xiii.] THE FROG. 253 



^. Scrape gently, with a scalpel, one of the pro- 

 minences in the roof of the frog's mouth be- 

 neath the eye-balls: mount the scrapings in 

 075 salt solution and examine with -J objec- 

 tive for individual ciliated cells ; note their 

 roundish form, granular protoplasm and nu- 

 cleus, and the group of cilia borne on one 

 end ; stain with iodine. 



b. Cartilage. 



i. Dissect out carefully the omosternal or xiphisternal 

 cartilage of a recently killed frog; mount in 075 J 

 sodic chloride solution and examine with ^ or | obj. 

 a. Large roundish granular cartilage-cells will 

 be seen imbedded in a structureless or very 

 finely granular matrix, which is more refrac- 

 tive next the cells than elsewhere, and causes 

 the appearance of a sort of halo round each. 



(3. In each cell lies a distinct granular round 

 nucleus or sometimes two nuclei, containing 

 large highly refractive molecules. 



y. If the preparation be carefully made, each 

 cell will at first completely fill the cavity of 

 the matrix in which it lies, but if it be kept 

 some time or be treated with distilled water 

 the cells contract and so leave a transparent 

 ring between their surface and the inside of 

 the cavity in which each lies. 



c. Bone. 



a. Examine a prepared transverse section of hard 

 bone (say a bit of the shaft of the humerus or 

 femur) with i inch obj. A bit of mammalian 



