318 



REPORT — 1846. 



neurapophysial relation to the sides of the prosencephalon, — we are led to 

 carry our inquiries into an earlier period of their development than that ad- 

 duced by Cuvier, as contravening their vertebral characters. Cuvier cites 

 the figure 2, in pi. xxxv. of the ' Osteogenia Fcetuum' of Kerkringius, as evi- 

 dence of his statement of the developmental characters of the ' spheuo'ide 

 anterieur." That figure, however, exhibits the condition of the bone, when, 

 although the median suture remains, each orbital ala has become anchylosed 

 with the posterior sphenoid, and is likewise directly perforated by the optic 

 nerve. The gelatinous cells of the anterior extremity of the notochord very 

 early retrograde to the basioccipital region of the basis cranii, and the noto- 

 chordal capsule alone is continued to the anterior extremity of the basis. 

 This is converted into cartilage, 



and the osseous particles which Fig. 26. 



ultimately constitute the anterior 

 sphenoid are deposited as follows : 

 first a centre or nucleus appears, 

 in each orbital ala, external to the 

 hole by which the optic nerve 

 passes through the primitive carti- 

 lage (fig. 26, A, io) ; soon after a 

 second nucleus (ib. B, io) is esta- 

 blished at the inner or mesial side 

 of each optic foramen : these cen- 

 tres form the foundation of the 

 neurapophyses or orbitosphenoids, 

 and ultimately coalesce around the 

 optic nerve, as Kerkringius has 

 depicted. But a third pair of ossi- 

 fic centres (ib. C, 9) is established 

 behind the optic foramina between 

 these and the basisphenoid (5). 

 This third pair unite together into 

 a single transverse bar (ib. D, 9), 

 before coalescing with the orbitosphenoids in front, or with the basisphenoid 

 behind, and that bar transitorily represents the centrum of the frontal vertebra. 

 To the objection that such supposed centrum is developed from two points 

 instead of one, the same reply may be made that was made before to a similar 

 objection raised by Cuvier against the general homology of the basisphenoid ; 

 which objection, as was then shown, would be equally valid against the uni- 

 versally admitted homology of the body or centrum of the atlas. 



The frontal neurapophyses manifest in their development, each from two 

 centres (fig. 26, B, C, io), a transitory mark of vegetative repetition analogous 

 to that which permanently characterizes the neurapophyses of the trunk-verte- 

 brae in the sturgeon. 



Thus the evidence of development, when complete, tells for, rather than 

 against the serial homology of the 'sphenoide anterieur ' of Cuvier with the 

 centrum and the neurapophyses of other vertebras ; and the more obvious 

 and important characters of relative position to the other bones of their own 

 segment, and to their homotypes in the contiguous segments, as well as to 

 prosencephalic segment and characteristic nerves, — which characters have 

 served to determine the special homologies of the coalesced bones in ques- 

 tion (9, 10) from man down to the fish, — concur with the developmental 

 characters in establishing their general homology as centrum and neur- 

 apophyses. 



Phases of development of the Human Sphenoid bone : 

 after Meckel. 



