1897J MICROSCOPICAL JOURNAL 13 



of the brain comparing them with the fish as you progress. 

 Tlie medulla oblougataKa most posterior and is seen to be 

 a continuation of tlie tissue of the spinal cord. On its 

 dorsal side there is a considerable open space. Crossing 

 which and in front of it is a narrow cerebellum. Directly 

 in front of these is a rounded mass (apparently single 

 but really paired) the optic lobes; and in front of these 

 again are two elongate masses the two cerebral hemispheres. 

 They are attached behind through the crura cerebri wnich 

 run under the optic lobes to the medulla; there is no 

 transverse nervous connection between the two hemis- 

 pheres {corpus callosum of the higher vertebrates). In 

 recognizing these parts you will })robably see some of 

 the cranial nerves running from the brain chiefly from 

 the medulla to various parts of the head. 



4. The Body Cavity. — Locate the wall of the body 

 cavity. It is limited i)Osteriorly by the cloaca, beliind 

 the level of the hind le<;s. Note the regidar cross fold- 

 ings in the side wall of the body; they are indications of 

 the limits of the sets of muscle fibres in the wall, and are 

 perhaps homologous with the myotomes in the fish. Cut 

 the wall of the body cavity open and find the cavity 

 within, draw the skin aside and note the pigmented 

 peritoneum which lines the space. Follow the cut forward 

 and as you reach the level of the front limbs note and dis- 

 sect out the two [tieces of cartilage which overlap in the 

 midventral line, they are the coracoid cartilages. Draw • 

 them aside and pin them out of the way. They will be 

 studied latter in connection with the front limb. 



Cut away the coracoid cartilages and continue to open 

 the body cavity forward to the throat. Follow it back- 

 ward toward the cloaca, in doing so you will come to a 

 similar ventral arch lieli)ing to hold the hind leg in place; 

 note that it is bony, dissect off the muscles and skin so . 

 as to disclose the pair of bones, and cut between them; , 



