THE SKELETON 179 



THE ENDOSKELETON 



422. A well-developed endoskeleton is found only in 

 Vertebrates, but the notochord of the lancelet may be regarded 

 as an endoskeleton of the simplest form. It is simply a rod of 

 large turgid cells with strong cell membranes. This rod ex- 

 tends lengthwise of the body, immediately under the spinal 

 cord. It is also found well developed and functional in the 

 round-mouth eels, but in other fishes the vertebrae are 



FIG. 93. Outline drawing of the lancelet (Branchiostoma) to show the position 

 of the notochord (N.C.) and the spinal cord (S.C.). 



formed around it and take its place, though traces of it remain 

 in the adult. In the higher Vertebrates it is formed in the 

 embryo, but all evidence of it disappears with the development 

 of the spinal column. 



423. The first evidence of a true internal skeleton occurs 

 in the round-mouth eels. Here small pieces of cartilage are 

 formed around the notochord and spinal cord. These pieces 

 do not unite to form vertebrae, but they are arranged in a series 

 segmentally. Beneath the brain and around the pharynx a 

 large number of similar cartilages occur. In the sharks and rays 

 and some other fishes like the sturgeon, the skeleton is also 

 cartilaginous, but better developed. There is a continuous 

 column of vertebrae and a skull. 



424. In the higher Fishes and all the higher classes of Verte- 

 brates the skeleton is also, at first, cartilage, but this is gradually 

 transformed into true bone. Certain parts of the skeleton 

 remain cartilaginous throughout life, even in the highest forms. 



425. Cartilage is composed of cells which secrete an ex- 

 tremely firm gelatinous substance. This substance is secreted 



