing characteristic of chordates, is present in the larval stage of ascidians, but not the adult: at metamorphosis, it will be resorbed along with the rest of the tail, whose only function is to get the larva to a nice spot to spend its entire adulthood.   (For a better view of the notochord forming, see this video.)

The differentiation of muscle cells is evident as the larval tail begins to flick and twitch in preparation for hatching.  Watch also at the tip of the tail: it looks like a fingernail grows from it.  This is an acellular fin created by secretions of the epidermis.  Without this fin, the ascidian tadpole's tail would likely provide little more propulsion than a cat's tail. 

— text by Katie Bennett & George von Dassow