Elements of biology, with special Elements of biology, with special reference to their rôle in the lives of animals elementsofbiolog00buch Year: 1933 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 a
Elements of biology, with special Elements of biology, with special reference to their rôle in the lives of animals elementsofbiolog00buch Year: 1933 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 Comp
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