ABSTRACT
THE ASCIDIANS stand midway between the invertebrate and vertebrate kingdoms. In the early development of many of them, a tadpole-like larva is formed, which is furnished with an axial notochord with an accompanying dorsal nerve-cord. These formations disappear or become greatly modified in the fmal metamorphosed adult, but in so far as the animal possesses them at all, it is to that extent entitled to be reckoned as a member of the chordates. It is more doubtful whether the ascidians should be regarded as exceedingly primitive members of the group in which the larva foreshadows the later evolutionary history, or as a specialised group of degenerate forms in which the chordate larva is the only remaining sign of a more glorious evolutionary past. Perhaps both views have something of the truth; the ascidians may represent a chordate stock which began to degenerate after only a short history of progressive evolution. In favour of this interpretation is the fact that in the very early development of the egg, which leads to the formation of the chordate larvae, they show many of the features which one would expect in a primitive, as opposed to a highly evolved, member of the Chordata. The eggs, in fact, bear a very striking resemblance to those of Amphioxus, which is undoubtedly a very primitive chordate. It will, indeed, be convenient to treat the early development of ascidians and Amphioxus together. The simplicity and clarity of their developmental processes, together with the primitive position of Amphioxus in the evolutionary scheme, has for long rendered these animals classical embryological material.
