INTRODUCTION 771 



salts, wastes, and water are the by-products. The veins, lymphatics, and 

 arteries convey these substances to the areas of eUmination as follows: 



( 1 ) Carbon dioxide and water are residues of carbohydrate metabolism. 

 The carbon dioxide and some of the excess water in the body are 

 discharged through the respiratory surfaces. 



(2) The products of protein breakdown together with excess water and 

 mineral salts are conveyed mainly to the kidneys and are eliminated 

 there. 



Exceptional areas exist for the elimination of some of the above-mentioned 

 materials. For example, a certain amount of salts, nitrogenous wastes, and 



Fig. 344 — (Continued) 



segmentation is lost. The pronephric duct is formed through coalescence of the outer 

 distal portions of the pronephric tubules (see tubules 3, 4, and 5). The coalesced portion 

 thus formed grows caudally to join the cloaca. The mesonephric tubules, however, 

 appropriate the pronephric duct in a secondary manner, growing outward to join this 

 duct (see tubules 10 to 12). The pronephric duct, after this appropriation, becomes the 

 mesonephric or Wolffian duct. 



Figs. 344C-F are diagrams of different types of renal units (nephrons) which appear in 



developing vertebrate kidneys. 



(C) This diagram represents a form of renal unit which we may designate as Type I. 

 It is a vestigial tubule which may or may not become canalized. Its chief function is 

 to initiate the formation of the pronephric duct. It is found in the pronephric kidneys 

 of elasmobranch fishes, reptiles, birds, and mammals and, to some extent, in the anterior 

 portion of the mesonephric kidneys of these groups. 



(D) This diagram represents a renal unit found typically in the pronephric kidneys 

 of larval forms such as that of the frog tadpole. It is designated as Type II. it possesses 

 a ciliated nephrostome connecting with the coelomic cavity and a secretory portion which 

 joins the pronephric duct. 



(E) This diagram is given to represent the typical form of renal unit found in the 

 earlier phases of mesonephric kidney development of lower vertebrates. It is called Type 

 III. It is found also in the pronephric kidney of Hypogeophis (Gymnophiona) (see 

 Brauer, '02). With some modifications it may represent a type of renal unit found in 

 the adult kidney of the urodele, Necturus maculosus (see fig. 345D). 



(F) The Type IV renal unit is similar to Type III but lacks the ciliated nephrostomal 

 connection with the coelomic cavity. It is the later renal unit of the mesonephric kidney 

 of most fishes and amphibia and the typical renal unit found in the mesonephric kidney 

 of reptile, bird, and mammalian embryos. With some elaboration it would represent 

 the nephron (renal unit) found in the metanephric kidney of reptiles, birds, and mammals. 



G.I., G.2., G.3., stages in development of the mesonephric tubule in the embryo of 

 Squalus ucanthias. G.l. and G.2. the tubule arises from the nephrotome in a segmental 

 fashion and appropriates the pronephric duct. G.3. a later mesonephric tubule. In the 

 latter tubule the nephrostomal connection with the coelomic cavity is lost. Observe that 

 the tubule empties into the collecting duct, an outgrowth of the mesonephric duct. The 

 early primitive segmental condition is lost and many tubules are formed in each body 

 segment. 



