EXISTING ORGANISMS— EMBRYOLOGY 55 



Higher Fishes. In the higher fishes similar parts appear, but 

 the cartilage is largely replaced by bone. To the chondrocranium, 

 which also becomes bony, are added other bones which form a dorsal 

 shell to enclose the head (Fig. 36) . These bones are called dermal 

 bones, and are not formed of cartilage at any stage. The vertebral 



Y/QSs^ /^JLjgy ^^at Hypostome 



Gastrovascular cavity t^^Ctt^ Ptch^p «^«^ltr^ ... 



^aaj/ir^^ Al^^^T J^ ^ Tentacle 



Polar bodies ^r-''^^^0 /-^^^ .^^^^^^^ 



Mature egg -|? ^ l~~'Z) W»>i3j /"/K-Xr^xif^ 



Blastula , 



Gastrula 1 



-Young eggs 



■ Basal disc 



Fig. 33. — A longitudinal section of Hydra. Not all of the structures shown 

 occur on one animal at the same time. (From Hegner.) 



column may remain much the same in form, although ossified. 

 The jaws, like the skull, are enclosed and strengthened by dermal 

 bones, and similar additions to the pectoral girdle occur. The 

 chief differences are the general tendency to ossification of the 

 cartilaginous structures and the addition of dermal bones. 



Other Classes. As the embryo develops in classes above the 

 fishes, the first evidence of skeletal structure after the notochord 

 is the formation of cartilages in various regions. In the head, for 



