The larva had been hatched three days, and was 4.8 mm. in length. The optic and auditory vesicles are supposed to be seen through the tissues. The letter _tv_ pointing to the base of the velum is where Scott believes the hyomandibular cleft to be situated. _c.h._ cerebral hemisphere; _th._ optic thalamus; _in._ infundibulum; _pn._ pineal gland; _mb._ mid-brain; _cb._ cerebellum; _md._ medulla oblongata; _au.v._ auditory vesicle; _op._ optic vesicle; _ol._ olfactory pit; _m._ mouth; _br.c._ branchial pouches; _th._ thyroid involution; _v.ao._ ventral aorta; _ht._ ventricle of heart; _ch._ notochord.] According to Scott (No. 87) a hyomandibular pouch forming the eighth pouch is formed in front of the pouch already defined as the hyobranchial. It disappears early and does not acquire gill folds[28]. The tissue forming the line of insertion of the velum appears to me to represent the mandibular arch. The grounds for this view are the following: (1) The structure in question has _exactly_ the position usually occupied by the mandibular arch. (2) There is present in late larvæ (about 20 days after hatching) an arterial vessel, continued from the ventral prolongation of the bulbus arteriosus along the insertion of the velum towards the dorsal aorta, which has the relations of a true branchial artery. [28] Scott informs me that he has been unable to find the hyomandibular pouch in larvæ larger than 4.8 mm. My material of the stages when it should be present is somewhat scanty, but I have as yet, very likely owing to the imperfection of my material, been unable to find Scott's hyomandibular pouch either in my sections or surface-views. Huxley describes this pouch as present in the form of a cleft in later stages; I have failed to find his cleft also. The vessel interpreted below as the branchial artery of the mandibular arch was only imperfectly investigated by me, and I was not sure of my interpretations about it. Scott however informs me by letter that it is undoubtedly present. On the ventral aspect of the branchial region is placed a sack (figs. 42, _h_, and 43, _th_), which extends from the front end of the branchial region to the fourth cleft. At first it constitutes a groove opening into the throat above (fig. 44), but soon the opening becomes narrowed to a pore placed between the second and third of the _permanent_ branchial pouches (fig. 43, _th_). In Ammocoetes[29] the simple tube becomes divided, and assumes a very complicated form, though still retaining its opening into the branchial region of the throat. In the adult it forms a glandular mass underneath the branchial region of the throat equivalent to the thyroid gland of higher Vertebrates. [29] Schneider (No. 85) states that in the full-grown Ammocoetes the opening is situated between the third and fourth pouches. This is certainly not true for the young larva.