Question

1.Describe how the primary germ layers arise and give 2 examples of an organ or tissue that each develops into

2. In addition to the fetus, what other structures must the initial “ball of cells” formed after fertilization develop into? Name two “extraembryonic” structures and explain their role in the pregnancy.

Answer 1

1. Primary germ layers are ectoderm, mesoderm and endoderm. During gastrulation, the embryo is in the form of by lamina disc with epiblast on top and hypoblast on bottom. There is a prochordial plate at the cranial end of hypoblast.

Cells of epiblast migrate inwards and downwards and then differentiate. This movement of cells form a primitive streak which forms a primitive node on its cranial end. Then the primitive streak elongate by the growth of embryo. Cells of epiblast which differentiate into endoderm displaces the cells of hypoblast. Cells which remain in the epiblast differentiate into ectoderm. Cells of epiblast that differentiates into we sodium has their way in between ectoderm and endoderm. This formation of a tree laminar disc ends the phase of of gastrulation.

Ectoderm gives rise to skin and its appendages and nervous system. Mesoderm gives rise to muscles, bones, cartilage and blood components. Endoderm gives rise to digestive tract and respiratory tract.

​​​​​​​2. In addition to foetus the cells also form amniotic membrane with a cavity, yolk sac, placenta, umbilical cord, chorion and allantoic membrane.

The two extraembryonic structures are as follows:

i) Amniotic cavity with fluid.

ii) Chorionic membrane.

Amniotic fluid which is present in the amniotic cavity protect the foetus by acting as a shock absorber in case of any injury to the abdomen and allows easy movements of foetus. Swallowed amniotic fluid helps in the formation of gastrointestinal tract.

​​​​​​​Chorionic membrane consists of blood vessels of foetus and chorionic villi with these blood vessels come in contact with maternal blood, thus involving in maternal-fetal exchange of nutrients and metabolites forming a feto-placental unit.