The caudal end of the embryo

Embryo

An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...

 is at first connected to the chorion

Chorion

The chorion is one of the membranes that exist during pregnancy between the developing fetus and mother. It is formed by extraembryonic mesoderm and the two layers of trophoblast and surrounds the embryo and other membranes...

 by a band of mesoderm

Mesoderm

In all bilaterian animals, the mesoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and endoderm , with the mesoderm as the middle layer between them.The mesoderm forms mesenchyme , mesothelium, non-epithelial blood corpuscles and...

 called the body-stalk, but with the formation of the caudal fold the body-stalk assumes a ventral position; a diverticulum

Diverticulum

A diverticulum is medical or biological term for an outpouching of a hollow structure in the body. Depending upon which layers of the structure are involved, they are described as being either true or false....

 of the yolk-sac extends into the tail fold and is termed the hind-gut.

The function of the body-stalk is later replaced by the umbilical cord

Umbilical cord

In placental mammals, the umbilical cord is the connecting cord from the developing embryo or fetus to the placenta...

.