Section 13. the Skull of the Frog Will Repay a Full Treatment, and Will

be dealt with by itself later. The vertebral column (Sheet 12) consists of nine vertebrae, the centra of which have faces, not flat, but hollow in front (pro-coelous), and evidently without epiphyses (compare the Rabbit). The anterior is sometimes called the atlas, but it is evidently not the homologue of the atlas of the rabbit, since the first spinal nerve has a corresponding distribution to the twelfth cranial of the mammal, and since, therefore, it is probable that the mammalian skull = the frog's skull + one (or more) vertebrae incorporated with it. Posteriorly the vertebral column terminates in the urostyle, a calcified unsegmented rod. The vertebrae have transverse processes, but no ribs.