Some of the Ascidians present what has been already described in other types (p. 57) as "alternation of generations." The discovery of this fact was made by the poet Adelbert von Chamisso. Some of his verses are known to English readers, for whom they were translated by Mary Howitt, a poetess whose writings were popular with our grandmothers, and deserved to be so. This is not the only case in which a poet has been also a zoologist: Goethe studied the science, and framed a theory regarding the vertebrate skull, which he regarded as consisting of a series of vertebræ. In this he was less fortunate than the Italian poet; for while Chamisso's observations were correct, and were confirmed by subsequent writers, Goethe's theory of the skull is anything but correct. It was made worse, too, by the speculations of subsequent writers, who attempted to follow it into detail, with the result of demonstrating its absurdity. CHAPTER XIV THE VERTEBRATA We have spoken of the Notochord as a structure which precedes the formation of the spinal column in Vertebrates. This needs a little more definite explanation. We all know that the spinal column of vertebrates is formed to protect the spinal cord. This protection is, however, an afterthought, so to speak, of the vertebrate structure; the lowest of all vertebrates is quite without it; and in the lower groups of fishes we may trace various steps of its formation. But in these cases where the spinal column is absent or incomplete, there is a large and well-developed notochord; and in the embryo of higher vertebrates, when the spinal column has not yet begun to be formed, the notochord is equally a conspicuous feature. It runs from the region known as the mid-brain, to the end of the tail, and lies throughout just beneath the spinal cord. Whatever its original use in the animal body may have been, it undoubtedly acts now as a support to the spinal cord, and indeed to the whole body. Bones, we must explain, do not exist either in the lower vertebrate, or in the early embryo. In the latter they are formed by degrees. The spinal cord and the notochord each begin to be surrounded by rings of cartilage or gristle, which by degrees is changed into bone. The rings surrounding the notochord, however, gradually encroach upon it and obliterate it. The place where it has been becomes the Centrum, or most solid part of each vertebra. The notochord at first is continuous, and has no division into successive parts; but when the bony spinal column is developed, it consists of a series of successive vertebræ. Each of them is made up of several parts, which by degrees become consolidated into the vertebræ.