Excretory and Reproductive Systems 



TERTIARY COLLECTING TUBULES 



85 



SECONDARY 



COLLECTING 



TUBULES" 



PELVI 



Fig. 83. The anterior expanded portion of the ureter in a 20 nun human embryo, 

 showing the beginning of formation of collecting tubules. Anterior is to the right. 

 (After Huber. Courtesy. Neal and Hand: "Comparative Anatomy," Philadelphia, 

 The Blakiston Company.) 



79, 86), the hollow of the double-walled cup being occupied by a dense 

 network of fine blood-vessels, the glomerulus. The arrangement serves 

 to bring a large quantity of blood into close relation to the lumen of the 

 tubule, thus favoring diffusion of waste into the tubule. The corpuscle 

 is situated near the nephrostome of a mesonephric tubule and on the 

 coiled excretory segment of a metanephric tubule. 



Some perplexing problems arise in the study of this series of suc- 

 cessive vertebrate kidneys. The reason for the existence of so much 

 complication in the performance of a relatively simple function is far 

 from obvious. It seems likely that some very primitive vertebrates may 

 have had a segmentally arranged series of renal tubules extending 

 through the entire length of the abdominal cavity, a "holonephros," 

 and that the several types of kidney found in modern vertebrates have 

 resulted from local accentuation within the holonephros, accompanied 

 by a tendency toward degeneration of its then weaker regions. But it 

 still does not become clear why there should have been so much 

 shifting in the locality to be accentuated. It might be urged that the 

 extreme rear of the abdominal cavity is the appropriate place for such 

 an organ as a kidney, but in most adult mammals the metanephros has 

 somehow come to lie more nearly in the middle region of the cav- 

 ity. The doubt as to the interpretation of the extreme posterior exten- 

 sion of the "mesonephros" of fishes and amphibians has led to the 

 suggestion that this somewhat ambiguous kidney should be called an 

 ""opisthonephros." 



