4i6 



NATURE 



{Sept 9, 1875 



those who have objected to speaking of cranial vertebrae have 

 put great importance on the point at which the chorda dorsalis 

 terminates, 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 certain 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. Looking at 

 the early embryo, the cerebro-spinal axis is seen to be one 

 continuous structure ; and the walls of the canal containing it 

 are likewise manifestly continuous, not at first distinguishable 

 into a spinal and a cranial portion. Looking at the adult 

 condition, in the higher classes the vertebrse of the tail are seen 

 dwindling into mere bodies developed round the chorda, 

 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 arches are enormously enlarged and have 

 additional centres of ossification, precisely as in the mammalian 

 thorax costal centres of ossification are found which do not exist 

 in the costal elements of cervical vertebras. 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 lainince, or none with 

 separate centres of ossification, as to object to its applicability to 

 segments of the skull because the chorda is absent, or the osseous 

 elements different in number from those found usually in the 

 segments of the trunk. 



However, it is gratifying to observe that among the most 

 recent additions to morphological anatomy there is a highly 

 suggestive paper by Prof. Huxley, appearing in the Royal 

 Society's " Proceedings" for December last, and entitled "Pre- 

 liminary Notes upon the Brain and Skull oiA^nphioxus lanceo- 

 latus," in which the learned Professor, who has for many years 

 been the most determined opponent to the mention of cranial 

 vertebrse, declares, so far as I can apprehend his meaning, that 

 the region of the head represents no less than fourteen segments, 

 all of which he terms protovertebra in Amphioxus. This deter- 

 mination of correspondences is made the more remarkable by 

 being followed up with a suggestion that the numerous proto- 

 vertebrse lying in front of the fourteenth in Amphioxus are repre- 

 sented only by muscles and nerves in the higher vertebrates. 



I hail this paper as being practically at last an ample acknow- 

 ledgment that there is no escape from admitting the correspon- 

 dence of the region of the head with the segments of the trunk : 

 but the details of the new theory scarcely seem convincing ; and 

 I might have preferred 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 vertebrate morphology. The argument 

 in this new theory runs thus : that the palate-curtain oi Amphioxus 

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

 curtain of the lamprey is attached below the ear ; that therefore 

 all the seven segments seen in front of the palate- curtain of 

 Amphioxus are represented by parts in front of the ear in the 

 lamprey and the other Vertebrata, Again, the branchial arches 

 of the higher Vertebrata are assumed to be of the nature of ribs, 

 and in none of the Vertebrata next shove Amphioxus "are there 

 more than seven pairs of branchial arches, so that not more than 

 eight myotomes (and consequently protovertebrae) oi Amphioxus, 

 in addition to those already mentioned, can be reckoned as the 

 equivalents of the parachordal region of the skull in the higher 

 vertebrates." Everything, observe, depends on the segment to 

 which the palate-curtain of Amphioxus belongs. Now I have 

 already pointed out to you that the segmentation of the vertebrate 

 body is not perfect ; and there is no method by which the alimen- 

 tary canal, of which the mouth and palate are the first part, can 

 be divided into segments corresponding with the cerebro-spinal 

 nerves. Most certainly we cannot judge that a portion of a 

 viscus belongs to a particular segment from its lying underneath 

 some other structure in definite relation, like the ear, to the 

 cerebro-spinal system ; for then should we be obliged to grant 

 that one-half or more of the heart belongs to segments in front 

 of the ear, since it is undoubtedly so situated in a chick of the 

 thirty-sixth hour. But the branchial arches are in front of the 

 heart, and, according to the theory which we are considering, 

 are behind the ear ; thus the principle assumed in the starting- 

 point of the theory is taken away. 



Again, it is important to observe that the branchial skeletal 

 arches cannot be ribs, for they lie internal to the primary circles 



of the vascular system formed by the branchial arteries and veins, 

 while the ribs are superficial to both heart and aorta. If the ribs 

 are represented at all in the branchial apparatus (and I doubt it 

 very much), it is by the cartilages superficial to the gills in 

 sharks, rays, and dog-fishes ; and it would seem impossible for 

 anyone who has dissected them to doubt that those cartilages 

 are homologous with the branchial skeleton of the lamprey, 

 which they somewhat resemble. In fact, if the external and 

 internal branchial openings of the lamprey be enlarged, its gills 

 are reduced to a form similar to those of the shark. 



