296 ON THE STRUCTURE OF THE VERTEBRATE SKULL. 



But the further investigations of embryologists have demon- 

 strated that the occipital segment of the skull is, developmentally, 

 as different from a vertebra as all the rest, seeing that, as 

 Kemak has more fully proved than any other observer, the 

 segmentation into "urwirbel," or proto-vertebrse, which is 

 characteristic of the vertebral column, stops at the occipital 

 margin of the skull — the base of which, before ossification, 

 presents no trace of that segmentation which occurs throughout 

 the vertebral column. By this third great step the vertebral 

 hypothesis of the skull seems to me to be altogether abolished ; 

 even though Professor Goodsir, whose thorough acquaintance 

 with embryology gives his opinions on these subjects great 

 weight, has endeavoured, in his learned aud ingenious essays, 

 to combine 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 moment, was 

 opened up by the observations of Arendt on the persistent 

 cartilaginous cranium of the Pike,* and by the subsequent 

 investigations 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 are — 

 Is there a clear line of demarcation between membrane bones 

 and cartilage bones? Are certain bones always developed 

 primarily from cartilage, while certain others as constantly 

 originate in membrane ? And further, if a membrane bone is 

 found in the position ordinarily 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 de- 

 velopment 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 Structura Singulari." 1822. Nesbitt, how- 

 ever, appears to have been the first to direct attention to the difference bit ween 

 membrane bones and cartilage bones. 



