CHAPTER LVI 

 THE SECRETION OF URINE 



IN unicellular organisms of low activity there is no special structure 

 for the excretion of the waste products of metabolism; the general 

 surface of the cell serves as a medium for their discharge. In one- 



COLLECT: TUBE. 



URETERIC 8UD 



NE.PHWIC BUD 



LOOP of 



HEINLE 



COLLECTING TUBES 



CROWING Er.o 

 (uret*-ic) 



NE.PHRIC TUBULE 



GLOMERULUS 



.COLLECT: TUBE: 

 (Uretcric) 



(B) 



FIG. 239. ILLUSTRATING THE DEVELOPMENT OF THE RENAL TCBCLK. (Keith, 



after G. C. Huber.) 



A , Growing end of collecting tubule with nephric bud attached to it. B, First stage 

 of development of nephric bud into nephric tubule. C, Fully developed r.'nal 

 tubule. The part formed from the ureteric bud is represented in outline ; the 

 part from nephric tubule is shaded. 



celled organisms of greater activity there exist special contractile- 

 vacuoles, which from time to time expel from the cell waste fluid, 

 which may contain solid particles. In such a fluid the presence of 

 uric acid has been demonstrated. 



