Class 12th Biology 2012 Set2 Delhi Board Paper Solution

Figure 'f' illustrates ovulation. It represents the ovulatory stage of oogenesis. Progesterone is the ovarian hormone released during ovulation. Follicle stimulating hormone (FSH) and Luteinizing hormone (LH) are the pituitary hormone released during ovulation. In anticipation of receiving the fertilised egg, the endometrium of the uterus gets thickened and also the blood supply to the endometrium increases. In the figure, (c) stage represents the secondary follicle and the (h) stage represents the          degenerating corpus luteum.

   5.  Labeled diagram of the human ovum prior to fertilization.

                                                                         OR

The megaspore mother cell undergoes mitosis to form two nuclei which migrate to opposite poles, forming a 2-nucleate embryo sac. Further mitotic divisions lead to the formation of 4- nucleate followed by 8-nucleate stages of the embryo sac. In these mitotic divisions, nuclear division is not followed by cell division.

After the 8-nucleate stage, cell walls are laid down and a typical female gametophyte or embryo sac is formed. Among the eight nuclei, six are enclosed by cell walls and organised into cells, while the emaining two nuclei (called polar nuclei) are situated above the egg apparatus in a large central cell. Out of the six cells, three are grouped at the micropylar end, and constitute the egg apparatus made up of two synergids and one egg cell. The other three cells are located at  the chalazal end, and are called antipodals. Thus, a typical angiosperm embryo sac after maturity is 8-nucleated and 7-celled.