NA TURE 



43: 



THURSDAY, MARCH 7, 1895. 



THE ANCESTRY OF THE VERTEBRATES. 

 Amphioxus and the Ancestry of the Vertebrates. By 

 Arthur VVilley, 15. Sc, Tutor in Biology, Columbia 

 College ; Balfour Student of the University of Cam- 

 bridge. With a preface by Henry Fairfield Osborn. 

 (Columbia University Biological Series, II.) (New 

 York and London : Macmillan and Co., 1 894.) 



THE observations on Amphioxus made before the 

 second half of the present century, amongst which 

 those of Johannes Mailer take a foremost place, showed 

 that this remarkable animal bears certain resemblances 

 to Vertebrates ; and since then its interest in this respect 

 has gradually become more apparent. The extent to 

 which our knowledge of its structure and development 

 has recently increased, is indicated by the fact that about 

 two-thirds of the papers dealing with Arnphioxus quoted 

 by Mr. Willey have appeared during the last ten years. 

 With the exception of the admirable account given by the 

 late Prof. Milnes Marshall, ast year, in his " Vertebrate 

 Embryology," most of the works relating to this form are 

 of a special nature, and to many not easily accessible. A 

 consecutive history of the more recent observations was, 

 therefore, greatly needed by those whose opportunities 

 did not peimit them to follow out the matter for them- 

 selves, and who will welcome a book written in an ex- 

 tremely lucid style by a naturalist who can speak with 

 authority on the subject. 



After giving an excellent description of the habits^ 

 anatomy, and development of Amphioxus, Mr. Willey 

 devotes a special section to the Ascidians. Then follows 

 a section on " the Protochordata in their relation to 

 the problem of Vertebrate descent," which includes an 

 account of the so-called " Hemichorda," together with a 

 number of details relating to larval forms, Xemertines, 

 and Vertebrates. On the basis of the facts stated, there 



points out, mere parallelisms are often difficult to dis- 

 tinguish from true affinities ; and even with the increase 

 in our knowledge during the past few years, we are stil 

 so much in the dark that it is hardly possible to do more 

 than accept provisionally any particular theory which 

 appears to be supported by the greatest number of facts. 

 Although Amphioxus and the Ascidians have long 

 been recognised as of extreme importance in helping to 

 throw light on the question of Vertebrate ancestry, their 

 many peculiarities are so difficult to explain that by 

 many zoologists they have been attributed to degenera- 

 tion—more especially in the case of the Ascidians — and 

 on this supposition these animals can be of little use in 

 helping us to any sound view as to the form and structure 

 of the proximate ancestor of the X'ertebrates. 



The more recent researches on the structure and 

 development of Amphioxus have, however, shown that, 

 specialised as this animal undoubtedly is, it can no longer 

 be so easily put on one side, and that it, at any rate, 

 almost certainly represents an extremely archaic form. 

 The general lines of its early development, for the know- 

 ledge of which we are mainly indebted to Kowalewsky 

 and Hatschek, are now a matter of every-day know- 

 ledge, and are almost universally accepted as the best 

 possible startmg-point for the study of Embryology gener- 

 ally. In the present notice we can only refer to some of 

 the more recent discoveries and theories as stated by 

 Mr. Willey. . 



The curious tongue-bars in the gill-slits, which are only 

 known to be represented elsewhere in Balanoglossus, 

 are supposed by Mr Willey to be the " functionally active 

 organs, of which the thymus of the higher forms is a 

 metamorphosed derivative." This view is supported by 

 reference to Dohrn's account of the development of 

 the thymus in Selachisms. In a note on p 42, the meta- 

 pleural folds of Amphioxus and the evolution of the 

 lateral fin-folds and lateral line of Vertebrates are made 

 the subject of a hypothesis which cannot be done justice 

 to in a few lines. The discovery of the e.xcretory tubules 

 by Boveri and Weiss, is one of the most interesting 



is a considerable amount of theoretical matter dealing 



with the complex question included in the title of the | points which have recently come to light with regard to 



book, in ihe discussion of which the author is not dog- Amphioxus ; and Mr. Willey supports Boveri's view that 



matic, and makes the distinction between fact and theory 

 clear to the reader. Of the very few inaccuracies we have 

 noticed, mention need only be made of the statements with 

 regard to the cilia on the ectoderm of craniate \'erte- 

 brates, on p. 176, and to the distribution of the ganglion- 

 cells in the nerve-cord of .\nnulatcs, on p. 260, both of 

 which require modification. The figures, which are 135 

 in number, are extremely good and clear, not a few of 

 them being taken from the author's original papers ; and 

 a good bibliography is given at the end. Altogether 

 we congratulate Prof. Osborn on the publication of the • 

 second of the series which is appearing under his 

 editorship. 



The problem of the ancestry of the Vertebrates has 

 been approached from many dilTerent points of view, and 

 some of the various theories advanced — such as the well- 

 known "Annelid theory '' of .Semper and Dohrn — have 

 been supported by so many facts and arguments as to 

 gain for them many adherents. Others, again, were less 

 fortunate, and cannot be said to have held the field at 

 all. In all such phylogenetic speculations, as Mr. Willey 

 Nj. 1323, VOL. 51 



in spite of their special peculiarities, they represent the 

 pronephric system of \ertebrates, and that the pro- 

 nephric ducts of the latter are partially homologous with 

 the atrial chamber of Amphioxus. This conclusion 

 is especially important as furnishing another argument 

 against the Annelid theory, to which, moreover, the 

 nervous system of .Amphioxus lends no support, although 

 its peripheral portion " can only be compared definitely, 

 at present, in its broader features, with that of the higher 

 Vertebrates." 



The remarkable asymmetry of the larva: of Amphioxus 

 has probably no ancestral significance, and Mr. Willey 

 concludes that it " is of no specific advantage whatever 

 to the larv.-v, but is merely a stage, which has been pre- 

 served in the ontogeny, of a topographical readjustment 

 of parts necessitated by the removal of the mouth from 

 its primitive mid-dorsal position in consequence of the 

 secondary forward extension of the notochord, which 

 has thus caused a virtual semi-rotation of the pharyngeal 

 region of the body. On the other hand, the forward 

 extension of the notochord is a distinct advantage in 



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