xii.] THE EXCRETORY ORGANS. 4 8i 



the Fifth Lesson of " Elementary Physiology," 6. Here it 

 may be added that the true kidneys are not the primitive 

 urinary organs, but that the first- formed structures of this 

 kind are what are called the Woolffian bodies or primordial 

 kidneys. These Woolffian bodies are formed one on each 

 side of the line of attachment of the alimentary canal to the 

 spine. They appear very early, each as a series of trans- 

 versely extended tubuli, and on the outer side of each is a 

 duct which extends post-axially and enters that primitive 

 chamber at the hind end of the body (the cloaca) into which 

 the termination of the alimentary canal also opens. 



The kidneys arise behind the Woolffian bodies, and inde-' 

 pendently of them ; and the ducts which pass from the kidneys 

 (the ureters] also terminate, independently of the termination of 

 the Woolffian ducts, in the same primitive chamber common 

 to the alimentary and urinary systems. 



The Woolffian bodies are gradually absorbed, and disap- 

 pear as the kidneys become developed. The latter organs 

 are at first smooth and oval, but soon become divided, each 

 into about fifteen lobes. These subsequently coalesce, but 

 even at birth the kidney shows signs on its surface of its 

 previous lobulated condition. 



10. In that man possesses distinct urinary or RENAL 

 ORGANS, he agrees with all Vertebrates, with the single 

 exception of the Lancelet, in which such parts have not 

 been yet certainly determined. 



In that Woolffian bodies are formed, man agrees with all 

 other Vertebrates with the exception just referred to. 



In that Woolffian bodies are subsequently replaced by true 

 kidneys, man agrees with all Mammals and all Sauropsi- 

 dans ; but it may be that Woolffian bodies persist, and by 

 themselves alone constitute the renal organs through the 

 whole of life, in Ichthyopsidans. 



The renal organ may exist as a very elongated body made 

 up of a series of bodies analogous to Malpighian corpuscles, 1 

 connected at short intervals, as in the Myxinoids. 



The urinary gland may extend on each side from the head 

 to the opposite end of the abdominal cavity, as in the Stur- 

 geon, and the ureters may join together before their termi- 

 nation, as in the same last-menti-oned animal. 



The two urinary glands may blend together behind the 

 pharynx, as in many osseous Fishes ; or at their posterior 



1 For a description of these corpuscles see the Fifth Lesson of " Elementary 

 Physiology," 9, Fig. 27. 



I I 



