C. From the third layer of the embryo, the entoderm, arises:—   The fat and the marrow.

They are developed from the entoderm of the third and fourth branchial grooves.

The inner layer we call the entoderm, the outer the ectoderm; and the "primitive mouth" is known as the blastopore.

Endoderm: the inner layer of the blastoderm in the embryo, giving origen to the mid-intestine and other visceral organs: see entoderm.

The entoderm, which has the appearance of being thickened because of the fact that the notochord has not yet completely separated from it, is continuous, through the blastopore, with the ectoderm.