464 THE DOGFISH chap. 



vascular sac on either side, pituitary body, crura cerebri, and, as far as 

 possible, the origins of the nerves. Sketch from below. 



2. On one side of the brain, cut into the olfactory lobe, prosence- 

 phalon, optic lobe, and cerebellum from above, so as to expose the olfac- 

 tory ventricle, lateral ventricle, optic ventricle, and cerebellar ventricle. 

 Then bisect the entire brain into right and left halves with n. sharp 

 scalpel, and examine the uninjured half in longitudinal section, noting 

 in addition to the parts mentioned above, the third ventricle, foramen of 

 Monro, iter, sxid fourth ventricle. Sketch. 



G. Transverse Sections. — Cut thick transverse sections of an en- 

 tire dogfish with a knife through — a. the anterior, and b. the posterior 

 part of the head (pharyngeal region) ; t. about the middle of the 

 body ; and d. the tail. Make out the relations of the various parts and 

 organs, and sketch the lateral half of each section. 



A more satisfactory method than this is to obtain a very young dog- 

 fish, not more than \ inch in diameter, and after cutting it transversely 

 into pieces about \ inch in thickness in the regions named above, stain, 

 imbed, and mount a few sections from each piece (see p. 1 36). These 

 can first be examined with a lens or with the low power of the micro- 

 scope, and then, by putting on the high power, important points in the 

 histology can be made out. In addition to the minute structure of the 

 tissues and organs described in Part I. of this book, the structure of the 

 notochord (p. 425), integumentary sense-organs (pp. 414, 418), dermal 

 teeth (p. 417), &c., should be studied, 



H. Side Dissection. — It is very instructive to supplement and 

 recapitulate your work on the anatomy of the dogfish by dissecting 

 another specimen from the side (compare Fig. 108), as in the case of the 

 crayfish. Open up the abdominal cavity as before, very slightly to the 

 left side of the middle line. Continue the cut' forwards through the 

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

 be injected at this stage. Then dissect off the left haJf 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 up 

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



