96 ON THE MORPHOLOGICAL CONSTITUTION OF 



are two on each side in certain cephalic sclerotomes in at least 

 fishes and reptiles. Professor Owen admits one neurapophysis 

 only on each side of his typical vertebra. He accounts for the 

 additional pair in the spine of the sturgeon on the principle 

 of "vegetative repetition," while the additional elements in 

 the neural arches of certain cephalic vertebrae he at one time 

 considered as parapophyseal, and latterly as diapophyseal 

 elements. But it appears to me that the principle of " vegeta- 

 tive repetition " is out of place in a morphological question ; 

 and a parapophysis cannot, according to Professor Owen's 

 archetype, be intercalated between a neurapophysis and a 

 neural spine, nor can a diapophysis become an independent 

 element. 



The superior or posterior spinous process, " neural spine," 

 or (as a more convenient general designation) metaneurapo- 

 physis, is developed in the mesial neural fibrous lamina. As 

 this element is situated in the mesial plane, it is potentially 

 double, and its right and left halves become depressed and 

 more or less flattened out in the cephalic sclerotomes. With 

 the neurapophysis it completes the neural arch. 



The cartilaginous or osseous elements developed in the 

 lateral and mesial haemal laminse of the fibrous sclerotome 

 constitute the haemal arch. The fundamental character of the 

 inferior or haemal arch, as I understand it, consists in this, 

 that its constituent elements take their rise at or close to the 

 inner surface of those " ventral laminae" or "folds" in the 

 embryo, which form the lateral and inferior walls of the vis- 

 ceral chamber. Every haemal arch, therefore, within the 

 antero-posterior range of the alimentary tube must, according 

 as it is more or less developed, necessarily inclose that tube 

 more or less completely. Accordingly, no arch within the 

 range of that tube, if it excludes the tube, can be considered 

 as a haemal arch, merely because it incloses great blood-vessels. 

 Again, before any arch beyond the range of the alimentary 



