MUSCLES AND ENDOSKELETON 49 



From the cross-section made above run a median sagittal 

 section for a short distance. A vertebra is of the shape of 

 an hourglass; diamond-shaped spaces are therefore found 

 between the vertebrae. These contain the gelatinous 

 notochord which is thus mostly restricted to the ends of 

 each vertebra. 



Drawing 27. Draw a vertebra in sagittal section. 



Cut a section of the vertebral column from the middle 

 part of the body. Clean one side down to the cartilage. 

 Haemal arches are not present in the body section, but 

 they are represented by a pair of small cartilages, trans- 

 verse processes, on the sides of the ventral part of the 

 centrum. Within the portion of the transverse cartilagi- 

 nous septum a cartilage, the rib, will be found. Notice 

 that between each neural arch there is an intercalary arch. 

 Find the exit of the spinal nerves. 



Drawing 28. Draw the cross-section of a body vertebra and a 

 side view showing the intercalary arch. 



2. The Skeleton. 



The skeleton of the skate is mostly cartilaginous except 

 for a few ossifications in the vertebrae, teeth, scales, and 

 elsewhere. Ossification is a replacement process, not a 

 calcification or stiffening of the cartilage. 



In the study of the skeleton use the demonstration skele- 

 ton, the figure, and your specimen. 



A. The Girdles. 



The pelvic girdle is the ischiopubic bar of cartilage 

 across the ventral side of the trunk between the pelvic 

 fins. Iliac processes project from its main body dorsally, 

 prepubic processes anteriorly. This girdle bears two ar- 

 ticulating facets. The posterior facet bears the metaptery- 



