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Show 296 ON THE. STRUCTURE OF THE VERTEBRATE SKULL. But the further investigations of embryologists have demonstrated that the occipital segment of the skull is, developmentally, as different from a vertebra as all the rest, seeing that, as Remak has more fully proved than any other observer, the segmentation into "urwirbel," or proto-vertebrre, whi~h. is characteristic of the vertebral column, stops at the ocCipital margin of the skull-the base of_ whic~, before ossification, presents no trace of that seg~enta~Ion whiCh occurs throughout the vertebral column. By this thud great step the vertebral hypothesis of the skull seems .to me to be altogether abo~ished; even though Professor Goodsir, whose thorough a~quaintance with embryology gives his opinions on these subJects great weight., has endeavoured, in his lear~ed and ingeniou~ essays, to c01nbine the facts of development with that hypothesis. IV. A fourth line of investigation, not bearing so directly upon the vertebral hypothesis, but still of great momen~, was opened up by the observations of Arendt on the persistent cartilao'inous cranium of the Pike,* and by the subsequent investi~ations of Von Bar, of Duges, of Reichert, of Agassiz, of Jacobson, Sharpey, Spondli, and Kolliker, and all the discussions which have taken place on the "primordial cranium" question. The problems attempted to be solved by these inquiries areIs there a clear line of demarcation between membrane bones and cartilage bones? Are certain bones always developed primarily from cartilage, while certain others as constantl.y oriainate in membrane? And further, if a membrane bone IS fou~d in the position ordinarii y occupied by a cartilage bone, is it to be regarded merely as the analogue, and not as the homologue, of the latter ? In other words, is histological .development as complete a test of homology as morphological development ? At present the course of investigation appears to me to tend towards giving an affirmative answer to these questions ; but much and careful observation is yet needed. * "De Capitis Ossei Esocis Lucii StructUl'a Singulari.'' 1822. Nesbitt, however, appears to have been the first to direct attention to the difference between membmne bones and cartilage bones. THE 'fHEORY OF THE VERTEBRATE SK.ULL. 2~)7 Having concluded this rapid historical sketch of the gradual growth of the true theory of the skull, it may be well if I state in a brief summary, what I conceive to be the present conditio~ of our knowledge respecting its structure and developm.ent :- 1. All crania result from the modification of the anterior part of that "primitive groove" of the embryo, the posterior part of which gives rise to the vertebral column ; and, at the very first, there is no discernible difference between that. part of the groove which will give rise to the vertebral column, and that from which the skull will be produced. 2. The first changes which take place, in both the cranial and the spinal regions of the primitive groove, are also precisely similar, the dorsal laminre growing up and uniting together in the middle line, so as to enclose a cavity which is, on the one hand, the primordial brain-case, and, on the other, the primordial spinal canal. So far, a unity of organization may · be predicated of both brain-case and spinal canal; but the braincase is not yet a skull, nor the spinal canal a vertebral column. 3. Beyond this point, the course of development of the cranial region differs absolutely from that of the spinal region. In the latter, that histological differentiation takes place which results in the formation of the proto-vertebrre, while in the skull no such process occurs. Again, the notochord extends throughout the whole length of the spinal column ; while, as soon as the skull is distinguishable, as such, the notochord ceases to extend beyond the middle of its floor, stopping immediately behind that part which lodges the pituitary fossa.* 4. Furthermore, when chondrification takes place in the spinal column, separate masses of cartilage are developed in each proto-vertebra; but, when chondrification commences in the base of the skull, it gives rise to a continuous body of cartilage, which never exhibits any trace of transverse division, ?r segmentation; but is always divided under the pituitary body Into two longitudinally-arranged crura, the "trabeculfl3 cran~£i." 5. Hence it follows that, though the primordial brain-case and the primordial spinal canal are identical in general plan of construction, the two begin to diverge as soon as the one puts * Amphioxus forms an exception, probably only apparent, to this generalization. |
