x PRACTICAL DIRECTIONS 481 



pectoral arch and backwards through the pelvic arch : the arteries may 

 be injected at this stage. Then dissect off the left half of each arch 

 with the corresponding fin, and cut away the left body- wall. Cut 

 through the skin in the mid-dorsal line ; and then, working from the 

 left side, remove the left half of the skull and visceral arches, 

 as well as the left side of the vertebral column, so as to expose 

 the brain and spinal cord. Remove the left kidney (and left ovi. 

 duct if your specimen is a female). Pin down under water, and tidy 

 your dissection so as to reduce it to a neat longitudinal section, in 

 which all the unpaired soft organs are left intact. Without tearing 

 the mesentery, pin out the liver, stomach, and intestine beyond the 

 ventral limits of the body-wall, so that the other abdominal organs are 

 not hidden. Follow out the relations of the various organs as before, 

 and sketch your dissection. 



It is important at this stage to refresh your memory of the anatomy 

 of the frog", and to compare it with the dogfish, by making a similar 

 dissection of it from the side (compare Fig. 7). Sketch. 



PRACT. ZOOL. 1 I 



