33° 



ZOOLOGY 



by the dermal bones. The following regions may be described 

 as typical: 



1. Head region (skull) embracing the cranium or brain case 

 and its associated ventral arches including the bones of the face. 



2. Cervical vertebrae, located in the neck and lacking ribs. 

 Usually the anterior one or two are considerably modified. 



-ect. 



i— -fries'. 



Fig. 157. Diagram similar in position and lettering to Fig, 156, at a later stage, e, central canal 



of spinal cord. 

 Fig. isS. Transverse section of an embryo Vertebrate at a stage later than Fig. 157. 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 the figures 156 to 158. — 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? 



3. Dorsal vertebrae, in the thoracic region and bearing well- 

 developed ribs which may unite with a ventral bone, the 

 sternum. 



4. Lumbar vertebrae, following the dorsal vertebrae and not 

 bearing ribs. 



5. Sacral vertebrae, usually a small number of vertebrae, fre- 

 quently fused into one piece with which the girdles of the 

 posterior appendages unite. 



