Meanwhile, in 1866 and 1871, Kowalevsky had communicated memoirs of even greater interest,[390] in which he showed that the simple Ascidians developed in an extraordinarily similar way to Amphioxus and hence to Vertebrates in general. His proof that Ascidians also develop on the vertebrate type aroused great interest at the time, and was naturally acclaimed by the evolutionists as a striking piece of evidence in favour of their doctrine. The systematic position of the Ascidians was at that time quite uncertain; they were grouped, as a rule, with the Mollusca, and certainly no one suspected that their well-known tailed larvae, first seen by Savigny, showed any but the most superficial analogy with the tadpoles of Amphibia. Kowalevsky's papers put a different complexion on the matter. In the first of them he showed how the nervous system of the simple Ascidian developed from ectodermal folds just as it did in Amphioxus and Vertebrates, how gill-slits were formed in the walls of the pharynx, and how there existed in the ascidian larva a structure which 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