Question

Which of the following statements regarding human development is correct?
A. Ovulation occurs at the primary oocyte stage.
B. Chorion develops from the inner cell mass.
C. Embryonic disc develops from the trophoblast.
D. Primitive streak is formed in the post part of the embryonic shield.

Answer 1

Hint: The epiblast, a single epithelial layer of the bilaminar embryonic disc that gives rise to the primitive streak, is the source of all embryonic material. The primitive streak shapes a longitudinal midline structure on the dorsal side of the developing embryo in the narrower caudal (posterior) area.

Complete Step by Step Answer:
When a follicle matures and becomes a secondary oocyte, the primary oocyte starts its primary meiotic division. Even though the second meiotic division has not occurred, the follicle splits and secondary oocytes are released shortly afterward in the Fallopian tube. Ovulation refers to the release of a secondary oocyte from the ovaries. This contradicts statement I. Therefore, option A is incorrect.
In mammals, the chorion is the outermost fetal membrane surrounding the embryo (amniotes). It forms from an outer fold on the exterior of the yolk sac, located outside the zona pellucida (in mammals).
The blastocyst grows in size, and the embryonic disc takes on an oval shape. This is preceded by cell ingression into the space between the epiblast/trophectoderm as well as the hypoblast. Ingressing cells form mesoderm, which moves rapidly into the extra-embryonic region backside to the embryonic disc.
 The primitive streak is a structure that forms in the early embryo. The embryonic disc becomes oval, then pear-shaped, with the broad end facing the anterior and the narrower region facing posteriorly. The primitive streak is a longitudinal midline structure that forms on the dorsal side of the developing embryo in the narrower posterior (caudal) region. The primitive streak initially broadens for half the length of the embryo. This appears in the human embryo by stage 6, around 17 days. This leads to the conclusion that the fourth statement is correct.
The correct option is D.

Note: The embryoblast is responsible for the formation of the bilaminar disc and amniotic cavity. The embryoblast divides into epiblast and hypoblast. They are collectively called the bilaminar disc. The process starts around the eighth day of pregnancy. The existence of the primitive streak will create bilateral symmetry, determine the site of gastrulation, as well as start the formation of the germ layer.