Immediately after ovulation, the mammalian egg is covered by a membrane known as the corona radiata.

The corona radiata is a layer of cells that surrounds the oocyte (immature egg) and is formed by the follicular cells that were surrounding the developing egg in the ovary. It is one of the protective layers that surround the oocyte as it travels through the fallopian tube towards the uterus.

Here are some key points about the corona radiata:

1. Formation:
- The corona radiata is formed during folliculogenesis, the process by which a follicle matures and releases an egg.
- As the follicle grows and matures, the granulosa cells that surround the oocyte form multiple layers.
- These layers of granulosa cells become the corona radiata.

2. Structure:
- The corona radiata consists of several layers of granulosa cells that are tightly packed together.
- These cells are rich in cytoplasm and are connected to each other by gap junctions, allowing them to communicate with the oocyte.

3. Function:
- The corona radiata serves as a protective barrier for the oocyte.
- It provides nourishment and support to the oocyte during its journey through the fallopian tube.
- The corona radiata also plays a role in the fertilization process. The cells of the corona radiata can be penetrated by sperm cells, allowing them to reach the oocyte for fertilization.

4. Dissipation:
- Once the oocyte is fertilized, the corona radiata starts to dissipate.
- This allows the fertilized egg, now called a zygote, to continue its journey towards the uterus for implantation.

In conclusion, immediately after ovulation, the mammalian egg is covered by the corona radiata, a protective layer formed by the granulosa cells surrounding the oocyte. This membrane provides nourishment and support to the oocyte and plays a role in the fertilization process. After fertilization, the corona radiata starts to dissipate, allowing the zygote to implant in the uterus.