The planum occipitale

Planum occipitale

That part of the squama occipitalis which lies above the highest nuchal lines is named the planum occipitale , and is covered by the Occipitalis muscle....

 [Fig. 3] of the squama of the occipital bone

Occipital bone

The occipital bone, a saucer-shaped membrane bone situated at the back and lower part of the cranium, is trapezoidal in shape and curved on itself...

 is developed in membrane, and may remain separate throughout life when it constitutes the interparietal bone; the rest of the bone is developed in cartilage.

The number of nuclei for the planum occipitale is usually given as four, two appearing near the middle line about the second month, and two some little distance from the middle line about the third month of fetal life.

The planum nuchale of the squama is ossified from two centers, which appear about the seventh week of fetal life and soon unite to form a single piece.

Union of the upper and lower portions of the squama takes place in the third month of fetal life.

An occasional center (Kerckring) appears in the posterior margin of the foramen magnum during the fifth month; this forms a separate ossicle (sometimes double) which unites with the rest of the squama before birth.

Each of the lateral parts begins to ossify from a single center during the eighth week of fetal life. The basilar portion is ossified from two centers, one in front of the other; these appear about the sixth week of fetal life and rapidly coalesce.

Mall states that the planum occipitale

Planum occipitale

That part of the squama occipitalis which lies above the highest nuchal lines is named the planum occipitale , and is covered by the Occipitalis muscle....

 is ossified from two centers and the basilar portion from one.

About the fourth year the squama and the two lateral portions unite, and about the sixth year the bone consists of a single piece. Between the 18th and 25th years the occipital and sphenoid

Sphenoid bone

The sphenoid bone is an unpaired bone situated at the base of the skull in front of the temporal bone and basilar part of the occipital bone.The sphenoid bone is one of the seven bones that articulate to form the orbit...

 become united, forming a single bone.

In other animals

In many animals these parts stay separate through life; e.g. in the dog as four parts: supraoccipital (= the squama); left and right exoccipital (= the lateral parts); basioccipital (= the basilar part).