There will have to be at least 300 units of Vitamin A present in the egg. This minimum amount must be given to the bird by way of enriched diet before the mating. This is also contained in codliver oil but plays a different role to that of Vitamin A. It will give enough strength for hatching. It is not sufficient simply for an egg to be fertilized. It is also important that the embryo hatches out of the egg at the right point in time.
Vitamin D gives the embryo enough strength to crack the shell and hatch out at the right time.
This vitamin is so essential that birds who do not receive any Vitamin D will lay eggs which later on cannot hatch, even when all other Vitamins and Minerals were given in the feed. The Vit D in the codliver oil can only take complete effect with UV treatment and if in addition raw milk, vegetables and greens are fed. In order to preserve Vitamin A the bottle containing the codliver oil should be stored in a cool and dark place. Therefore, codliver oil is being traded only in brown bottles. As the content of the bottle reduces, the remaining oil will be affected by the oxygen contained in the air-space above the oil in the bottle and the Vitamin A content will be reduced by oxydation.