527 



tire centrum ossified .from this source, without any independent 

 points of ossification " (p. 88). That is to say, the centrum is in 

 these cases an exogenous process of the neurapophyses. We see, 

 then, that these so-called typical elements of vertebne have no 

 constant developmental character by which they can be identified. 

 Not only are they undistinguishable by any specific test from other 

 bones not included as vertebral elements ; not only do they fail to 

 show their typical characters by their constant presence ; but, 

 when present, they exhibit no persistent marks of individuality. 

 The central element may be ossified from six, four, three, or two 

 points ; or it may have no separate point of ossification at all : 

 and similarly with various of the peripheral elements. The whole 

 group of bones forming the " ideal typical vertebra" may severally 

 have their one or more ossific centres ; or they may, as in a mam- 

 mal's tail, lose their individualities in a single bone ossified from 

 one or two points. 



Another fact which seems very difficult to reconcile with the 

 hypothesis of an " ideal typical vertebra," is the not infrequent 

 presence of some of the typical elements in duplicate. Not only, 

 as we have seen, may they severally be absent ; but they may seve- 

 rally be present in greater number than they should be. When we 

 see, in the ideal diagram, one centrum, two neurapophyses, two 

 pleurapophyses, two haemapophyses, one neural spine, and one 

 hasmal spine, we naturally expect to find them always bearing to 

 each other these numerical relations. Though we may not be 

 greatly surprised by the absence of some of them, we are hardly 

 prepared to find others multiplied. Yet such cases are common. 

 Thus the neural spine " is double in the anterior vertebra? of somo 

 fishes " (p. 98). Again, in the abdominal region of extinct saurians, 

 and in crocodiles, " the freely-suspended hremapophyses are com- 

 pounded of two or more overlapping bony pieces " (p. 100). Yet 

 again, at p. 99, we read " I have observed some of the expanded 

 pleurapophyses in the great Testudo elephantopus ossified from two 

 centres, and the resulting divisions continuing distinct, but united 

 by suture." Once more " the neurapophyses, which do not advance 

 beyond the cartilaginous stage in the sturgeon, consist in that fish 

 of two distinct pieces of cartilage ; and the anterior pleurapophyses 

 also consist of two or more cartilages, set end on end" (p. 91). 

 And elsewhere referring to this structure, he says : 



" Vegetative repetition of peri vertebral parts not only manifests itself in 

 the composite neurapophyses and pleurapophyses, but in a small accessory 

 (interneural) cartilage, at the fore and back part of the base of the neura- 

 Dophysis ; and by a similar (interhtemal) one at the fore and back part of 

 most of the parapophyses " (p. 87). 



Thus the neural and haemal spines, the. neurapophyses, the plea- 

 Vo:.. II. S3 



