258 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



toneum, one on each side of the vertebral column and 

 on a level with the last dorsal and the upper two or 

 three lumbar vertebrae. They are held in position 

 by the renal vessels, by a loose areolar tissue that 

 surrounds them which contains much fat, and by 

 the abdominal pressure. Each kidney measures 

 four inches in length, two and one-half inches in 

 breadth, and one and one-fourth to one and one- 

 half inches in thickness. 



Their developmental history is rather complex 

 and can be but briefly given here. It involves the 

 history of the pronephros, mesonephros, and meta- 

 nephros, three sets of excretory organs which re- 

 place each other in the sequence in which they are 

 mentioned. 



1. The pronephros, or head kidney, develops in 

 connection with the nephrotomes of the first three or 

 four embryonic somites. These nephrotomes unite 

 with a longitudinal duct, the pronephric duct, 

 which opens posteriorly into the cloaca. With this 

 organ fluid from the celomic or peritoneal cavity 

 can be eliminated, and also waste products from the 

 blood, as a tuft of blood capillaries or glomerulus 

 is present near the peritoneal opening of each nephro- 

 tome. This kidney is exceedingly rudimentary in 

 mammals and functional only in larval stages of 

 amphibians and in bony fishes. 



2. The mesonephros, or Wolffian body, develops 

 in connection with the nephrotomes posterior to 

 those that form the pronephros. The pronephric 

 duct becomes the Wolfflan duct' and drains from the 

 peritoneal cavity in the same manner as in the head 



