Q.What is the difference between a foetus and an embryo?

A healthy foetal heart beats, on average, at 120-160 beats per minute -- noticeably faster than an adult's resting heart rate.

During foetal development, the heart begins beating early in gestation and its rate is routinely monitored (by Doppler ultrasound or foetal heart-rate monitors) as an indicator of foetal well-being. The standard reference range used clinically and in NCERT/TN biology texts is 120-160 beats per minute; a rate persistently outside this range can signal foetal distress. The narrower ranges given in the other options do not correspond to the standard reference range.

Gastrulation converts the single-layered blastula into a three-germ-layered gastrula (ectoderm, mesoderm, endoderm).

During early embryonic development, the zygote undergoes cleavage to form a multicellular, single-layered hollow blastula (option b — this is cleavage, not gastrulation). The blastula then undergoes extensive cell movements — invagination, involution, and epiboly — that rearrange its cells into three distinct primary germ layers: outer ectoderm, middle mesoderm, and inner endoderm; this reorganisation is gastrulation, and the resulting embryo is called a gastrula. Option (c), attachment of the blastocyst to the uterine wall, is implantation, which occurs around day 7 after fertilisation, a separate event from gastrulation. Option (d), formation of specific organs from the primary germ layers, is organogenesis, which occurs after gastrulation is complete. Therefore only option (a) correctly describes gastrulation.

The polar trophoblast cells (Rauber's layer) — the trophoblast cells overlying the embryonal (inner cell) mass.

During blastulation, the trophoblast cells directly overlying the inner cell mass are called polar trophoblast (Rauber's layer).

As the mammalian blastocyst forms during blastulation, the outer layer of cells (trophoblast) differentiates regionally: the cells that lie directly over the inner cell mass (embryonal knob), which will later contribute to the developing embryo, are called the polar trophoblast (also known as Rauber's layer), while the trophoblast cells surrounding the rest of the blastocoel (away from the inner cell mass) are called mural trophoblast. The polar trophoblast plays an important role in implantation and later contributes to extra-embryonic membranes.

The foetoscope is the classic tool for auscultating (listening to) the foetal heartbeat during late pregnancy and labour.

A foetoscope is a specialised stethoscope-like device placed on the mother's abdomen to detect and monitor the foetal heart sounds and rate, traditionally used by birth attendants to track foetal wellbeing as labour progresses (modern practice has largely moved to electronic/Doppler foetal monitors, but the foetoscope remains the classic instrument for this purpose taught at this level). EEG records brain electrical activity (not relevant to monitoring a foetus in labour), while amniocentesis and CVS (chorionic villus sampling) are invasive prenatal diagnostic procedures used earlier in pregnancy to detect genetic/chromosomal abnormalities, not to monitor heart rate during labour.

(c) Heart — the heart, blood vessels, muscles and skeleton develop from the mesoderm germ layer; the retina and enamel are ectodermal, and the liver is endodermal.

The heart is mesodermal in origin.

During embryonic development, the three primary germ layers give rise to specific sets of organs. The mesoderm (middle germ layer) gives rise to the circulatory system (heart, blood vessels, blood), muscles, bones/skeleton, kidneys, and the dermis of skin. In contrast, the retina (part of the eye, an outgrowth of the brain) and the enamel of teeth are ectodermal derivatives, while the liver (and other digestive glands) develops from the endoderm. Among the given options, only the heart is mesodermal.

Correct answer: (d) progesterone

Progesterone maintains the uterine lining in early pregnancy; its deficiency can lead to loss of that lining and abortion.

Progesterone, secreted initially by the corpus luteum and later by the placenta, is essential for maintaining the endometrium (uterine lining) in a thickened, vascularised, secretory state suitable for implantation and continued support of the developing embryo, and it also suppresses uterine contractions and further ovulation during pregnancy. If progesterone levels are insufficient during the first trimester, the endometrium cannot be adequately maintained, which can result in its breakdown and consequent abortion/miscarriage. Aldosterone regulates sodium/water balance, testosterone is the primary male sex hormone, and oestrogen, while also important in pregnancy, is not primarily responsible for maintaining the endometrial lining in the way progesterone is — hence progesterone deficiency is the answer most directly linked to first-trimester abortion.

Correct answer: (c) Rh-ve

Erythroblastosis foetalis arises when an Rh-negative mother carries an Rh-positive foetus and develops anti-Rh antibodies.

Erythroblastosis foetalis (haemolytic disease of the newborn) occurs when an Rh-negative mother is carrying an Rh-positive foetus (having inherited the Rh antigen from an Rh-positive father). During delivery (or any event causing fetomaternal blood mixing), some Rh-positive foetal blood cells enter the mother's Rh-negative circulation, stimulating her immune system to produce anti-Rh antibodies. These antibodies do not usually affect the first pregnancy but, in a subsequent pregnancy with another Rh-positive foetus, can cross the placenta and destroy the foetal red blood cells, causing severe anaemia, jaundice or even foetal death. This condition therefore specifically requires the mother to be Rh-negative.