xii PHYLUM CHORDATA 335 



in the Rabbit, very long in the Lizard, lies behind the sacral. 

 The ribs connected with the thoracic vertebrae are slender 

 curved rods, which lie in the side-walls of the anterior part 

 of the trunk ; the most anterior of them with their 

 continuations, the sternal ribs, form, on either side, half-hoops 

 extending from the spinal column dorsally, to the sternum 

 ventrally. The sternum or breast-bone, absent in the Dog- 

 fish, lies in the middle of the wall of the ventral region of 

 the trunk. In the Lizard it is a rhomboidal plate of cartilage : 

 in the Rabbit it is bony, and divided up into a number of 

 segments known as sternebrce. 



In the embryo of each of the three forms used as illus- 

 trations, the spinal column passes through a stage in which 

 it consists merely of a continuous cylindrical rod of cells 

 the notochord, corresponding to the notochord of Amphioxus 

 and enclosed in a sheath. In some Craniates it never passes 

 much beyond this stage. But in the great majority the 

 notochord becomes enclosed in a sheath of cartilage, which 

 becomes -divided up into a number of segments. Eventually 

 ossification sets in, and a series of completely-formed bony 

 vertebrae, such as we find in the Lizard and Rabbit, 

 become developed. 



As already mentioned, the spinal column does not ex- 

 tend into the head region. The skeleton of this region is 

 the complex cartilaginous or bony structure known as the 

 skull. The chief part of this is a case, the cranium, in the 

 interior of which the brain is lodged, and the walls of which 

 afford support to three pairs of organs of special sense the 

 nasal or olfactory organs in front, the eyes in the middle, 

 and the ears or auditory organs behind. The cavity of the 

 cranium opens behind by a rounded foramen, the foramen 

 magnum, into the anterior end of the neural canal enclosed 

 by the neural arches of the vertebrae ; and the posterior 



