The elaborate hardware surrounding the Corella embryo is made up of three parts: the follicle cell layer on the outside, looking like inflated sacs; a chorion in the middle; and test cells, which are the busy, migratory cells that cohabit the chorion with the egg. 

As the movie begins, a sperm nucleus near the top of the screen is trying to get together with the nucleus of the egg.  How do we know?  Because the egg’s nucleus – called the female pronucleus – goes whizzing upward to meet it.  The fused pronuclei, now the zygote nucleus, then migrate to the middle of the cell and undergo mitosis.

Cleavage in Corella and other ascidians is called bilateral because the early embryo exhibits a very definite mirror-symmetric cleavage pattern: the only plane of symmetry coincides with the anterior-posterior axis.  The first three divisions are approximately equal, resulting in an eight-cell embryo in which all cells are roughly the same size.  (Watch for the transient and beautiful apparition of all four nuclei in the same focal plane, between second and third cleavage!)  

Two of the eight cells undergo a highly-unequal division at fourth cleavage, resulting in two cells very much smaller than their sisters.  These happen to be located at the very posterior end of the embryo.  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.

So what is all that stuff around the egg for?  It reflects the habitat and habits of this animal.  Corella inflata is a transparent-bodied solitary tunicate (sea squirt) that is found attached to floats and docks in the harbor.  It hangs with both siphons pointing down.  C. inflata is unusual among solitary ascidians because it broods its young: gametes are released into an enlarged “atrial chamber” around the atrial siphon (where water goes out).  Embryos are retained there throughout development.  As in other ascidians, the outermost cellular layer surrounding eggs and embryos is made up of follicle cells, but in C. inflata the follicle cells become large vacuoles which provide buoyancy to the developing embryos which conseqeuntly float within the atrial chamber of the parent. 

— text by Katie Bennett & George von Dassow