772 THE EXCRETORY AND REPRODUCTIVE SYSTEMS 



water pass off through the sweat glands of mammals; water and possibly 

 small quantities of salts and wastes find riddance through the tongue's surface 

 and oral cavity of dogs; and the salt-excretory glands in the gills of teleost 

 fishes remove excess salt materials from the blood, together with small 

 amounts of nitrogenous substances. On the whole, however, the kidneys 

 function to eliminate most of the nitrogenous residues and excess water, to- 

 gether with salt ions of various kinds, particularly those of chloride, sulfate, 

 sodium, and potassium. The dispatch of salt ions by the kidneys is all important 

 in maintaining the correct salt balance in the blood stream. 



3. Basic Embryonic Tissues Which Contribute to the 

 Urogenital Structures 



The basic, embryonic, cellular areas which contribute to the formation of 

 the excretory and reproductive structures are as follows: 



(1) the nephrotomic plate (intermediate-cell-mass mesoderm) (fig. 344A). 



(2) the adjacent coelomic tissue, underlying the nephrotomic plate during 

 its development, 



(3) the entodermal lining and surrounding mesoderm at the caudal end 

 of the digestive tube, and 



(4) the ectoderm of the integumentary areas where the urogenital openings 

 occur. 



(5) primordial germ cells. 



B. Development of the Excretory System 



1. General Description 

 The excretory system is composed of the following: 



( 1 ) a series of excretory units, known as nephric units or nephrons, 



(2) the kidney, a structure in which the nephrons are grouped together, 



(3) a series of collecting ducts from a particular region of the kidney, 

 which join the nephric units on the one hand and a main excretory 

 duct on the other, and 



(4) the cloaca (or its derivative, the urinary bladder) and a passageway 

 to the external surface of the body (figs. 345A, B, D; 348G, D). 



a. Types of Kidneys Formed During Embryonic Development 



The kidney in Greek is called nephros and in Latin, ren. The words nephric 

 and renal are adjectives, pertaining to the kidney but differing etymologically. 

 By adding a prefix to the word nephros, various types of kidneys are denoted 

 as follows: 



(1 ) Holonephros is a word that was introduced by Price ( 1896) and des- 

 ignates a kidney derived from the entire nephrotomic plate in which 

 a single nephron (nephric unit) arises from each nephrotome. (The 



