EXCRETION, REPRODUCTION 21 7 



slender convoluted tube which in the primitive condition has a 

 funnel like the nephridium, but in the mature condition of the 

 mammal it is closed. Instead, however, a considerable part of 

 its wall is closely applied to complex knots and networks of 

 blood capillaries from which the secreting cells of the tubule 

 extract the nitrogen waste matter. The kidney tubules all 

 open into a common chamber from which a duct, the ureter, 

 leads to a reservoir, the urinary bladder. The nitrogenous 

 wastes leave the tissues with the lymph and thus are carried 

 back into the general circulation. In this way they reach the 

 kidneys. The chief waste drawn from the blood by the kidneys 

 is urea, CON2H4, but there are a number of other substances 

 eliminated in much smaller volume. 



493. The liver of the higher animals seems to have several 

 functions, one of which is excretion. The bile secreted by the 

 liver is a complex substance and its significance is not fully 

 understood. Its function in digestion is probably only a 

 secondary one. It contains waste matters taken from the blood 

 and these are eliminated through the intestine. 



494. The white blood corpuscles also assist in ridding the 

 body of useless or deleterious substances. 



REPRODUCTION 



495. Under favorable conditions an ama'ba will occasion- 

 ally divide into two similar parts. These parts then continue 

 to grow and after a time they also divide. This phenomenon 

 is one of the characters of the living cell. The impulse to divide 

 does not seem to depend upon any special external stimulus. 

 It is the normal consequence of growth under favorable condi- 

 tions. The process of division requires from a few minutes to 

 an hour from beginning to completion, and may be repeaterl 

 after a number of hours to several days. 



496. With regard to the details of the process of division there 

 are two types. In one case it is much more complicated than 



