or mulberry-shaped cluster of cells, is formed. Fluid gathers inside
it, and thus we get once more a globular vesicle (the blastula); the
wall of this is a single stratum of cells, the blastoderm. A real
gastrula (a simple bell-gastrula) is formed from the blastula by
invagination, in the same way as in the amphioxus.

Up to this there is no definite ground in the embryology of the
Ascidiae for bringing them into close relationship with the
Vertebrates; the same gastrula is formed in the same way in many other
animals of different stems. But we now find an embryonic process that
is peculiar to the Vertebrates, and that proves irrefragably the
affinity of the Ascidiae to the Vertebrates. From the epidermis of the
gastrula a medullary tube is formed on the dorsal side, and, between
this and the primitive gut, a chorda; these are the organs that are
otherwise only found in Vertebrates. The formation of these very
important organs takes place in the Ascidia-gastrula in precisely the
same way as in that of the Amphioxus. In the Ascidia (as in the other
case) the oval gastrula is first flattened on one side--the subsequent
dorsal side. A groove or furrow (the medullary groove) is sunk in the
middle line of the flat surface, and two parallel longitudinal
swellings arise on either side from the skin layer. These medullary
swellings join together over the furrow, and form a tube; in this
case, again, the neural or medullary tube is at first open in front,
and connected with the primitive gut behind by the neurenteric canal.
Further, in the Ascidia-larva also the two permanent apertures of the
alimentary canal only appear later, as independent and new formations.
The permanent mouth does not develop from the primitive mouth of the
gastrula; this primitive mouth closes up, and the later anus is formed
near it by invagination from without, on the hinder end of the body,
opposite to the aperture of the medullary tube.