376 HISTORY OF THE HUMAN BODY 



onic life. The metanephrotic system is not laid down at first 

 in the form of nephridia as in the other cases, but arises as a 

 blind canal or evagination from the mesonephrotic duct near 

 its lower end. This evagination, medial at its origin, comes 

 to lie dorsal to the mesonephrotic duct and develops anteriorly 

 until it comes in contact with the dorsal wall of the ccelomic 

 cavity, where it meets a mass of indifferent cells proliferating 

 from it. From the differentiation of this cell mass develop 

 numerous nephridia of a type similar to but in certain charac- 

 ters distinct from either of the other types, and from the re- 

 peated branching of the anteriorly growing canal there de- 

 velop collecting tubules with which the nephridia unite. The 

 expanded bases of the terminal branches of the tube form a 

 pocket or pelvis, which collects the fluid from the tubules. The 

 nephridia and collecting tubules form together the definitive 

 kidney, the metanephros, while the main tube, beginning with 

 its expanded pelvis, becomes the ureter. 



Each elementary unit of the metanephros, a metanephri- 

 dium, is like that of the previous system without the nephro- 

 stome. The connection with the circulatory system through 

 the glomeruli, which when first introduced was clearly a sec- 

 ondary function of the nephridia, becomes in the mesone- 

 phros of primary importance through the development of a 

 Bowman's capsule, and in the metanephros all direct connec- 

 tion with the ccclom is- given up. Here,* in addition to the 

 association with the circulatory system through the Bowman's 

 capsules, the tubules themselves become very long and at- 

 tenuated, and, as they are accompanied by a rich network of 

 capillaries, they are enabled to extract the waste products 

 through their entire length as well as at the localized renal 

 corpuscles. 



This history of the development of the metanephrotic sys- 

 tem, so different in origin from that of the other two, and yet 

 so similar in its results, has led to much speculation. It can- 

 not be supposed for a moment that nephridia so nearly alike 

 as those of the meso- and meta-nephros can have developed 

 independently, for that would involve also an independent 



