THE ORGANS OF THE INTERMEDIATE LAYER OR MEKENOHYME. 599 



The signification of the primitive segments consists, if not exclu- 

 sively, at least principally, in this, that they are the fundaments of 

 the musculature of the body. But in the arrangement of the muscu- 

 lature is expressed the original and oldest segmentation of the vertebrate 

 body. It is present even in Amphioxus and the Cyclostomes. The 

 segmentation of the vertebral column, on the contrary, toas acquired 

 much later, and has resulted, as was explained above, from a necessary 

 dependence on the segmentation of the musculature. A primary 

 segmentation of the vertebral column as understood by Rbmak and 

 his followers has never existed, for the cartilaginous vertebrae are 

 formed from an unsegmented mass of tissue enveloping the chorda — 

 from the skeletogenous layer. One cannot speak of a segmentation 

 of the vertebral column until the beginning of the process of chon- 

 drification, by reason of which alone it became necessary. 



Even before the cartilaginous vertebral column has been completely 

 established, it enters in Mammals upon the third stage, which begins 

 in Man at the end of the second month. 



The ossification of every cartilage takes place in the main in a 

 corresponding, typical manner. Blood-vessels at one or several 

 places grow from the surface into its interior, dissolve the matrix of 

 the cartilage of a limited region, so that there arises a small cavity 

 filled with vascular capillaries and marrow-cells. In the vicinity of 

 this salts of hme are deposited in the cartilage. By a portion of the 

 proliferated medullary cells, which become osteoblasts, bone substance 

 is then secreted (fig. 326 w). In this manner there arises in the 

 midst of the cartilaginous tissue a so-called bone nucleus or centre of 

 ossification, around which the destruction of the cartilage and its 

 replacement by osseous tissue advance further and further. 



The places where the separate bone nuclei are formed, as well as their 

 number, are tolerably uniform for the different cartilages. 



In general the ossification of each vertebra proceeds from three 

 points. At first a centre of ossification is established in the base of 

 each half of the vertebral arch, to which there is added somewhat 

 later a third centre in the middle of the body of the vertebra. In 

 the fifth month the ossification has advanced up to the surface of 

 the cartilage. Each vertebra is now distinctly composed of three 

 pieces of bone, which for a long time continue to be joined to one 

 another by bridges of cartilage at the base of each half of the arch 

 and at the union of the latter with the vertebral spines. The last 

 remnants of cartilage do not ossify until after birth. During the 

 first year with the development of a bony spinous process the halves 



