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THE NEW INDIVIDUAL 



Part IV 



Fig. 19.14. Photograph of human embryo and sacs, in the eighth week of de- 

 velopment; the chorion has been cut away to show the embryo, about one half inch 

 long. The two sacs, amnion and chorion, are roomy and fluid-filled. In its natural 

 position, the whole chorion is covered by the tissue of the uterine wall in which it 

 first became embedded. The exchange of gases, food and waste between the blood 

 of the mother and embryo occurs through the walls of the finger-like villi of the 

 chorion that look so feathery in this figure. The left eye, hand, and leg of the em- 

 bryo are clearly recognizable. (Courtesy, Department of Embryology, Carnegie 

 Institute of Washington.) 



The Chorion and Associated Membranes. The life processes of the embryo 

 depend upon the chorion with its specialized part the placenta in mammals 

 and in reptiles and birds with its associated sac the allantois. In birds, the 

 amnion and chorion arise simultaneously from a fold of the extended body 

 wall that first appears in front of the head and then encircles the embryo 

 with its edges closing together as if pulled by a drawstring. The inner part 

 of the fold becomes the amnion, the outer part forms the chorion. They are 

 united for a short time at the meeting place of the folds but the delicate join- 



