. Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus. Vertebrates -- Embryology; Comparative embryology. 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 ( two fertilized eggs) twins implanted close together. Observe two chorionic vesicles, an
. Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus. Vertebrates -- Embryology; Comparative embryology. 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 ( 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. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Nelsen, Olin E. (Olin Everett), b. 1898. New York, Blakiston
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