STRUCTURE OF THE VERTEBRATES 



103 



velopmental tj'pes in the liigher vertebrates. The oldest, from 

 the standpoint of evolution, are the dermal bones which are 

 homologous with the dermal denticles of the dogfish, and in all 

 cases develop in the mesodermal layers of the skin. The others 

 are replacement bones which originate in cartilage. The carti- 

 lage is not transformed but is first destroyed, and bone is laid 

 down by bone building cells. This process is very gradual, much 

 skeletal cartilage being left in the new-born human, and the 

 replacement in the long bones is not complete until growth has 

 ended. Especially in the lower vertebrates, large regions are 

 never completely ossified. 



C. Fluid Tissues. The blood of the vertebrate is composed 

 of a fluid medium with cells, or corpuscles, floating in it as dis- 

 crete units. The fluid is the plasma. As most of the corpuscles 

 arise in the long bones, these tissues are often included as fluid 

 connective tissues. 



Blood corpuscles are of two main types: (1) red cells or 

 erythrocytes, containing the haemoglobin which has an affinity' 

 for oxygen; and (2) the white cells, or leucocytes, which are 

 ameboid in nature and assist in freeing the body of foreign par- 

 ticles. A more detailed description is given in the chapter on 

 the vascular system. 



D. Muscle Tissues are highly specialized for a contractile 

 function. In the higher vertebrates, at least, they lose the ability 



Smooth 







Striated 

 Fig. 44. Muscle Tissues. 



Cardiac 



