72 Vertebrate Embryology 



largely converted into muscle, the somato- 

 pleure and ectoblast forming the body wall, 

 while the splanchnopleure and entoblast form 

 the wall of the digestive tract. 



By the separation of the somatopleure and 

 splanchnopleure the coelom is greatly enlarged, 

 and, at the same time, a small portion is sepa- 

 rated from the anterior end as the pericardial 

 cavity. 



The Development of the Skeleton 



" The vertebral column. — The earliest skeletal struc- 

 ture, 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 blastopore 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 (Figs. 12 and 13, N^. 



" 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 longitudinal 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 



