THE ASCIDIAN THEORY 271 



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 ecto- 

 dermal 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. 



Kowalevsky's results were accepted by Haeckel, Gegen- 

 baur, Darwin, 1 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 2 in 

 1870, later by van Beneden and Julin 3 and numerous other 

 workers ; they were adversely criticised by Metschnikoff 4 

 and von Baer, 5 as well as by H. de Lacaze-Duthiers and 

 A. Giard. 6 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 for- 



1 Descent of Man, i., p. 205, 1871. 



2 Arch. f. mikr. Ana?., vi., 1870, and viii., 1872. 



3 Archives de Biologie, 1884, 1885, and 1887. 



4 Bull. Acad. Set. St Petersbourg (Petrograd) xiii., 1869, and Zeits. 

 f. iviss. Zool., xxii., 1872. 



5 Mem. Acad. Set. St Petersboiirg (Petrograd) (7), xix., 1873. 



6 Giard, Arch. zool. cxper. gen., i., 1872, and Lacaze-Duthiers, ibid.) 

 iii., 1874. 



