B. Older embryo after the closure of the medullary canal. _oc._ optic vesicle. Behind the optic vesicle are seen two visceral arches.] [FIG. 78. LATERAL VIEW OF AN ADVANCED EMBRYO OF BOMBINATOR. (After Götte.) _a._ mid-brain, _a´._ eye; _b._ hind-brain; _d._ mandibular arch; _d´._ Gasserian ganglion; _e._ hyoid arch; _e´._ first branchial arch; _f._ seventh nerve; _f´._ glossopharyngeal and vagus nerve; _g._ auditory vesicle; _i._ boundary between liver and yolk-sack; _k._ suctorial disc; _l._ pericardial prominence; _m._ prominence formed by the pronephros.] The rudiments of the mandibular (_d_), hyoid (_e_), and first branchial (_e´_) arches project as folds at the side of the head, but the visceral clefts are not yet open. Rudiments of the proctodæum and stomodæum have appeared, but neither of them as yet communicates with the mesenteron. Below the hyoid arch is seen a peculiar disc (_k_) which is an embryonic suctorial organ, formed of a plate of thickened epiblast. There is a pair of these discs, one on each side, but only one of them is shewn in the figure. At a later period they meet each other in the middle line, though they separate again before their final atrophy. They are found in the majority of the Anura, but are absent according to Parker in the Aglossa (Pipa and Dactylethra (fig. 83)). They are probably remnants of the same primitive organs as the suctorial disc of Lepidosteus. [FIG. 79. TRANSVERSE SECTION THROUGH A VERY YOUNG TADPOLE OF BOMBINATOR AT THE LEVEL OF THE ANTERIOR END OF THE YOLK-SACK. (After Götte.) _a._ fold of epiblast continuous with the dorsal fin; _is^x._ neural cord; _m._ lateral muscle; _as^x._ outer layer of muscle-plate; _s._ lateral plate of mesoblast; _b._ mesentery; _u._ fold of the peritoneal epithelium which forms the segmental duct; _f._ alimentary tract; _f´._ ventral diverticulum which becomes the liver; _e._ junction of yolk-cells and hypoblast-cells; _d._ yolk-cells.] The embryo continues to grow in length, while the tail becomes more and more prominent, and becomes bent round to the side owing to the confinement of the larva within the egg-membrane. At the front of the head the olfactory pits become distinct. The stomodæum deepens, though still remaining blind, and three fresh branchial arches become formed; the last two being very imperfectly differentiated, and not visible from the exterior. There are thus six arches in all, viz. the mandibular, the hyoid and four branchial arches. Between the mandibular and the hyoid, and between each of the following arches, pouches of the mesenteron push their way towards the external skin. Of these pouches there are five, there being no pouch behind the last branchial arch. The first of these will form the hyomandibular cleft, the second the hyobranchial, and the third, fourth and fifth the three branchial clefts.