54 COMPARATIVE ANATOMY 



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 yolk mass, a yolk-sac also, although 

 devoid of yolk, is formed. This is usually interpreted as a relic of reptilian 

 ancestors. The allantoic sac becomes greatly expanded, more or less 

 enwrapping 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. 89). From the conjoined 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 the 

 maternal blood vessels are so close together that materials readily diffuse 

 from one blood to the other. Dissolved food substances and oxygen pass 

 from the maternal to the fetal blood; waste materials and certain special 

 fetal substances of hormone nature pass from the fetal to the maternal 

 blood. By this placental relation between mother and young the nutrition 

 and respiration of the young animal are provided for through the usually 

 long period of intra-uterine development. 



Mammals show many variations in the mode of origin and details of 

 structure of the placenta. The marsupial mammals (Metatheria; the 

 kangaroo and its alHes) produce only a weakly developed and briefly 

 temporary placenta or none at all. Accordingly the development of the 

 young cannot proceed beyond what is made possible by the initial small 

 yolk supply plus what nutritive material may be absorbed by the embryo 

 and its investing membranes directly from the neighboring uterine tissues 

 and fluids. The young marsupial is therefore necessarily born at an early 

 fetal stage and while very small. The deficiency of the intra-uterine 

 arrangements is compensated for by the marsupium, a pouch formed by 

 a fold of abdominal skin. The mammary glands are within this pouch. 

 The very immature and quite helpless new-born young (in the great 

 kangaroo, Macropus major, being only about one inch long) is transferred 

 to the marsupium by the mother. The young becomes attached to one 

 of the mammary nipples and feeds passively, the milk being pumped in 

 by contraction of muscle about the mammary gland. This "mammary 

 fetus" inhabits the marsupium for a time which is usually much longer 

 than its period of intra-uterine development. For example, in the great 

 kangaroo the period of intra-uterine gestation is between five and six 



