98 



COMPARATIVE ANATOMY 



formed longitudinal duct which, as the pronephros degenerates, then 

 serves at least in part as the mesonephric or "Wolffian duct. In Anamnia 

 each mesonephric tubule has a ciliated nephrostome opening into the 

 coelom (Fig. 72). Near the nephrostome the tubule gives rise to a cup- 

 shaped expansion (Bowman's capsule). The hollow of the cup is occa- 

 sioned by ingrowth of a dense network of fine blood vessels, the glomerulus. 

 The capsule and glomerulus together constitute a renal (or Malpighian) 

 body or corpuscle. The part of the tubule between the renal body and 

 the mesonephric duct eventually becomes much elongated, coiled and 

 locally differentiated. At first there is but one pair of tubules per seg- 

 ment. Later additional tubules are ordinarily formed in each segment 

 by a process of branching from the primary tubules. 



Fig. 72. — Diagram of renal (Malpighian) corpuscle, a, artery; b. Bowman's 

 capsule; gl, glomerulus; n, nephrostome; t, nephridial tubule; v, vein. (From Kingsley, 

 Comparative Anatomy of Vertebrates.) 



In amniote vertebrates, in which the mesomere material ("inter- 

 mediate mesoderm") is not segmented and not hollow, the pronephric 

 and mesonephric tubules ordinarily do not possess nephrostomes or, at 

 most, exhibit merely vestiges of them. 



The amniote metanephros has outlet by way of a duct, the ureter, 

 which develops as a forward-growing branch from the cloaca! end of the 

 mesonephric duct of the same side of the embryo. The tubular structures 

 of the metanephros are formed partly from mesomere material lying 

 posterior to the mesonephros and largely by outgrowth from the anterior 

 end of the ureter. These tubules produce renal corpuscles similar to 

 those of the mesonephros but do not possess nephrostomes. In the 

 absence of nephrostomes drainage of waste from the coelom does not 

 occur and the function of excretion must be confined to the renal cor- 

 puscle, where the glomerulus brings blood vessels into close relation to 

 the lumen of a kidney tubule, and to other vascular regions of the tubule. 



The adult kidney consists of the entire system of tubules — meso- 

 nephric or metanephric — of one side of the embryo, increased to great 

 number by the branching process, each tubule tremendously elongated 

 and much coiled, the tubules bound together by connective tissue with 



