Read the passage and answer the following question:
The blastomeres in the blastocyst are arranged into an outer layer called trophoblast and an inner group of cells attached to trophoblast called the inner cell mass. The trophoblast layer then gets attached to the endometrium and the inner cell mass gets differentiated as the embryo. After attachment, the uterine cells divide rapidly and covers the blastocyst. As a result, the blastocyst becomes embedded in the endometrium of the uterus . This is called implantation and it leads to pregnancy. After implantation, finger-like projections appear on the trophoblast called chorionic villi which are surrounded by the uterine tissue and maternal blood. The chorionic villi and uterine tissue become interdigitated with each other and jointly form a structural and functional unit between developing embryo (foetus) and maternal body called placenta. The placenta facilitate the supply of oxygen and nutrients to the embryo and also removal of carbon dioxide and excretory/waste materials produced by the embryo. The placenta is connected to the embryo through an umbilical cord which helps in the transport of substances to and from the embryo. Placenta also acts as an endocrine tissue and produces several hormones like human chorionic gonadotropin (hCG), human placental lactogen (hPL), estrogens, progestogens, etc.
Which of the following statements about the embryo after implantation is incorrect?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → The mesoderm appears before the ectoderm and the endoderm are formed.
The passage stops at the placenta and its hormones; it does not describe how the embryo itself develops. After implantation, the inner cell mass (the embryo) forms the three germ layers. The ectoderm and endoderm form first, and the mesoderm appears later between them.
Option (1) Immediately after implantation, the inner cell mass differentiates into an outer ectoderm and an inner endoderm. — A correct statement. These two layers are the first to form from the inner cell mass, right after implantation.
Option (2) The mesoderm appears before the ectoderm and the endoderm are formed. — The incorrect statement. The order is reversed. The mesoderm appears after the ectoderm and endoderm, in the space between them. It cannot lie between two layers that do not yet exist.
Option (3) The ectoderm, mesoderm and endoderm give rise to all tissues (organs) in adults. — A correct statement. Every adult tissue and organ comes from one of these three germ layers.
Option (4) After one month of pregnancy, the embryo’s heart is formed. — A correct statement. The heart forms early, after the first month of pregnancy. From then on, the heart sound can be heard through a stethoscope.
NCERT Extract:
Pregnancy and Embryonic Development – “… Immediately after implantation, the inner cell mass (embryo) differentiates into an outer layer called ectoderm and an inner layer called endoderm. A mesoderm soon appears between the ectoderm and the endoderm. These three layers give rise to all tissues (organs) in adults. … In human beings, after one month of pregnancy, the embryo’s heart is formed. The first sign of growing foetus may be noticed by listening to the heart sound carefully through the stethoscope.”