208 OUTLINES OF CHORDATE DEVELOPMENT 



structures of the tadpole, merely indicating the trend of later 

 development. The later history of the skeleton falls largely 

 without our province. 



1. The Vertebral Column 



The formation of the notochord has been described pre- 

 viously, but its differentiation deserves a further word. The 



chorda cells early become flattened 

 antero-posteriorly, and about the time 

 the embryo begins to elongate, vacuoles 

 appear within the protoplasm of the 

 cells and also between adjacent cells. 

 The chorda becomes surrounded by 

 three sheaths. The primary or elastic 

 sheath is formed on the surface of the 

 chorda by the action of the super- 

 ficial chorda cells. The secondary or 

 fibrous sheath is formed within the pri- 

 mary sheath by the chorda epithelium, 

 which is composed of a layer of cells 



column in the body region siderably later a skeletogenous sheath is 



of a larva of Xenopus capen- -i i i , i , i i ,-1 



ma. After Schauinsiand. c, laid down outside the primary sheath; 

 Notochord; d dorsal ver- this is f orme d by the sclerotomal out- 



tebral cartilaginous arch; i, 



inner chorda sheath of scle- growths of the SOmiteS, whose forma- 



chlr^a^h^ath! r rperichon- tion was described above (Fig. 79). 

 drial connective tissue; v, The skeletogenous layer is continued 



ventral (hypochordal) ver- 



tebral cartilage. dorsally around the nerve cord, and 



it also extends a short distance later- 

 ally from the chorda, between successive myotomes. 



The vertebral column is formed within this skeletogenous 

 layer. First there appears (about 15 mm.) a metameric series 

 of cartilages, along the dorso-lateral surfaces of the chorda 

 in the base of the neural arch. A metameric series of cartilages 

 appears also in the skeletogenous layer along the median ven- 

 tral surface of the chorda (Fig. 79). The cartilages of the 

 dorsal series become united on each side, so that a pair of con- 





