h in position and mode of development was the strict homologue of
the vertebrate notochord. In his second paper he entered into much more
detail, and published some excellent figures, often reproduced since
(see Fig. 13), but the proof of the affinity between Vertebrates and
Ascidians was in all essentials complete in his paper of 1866.

[Illustration: FIG. 13.--Development of the Ascidian Larva. (After
Kowalevsky.)]

Kowalevsky's results were accepted by Haeckel, Gegenbaur, Darwin,[391]
and many others as conclusive evidence of the origin of Vertebrates
from a form resembling the ascidian tadpole; they were extended and
amplified by Kupffer[392] in 1870, later by van Beneden and Julin[393]
and numerous other workers; they were adversely criticised by
Metschnikoff[394] and von Baer,[395] as well as by H. de
Lacaze-Duthiers and A. Giard.[396] Lacaze-Duthiers and von Baer both
held fast to the old view that Ascidians were directly comparable with
Lamellibranch molluscs; they denied the homology of the ascidian
nervous system with that of Vertebrates, von Baer being at great pains
to show that the ascidian nerve-centre was really ventral in position.
He pointed out also that the "notochord" was confined to the tail of
the ascidian larva. Giard's attitude was by no means so
uncompromising, and the criticisms he passed on the Kowalevsky theory
are both subtle and instructive. He admits that there exists a real
homology between, for instance, the notochord of Vertebrates and that
of Ascidians. "But," he adds, "it is too often forgotten that homology
does not necessarily mean an immediate common origin or close
relationship. There exist, doubtless, homologies of great atavistic
importance--I consider as such, for example, the formation of the
cavity of Rusconi [the archenteron] in Ascidians and lower
Vertebrates. But there are also adaptive and purely analogical
homologies, such as the interdigital palmation of aquatic birds,
amphibians and mammals. These are not purely analogous organs, for
they can be superposed one on another, which is not the case with
simply analogous structures (the bat's wing, for example, cannot be
superposed on the bird's wing); they are homologous formations,
resulting from the adaptation of the same fundamental organs to
identical functions. Such is, in my opinion, the nature of the
homology existing between the tail of the ascidian tadpole and that of
Amphioxus or of young amphibians. The ascidia