A section through the posterior part of the head of an embryo of 30 hours is represented in fig. 110. The enlarged part of the neural tube, forming the hind-brain, is shewn at (_hb_). It is still connected with the epidermis, and at its dorsal border an outgrowth on each side forming the root of the vagus nerve is present (_vg_). The notochord (_ch_) is seen below the brain, and below this again the crescentic foregut (_al_). The commencing heart (_ht_), formed at this stage of two distinct tubes, is attached to the ventral side of the foregut. On the dorsal side of the foregut immediately below the notochord is seen a small body (_x_) formed as a thickening of the hypoblast. This may possibly be a rudiment of the subnotochordal rod of the Ichthyopsida. [FIG. 108. EMBRYO OF THE CHICK BETWEEN 30 AND 36 HOURS VIEWED FROM ABOVE AS AN OPAQUE OBJECT. (Chromic acid preparation.) _f.b._ front-brain; _m.b._ mid-brain; _h.b._ hind-brain; _op.v._ optic vesicle; _au.p._ auditory pit; _o.f._ vitelline vein; _p.v._ mesoblastic somite; _m.f._ line of junction of the medullary folds above the medullary canal; _s.r._ sinus rhomboidalis; _t._ tail-fold; _p.r._ remains of primitive groove (not satisfactorily represented); _a.p._ area pellucida. The line to the side between _p.v._ and _m.f._ represents the true length of the embryo. The fiddle-shaped outline indicates the margin of the pellucid area. The head, which reaches as far back as _o.f._, is distinctly marked off; but neither the somatopleuric nor splanchnopleuric folds are shewn in the figure; the latter diverge at the level of _o.f._, the former considerably nearer the front, somewhere between the lines _m.b._ and _h.b._ The optic vesicles _op.v._ are seen bulging out beneath the superficial epiblast. The heart lying underneath the opaque body cannot be seen. The tail-fold _t._ is just indicated; no distinct lateral folds are as yet visible in the region midway between head and tail. At _m.f._ the line of junction between the medullary folds is still visible, being lost forwards over the cerebral vesicles, while behind may be seen the remains of the sinus rhomboidalis, _s.r._]