REPRODUCTION. 699 



number, and receive the ultimate branches of the umbilical arteries. The 

 maternal portion consists primarily of the decidua basilaris. As gestation 

 advances the placental villi rapidly increase in size and number, and re- 

 ceive the branches of the umbilical arteries. At the same time the decidua 

 basilaris becomes hypertrophied and vascular. With the continued growth 

 and development of these two structures they gradually fuse together and 

 finally become inseparable. In accordance with the needs of the embryo, 

 the decidua basilaris and its contained blood-vessels undergo certain histo- 

 logic changes which result in the formation of large cavities, sinuses, or 

 lakes, into which the blood of the uterine vessels is emptied. As the pla- 

 centa develops, the structures separating the blood of the mother from that 

 of the child gradually become modified until they are represented by a thin 

 cellular or homogeneous membrane. The conditions now are such as to 

 permit of a free exchange of material between the mother and child. Whether 

 by osmosis or by an act of secretion, the nutritive materials of the maternal 

 blood pass through the intervening membrane into the fetal blood on the 

 one hand, while waste products pass in the reverse direction into the maternal 

 blood on the other hand. Inasmuch as oxygen is absorbed and carbon 

 dioxid exhaled by the same structures, the placenta is to be regarded as 

 both an absorptive and a respiratory organ. So long as these exchanges are 

 permitted to take place in a normal manner the nutrition of the embryo is 

 secured. 



The Nutritive Supply of the Embryo. The growth and development 

 of the embryo from the period of fertilization to the period of birth require 

 a continuous and ever increasing supply of food materials. This is derived 

 from several sources and requires for its utilization, the development, in 

 different classes of animals, of specialized forms of the circulatory apparatus, 

 the relative importance of with varies in accordance when the source of 

 the food supply. These are known as the vitelline, the allantoic, and the 

 placental circulations. All these forms are present at successive stages in 

 the development of the human embryo but only the last is of major 

 importance. 



As the ovum passes down the oviduct it imbibes its nutritive material 

 from the mucosa. When it lodges itself in the uterus it probably receives 

 additional material in the same way. The period during which it does so 

 is, however, very limited. 



The Vitelline Circulation. The vitelline circulation, which in oviparous 

 animals, e.g., the chick, is of primary importance because of the large 

 amount of food stored in the vitellus or yolk, is in mammals of relatively 

 slight importance because of the limited supply of food in the vitellus. It 

 is nevertheless present in early stages. 



The Allantoic Circulation which in oviparous animals is also of primary 

 importance in the latter half of the incubation period both as an absorption 

 and respiratory apparatus is also present in mammals to a slight extent, 

 but it is merely a transition stage in the development of placental circulation. 



The Placental Circulation. The development of the fetal or placental 

 circulatory apparatus by which the fetus obtains its food supply and neces- 

 sary oxygen and frees itself from carbon dioxid has been alluded to in a 

 foregoing paragraph relating to the formation of the placenta. After the 



