824 DEVELOPMENT. [CH. LVIII. 



Development of the Framework of the Body. 



The notochord is a primitive vertebral column which, unlike 

 the true vertebral column that replaces it, is a single rod. In 

 ampbioxus and the lampreys, however, it remains in the adult as 

 a permanent skeletal support. In structure it closely resembles 

 cellular cartilage (p. 57), and is enclosed in a sheath. It is 

 composed of a very insoluble proteid-like matter, which is, however, 

 not collagen. Collagen, and gelatin which is formed from collagen 

 by boiling, are characteristic of true connective tissues ; these are 

 formed from mesoblast; the notochord is hypoblastic. It contains 

 also, like all embryonic tissues, a large quantity of glycogen. Its 

 place is ultimately occupied by the vertebral bodies, but traces of 

 it are found even in the adult in the centres of the intervertcbral 

 discs. 



The protovertebrae or protovertebral somites form the 

 vertebrae and other structures as well. Each divides vertically 

 into two parts, an inner and an outer. It is the inner division 

 that forms the vertebrae ; the outer division is called the museulo- 

 cutaneous plate, and it is continued into the general mesoblast 

 which divides into the splanchnopleur and somatopleur with the 

 pleuro-peritoneal cavity between them. 



The inner portion of each pair of protovertebrae grows around 

 the notochord and in time almost obliterates it ; this forms the 

 body of the vertebra; it also grows around the primitive spinal 

 cord, and so forms the neural arch of the vertebra. This part of 

 the protovertebra is more distinctly separated from the other 

 segments of the protovertebral column from the first, and so 

 allows the spinal nerves which are sprouting out from the spinal 

 cord to leave the spinal cord for the body walls. 



At first, all these parts are composed of protoplasmic embryonic 

 cells, but as development progresses the cells become specialised 

 in function and structure, some becoming cartilage cells, others 

 muscular fibres, &c. At a later date still, the cartilaginous 

 vertebrae are replaced by bone. 



The vertebra do not exactly correspond in their position to the 

 protovertebrae ; each vertebra is developed from the contiguous 

 halves of two protovertebrse. The original segmentation of the 

 protovertebrse disappears, and a fresh subdivision occurs in such a 

 way that the intervertebral disc is developed opposite the centre 

 of each protovertebra. In the musculo-cutaneous plate are 

 developed the muscles and true skin of the body wall, and the 

 ribs. 



