352 



THE VERTEBRATE BODY 



below the bladder, and the urethra, therefore, carries both urine and the 

 semen containing the sperms. 



Physicians often analyze the chemical nature of urine as a means of 

 diagnosis of disease. This gives valuable data about the function of 

 other organs as well as the kidneys. For example, sugar in the urine is 

 frequently a sign of disease of the pancreas called diabetes. In addi- 

 tion to the urea and uric acid present in normal urine, the excess salts 

 in the diet are discharged by way of the kidneys. These include salts 



Cortex 



Medulla 



Renal papilla 



Renal sinus 



Ureter 



Fig. 24.3 Diagram of a section through a kidney and its connecting ureter. 



of sodium, potassium, calcium, and magnesium ; all of which are present 

 in greater quantities in the urine than in the blood. 



The Reproductive System. The frogs are the first of the chordates 

 to adapt themselves to life out of the water. They have been fairly suc- 

 cessful in every system except reproduction, for in this system they are 

 like the water animals. Water is required to transfer the sperm to the 

 eggs and since the eggs cannot hatch on the land, they must be laid in 

 the water. 



Frogs are ordinarily individualists and pay little attention to other 

 members of their race, except perhaps to eat them if one small enough to 

 swallow is found. However, during the spring they become gregarious 



