THE SKELETON 139 



free surfaces of both layers, i.e., the cells lining the body cavity, 

 become the peritoneum, or coelomic epithelium, from which, ua 

 we have already seen, the ovaries and testes are formed. 



M. Development of the Skeleton. 

 1. The Vertebral Column. 



The earliest skeletal structure, and for a time the only one, 

 is the notochord, the development of which from the hypoblast 

 of the mid-dorsal wall of the mesenteron has already been 

 described. It forms a cellular rod extending from the blasto- 

 pore to the pituitary body ; and as the tail is formed, it extends 

 back into it. The notochord consists of vacuolated cells, filled 

 with fluid, and is invested by a delicate structureless sheath. 



About the time of appearance of the hind legs, a delicate 

 skeletal tube, at first soft but soon becoming cartilaginous, is 

 formed round the notochord from the mesoblast. This tube 

 grows upwards at the sides of the spinal cord, as a pair of longi- 

 tudinal ridges, with which a series of cartilaginous arches, which 

 appeared at the sides of the spinal cord at a slightly earlier 

 stage, very soon become continuous. 



By the appearance of transverse lines of demarcation, the 

 cartilaginous sheath of the notochord becomes cut up into a 

 series of nine vertebrae, followed by a posterior unsegmented 

 portion, which later becomes the urostyle. This transverse 

 division does not affect the notochord, which remains as a 

 continuous structure until the complete absorption of the tail 

 at the end of the metamorphosis. 



Shortly after the metamorphosis thin rings of bone, slightly 

 constricted in their centres, so as to be hourglass-shaped in 

 section, are developed in the membrane investing the cartila- 

 ginous sheath of the notochord : these correspond with the nine 

 vertebras already present, and form the first rudiments of the 

 vertebral centra. 



In the intervertebral regions, between the successive bony 

 rings, annular thickenings of the cartilaginous sheath occur, 

 which grow inwards so as to constrict and ultimately obliterate 

 the notochord. Each of these interyertebral rings becomes, 

 after the metamorphosis, divided into an anterior and a posterior 

 portion, which fuse with the bony centra of adjacent vertebrae, 

 and ossify to form their articular ends. 



From the circumference, and from the articular ends of each 



