The infra-infundibular nervous mass is naturally confined to the ventral side of the epithelial tube, because it represents the infra-oesophageal ganglia, situated as they are on the ventral side of the cephalic stomach, and, owing to the size of the stomach, they could not enclose it by dorsal growth, as they do in the case of the formation of the spinal cord (Fig. 21, B (1)). Still these nervous masses have grown dorsalwards, have commenced to involve the walls of the cephalic stomach even in the lowest vertebrate, as is seen in Ammocoetes, in which animal a ventral portion of the epithelial bag has been evidently compressed and its lumen finally obliterated {46}by the growth of the nerve-masses on each side of it. Throughout the whole vertebrate kingdom this obliterated portion still leaves its mark as the _raphé_ or seam, which is so characteristic of the infra-infundibular portion of the brain. [Illustration: FIG. 22.--HORIZONTAL SECTION THROUGH THE BRAIN OF AMMOCOETES. _Cr._, membranous cranium; _I_, olfactory nerves; _l.v._, lateral ventricles; _gl._, glandular tissue which fills up the cranial cavity.] Here, again, it is seen how simple is the explanation of a peculiarity which has always puzzled anatomists--why should there be this seam in the infra-infundibular portion of the brain and not in the supra-infundibular or in the spinal cord? The corresponding compression in the upper brain-region forms the lateral ventricles, as is seen in the accompanying figure of the brain of Ammocoetes (Fig. 22). [Illustration: FIG. 23.--SECTION THROUGH RHOMBOIDAL SINUS OF BIRD.]