The placenta consists of two parts:

a larger fetal part derived from the                             and a smaller maternal part developed from the                        .

The two parts are held together by stem chorionic villi that attach to the                                      surrounding the chorionic sac, which attaches the sac to the                         .

The principal activities of the placenta are                (synthesis of glycogen, cholesterol, and fatty acids),                    gas exchange (oxygen, carbon dioxide, and carbon monoxide), transfer of nutrients (vitamins, hormones, and antibodies), elimination of waste products, and endocrine secretion(e.g.          ) for maintenance of pregnancy.

The fetal circulation is separated from the maternal circulation by a thin layer of extrafetal tissues, the                              .

This permeable membrane allows water, oxygen, nutritive substances, hormones, and noxious agents to pass from the mother to the embryo/fetus.

Excretory products pass through the                       from the fetus to the mother.

The fetal membranes and placentas in multiple pregnancies vary considerably, depending on the derivation of the embryos and the time when division of embryonic cell occurs.

The common type of twins is DZ twins, with two              , two             , and two               that may or may not be fused.

MZ twins, the less common type, represent approximately            of all twins;

they are derived from one zygote. MZ twins commonly have         chorion,         amnions, and         placenta.

Twins with one amnion, one chorion, and one placenta are always monozygotic, and their                            are often entangled.

Other types of multiple birth (e.g. triplets) may be derived from one or more zygotes.

The umbilical vesicle and allantois are vestigial structures; however, their presence is essential to normal embryonic development.

Both are early sites of                        and both are partly incorporated into the embryo.

                               also originate in the wall of the umbilical vesicle.

The                         and                      are vestigial structures; however, their presence is essential to normal embryonic development.

Both are early sites of           formation and both are partly incorporated into the embryo.

 Primordial germ cells also originate in the wall of the                         .

The amnion forms and amniotic sac for amniotic fluid and provides a covering for the                       .

The amniotic fluid has three main functions:

to provide a                            for the embryo/fetus, to allow room for                           ,

and to assist in the regulation of fetal                               .