474 



Prof. W. K. Parker. 



[Mar.. 22, 



On the Intercentra of Birds. 



I have not spot en of the neural arches as actually distinct from the 

 centra ; they are, as bony tracts, for a time, but the great heat aud 

 haste of the development of an embryo bird causes many essentially 

 distinct parts to be converted into hyaline cartilage continuously ; 

 such distinct morphological regions, however, are very apt to assert 

 their independence for a few weeks during the growth of the young 

 bird, and although separate osseous centres in a continuous tract of 

 hyaline cartilage are apt to be very inconstant as to the share they 

 take in the work, yet, on the whole, in default of the primary seg- 

 mentation of the cartilage, they are very valuable landmarks. 



In a survey of this subject from below upwards, it is well known 

 that the neural arches come before the centra ; that establishes their 

 independence and importance. 



It is very difficult to put this matter into a small compass, and to 

 show throughout the whole of the Vertebrata what parts of a ver- 

 tebra are important autogenous " elements " and what are . mere 

 apophyses or outgrowths. The old pre-embryological nomenclature fails 

 us here, entirely. 



Nothing newer and nothing better has been said upon this subject 

 than by Baur, whose wide acquaintance with the extinct forms that 

 lie between Birds above, and Fishes below, makes him, on the whole, an 

 excellent guide. 



In some " General Notes " [extracted from the ' American Natu- 

 ralist,' October, 1887, pp. 942—945] Dr. Baur (p. 945) gives his 

 " results " as follows : — 



" 1. That the ribs are intervertebral. 



" 2. The ribs are originally one-headed and connected with well- 

 developed intercentra. 



"3. All forms and connexions of the other ribs can be derived 

 from that condition. 



" 4. The lower arches of the caudal vertebrae are either formed by 

 true ribs, the oldest fishes (Granoidei, Dipnoi), or by processes of the 

 intercentra (Teleostei, Stapedifera). 



" 5. The connexion between the Dipnoi and the Stapedifera is 

 still missing. 



" 6. Some remarks on the nomenclature of the elements of the 

 vertebral column : — 



"Owen's names, ' neurapophysis ' and ' pleurapophysis,' are not 

 correct ; the neural and pleural arches are no processes of the ver- 

 tebrae, but are distinct parts. 



" The two elements composing the neural arch ought to be called 

 the ' neuroids,' the two elements composing the pleural arch the 

 ' ' pleuroids.' 



