Ontogeny doesn't represent phylogeny means it doesn't relate with evolutionary relationship on the contrary it represents developmental history while phylogeny represents evolutionary relationship.

There is no evidence of the existence of gills during embryogenesis of mammals.

This statement is incorrect. During embryogenesis, mammals do exhibit the presence of gills. However, these gills are not functional and do not persist in the adult stage. This phenomenon is known as the branchial apparatus or pharyngeal arches.

Embryogenesis in mammals involves the development of various structures and organs. The branchial apparatus is a transient structure that appears early in development and gives rise to several important structures like the jaws, muscles, nerves, and blood vessels of the face and neck region.

The branchial apparatus consists of a series of paired arches, and each arch contains specific components. The first arch gives rise to structures such as the maxilla and mandible, while the second arch forms the hyoid bone. The third, fourth, and sixth arches contribute to the development of structures in the throat region.

One of the key features of the branchial apparatus is the presence of gill-like structures. These gills, called pharyngeal pouches, are present in the early stages of embryonic development in mammals. They are lined with specialized epithelial cells that resemble the gill epithelium found in fish and amphibians.

However, in mammals, these gills are not functional and undergo significant transformations during development. The first pharyngeal pouch gives rise to the middle ear cavity and the Eustachian tube, while the second pouch forms the palatine tonsils. The third and fourth pouches contribute to the development of the thymus and parathyroid glands, respectively.

Therefore, while there is evidence of the existence of gills during embryogenesis of mammals, these gills are not functional and are eventually transformed into other structures essential for mammalian development.

In summary, option a) "There is no evidence of the existence of gills during embryogenesis of mammals" is incorrect. Mammals do have gill-like structures during their embryonic development, but these gills are not functional and undergo transformation to form other important structures.