256 report— 1846. 



two neurapophyses and the two parapophyses ; but the terminal concave plates 

 of the centrum are separately ossified. They coalesce with the intermediate 

 part of the centrum, which is sometimes completely ossified, but commonly a 

 communicating aperture is left between the two terminal cones; and in 

 many cases, the plates by which calcification attains the periphery of the 

 body leave interspaces permanently occupied by cartilage, forming cavities 

 in the dried vertebra, especially at their under part, or giving a reticulate 

 surface to the sides of the centrum. The expanded bases of the neur- and 

 par-apophyses usually soon become confluent with the bony centrum ; some- 

 times first expanding so as wholly to inclose it, as, for example, in the tunny, 

 where the line of demarcation may always be seen at the border of the arti- 

 cular concavity, though it is quite obliterated at the centre, as a section 

 through that part demonstrates. 



Muller correctly distinguishes a 'central' from a 'peripheral' (cortical) part 

 or seat of the ossification of the vertebral bodies of fishes. The peripheral 

 ossification which takes its rise from the outer layer of the fibrous sheath of 

 the notochord sometimes extends into broad plates beneath the anterior ver- 

 tebrae of the trunk, and tends to fix or anchylose a certain number of them ; 

 when they are commonly represented by the partially distinct central parts 

 of the bodies, together with the neur- and par- and pleur-apophyses. 



The batrachia follow closely the stages above-cited in fishes ; the centrums 

 being arrested at the biconical stage in the perennibranchiates, but converted 

 into ball-and-socket vertebras by the ossification of the interposed gelatinous 

 ball* and its adhesion, either to the fore-part of the centrum (Pipa, Sala- 

 mandra), or the back part (Rana, Bufo). The mode of ossification of the 

 centrum varies somewhat in batrachia. Mullerf describes annular ossifi- 

 cations in the sheath of the notochord of the Rana temporaria and R. escu- 

 lenta, which support, at first, the neurapophyses. Duges, apparently in- 

 fluenced by M. Serres' so-called 'law of centripetal development,' describes 

 two cartilaginous nuclei, side by side ; but the more obvious and better-de- 

 termined development of the vertebrae of fishes gives no countenance to this 

 bilateral beginning of ossification of the centrum as a general law. The first 

 distinct bony nucleus in the centrum observed by Duges was bilobed, and 

 afterwards cubical; but excavated before and behind, as well as beneath %. 

 The ossification of the centrum is completed by an extension of bone from 

 the bases of the neurapophyses, which effect, also, the coalescence of these 

 with the centrum. In Pelobates fuscus, and Pelobates cultripes, Muller found 

 the entire centrum ossified from this source, without any independent points 

 of ossification. 



The vertebrae of the tail of the larvae of the anourans are represented di- 

 stinctly only in the aponeurotic stage. Even when the change to cartilage 

 takes place, the tendency to coalescence has begun to operate, and only two 

 long neurapophyses are established on each side : the ossification of these 

 plater, extends into the fibrous sheath of the remnant of the coccygeal noto- 

 chord, and they coalesce when the perishable parts of the tadpole-tail have 

 been absorbed, and the fore- and hind-legs developed, constituting the long, 

 often hollow, and inferiorly grooved coccygeal bony style. 



In saurians, birds and mammals, the notochord is inclosed by cartilage 

 before ossification begins ; which cartilage is continuous with the cartilagi- 

 nous neurapophyses §. In birds, the two histological processes, chondrifica- 



* Dutrochet, Memoires pour servir a l'Histoire Nat. et Physiol, des Animaux, &c, t. ii. 

 p. 302. 1837. 



t Neurologie der Myxinoiden, 1840, p. 69. 



X Recherches sur les Batraciens, 1835, 4to, p. 106. 



§ Muller, Vergleichende Anatomie der Myxinoiden, Neurologie, 1840, p. 74. 



