CHOEDATA 345 



forms at least a part of it returns twice, passing from the 

 heart to the lung, then back to the heart, and thence to the 

 system and back to the heart again. In Amphibia and reptiles 

 the blood from the lung and from the system mix somewhat 

 in the heart, because the partition between the right and left 

 sides is not complete, but in birds and mammals the two sides 

 of the heart are completely separated and the pure and impure 

 blood are not allowed to mix (see Figs. 169, 170). 



360. Excretion. — We have seen that carbon dioxid, one 

 of the waste products of the protoplasmic activity, is eliminated 

 through the lungs and skin. Water is similarly excreted. The 

 most important remaining waste {e.g., urea) contains nitrogen. 

 This is taken from the blood by means of the kidneys, a pair 

 of organs very complicated both as to structure and develop- 

 ment. They lie near the middle line of the body at the back 

 of the body cavity. Each gland represents a large number 

 bi nephridia or tubules similar in some respects to the seg- 

 mental organs of worms (Fig. 35), but much more compli- 

 cated. The kidneys are always well supplied with blood. 

 In fishes, amphibians, reptiles, and birds both arteries and veins 

 carry blood to the kidney; in mammals, only arteries. The 

 excretion, more or less in solution in water, is poured by the 

 tubules into a duct — the ureter — which may be the final outlet ; 

 or the ureters may empty first into a urinary bladder, which 

 has its own outlet (the urethra). 



361. Reproduction. — With certain exceptions among the 

 fishes the sexes are separate in the vertebrates. In development 

 the primary germ cells arise, in several groups of the vertebrates 

 (probably in all), from the entoderm of the intestine (mesen- 

 teron) and migrate into the germinal (mesodermal) epithelium 

 to the future position of the gonads. The eggs vary in size 

 from i^foo of an inch in mammals to 5 inches (ostrich), or more 

 in some extinct birds. The outlets for the ova and spermatozoa 

 {oviducts and vasa deferentia) are modified portions of the embry- 

 onic excretory and kidney ducts. Throughout the group there 

 is a close connection between the excretory and the reproductive 

 organs. The oviducts may have special glands for depositing 

 nutritive or protective material about the egg before or after 



