112 ON THE MORPHOLOGICAL CONSTITUTION OF 



Second general form of Deterioration of the Sclerotome at 

 the front of the Head. The character of this form of deteriora- 

 tion may be best observed in the intermaxillary or vomerine 

 sclerotome of the osseous fish. Instead of being reserved for 

 the purpose of forming portions of nostrils, the neural space 

 no longer required for the lodgment of a segment of the neural 

 axis disappears entirely, the neurapophyses being at the same 

 time generally absent. The centrum may also disappear, or 

 may exist in the form of a cartilaginous nodule ; a pair of 

 neurapophyses may therefore form the entire sclerotome. 

 These hsemapophyses generally extend outwards and down- 

 wards from one another, or from the centrum if it exists at 

 the mesial plane. They form together, therefore, an arch sus- 

 pended at its centre, with its piers unsupported. The hsema- 

 pophyses of the two sclerotomes immediately behind form re- 

 spectively two arches, the maxillary and palatal, suspended 

 by their centres from the base of the skull. The centres of 

 these two arches are, however, morphologically their approxi- 

 mated piers, the actual centres ; their sternal or haemal con- 

 junctions are not completed in the osseous fish, in conse- 

 quence of the non-formation of the nasal fossae. These three 

 incomplete haemal arches retain their embryonic form of im- 

 perfect visceral laminae. They do not bridge across to form 

 a palate, and therefore the first complete haemal arch in the 

 osseous fish is the mandibular. The palate in it is, therefore, 

 like that of the mammal, morphologically a portion of the ex- 

 ternal surface of the animal. But they differ from one another 

 in this respect, that the palate of the fish is a primary, that 

 of the mammal a secondary surface. 



Number of Sclerotomes in the Vertebrate Head. It has 

 tended not a little to throw discredit on the vertebral theory 

 of the skull, that its advocates have differed much as to the 

 number of its constituent vertebrae. I am inclined to think 

 that these discordant views are the result of a tendency in 



