**C. Development**1. **Label the following embryo development stages and cells**: zygote, blastocoel, embryoblasts, blastomeres, morula, blastocyst, zona pellucida, trophoblasts. - **Diagram Descriptions**: 1. **First Image (Zygote)** - Illustration of a single cell with a prominent nucleolus. - Labeled as "Stage: Zygote." 2. **Second Image** - Two-celled structure. - Labeled with "zona pellucida." 3. **Third Image (Morula)** - A cluster of cells resembling a mulberry. - Labeled as "Stage: Morula." - Cells are indicated as "blastomeres." 4. **Fourth Image (Blastocyst)** - A hollow, spherical structure with an outer layer and clusters inside. - Labeled with "trophoblast," "blastocoel," and "embryoblast." - Labeled as "Stage: Blastocyst."These diagrams illustrate the key stages and structures during early embryonic development.

Introduction Life starts from single cell called Zygote. Zygote is the single diploid cell resulted from the fusion of haploid male (Sperm) and female (Ova) gametes. There are usually four stages of development of zygote: Fertilization (Occurs in Fallopian tube) Cleavage (Start after fertilization of the ova) Implantation (occurs in the lining of Uterus) Gastrulation (Formation of germ layers)Fertilization Fertilization refers to the process when there is fusion of male and female gametes which leads to the formation of diploid cell called Zygote. Fertilization can occur either inside the female body (Internal fertilization) or outside the female body (External fertilization). In humans this fertilization occurs inside the female genital tract in the Fallopian tubes. Sperms (male gametes) when released during copulation travels through the vagina and uterus and get fused to female egg (Ova) in the fallopian tubes. This stage of development is called Fertilization of eggs. Cleavage Cleavage refers to the rapid mitotic cellular divisions without significant growth. During the cleavage zygote undergo rapid cell division. These cells have either very short or no growth phase G-phase of mitotic cell division. In cleavage single celled Zygote undergo cell division and produce two celled which further divides and forms 4 celled stage which later develops into 8 celled stage. These 8 celled stage is called Morula which is like solid ball of cells. After several cell division morula develops into Blastula stage. Blastula is around 32 cell stage which is hollow ball consist of cavity called Blastocoel. This blastula stage can have up to 100 cells. Later on, this blastula develops into Gastrula. Implantation Implantation refers to the attachment of growing embryo to the uterus wall. Implantation usually occurs at blastula stage. Blastocyst gets attached to the endometrium (inner lining) of Uterus. After the implantation further process of development occurs such as Gastrulation. Here, after implantation the formation of special organ called Placenta occurs. Placenta helps in nutritional and gaseous exchange between the developing foetus and the mother. The foetus is connected to the placenta via Umbilical cord. Gastrulation The process of formation of gastrula from blastula is called Gastrulation, during gastrulation cells organize themselves in the layers. Cells shows migration and arranged themselves in three germ layers termed as Ectoderm, mesoderm and Endoderm. These three-germ layers then later give rise to the formation of different organs. For example, ectoderm leads to formation or cornea of eyes, brain etc. while mesoderm forms heart, connective tissues, muscles etc, endoderm forms visceral organs like digestive tract etc. Therefore, the First figure is of Zygote contain a single cell, second figure is of Two celled stage (Zygote after single cell division) Third figure represents the morula stage as it is solid mass of cells after cleavage. These cells are called as Blastomeres. Fourth figure represents Blastocyst stage which is filled with blastocoel cavity having the cell mass which is known as Inner cell mass and a single layer of cells known as Trophoblast cells.