326 



ZOOLOGY. 



often becomes almost obliterated by the developing vertebrae. 

 To each vertebra may be attached a pair of ribs, which protect 

 the ventral structures, somewhat as the neural arch protects 

 the nerve cord. The ribs of fishes and of the higher forms are 

 not considered to be homologous structures (Figs. 157, 158). 



FIG. 156. 



FIG. 155. Diagram similar in position and lettering to Fig. 154, at a later stage. 



c, central canal of spinal cord. 



FIG. 156. Transverse section of an embryo Vertebrate at a stage later than Fig. 155. 

 m, mesentery; sk., the beginning of the mesodermal skeleton which surrounds the 

 notochord (n.c.), and in part the spinal (nerve) cord, n. 



Questions on Figs. 154 to 156. How does the mesoderm originate in 

 vertebrates? Trace its gradual growth and differentiation in the figures. 

 What two principal portions are to be distinguished? How does the 

 notochord arise? How the spinal cord? What is the source of the cavity 

 of the spinal cord? From which of the three layers does the protecting 

 skeleton arise? What does the mesentery connect? What other organs 

 might be expected in the ccelom, if it were the purpose to make a 

 complete diagram of the visceral organs ? 



The axial skeleton varies from this typical condition in dif- 

 ferent parts of its course. In the head region, for example, 

 the nervous cord is immensely enlarged and the neural arches 

 are much modified, being replaced by plates and supplemented 

 by the dermal bones. The following regions may be described 

 as typical : 



