FORMATION OF PROTECTIVE EMBRYONIC MEMBRANES 



923 



PL4CENTA 



Fig. 377. Human placentation. (A) Condition at about 4 weeiis. (B) About six 

 weeks. Villi disappearing on one side, while those of chorion frondosum are enlarging. 

 (A and B redrawn from Corner, 1944, Ourselves Unborn, Yale University Press, New 

 Haven, Conn.) (C) Placental relationships in dizygotic (i.e. two fertilized eggs) twins 

 implanted close together. Observe two chorionic vesicles, and two placentae. (D) Pla- 

 cental relationships in monozygotic (one fertilized egg) twins. Observe one chorionic 

 vesicle, two amnions, and one placenta. (C and D redrawn from Dodds, 1938, The Essen- 

 tials of Human Embryology, John Wiley & Sons, New York.) 



rionic vesicle remains within the lumen of the uterus (see Wislocki and 

 Streeter, 38). 



5. Implantation of the Pig Embryo 

 As in the human the passage of the cleaving egg of the pig through the 

 Fallopian tube is slow, consuming about 2V2 days. When the egg reaches the 

 uterus it still is surrounded by the zona pellucida and developmentally is in an 

 advanced state of cleavage or early blastocyst formation (fig. 145H). It re- 

 mains free in the uterine horn for about 6 to 7 days. During this period the 

 blastocyst enlarges and elongates at a rapid pace, particularly during the sixth 



