104 



Basic Structure of Vertebrates 



tudinal supporting axis for the body, and its surfaces are thickly beset 

 with the attachments of the numerous muscles which effect the bending 

 movements of the body and of some muscles which move the limbs. 

 In all of the major movable parts of a vertebrate — the trunk, tail, 

 and paired limbs — skeletal structures constitute the axis of the part. 

 The muscles are disposed around the skeletal axis. The head can be 

 moved as a whole only in relation to the trunk. Even in such minor 

 parts as digits, the skeleton is axial and the muscles are external to it. 



Skeleton of Notochordal Axis 



There are vertebrates without backbone or vertebral column, but 

 every vertebrate possesses a skeletal structure which is the chief me- 

 chanical axis of the body. 



One of the earliest organs to develop in a vertebrate embryo is the 

 notochord, which appears as a median longitudinal cylindric rod of 

 cells lying dorsal to the embryonic digestive cavity and ventral to the 

 developing central nervous organs (Figs. 100, 138). In later stages each 



Fig. 100. (A) Diagrammatic transverse section of the body of a vertebrate 

 embryo at an advanced stage. The muscle-forming myotome is beginning to 

 extend into the ventral body-wall of the embryo, (c) Coelorn; (g) genital ridge; 

 (m) muscle derived from myotome; (mc) myocoele; (p) peritoneum; (pd) prone- 

 phric duct; (so) somatic layer (dermatome) of somite; (i>) advancing ventral border 

 of myotome. The finely dotted areas are occupied by mesenchyme. 



(B) Diagrammatic transverse section of the body of an adult vertebrate, (av) 

 Aorta; (c) coelom; (e) ectoderm; (ep) epaxial (dorsal) muscle; (g) gonad; (ha) 

 hemal rib; (hp) hypaxial (ventral) muscle; (i) intestine; (mes) mesentery; (n) 

 kidney; (o) omentum; (p) somatopleure; (r) rib; (so) (sp) splanchnopleure; (») 

 centrum of vertebra and, above it, neural arch containing spinal cord. (Courtesy, 

 Kingsley: "Comparative Anatomy of Vertebrates," Philadelphia, The Blakiston 

 Company.) 



