The fundamental assumption of the Annelid theory, that dorsal and ventral surfaces are morphologically interchangeable, seemed rather bold to many zoologists, and Gegenbaur[417] voiced a common opinion when he rejected as unscientific the comparison of the ventral nerve cord of Articulates with the dorsal nervous system of Vertebrates.
The Balanoglossus theory of Vertebrate descent also belongs, at least in its first form, to the earlier group of evolutionary speculations. The gill-slits of Balanoglossus were discovered by Kowalevsky as early as 1866.[418] Tornaria was discovered by J. Müller in 1850, but by him considered an Asterid larva; its true nature as the larva of Balanoglossus was made out by Metschnikoff in 1870, who also remarked upon its extraordinary likeness to the larvæ of Echinoderms.[419] That it had some relationship with Vertebrates was recognised by Semper, Gegenbaur and others, but the full working-out of its Vertebrate affinities is due to Bateson.[420]
Bateson broke completely with the Dohrn-Semper view that the metamerism of Articulates and Vertebrates must be put down to inheritance from a common ancestor. He held that metamerism was merely a special manifestation of the general property of repetition, common to all living things (cf. Owen's "vegetative force"), and that accordingly "however far back a segmented ancestor of a segmented descendant may possibly be found, yet ultimately the form has still to be sought for in which these repetitions had their origin" (p. 549). The meaning of the phenomenon was obscure, but he was convinced that the explanation was not to be found in ancestry. "This much alone is clear," he wrote, "that the meaning of cases of complex repetition will not be found in the search for an ancestral form, which, itself presenting this same character, may be twisted into a representation of its supposed descendant. Such forms there may be, but in finding them the real problem is not even resolved a single stage; for from whence was their repetition derived? The answer to this question can only come in a fuller understanding of the laws of growth and of variation, which are as yet merely terms" (pp. 548-9). It was in following up this line of thought that Bateson produced his monumental Materials for the Study of Variation (1894).
He found a strong positive argument for his theory that Vertebrates are descended from unsegmented forms in the fact that the notochord arises as an unsegmented structure. With the notochord he homologised the supporting rod in the proboscis of Balanoglossus, which like the notochord arises from the dorsal wall of the archenteron, and has a vacuolated structure. The gill-slits of Balanoglossus, with their close resemblance in detail to those of Amphioxus, Bateson also used as an argument in favour of the phylogenetic relationship of the Enteropneusta and Vertebrata, together with the formation from the ectoderm of a dorsal nerve tube.
Bateson's views attracted considerable attention, and were thought by many to lighten appreciably the obscurity in which the origin of Vertebrates was wrapped. Thus Lankester wrote in his article on Vertebrates[421] in the Encyclopedia Britannica:—"It seems that in Balanoglossus we at last find a form which, though no doubt specialised for its burrowing sand-life, and possibly to some extent degenerate, yet has not to any large extent fallen from an ancestral eminence. The ciliated epidermis, the long worm-like form, and the complete absence of segmentation of the body-muscles lead us to forms like the Nemertines. The great proboscis of Balanoglossus may well be compared to the invaginable organ similarly placed in the Nemertines. The collar is the first commencement of a structure destined to assume great importance in Cephalochorda and Craniata, and perhaps protective of a single gill-slit in Balanoglossus before the number of those apertures had been extended. Borrowing, as we may, the nephridia from the Nemertines, and the lateral in addition to the dorsal nerve, we find that Balanoglossus gives the most hopeful hypothetical solution of the pedigree of Vertebrates."
Much doubt was cast upon the Chordate affinities of the Enteropneusta by Spengel in his monograph of the group,[422] but when the development of the cœlom came to be more thoroughly worked out in Balanoglossus and Amphioxus, the striking resemblance in this respect between the two forms gave additional support to the Batesonian view.[423]
[386] The stages in the development of microscopical technique are well summarised by R. Burckhardt, Geschichte der Zoologie, p. 121, Leipzig 1907.
[387] "Entwickelungsgeschichte des Amphioxus lanceolatus," Mém. Acad. Sci. St Pétersbourg (Petrograd) (vii.), xi., No. 4, 1867, 17 pp., 3 pls.
[388] "Weitere Studien ü. die Entwickelungsgeschichte des Amphioxus lanceolatus," Arch. für mikr. Anat., xiii., pp. 181-204, 1877.
[389] Particularly by Hatschek (1881) and Boveri (1892).