Fertilization and embryo development up to blastocyst formation

Correct answer: B. Sperm and secretions from accessory reproductive glands

Explanation: Semen is the ejaculated mixture of spermatozoa and fluids secreted by accessory reproductive glands, especially the seminal vesicles, prostate gland, and bulbourethral glands. These fluids provide nutrients, suitable chemical conditions, lubrication, and a medium for sperm transport. Urine, blood, or hormones alone do not constitute semen, so option B is correct.

Correct answer: A. Zygote

Explanation: Fertilisation is the fusion of a haploid sperm and a haploid secondary oocyte. This fusion restores the diploid chromosome number and produces the single first cell of the new organism, called the zygote. The zygote then undergoes cleavage to form a morula, followed by a blastocyst; gastrulation occurs later. Therefore option A is correct.

Correct answer: A. Cleavage

Explanation: Cleavage is the series of rapid, repeated mitotic divisions that begins after fertilisation. These divisions increase the number of cells, called blastomeres, without causing a corresponding increase in the overall size of the early embryo. Cleavage produces stages such as the morula and blastocyst. Gastrulation reorganises cell layers later, implantation is attachment to the uterus, and fertilisation has already occurred; therefore A is correct.

Correct answer: A. Morula

Explanation: The morula is the solid, compact, mulberry-like stage produced after several cleavage divisions of the zygote. It consists of a cluster of blastomeres. As development continues, fluid accumulates inside this cell mass and a cavity forms, producing the blastocyst. The zygote is the original single cell, while gastrula is a later developmental stage; therefore option A is correct.

Correct answer: A. Trophoblast

Explanation: The trophoblast is the outer cellular layer of the blastocyst. It surrounds the blastocoel, participates in attachment and implantation, and later contributes substantially to placenta formation. The inner cell mass lies inside and develops into the embryo proper. An acrosome is a cap-like structure on a sperm head, while the endometrium is the uterine lining, not a blastocyst layer. Thus, option A is correct.

Correct answer: A. Zygote

Explanation: Fertilization is the fusion of two haploid gametes: a sperm and the female gamete, usually the secondary oocyte. Their nuclei unite and restore the diploid chromosome number, producing the first cell of the new individual, called a zygote. The placenta develops later from embryonic and maternal tissues, while the corpus luteum and ovarian follicle are ovarian structures. Therefore, option A is correct.