Biology · Human Reproduction · NEET
Cleavage is the set of rapid mitotic (equal, mitotic) divisions of the zygote right after fertilisation. The zygote divides into 2, 4, 8, 16 cells. These daughter cells are called blastomeres. Important for NEET: cleavage divisions do NOT grow the embryo. The blastomeres get smaller each time because the whole thing is still trapped inside the zona pellucida, so the total size stays about the same as the zygote.
A morula is the embryo stage with about 8 to 16 blastomeres, packed like a solid ball. NCERT says the embryo with 8 to 16 blastomeres is called a morula. The name comes from the word for mulberry because it looks like a small berry. There is no fluid cavity inside a morula yet.
A morula is a solid ball of 8 to 16 blastomeres with no cavity. As the morula keeps dividing and moves into the uterus, it becomes a blastocyst. The blastocyst has a hollow fluid-filled cavity and its cells are now arranged into two groups: an outer trophoblast layer and an inner cell mass. So morula = solid, blastocyst = hollow with trophoblast and inner cell mass. This is a very common NEET single-line trap.
Both are found in the blastocyst. The trophoblast is the outer layer of cells; it attaches to the endometrium (uterus lining) and later helps form the placenta. The inner cell mass is a group of cells attached to the inside of the trophoblast; it differentiates into the actual embryo (and contains stem cells). Simple rule: Trophoblast = outer, attaches, becomes placenta; Inner cell mass = inner, becomes the baby.
Fertilisation happens in the ampulla of the oviduct. Cleavage begins as the zygote slowly moves through the oviduct towards the uterus. The morula also forms while still moving. The morula then transforms into a blastocyst as it moves further and reaches the uterus. So the journey is: oviduct (cleavage, morula) then uterus (blastocyst then implantation).
No. This is a classic NEET point. During cleavage the number of cells increases but the overall size does not increase. The zona pellucida still surrounds the embryo and holds it, so each blastomere is smaller than the one before. Growth in size starts only later, after implantation.
Assertion A: Endometrium is necessary for implantation of blastocyst. Reason R: In the absence of fertilization, the corpus luteum degenerates that causes disintegration of endometrium. Choose the correct answer.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The morula comes first. Order: zygote, then cleavage to blastomeres, then morula (8 to 16 cells), then blastocyst.
Blastomeres are the small daughter cells produced by cleavage of the zygote. The zygote divides to give 2, 4, 8, 16 blastomeres.
The inner cell mass becomes the embryo and contains stem cells. The outer trophoblast attaches to the endometrium and helps form the placenta.
Yes. The trophoblast attaches to the endometrium, and after implantation its chorionic villi help form the placenta.
Because each blastomere has the same diploid chromosome number as the zygote. Cleavage is mitosis, so no reduction in chromosome number happens.
