REPRODUCTION 35 



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 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. 75). 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 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 means of this placenta, intervening 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 allies) 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 trans- 

 ferred 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 "mam- 

 mary 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 weeks, but the young kangaroo is carried in the pouch and nour- 

 ished by mammary glands for about eight months. 



