REPRODUCTION AND DEVELOPMENT IN CHORDATES 



which is formed has a protective function comparable to that in reptiles and 

 birds (Fig. 5.255). The chorion, however, is the layer next to the tissues 

 of the mother, and in man, for example, becomes concerned directly with 

 nutrition, gas exchange, and excretion. An allantois appears during the 

 development of the human embryo but has no function. A rich supply of 

 blood vessels in the chorion is connected with the vessels of the embryo's 

 body by way of vessels in the umbilical cord (Fig. 5.26). The chorion is 

 covered with villi, or finger-like processes, that extend into blood-filled spaces 

 in the uterine wall (Figs. 5.25/1 and 5.27). That part of the wall of the 

 uterus in which the young human embryo becomes embedded, together with 

 extensions of the chorion, constitutes the placenta. In the placenta the 

 blood of the embryo is everywhere separated from the blood of the mother 

 by the cells of the chorion, through which diff^usion of nutrients, oxygen, and 

 waste products occurs. There is no mechanism for regular exchange of blood 

 between mother and embryo, although a certain amount of seepage probably 

 occurs. In this connection it should be stated that the somewhat widespread 

 ideas of prenatal impressions are entirely without foundation. The attach- 

 ment between mother and embryo is such that diffusible substances carried 

 by the blood can pass from one to the other. There is, however, no mech- 

 anism for the transfer of emotional conditions, unless an effect on nutrition 

 might be so produced, and no nervous connections exist whereby reactions of 

 the mother to unpleasant sights can aff"ect the embryo. Only metabolic con- 

 ditions can be reflected by the development of the embryo. In very few 



Uterine vein 



Placental septum 



Uterine blood space 



Uterine 

 artery 



Uterine 

 wall 



Fig. 5.27. A portion of the human placenta, in section, showing the relation between the 

 capillaries of the embryo, which are continuous with the umbilical arteries and veins, and 

 the blood spaces of the mother, which are continuous with the uterine arteries and veins; 

 diagrammatic. (Redrawn from L. B. Arey, Developmental Anatomy, copyright 1930 by W. B. 

 Saunders Co., printed by permission.) 



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