242 Basic Structure of Vertebrates 



specialization and efficiency in birds. Correlated with the greatly 

 increased protection afforded during development, relatively few eggs 

 are produced. 



Primitive mammals, as indicated by such surviving examples 

 as the duckbill and the spiny anteater, must have retained reptilian 

 methods of reproduction. The duckbill, a burrowing animal, deposits 

 the eggs (usually two) in the burrow. The anteater, producing usually 

 only one egg in a season, places the egg in a fold of abdominal skin, a 

 temporary marsupium, where it is carried and incubated by the 

 warmth of the body until the young hatches. The embryos of these 

 two mammals develop amnion, chorion, allantois, and allantoic and 

 yolk-sac circulations essentially as do reptiles. The one new thing 

 which these animals do is to provide the young with a convenient 

 source of food to serve for a time immediately after hatching. Milk, 

 produced by mammary glands (see Fig. 447) developed in and by the 

 abdominal skin, serves to prolong the period of dependence on maternal 

 food. 



All known existing mammals except the duckbill and spiny ant- 

 eater are viviparous. The minute eggs contain so little yolk that they 

 could never pass beyond the very early stages of development unless 

 additional food material were somehow provided. In the great majority 

 of mammals this is done by means of an organ which is one of the most 

 characteristic features of a mammal. The egg, liberated from the ovary 

 and fertilized, becomes caught and lodged in the superficial tissue of 

 the uterine wall. Here it passes into the early phases of development 

 and very shortly gives rise to an amnion, a chorion, and an allantois, 

 essentially similar to those structures as developed in reptiles and 

 birds. Curiously, in spite of the absence of any considerable amount of 

 yolk, a yolk-sac is formed although devoid of yolk. This is usually 

 interpreted as a relic of reptilian ancestors. The allantoic sac becomes 

 greatly expanded, more or less wrapping itself around the embryo, 

 and certain regions of it fuse with the adjacent chorion and enter into 

 a very peculiar relation to the uterine wall (Fig. 241). From the con- 

 joined allantoic and chorionic membranes grow out slender extensions 

 (villi) which penetrate more or less deeply into the adjacent uterine 

 wall. They may become more or less branched. These villi are highly 

 vascular, fetal blood circulating in them under the drive of the fetal 

 heart. The surrounding uterine tissue is likewise highly vascular. There 

 is, however, no open communication between the blood-vessels of the 

 villi and those of the uterine wall. But the fetal and maternal vessels 

 are so close together that materials readily diffuse from one blood 

 to the other. Dissolved food-substances and oxygen pass from the ma- 

 ternal to the fetal blood; waste materials and certain special fetal 



