782 THE EXCRETORY AND REPRODUCTIVE SYSTEMS 



the developmental period from 8 to 10 days its tubular system is exceedingly 

 complex compared to that shown in figure 345G. After this period, it begins 

 to degenerate, and its function is taken over by the developing metanephric 

 kidney. 



d. Mammal 



As in the chick, the mesonephric kidney in many mammalian embryos is 

 a prominent excretory structure. However, in the rat, mouse, and certain 

 other mammals its function as an excretory organ is dubious, probably result- 

 ing from the fact that the placental connection in these forms is sufficiently 

 intimate to assume excretory functions. In the 10-mm. pig (figs. 261, 262), 

 it is a prominent structure, filling a considerable part of the coelomic cavity 

 on either side. In the human embryo, the condition is intermediate between 

 that of the pig and rat. It possibly functions as an excretory structure in the 

 human embryo. 



The renal unit or mesonephric tubule which is evolved within the nephro- 

 genic cord is similar to that of the bird. It develops from a condensed mass 

 of epithelium within the nephrotomic plate (nephrogenic cord). This con- 

 densed, S-shaped mass elongates, becomes canalized, and joins the meso- 

 nephric duct. The mesial end of the tubule, in the meantime, develops a 

 Malpighian body with its glomerules and vascular connections. The glandular 

 tube is a highly coiled affair and is associated intimately with the veins as 

 indicated in figure 344F. Collecting ducts, arising as evaginations of the meso- 

 nephric duct similar to those in the chick mesonephros, are formed. 



5. Development of the Metanephric Kidney 



The metanephric kidney is the later embryonic and adult form of the renal 

 organ in reptiles, birds, and mammals. As observed above, the mesonephric 

 kidney involves three structures: 



( 1 ) the urinary or Wolffian duct, 



(2) a series of collecting ducts which evaginate from the mesonephric or 

 Wolffian duct to connect with the renal units, and 



(3) the nephrons or renal units. 



These same relationships are present in the developing metanephric kidney. 



Fig. 347. Urogenital system relationships in various vertebrates. (A) Reconstruction 

 of 20.6 mm. embryo of Squalus acanthias. (Redrawn from Scammon, 1911, Ciiap. 12, 

 Normentafeln Entwichiungsgeschichte der Wirbeltiere, by F. Keibei, G. Fischer, Jena.) 



(B) Left side view of dissection of male pickerel, Esox Indus, showing reproductive 

 and urinary ducts and absence of a cloaca. (Redrawn from Goodrich, 1930, Studies on 

 the Structure and Development of Vertebrates, Macmillan and Co., Limited, London.) 



(C) Male reproductive system, ventral aspect, of the pigeon. (Redrawn from Parker, 

 1906, Zootomy, Macmillan and Co., Limited, London, The Macmillan Co., N. Y.) 



