30 



ANATOMY OF VERTEBRATES. 



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2g-a -OL-«-=;* 



The different elements of the pri- 

 mary segments are distinguished by 

 peculiar markings : — 



The neurapophyses by diago 

 nal lines, thus — 



The diapophyses by vertical 

 lines — 



The parapophyses by horizon- 

 tal lines — 



The centrum by decussating; 

 horizontal and vertical lines — 



The pleurapophyses by diago- 

 nal lines — 



The appendages by dots — . . . . . 

 The neural spines and haemal spines 

 are left blank. 



In certain segments the elements 

 are also specified by the initials of 

 their names : — 



?is is the neural spine. 

 n is the neurapophysis. 

 pi is the pleurapophysis. 

 c is the centrum. 

 h is the haemapophysis, also indi- 

 cated by the numbers 21, 29, 



44, 52, 58, 63, 64. l 



hs is the haemal spine. 



a is the appendage. 

 The centrum is the most constant 

 vertebral element as to its existence, 

 but not as to its ossification. There 

 are some living fishes, and formerly 

 there were many, now extinct, in 

 which, whilst the peripheral elements 

 of the vertebra become ossified, the 

 central one remains unossified ; and 

 here a few words are requisite as to 

 the developement of vertebrae. 



§ 16. Developement of vertebra, — 

 The central basis of the neuroskeleton 

 is laid down in the embryo of every 

 vertebrate animal as a more or less 



1 See •Table of Stkonyjis, Special Homologies,' for the names of the bones 

 indicated by numbers. 



