1. Remove the skin and muscles from the dorsal surface of the back to show the nine vertebræ and the urostyle. Make a drawing showing the vertebræ with their transverse processes, and the urostyle attached to the posterior end of the 9th vertebra. [Illustration: FIG. 14.--Vertebral Column of Xenopus and Rana.] The 2nd, 3rd and 4th vertebræ bear long transverse processes curved backwards, those of the 3rd and 4th being tipped with small cartilaginous ribs. The transverse processes of the 5th, 6th, 7th and 8th vertebræ are slender and project forwards and outwards. The 9th vertebra has long flat wing-like processes attached to which are the ilia of the pelvic girdle. The urostyle is fused to the 9th vertebra. [_In the Frog the transverse processes are produced outwards and they do not bear cartilaginous ribs. The urostyle is attached to the 9th vertebra by two facets._] Note the intervertebral foramina for the exit of the nerves from the spinal cord, and the intervertebral discs. 2. Remove, examine, and draw the 1st or atlas vertebra, note its ring-like form, absence of transverse processes, the concave facets by which it articulates with the two occipital condyles of the skull. 3. Remove, examine, and draw the 3rd vertebra and note the opisthocœlous centrum, neural arch and neural spine, anterior and posterior zygapophyses, the transverse processes, and the cartilaginous ribs. [_The centrum is procœlous in the Frog and there are no ribs._] 4. Remove and examine the 8th vertebra and note that the centrum is opisthocœlous. Make a drawing of a section showing the centrum. [_In the Frog the centrum is amphicœlous._] 5. Remove and examine the 9th vertebra or sacrum, and note that the centrum is slightly convex on the anterior surface. The urostyle is fused to the posterior surface of the 9th vertebra. Note also the long flat wing-like transverse processes. [_In the Frog the centrum is also convex anteriorly and on the posterior surface there are two convex facets for the articulation of the urostyle._] _Sixth Day._ =Skeletal System= (_continued_). C. The Skull. 1. Remove the skin and other tissues from the top of the skull and observe the large fronto-parietals, nasals, supra-ethmoid, premaxillæ, maxillæ, squamosals, exoccipitals, and pro-otics. The quadrato-jugals are very inconspicuous. The extrastapedial plate of cartilage is well marked and the bony columella may be seen lying underneath it. The sides of the skull are ossified and may represent the orbitosphenoids and alisphenoids of higher types. The supra-occipital consists of a piece of cartilage. [Illustration: FIG. 15.--Dorsal view of Skull of Xenopus and Rana.]