XII. THE COMPARATIVE ANATOMY OF THE UROGENITAL SYSTEM 



The excretory organs of vertebrates consist of a pair of kidneys and their ducts and the 

 reproductive system of a pair of gonads or sexual glands and their ducts. These two systems 

 are generally closely united, since the reproductive organs use the ducts of the kidneys tc 

 convey their products to the exterior. Hence it is customary to consider the two systems 

 together as the urogenital (urinogenital) system. . 



A. EMBRYONIC ORIGIN AND EVOLUTION OF THE UROGENITAL SYSTEM 



i. Development of the kidneys. — The kidneys or excretory organs arise from the meso- 

 • mere; this it will be recalled is the small middle region of the mesoderm. Within the mesomere 

 there are first produced the tubules of the kidneys. These tubules arise through a hollowing 

 out of the tissue of the mesomere. The tubules of the kidney belong to the category of ducts 

 known as coelomoducts. 1 Each excretory tubule opens into the coelom (cavity of the hypomere) 

 by means of a ciliated funnel-shaped opening called a nephrostome, pursues a more or less 

 winding course in the mesomere and finally terminates in the collecting duct. At first there 

 is a pair of tubules to each segment, but later they increase greatly in number and become 

 compacted together into a definite body which is called a kidney. In connection with the 

 kidneys there are developed peculiar networks of capillaries which are known as glomeruli. Each 

 glomerulus consists of a knot of capillaries into which blood is brought by an artery and 

 conveyed away by a vein. This arrangement evidently serves to bring the blood into closet 

 contact with the cells of the kidneys. The glomeruli come into close relationship with the 

 kidney tubules. 



2. The kidneys and their ducts. — We have thus far spoken of the kidneys as if the kidneys 

 of all vertebrates were homologous organs. This is not, however, the case. On the contrary, 

 in the evolution of the vertebrates there have been present three more or less distinct excre- 

 tory organs, which are similar in structure and arise in a similar manner from the mesomere 

 but are nevertheless not identical (Fig. 60) . 



a) The pronephros: The first kidneys to appear in the chordate phylum are the paired 

 pronephroi (singular, pronephros) . The pronephroi arise from the anterior end of the mesomere, 

 far forward in the body and extend over several segments (Fig. 60A). They are composed oi 

 tubules, each pronephros possessing one tubule to each segment of the body. Each tubule 

 consists of a simple curved tube, opening at one end into the coelom by a nephrostome and at 

 the other end into a common duct, the pronephric duct, which collects from all of the tubules. 

 The pronephroi are present in the adult stage of only cyclostomes and a very few fishes; they 

 appear, however, as transient structures during the development of all other vertebrates. 

 In most fishes and Amphibia they are probably functional as excretory organs during embryonic 

 stages. In reptiles, birds, and mammals they appear during early embryonic development, 

 but do not develop completely, do not function, and soon disintegrate and disappear. Glome- 

 ruli are generally present in connection with the pronephroi but may be absent even ir 



1 It was formerly supposed and is commonly taught that the tubules are homologous with the 

 nephridia of the earthworm and other oligochaetes. It has been shown, however, that the nephridia 

 are of ectodermal origin, while the tubules of the vertebrate kidney are of mesodermal origin. It is 

 therefore desirable to adopt the terminology of Goodrich, designating as nephridia such excretor> 

 or genital ducts as are homologous with the nephridia of the earthworm; while those genital or .excre- 

 tory ducts or tubules which arise in the coelomic wall are named coelomoducts. 



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