140 THE STORY OF ANIMAL LIFE 



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 

 vertebrae. Each of them is made up of several 

 parts, which by degrees become consolidated into 

 the vertebras. 



The lowest member of the vertebrate group, 

 separated in fact from the true vertebrates and 

 placed in a lower division all by itself, is the 

 little animal called the Lancelet or Amphioxus. 

 It is often spoken of as a "fish"; but it is only 

 by a stretch of our courtesy that it can receive 

 that name, being an animal of a much lower form 

 than the fishes. It was discovered in 1834, in the 

 Mediterranean, and described as a fish ; but it 

 had previously been discovered in 1778, by a Ger- 

 man naturalist who described it as a slug. The 

 latter was misled by its external shape. He had 

 not the advantage of the modern methods of pre- 

 paring animals for examination under the micro- 

 scope; in these days, Amphioxus is cut into suc- 

 cessive slices along its whole length, and each of 

 these carefully magnified, so that no detail of 

 structure is lost. The Amphioxus burrows in the 



