ADDRESS. *CU1 



of Kowalevsky in 18G6, it is to to. found among the Invertebrates in the 

 larva of the Asoidia*. 



In Amphioxus and the Cyclostomatous Fishes the notochord, growing with 

 the rest of the body into a highly developed form, acts as a substitute for the 

 pillar of the bodies of the vertebrae, no vertebral bodies being developed ; but 

 in Cartilaginous and Osseous Fishes various gradations of cartilaginous and 

 osseous structures come to surround the notochord and give rise to the simpler 

 forms of vertebral bodies, which undergo more and more distinct development 

 in the higher vertebrates. In all instances the substance forming the vertebral 

 bodies is deposited on the surface of or outside the notochord and its sheath, 

 so that this body remains for a time as a vestigial structure within the 

 vertebral bodies of the higher animals. 



The observations of Kowalevsky with respect to the existence of a notochord 

 in the Ascidia, which have been confirmed by Kupfer and others, have pro- 

 duced a change little short of revolutionary in embryological and zoological 

 views, leading as they do to the support of the hypothesis that the Ascidian 

 is an earlier stage in the phylogenetic history of the mammal and other 

 Vertebrates. The analogy between the Amphioxus and Ascidian larva is 

 certainly most curious and striking as regards the relation of the notochord 

 to other parts ; and it is not difficult to conceive such a change in the form 

 and position of the organs in their passage from the embryonic to the adult 

 state as is not inconsistent with the supposition that the Yertebrates and 

 the Ascidia may have had a common ancestral form. Kowalevsky's discovery 

 opens up at least an entirely new path of inquiry ; and necessitates the modi- 

 fication of our views as to the entire separation of the Yertebrates from the 

 other groups of animals, if we do not at once adopt the hypothesis that 

 through the Ascidian and other forms the origin of the Yertebrates may bo 

 traced downwards in the series to the lower grades of animal organization. 



The notochord extends a short way forward into the cranial basis ; and an 

 interesting question here presents itself, beginning with the speculations of 

 Goethe and Oken, and still forming a subject of discussion, whether the series 

 of cranial or cephalic bones is comparable to that of the vertebra. On the 

 whole it appears to me that it is consistent with the most recent views of tho 

 development and anatomy of the head to hold the opinion that it is composed 

 of parts which are to some extent homologous with vertebral metameres f. 



The history of the formation of the vertebral column presents an interesting 

 example of the correspondence in the development of the individual and the 

 race, in that all the stages which have been referred to as occurring in the 

 gradual evolution of the vertebral column in the series of Yertebrates are 



* Mem. de l'Acad. de St. P6tersbourg, vol. x. 



t See the interesting and valuable memoirs of W. K. Parker, " On the Anatomy and 

 Development of the Vertebrate Skull," in Trans, of Eoy. Soc., the researches of Gegenbaur, 

 Mihalkovics, and more particularly the memoir by F. M. Balfour, " On the Development 

 of the Elasmobranchs," in tho Journ. of Anat. and Physiol, vols. x. and xi. 



