xxvi PREFACE. 



skeleton, affirmed tliat to learn their true number in any given 

 species we must go to the first osseous centres as these are 

 manifested in the fostus ; ' and Geoffroy St. Hilaire^ concurred 

 in this view. 



In the cartilages called ' epiphysial,' that eke out the ends 

 and margins of bones, ossification begins later than does that of 

 the bone itself. The times of appearance of the osseous nucleus 

 in the coracoid process and acromion of the human scapula well 

 exemplify this difference; in the coracoid, e.g., at the first year, 

 in the acromion at the fifteenth year. Embryology teaches the 

 facts but affords not the reason. 



Special homology shows that the coracoid is a distinct bone, the 

 acromion a mere jjrocess, in the vertebrate series. General ho- 

 mology gives the ground of the distinctness — the coracoid being 

 the hajmapophysis of the haemal arch of which the scapula proper 

 is the pleurapophysis. In most mammals this haemapophysis is 

 stunted and terminates freely, like that of the last (floating) rib. 

 In Monotremes it attains and articulates with its hsemal spine, as 

 in the ' true rib,' and keeps this normal extent and condition 

 through all the lower vertebrates. It is the typical state of the 

 coracoid, which is departed from in all vertebrates above Mono- 

 tremes : but such typical state is not passed through in the 

 covirse of their development. As in that of other modified hfemal 

 arches, the maxillary, e.g., so in the scapular arch, the special con- 

 dition of the aborted hajmapophysis is gained directly, not through 

 any intervening transitory manifestation of the general character. 

 So far is embryology from being a criterion of homology. 



In regard to what I have reckoned a ' seventh way of ana- 



' ' Pour ayoir le veritable nonibre des os de chaquc esp^oe, il faut remonter jusqu'aux 

 premiers noyaux osseux tels qii'ils se montrent dans le fcetus.' — Lcfons (TAnatomk Cmn- 

 })ark, 8to. ed. 1835, torn. i. p. 120. 



■■^ ' Ayant imagine de compter antaut d'os qu'il y a de centres d'ossification distincts, 

 et ayant cssaye de suite eette manicre de faire, j'ai eu bieu d'apprecier la justesse de 

 cette lAte.'— Annates du Museum, torn. x. p. 344. See, however, the remarks on this 

 point in my ' Lectures on the Comparative Anatomy of the Vertebrate Animals,' Svo. 

 1846, p. 37, et seq. 



