50 THE FROG 



in the frog and in most higher vertebrates they sink beneath 

 the skin and come into closer relation to the more deeply 

 placed cartilaginous skeleton. In the frog many of the bones 

 of the skull, and in addition the clavicles, are membrane 

 bones. 



Recent opinion has tended to diminish the importance of 

 the distinction between cartilage bones and membrane bones, 

 since it is held that primitively all bone-forming cells origi- 

 nated in the skin; in the phylogenetic development of the 

 skeleton the cartilage bones were formed by cells (osteo- 

 blasts) that migrated inward, invading the cartilage. 



Obtain a very thin slice of clear, uncalcified cartilage 

 (hyaline cartilage) cut from the head of the femur of the 

 frog, mount it in a drop of water on a slide and cover it 

 with a cover glass; examine it first with the low and then 

 with the high power of the microscope. Observe the clear 

 matrix or intercellular substance, and the cell spaces each 

 containing one or more cells. Sketch a small portion, repre- 

 senting each cell about fifteen millimeters in diameter. 



Study a prepared slide showing cross-sections taken 

 through the middle of the femur of the frog; the material 

 has been decalcified by the action of an acid, and subse- 

 quently stained. In a section observe under low power the 

 broad outer ring or zone of compact bone encircling an inner 

 region of marrow. Examine the bony tissue under high 

 power and observe the bone cells, each in a small space 

 called a lacuna; in what direction does the longer dimension 

 of the bone cell extend? In a favorable preparation careful 

 focusing will reveal the canaliculae, minute tubules which 

 extend from the lacunae in a radial direction. Look for 

 faint striations parallel to the circumference of the section, 



