REPRODUCTION 751 



(fixed mill) or remain free {floating mill). At the edge of the placental area 

 very few villi develop, leaving a circular channel called the marginal sinus. 

 This attachment of villi becomes marked from the third month on and 

 this is considered the beginning of placentation. From this time on to term 

 there is merely an increase in number of villi and vessels with thus a cor- 

 responding increase in the size of the placenta. (Fig. 330.) 



The Placenta. Of all the embryonic structures the placenta is the 

 most important. It is formed by the end of the third month, after which it 

 gradually increases in size up to the end of the eighth month, by which 

 time it is fully developed. It then measures from 1 8 to 24 cm. in diameter 

 and weighs from 400 to 600 grams. It is most frequently attached to the 

 upper and back part of the uterine wall. Though exceedingly complex 

 in structure it consists essentially of two portions, a fetal and a maternal. 



The fetal portion consists primarily of those villi on the chorion in relation 

 with the decidua basilaris. These structures gradually increase in size and 

 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 which varies in accordance with 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 



