CHAPTER XLV. 



THE SKELETAL SYSTEM 



The notochord extends from the blastopore to the pituitary body, 

 as a rod of vacuolated cells rilled with fluid, around which three layers 

 or sheaths form. 



The primary, or elastic sheath, is an outgrowth of superficial cells 

 of the notochord itself and forms the superficial surface sheath of the 

 notochord. 



The secondary or fibrous sheath is formed between the primary 

 sheath and the notochord also by cells from the notochord itself. 



The third, or skeletogenous sheath, forms on the outside of the pri- 

 mary layer at a later period as a thin sheath which is formed by the 

 sclerotomes. The sclerotomes, it will be remembered, are outgrowths 

 from the somites. This skeletogenous layer extends dorsally, entirely 

 around the neural tube, and laterally, from the notochord, in between 

 the successive myotomes. It is in this skeletogenous layer that the ver- 

 tebrae are to be formed. 



When the tadpole is about fifteen millimeters long, a series of 

 metameric cartilages can be observed along the medio-ventral surface of 

 the notochord. They lie in the skeletogenous sheath (Fig. 352, cs). 

 These segments fuse longitudinally to form a pair of dorsal and ventral 

 strips which extend along the entire notochord. 



These strips now become metameric by constrictions of fibrous tissue 

 which form rings. The rings are the beginnings of the inter-vertebral 

 ligaments, which, just as in the chick, appear opposite the middle of each 

 mesodermal segment. 



The mesodermal segments become the vertebrae, so that the liga- 

 ments which form as separate segments between the vertebrae, are able 

 to act on both the vertebra lying immediately anterior and immediately 

 posterior to each mesodermal segment, after the muscle has developed 

 in connection with these ligaments (Figs. 305, 352). 



The notochord becomes segmented and surrounded by cartilage, the 

 notochordal segments form the soft centrum of the vertebrae, and prob- 

 ably also portions of the intervertebral discs. 



The ventral cartilages now grow around the sides of the notochord 

 and meet to fuse with the dorsal series. 



The transverse processes of the vertebrae grow out from the ventral 

 cartilages. It is toward the lateral ends of these that the transverse 

 processes of the ribs later develop. 



The neural arch is formed from outgrowths of the dorsal series 

 which grow inward beneath the neural cord and also dorsally and ven- 