There is nothing in this, however, which interferes seriously 

 with the proposed theory of the skull. It is merely a point in 

 the argument which I have thought right to clear. More im- 

 portant it is to remark that, on the supposition that numerous 

 protovertebrae are represented in the region of the head, there 

 arc most serious difficulties interfering with the idea that they 

 are, as Prof. Huxley states, "represented only by muscles 

 and nerves in the higher Vertebrata," and that there is any 

 correspondence between " the oculo-motor, pathetic, trigeminal, 

 and abducens nerves with the muscles of the eye and jaws " and 

 the regular nerves and muscle-segments of the fore part of 

 Amphioxus. Even in the lamprey the eye-balls are supplied 

 with muscles similar to those to which, in other vertebrates, the 

 oculo-motor, pathetic, and abducens are distributed ; and I find 

 in the large species that, notwithstanding this, the series of 

 regular muscle-segments is continued over the head, not indeed 

 in the same way as in Mixine, but in a highly instructive and 

 curious manner. After further dwelling upon this point. Prof. 

 Cleland said : — 



It may be noticed as a wholesome symptom in anatomical 

 speculation, that the new theory which has led to these remarks 

 is founded on arguments drawn altogether from comparison of 

 different species, and not from embryology, a very remarkable 

 circumstance as coming from one who so lately as last autumn 

 reiterated in this Section his slowness to believe in reasonings 

 founded on adult forms, and even on "later development." 

 The wisest know so little, that humanity must be content to 

 gather information from every possible source, and leave no set 

 of ascertained facts out of view in attempting to arrive at 

 generalisations. If we had before us all the adult anatomy of 

 every species that ever lived on the earth, we should only then 

 have the record completed from which to frame a full system of 

 morphology ; and as matters stand we must translate embryolo- 

 gical phenomena with the aid of the series of adult forms, as 

 well as translate the teachings of the adult series with the aid of 

 embryology. 



Falling back on my proposition, that the segments of the 

 vertebrate body are nowhere complete, and that segmentation ;at 

 one depth may exist to a greater extent than at another, I may 

 mention certain embryological phenomena in the brain, which 

 have received too little attention, and which to some extent 

 warrant belief in a larger number of segments in the head than 

 is usually admitted ; although I do not see that they are 

 necessarily at variance with that theory of seven segments in 

 every ossified skull which I indicated in 1862. In the chick, in 

 the middle of the second day of hatching, already is the third 

 cerebral vesicle divided into a series of five parts, separated by 

 slight constrictions, the first part larger than those which 

 succeed, and the last part narrowing to the spinal cord. The 

 auditory vesicle lies opposite the constriction between the fourth 

 and fifth parts. At the end of the second day and during the 

 third, these divisions assume dimensions which give them a 

 general appearance exceedingly similar in profile to the proto- 

 vertebrse of the neck. In the following day they exhibit a more 

 complex appearance, and after that the first compartment alone 

 remains distinct as cerebellum, while the divisions between the 

 others disappear in the thickening of the cerebral walls. In 

 their first two stages, Mr. Huxley, whom I have already referred 

 to so often, has figured these crenations, but he has not, so far 

 as I know, described them, 



I may also direct attention to another embryological point, 

 to which I referred last year at Belfast as a probability. I speak 

 now from observation. That which is termed the first cerebral 

 vesicle in the early part of the second day of hatching of the 

 chick, is an undifferentiated region of the brain from which a 

 number of parts emerge successively from behind forwards. As 

 early as the thirty-sixth hour the optic nerves can be traced, 

 separated from the rest of the vesicle by distinct elevations of 

 the floor of the brain, reaching inwards to the constriction 

 between the first and second vesicles : and as fearly as this date 

 the first trace of bifidity of the brain in front may be discerned— 



