978 HUMAN FETAL MEMBRANES. PLACENTA. FETAL CIRCULATION. 



(placenta prasvia). The last position is a dangerous one, because rupture of the 

 vessels at birth may cause death of the mother from hemorrhage. Implantation 

 of the ovum in the cervical canal is extremely rare. 



The umbilical cord may be inserted either into the center of the placental 

 disc (central insertion) or more toward the border (marginal insertion) ; or the 

 cord may be attached to the chorion laeve, so that the vessels must pass to the 

 placenta through the thin chorion laeve (velamentous insertion). Rarely there 

 is an accessory placenta separated from the placenta proper (placenta suc- 

 centuriata). Kolliker designates as marginate placenta one that has villi only at 

 its center. If the placenta consists of two halves it is known as duplex or bipartite 

 (constant in the apes of the old world). 



The umbilical cord (mature, from 48 to 60 cm. long and from n 

 to 1 8 mm. thick) is covered by the amniotic sheath. The vessels make 

 about 40 spiral turns (beginning after the middle of the second month) 

 passing from the embryo from left to right toward the placenta: they 

 consist of two arteries with a well-developed muscular coat and one 



FIG. 380. Section through the Uterus and the Attached Placenta at the Thirtieth Week (after Ecker): a, root and 

 insertion of the umbilical cord; b, amniotic covering of the umbilical cord; c, chorion; d, d, fetal portion of 

 the placenta; e, e, uterine wall; /, /, villous radiation forming the framework of the fetal placenta; g, g, de- 

 cidua; *, h, processes of the decidua penetrating into the fetal placenta; i, L branches of the uterine artery 

 tp, an artery entering the placenta: k, k, k, k, uterine veins. 



(left) umbilical vein. Both arteries anastomose in the placenta. In 

 addition, the cord contains the continuation of the urachus, the ento- 

 dermal portion of the allantois (Fig. 376, VIII, a), which, persisting 

 until the second month, is often atrophied later. The omphalomesen- 

 tenc duct can still be dissected at the time of birth near the umbilical 

 vesicle as a filamentous pedicle (VIII, D) of the umbilical vesicle (N) that 

 persists and as a rule is situated beyond the placental border. The ves- 

 icle contains in its interior small villi, squamous epithelium, and the 

 obliterated vessels of the first circulation. Persistent, though diminu- 

 tive omphalomesenteric vessels are rare. Wharton's jelly, a gelatinous 



