THE NATURALIST S OCCUPATION. 3/ 



a number of segments, each of which is the structural 

 equivalent of a vertebra. The idea was suggested by 

 the sutural lines in the mammalian skull, which appeared 

 to mark the boundaries of successive segments. Thus 

 arose the so-called "vertebral theory" of the skull, 

 which was widely accepted and which was made the 

 corner stone of Richard Owen's great work on the com- 

 parative anatomy of vertebrates. If the skull could be 

 regarded as three, four, or more modified vertebrae, it 

 followed that the brain might be considered as a portion 

 of the spinal column, and that the cranial nerves were 

 the equivalents of the spinal nerves. It was impossi- 

 ble to settle these questions by compartive anatomy, and 

 the assistance of embryology was invoked. The dis- 

 covery by Jacobson, that the bony skull is preceded in 

 development by a so-called ^^ primordial cranium^' con- 

 sisting of a cartilaginous case, which, although a direct 

 continuation of the cartilaginous basis of the backbone, 

 yet differs from it in not being divided into segments, 

 and the fact that the adult skull is really a double brain 

 case, the inner portion representing the primordial cra- 

 nium and its derivatives, or by bone that has replaced it, 

 while the outer portion consists of the so-called dermal 

 bones that have been added externally and secondarily, 

 have been used with great force by Huxley, against the 

 vertebral theory of the skull. If the skull ever con- 

 sisted of segments comparable with the vertebrae, the 

 proof of this should appear in the primordial cranium, 

 as it is found to-day in the lower fishes, or in the course 

 of its development. 



Since the time of Goethe and Oken, we have learned 

 the important lesson, that the place to look for primitive 



