(i) Nucleus of spermatid shrinks to form sperm head.
(ii) Acrosome is formed from the glomerulus of spermatid and rests like a cap on the head of the spermatozoa.
(iii) Mitochondria of spermatids form the nucleus of the sperm.
(iv) Centriole of spermatids forms axial filament and tail of sperm.

The process of transformation of spermatids into spermatozoa is called spermiogenesis. Viable and motile sperm are called spermatozoa.

The term spermatogenesis is formed from the Greek words sperma meaning sperm and genesis meaning origin. It means sperm production. The process by which sperm are produced in the testicles of sexually fertile males is called spermatogenesis. Sperm arise from germinal epithelial cells located in the seminiferous tubules. Somatic or Sertli cells provide nutrients to the sperm. Spermatogenesis process takes 60-70 days.

The term spermatogenesis is formed from the Greek words sperma meaning sperm and genesis meaning origin. It means sperm production. The process by which sperm are produced in the testicles of sexually fertile males is called spermatogenesis. Sperm arise from germinal epithelial cells located in the seminiferous tubules. Somatic or Sertli cells provide nutrients to the sperm. Spermatogenesis process takes 60-70 days.
Spermatogenesis is a continuous process. It is described under 4 headings-
1. Multiplication phase: Spermatozoa contain primary germinal cells. Primary germinal cells divide repeatedly to form new cells in the process of mitosis. As a result, the number of cells increases. This is called the numerical growth phase. The resulting cells are called spermatogonia or sperm mother cells.
2. Growth phase: Each spermatogonium grows in size by receiving nutrients. Its nucleus is also enlarged in size. In this condition the cells are called primary spermatocytes.
3. Maturation phase: Primary spermatocytes divide by meiosis to form haploid cells. These are called secondary spermatocytes. Each secondary spermatocyte divides by meiosis to form two spermatids. As a result, 4 spermatids are produced from one primary spermatocyte.
4. Spermiogenesis: The process of transformation of spermatids into spermatozoa is called spermiogenesis. Viable and motile sperm are called spermatozoa. Changes in spermiogenesis are:
(i) Nucleus of spermatid shrinks to form sperm head.
(ii) Acrosome is formed from the glomerulus of spermatid and rests like a cap on the head of the spermatozoa.
(iii) Mitochondria of spermatids form the nucleus of the sperm.
(iv) Centriole of spermatids forms axial filament and tail of sperm.

The word gametogenesis is formed from the Greek words gamos meaning germ cell and genesis meaning origin. It means gamete or gamete formation. Gametogenesis is the process by which gametes or germ cells are produced in sexually fertile individuals. Gametogenesis is of two types. namely-
1. Spermatogenesis
2. oogenesis

In the phase in which blood, mucus, endometrium and unfertilized ovum are destroyed and released into the vagina is called menstruation. If the egg is not fertilized within 36 hours of ovulation, it is destroyed and menstruation occurs. Its shelf life is 4-5 days.

The phase in which the fully formed endometrium contains the blastocyst is called the premenstrual phase. Its shelf life is 15-28 days. During this time progesterone and estrogen hormones are secreted from corpus luteum, nutrients (glycogen, lipids) increase in endometrial cells, endometrium becomes 5-6 mm thick and contains blastocyst.

The phase in which the endometrium grows and ovulation occurs is called the proliferative phase. Its duration is 7th-14th day. During this time estrogen hormone is secreted, ovarian follicles transform and endometrium becomes 3-4 mm thick.

After the end of one menstrual cycle, the endometrium returns to its normal state before another cycle begins. Its duration is 3rd-6th day after onset of menstruation. During this time, FSH and LH hormones are secreted, ovarian follicle growth accelerates and the endometrium becomes 1 mm thick.

Menstrual is the discharge of blood, mucus, endometrium and unfertilized ovum from the uterus after the destruction of an average of 28 days (24-32) in the sexual life of an adult woman. Menstrual cycle is the periodic occurrence of menstruation. The first cycle is called menarche. Menstrual cycle starts at the age of 12-15 years and continues till the age of 45-50 years. It has gonadotropic hormone effect.