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 ascidian larva, having no cilia and being necessarily motile, requires for the insertion of its muscles or contractile organs ... a central flexible axis, a true chorda dorsalis analogous to that of Vertebrates" (pp. 278-9). This point of view is strengthened by the fact that in Molgula, studied by Lacaze-Duthiers, the embryo is practically stationary, and forms no notochord, nor ever develops sense-organs in the cerebral vesicle.
Giard's general conclusion is that "the true homology with Vertebrates ceases after the formation of the cavity of Rusconi and the medullary groove: the homologies established by Kowalevsky for the notochord and the relations of the digestive tube and nervous systems are not atavistic, but adaptive, homologies" (p. 282). There is accordingly no close genetic relationship between Ascidians and Vertebrates.
Giard's criticisms did not avail to check the vogue of the new theory, which soon became an accepted article of faith in most morphological circles.[397] The fall of the Ascidians from their larval high estate provided the text for many a Darwinian sermon.
Some years after the genetic relationship of Ascidians and Vertebrates had been established, a rival theory of the origin of Vertebrates made its appearance—a theory which was practically a rehabilitation in a somewhat altered form of the old Geoffroyan conception that Vertebrates are Arthropods walking on their backs. This was the so-called Annelid theory of Dohrn and Semper. Both Dohrn and Semper started out from the fact that Annelids and Vertebrates are alike segmented animals, and it was an essential part of their theory that this resemblance was due to descent from a common segmented ancestor. Both laid great stress on the fact that the main organs in Vertebrates are arranged in the same way as in an Annelid lying on its back, the nervous system being uppermost, the alimentary system coming next, and below this the vascular.
Dohrn's earlier views are contained in the fascinating little book published in 1875, which bears the title Der Ursprung der Wirbelthiere und das Princip des Functionswechsel (Leipzig). He followed this up by a long series of studies on vertebrate anatomy and embryology,[398] in which he modified his views in certain details. We shall confine our attention to the first sketch of his theory.
If the Vertebrate is conceived to have evolved from a primitive Annelid which took to creeping or swimming ventral surface uppermost, a difficulty at once arises with regard to the relative positions of the "brain" and the mouth. In Vertebrates the brain, like the rest of the nervous system, is dorsal to the mouth and the alimentary canal; in an inverted Annelid, however, the brain is ventral to the mouth and is connected with the dorsal nerve cord by commissures passing round the œsophagus. It would seem, therefore, that the primitive Vertebrate must have acquired either a new brain or a new mouth. Dohrn took the latter view. He supposed that the original mouth of the primitive ancestor lay between the crura cerebelli in the fossa rhomboidea, and that in Vertebrates this mouth has been replaced functionally by a new ventrally placed mouth, formed by the medial coalescence of a pair of gill-slits.[399] Probably the two mouths at one period co-existed, and the older one was ousted by the growing functional importance of the newer mouth.
The gill-slits were considered by Dohrn to be derived from the segmental organs of Annelids, which were present originally in every segment of the primitive ancestor. The gills were at first external, like the gills of many Chætopods at the present day. For their support cartilaginous gill-arches naturally arose in the body-wall, and the superficial musculature became attached to these bars. "There existed in all the segments of the Annelid-ancestors of Vertebrates gills with cartilaginous skeleton and gill-arches in the body wall. Each gill had its veins and arteries, each had its branch of the ventral nerve-cord, and between each successive pair of gills a segmental organ opened to the exterior" (p. 14, 1875). The paired fins and limbs of the Vertebrate arose by the functional transformation of two pairs of these gills. The anterior gills became the definitive internal gills of the Vertebrate, for they gradually shifted into the mouths of the anterior segmental organs, which had already acquired an opening into the pharynx and had been transformed into true gill-slits. The posterior gills degenerated and disappeared, but their arches remained as ribs. Gill-arches and ribs were accordingly homologous structures and formed a parietal skeleton. The vertebrate anus, like the mouth, was probably secondary and formed from a pair of gill-slits, the post-anal gut of vertebrate embryos hinting that the original anus was terminal as in Annelids. The unpaired fins of fish were originally paired and possibly arose from the coalescence of rows of parapodia. Dohrn assumed also that the primitive Annelid ancestor must have possessed a notochord to give support in swimming.
If Vertebrates arose from primitive Annelid ancestors, how account for Amphioxus and the Ascidians, which seem to be the most primitive living Vertebrates and yet show no particular annelidan affinities? Dohrn tries to answer this awkward question by showing that these forms are not primitive but degenerate. He points out first that Cyclostomes are degenerate fish, half specialised and half degraded in adaptation to a parasitic mode of life. He thinks that if an Ammocoetes were to become sexually mature and degenerate still further, forms would result which would resemble Amphioxus, and ultimately, if the process of degeneration went far enough, larval Ascidians. Amphioxus therefore might well be considered an extremely simplified and degenerate Cyclostome, and the ascidian larva the last term of this degeneration-series. Both Amphioxus and the Ascidians would accordingly be descended from fish, instead of fish being evolved from them.