The shading is due to an increased growth of cells, because here the three germinal layers—embryonic ectoderm, mesoderm, and entoderm—are in contact.

The lateral parts of the gland are developed from the entoderm of the fourth visceral clefts, and, joining the median part, lose their pharyngeal connexion.

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