TRANSACTIONS OF THE SKCTIOXS. 139 



jectingto a time-honoured phrase. It is questionable if any one who ever used the 

 convenient term " vertebral theory " meant to indicate more than a certain com- 

 munity of plan on which were built the segments of the skull ns well as those of 

 the spinal column ; that, in fact, the two constituted one complete chain, of which 

 the first few segments were so different from the rest that, till Okeu pointed the 

 fact out, it was not recognized that thej' were segments lying in lineal continuitj^ 

 with the rest. But the matter has recently stood thus : — that to some minds, in the 

 imperfect state of our knowledge, one thing seemed essential to a segment compa- 

 raWe with the rest, and to others something else seemed requisite ; and the oddity 

 of the position of affairs is this, that the objectors to the phrase " vertebral theoiy " 

 have been as crotchet}^ in setting up imaginary essentials to a segment as their 

 neighbours. On the one side we were taught to expect certain definite osseous 

 elements in each segment, to which definite names were given ; while, on the other, 

 in opposition schemes, centres of ossification have been built on as matters of pri- 

 mary consequence, altliough a glance at the modifications in the vertebral column 

 proper might convince any one that they are things of the very slightest impor- 

 tance morphologically. Also those who have objected to speaking of cranial ver- 

 tebras have put great importance on the point at which the chorda dorsalis termi- 

 nates, although it has been long known that in one animal the chorda dorsalis runs 

 right on to the front, that in others it fails to enter the skull at all, while in the 

 majority it passes for a cei-tain distance into the base. Johannes Miiller, on such 

 grounds, concluded, thirty years ago, that the presence of chorda dorsalis was not 

 necessary to constitute a cranial vertebra ; and there seems no reason to doubt that 

 he was right. Looldng at the early embryo, the cerebro-spiual axis is seen to be 

 one continuous sti-ucture ; and the walls of the canal containing it are likewise mani- 

 festly continuous, not at fii'st distinguishable into a spinal and a cranial portion. 

 Looking at the adult condition, in the higher classes tlie vertebrae of the tail are 

 seen dwindling into mere bodies developed round the chorda dorsali,'^, and giving 

 off' rudimentary processes without separate centres of ossification, while towards 

 the head the bodies diminish and the arches enlarge ; and in the skull the chorda, 

 round which the bodies in the rest of the column are developed, comes to an end, 

 and the neural fvrches are enormously enlarged and have additional centres of ossi- 

 fication, precisely as in the mammalian thorax costal centres of ossification are 

 foimd which do not exist in the costal elements of cervical vertebrte. It would 

 therefore be quite as justifiable to object to the term vertebra as applied to a joint 

 of the tail because it has no kimince, or none with separate centres of ossification, 

 as to object to its applicability to segments of the skull because the chorda is ab- 

 sent, or the osseous elements difterent in number from those found usu.ally in the 

 segments of the trunk. 



However, it is gi-atifjang to observe that among the most recent additions to 

 morphological anatomy there is a highly suggestive paper by Professor Huxley, 

 appearing in the Royal Society's ' Proceedings ' for December last, and entitled 

 " Preliminary Notes upon the Brain and Skull of Amphioxits lanccolatus" in whicli 

 the learned Professor, who has for many years been the most determined opponent 

 to the mention of cranial vertehrre, declares, so far as I can apprehend his meaning, 

 that the region of the head represents no less than fourteen segments, all of which 

 ho terms protovoiebree in Amphioxus. This determination of correspondences is 

 made the more remarkable by being followed up with a suggestion that the nume- 

 rous protovertebrfe lying in front of the fom-teenth in Amphioxus are represented 

 only by muscles and nerves in the higher vertebrates. 



I hail this paper as being practically at la=t an ample acknowledgment that 

 there is no escape from admitting the correspondence of the region of the head with 

 the segments of the trunk : but the details of the new theory scarcely seem con- 

 vincing ; andll might have prefen-ed to leave its discussion to others, were it not 

 that the notions which it opens up are far too important to allow it to be passed 

 over in any account of the present state of opinion on the subject of vertclirate 

 morphology. The argument in this new theory runs thus : that the palate-curtain 

 of Amphioxus is homologous with that of the lamprey, and that the palate-curtain 

 of the lamprey is attached below the ear ; th.at therefore all the seven segments 

 seen in front of the palate-curtain of Amphioxus are represented nv parts in front 



"11* 



