CHORDATA. 



325 



of the turtle). A difference between the internal and the 

 external skeleton is in the fact that bone of the former is 

 typically formed in and around cartilage, whereas in the latter 

 there is no cartilage. The internal skeleton consists of two 



FIG. 154. 



FTC. 154. Diagram of transverse section through embryo of a Vertebrate, showing 

 the mode of origin and the relations of Ihe notochord, nervous cord, ectoderm, ento- 

 derm and mesodeim (see also Fig. 13). coe., coelomic pouches; ect., ectoderm; ent., 

 entoderm; g, lumen of the gut; iv., invagination of ectoderm which forms the nerve 

 cord (see c, in succeeding figures); mes 1 ., somatic or body mesoderm; mes 2 ., splanchnic 

 mesoderm, that portion of the mesoderm which becomes allied with the entoderm; n, 

 the ne.rve (spinal) cord; n.c., notochord, arising by an outpocketing of the entoderm. 



portions, (i) the axial, embracing the vertebral column, and 

 (2) the appendicular, or that supporting the appendages. 



341. Axial Skeleton. In its simplest condition this con- 

 sists of the notochord which it will be remembered is derived 

 from the entoderm and lies between the alimentary canal and 

 the spinal cord (Fig. 154). In the true vertebrates, cells aris- 

 ing from the mesodermal pockets on either side (Fig. 156) 

 produce a continuous skeleton- forming sheath about the noto- 

 chord. From the cells of this sheath are developed, finally, 

 rings of cartilage or bone about the notochord (centrum; 

 plural, centra, Fig. 157, c} and about the spinal cord (spinous 

 processes, Fig. 157, na). These, with certain other growths, 

 constitute the typical vertebrae. In this process the notochord 



