Polyspermy means an egg has been fertilized by more than one sperms. The zona pellucida is modified by proteases. The proteases destroy the protein link between the cell membrane and the vitelline membrane, remove any receptors that other sperm have bound to, and help to form the fertilization membrane that prevents polyspermy.

Immediately after implantation, the inner cell mass of the embryo differentiates into an outer layer, called ectoderm and an inner layer, called endoderm.

The placenta secretes hormones like human gonadotropins, human placental lactogen, oestrogen and progesterone that are necessary to maintain pregnancy. Due to the secretion of these hormones, it is called an endocrine gland.

Differences between spermatogenesis and spermiogenesis are

Amoeba, Euglena; blastos.

1. Increase in size due to synthesis and accumulation of yolk.
2.Nucleus becomes bloated with nucleoplasm to form germinal vesicle.
3. Increase in number of mitochondria, and amount of ER and Golgi body.
4.Secretion of egg envelopes like vitelline membrane, zona pellucida etc.
5.Reductional division (meiosis-I) to transform diploid primary oocyte to haploid secondary oocyte and release of polar body.
6.Development of antrum with liquor folliculi in membrane granulosa.

Functions of Placenta.  The placenta serves primarily as an organ that permits the interchange of materials carried in the blood of mother and foetus.The main functions are:
1.Nutrition. Supply of nutrient material to foetus.
2.Respiration.Supply of \({ O }_{ 2 }\) to foetus and receives \({ CO }_{ 2 }\) back from it.
3.Excretion.Fluid nitrogenous waste products escape through the placenta.
4.Barrier. The placenta is a barrier like semipermeable membrane.
5.Storage. The placenta stores fat, glycogen and iron for the embryo the formation.

Foetal ejection reflex is neuroendocrine mechanism initiated by fully formed foetus and placenta which helps in parturition. It operates as below:
1.Foetus secretes hormone from its adrenal glands. These hormone stimulate the posterior pituitary of the mother to release oxytocin.
2.Oxytocin causes forceful contraction of smooth muscles of myometrium called labour pains, which further stimulates more secretion of oxytocin.
3.The stimulatory reflex between uterine contraction and oxytocin secretion continues which causes more forceful contraction of uterine muscles.
4.Uterine contractions pushes the young gradually out through the dilated cervix(caused by relaxin) and vagina.

Gametogenesis \(\rightarrow \) Insemination \(\rightarrow \) Fertilisation \(\rightarrow \) Implantation \(\rightarrow \) Gestation \(\rightarrow \) Parturition .