THE ANIMAL CLIMAX-THE CHORDATES 337 



rise to the present mammalian population nourishment from the fluids of the mother, 



of the world. This was done by a temporary fusion of 



The presence of hair in place of scales the allantois and other membranes to 

 was an accessory structure necessary in a the uterine wall of the mother, forming 

 warm-blooded animal. Sweat glands aided a placenta (p. 533). At first the placenta 

 in regulating body temperature, which is must have been a very primitive affair and 

 nearly constant as distinguished from other only partially satisfactory in performing 

 animals, except birds, which have tempera- this important function of nourishing the 

 tures varying with the external environ- young. Consequently, the young were born 

 ment. In addition, the heart was a double in a very immature state and needed extra- 

 one: one for circulation through the body, uterine care. This was furnished by the use 

 the other for circulation through the lungs, of a belly pouch, called a marsupium, in 



One of the more important characteris- which the young could not only be pro- 

 tics that has contributed to the success of tected from the cold but also could receive 

 mammals is their method of caring for nourishment from the mammary glands 

 their genital products and subsequent off- through nipples located inside the marsu- 

 spring. All reptiles lay large, yolk-packed pium ( Fig. 13-56 ) . Later in their evolution, 

 eggs, whereas mammals typically have very more and more time was spent in the uterus 

 small eggs which are fertilized internally in and less in the marsupium until the latter 

 the female and go through a great part of was finally discarded as unnecessary. This 

 their early life inside the body of the is a possible explanation of how the pres- 

 mother. Although internal hatching of eggs ent mammalian reproductive system came 

 is not uncommon in lower forms (reptiles into being; it is recapitulated in the mam- 

 and fish ) , receiving nourishment from the mals of today from the duckbill to man. 

 mother during the process of development In certain parts of the world such as 

 is only rarely found ( for example, placental Australia, which were isolated during the 

 shark) in the animal kingdom. These early time the mammals were evolving (Eo- 

 mammals apparently found that it was bet- cene), there survives to this day two dif- 

 ter to have fewer offspring and retain them ferent types of primitive mammals which 

 longer within the body in order to give might help to bear out the narrative of the 

 them greater protection while allowing preceding paragraph. They belong to the 

 them, at the same time, to develop to a group known as Monotremes, which means 

 more advanced state, than to lay the eggs "one opening," so named because both the 

 as the reptiles did and rely on chance for urogenital and intestinal tracts open into 

 subsequent maturity. Furthermore, since a common cavity, the cloaca (a reptilian 

 mammals are so much more highly organ- character ) , with only one external opening, 

 ized, longer time was needed to develop the The spiny anteater (Echidna) and the 

 young to a stage where it could care for well-known platypus or duckbill (Ornitho- 

 itself . The transition took place very slowly rhynchis. Fig. 13-57 ) are the two examples 

 and over a long period of time. Even that remind us of what must have hap- 

 though fossil remains offer little real proof pened many millions of years ago when the 

 about such things, it is reasonably certain earliest mammals were struggling with this 

 that this process of caring for the young problem of caring for tlieir offspring. These 

 within the mother started very early in the animals lay large yolked eggs like the rep- 

 evolution of mammals. tiles and birds and they possess bills like 

 If the embryos were to remain within the the latter. They are partially warm-blooded 

 mother, it was necessary first of all that and the duckbill incubates its two eggs 

 some means be devised for them to receive ( Fig. 13-56 ) . When the eggs hatch the 



