REPRODUCTION AND DEVELOPMENT 



277 



being within the tgg and independent of the maternal food supply. 

 In other viviparous forms, for example in certain viviparous fishes, 

 the quantity of yolk available appears to be insufficient for the proc- 

 esses of development and the young embryo develops organs that 

 adhere to the ovary or wall of the oviduct and absorb foods from 

 the blood stream of the adult. In the mammals, the egg is almost 

 devoid of yolk and other reserve foods, the major portion of its 

 nutriment being derived from the blood stream of the mother dur- 

 ing development. For this function as .well as for obtaining an 

 oxygen supply and for ridding 

 the developing embryo of the 

 wastes of metabolism a special 

 organ is developed, consisting 

 in large part of membranes 

 arising from the embryo but 

 also involving modifications of 

 the maternal uterine wall. This 

 organ is the placenta. A true 

 placenta is developed solely in 

 the higher mammals; hence 

 the class Mammalia include 

 both PLACENTAL and aplacental 

 mammals, grouped according to whether or not a placenta is formed. 

 Man is a placental mammal; marsupials (for example, the opossum) 

 and monotremes (example, the duckbill) are aplacentals. 



The Human Embryo and Its Nutrition. In the placental 

 mammals the transfer of food and oxygen from the blood stream 

 of the mother to that of the embryo takes place through the mem- 

 branes of the placenta. The human placenta is a disk-shaped struc- 

 ture (Fig. 189). The face of the disk apposed to the wall of the 

 uterus has fronds which penetrate deeply into the tissues of the 

 uterine wall, bringing the blood vessels of the embryo into intimate 

 contact with the blood of the mother. The maternal and embryonic 



UMBILICAL CORD 



Fig. 189. — A human placenta. 

 (After McMurrich: Development of 

 the Human Body, published by P. 

 Blakiston's Son and Company.) 



