How a bird egg is formed

The yolk forms in the ovary. It is the food store for the embryo, rich in fat and protein. When the yolk is ready it is released and enters the oviduct, the tube where the rest of the egg is added.

In the first stretch of the oviduct the egg white, or albumen, is laid down around the yolk. The white is mostly water and protein. It cushions the yolk and supplies more food.

Yolk, then white

Twisted cords of thicker white, the chalazae, keep the yolk near the centre. You can see them in a cracked egg. They are part of the egg, not a fault.

Further along, two membranes are wrapped around the white. Those membranes become the skin you peel off a boiled egg. At the broad end they later separate to form the air space.

The shell comes last

The shell is added in the shell gland. It is made of calcium carbonate crystals, with thousands of tiny pores so the developing chick can exchange gases. A bloom, a thin outer coat, helps keep bacteria out of a newly laid egg.

The whole passage takes about a day in a domestic hen. Fertilisation, if it happens, occurs before the white and shell are added. An unfertilised egg is built in the same way. It simply contains no embryo.

Yolk, then white

A bird egg is built in stages inside the hen: yolk, white, membranes, and finally a shell. The yolk forms in the ovary. It is the food store for the embryo, rich in fat and protein.

Put those two statements side by side. The first sets the condition. The second says what happens once that condition is met. How a bird egg is formed needs both. Drop the first and the second has nothing to explain. Drop the second and the first is only the scene.

The shell comes last

It is the food store for the embryo, rich in fat and protein. When the yolk is ready it is released and enters the oviduct, the tube where the rest of the egg is added.

The order is the explanation. What the first sentence names has to be there before the result in the second sentence can show up. The piece does not ask you to accept the result first and hunt for a cause afterwards.

A check you can make

When the yolk is ready it is released and enters the oviduct, the tube where the rest of the egg is added. In the first stretch of the oviduct the egg white, or albumen, is laid down around the yolk.

A fair check is sequential. Confirm the first statement. Then see whether the second is the result of it, rather than a separate fact parked in the same piece. If the result shows up without the condition, this account does not cover what you are looking at.

The limit of the claim

In the first stretch of the oviduct the egg white, or albumen, is laid down around the yolk. The white is mostly water and protein.

The two statements do different jobs. One says what is there, what it is made of, or what acts on it. The other says the change that follows. The title names the change. It is not an inventory of parts.

The same steps, in order

The white is mostly water and protein. It cushions the yolk and supplies more food.

Nothing in the second statement cancels the first. It carries the first forward or marks the limit of it. Remember only the result, and the cause goes missing. Remember only the cause, and you will not know what the piece says happens next.

Why the order matters

It cushions the yolk and supplies more food. Twisted cords of thicker white, the chalazae, keep the yolk near the centre.

Read them as one path. Start at the first full stop and do not skip to the outcome. The words between the two statements are the mechanism. The title is the name of that path, not a second claim.