Ovum or female gamete is a larger nutritive cell. This is filled by yolk. Like an animal cell it also contains all cell organelles. Its size varies in different animals. Ovum is generally round/spherical in shape. It is covered by a thin membrane known as plasma membrane. Beside this, it is also covered by other covering membranes known as egg membrane.
These are divided into three-
1. Primary egg membrane- Secreted by ovum itself.
2. Secondary egg membrane- Secreted by ovarian tissue like follicular membrane.
3. Tertiary egg membrane- Secreted or generated by various regions of uterus or oviduct.
These membranes provide nutrition and protection for developing embryo.
Principal components of yolk are proteins, phospholipids and fats. On the basis of these components yolk can be distinguished as protein yolk and fatty yolk. Yolk can be found in the form of granular yolk (Echinodermeta), yolk platelets (amphibian) and yolk spheres (Birds) . Differentiation in oocyte establishes polarity of ovum (Animal and Vegetal pole). Pigment granules have no role in developmental process.
Author:-Dr. Priyanka Dadupanthi
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