_rf._ medullary groove; _mp._ medullary plate; _rw._ medullary fold; _h._ epiblast; _dd._ hypoblast; _dd´._ notochordal thickening of hypoblast; _sp._ undivided mesoblast; _hp._ somatic mesoblast; _dfp._ splanchnic mesoblast; _ph._ pericardial section of body cavity; _ahh._ muscular wall of heart; _ihh._ epithelioid layer of heart; _mes._ lateral undivided mesoblast; _sw._ fold of hypoblast which will form the ventral wall of the pharynx; _sr._ commencing throat.] Before the ninth day is completed great external changes are usually effected. The medullary groove becomes closed for its whole length with the exception of a small posterior portion. The closure commences, as in Birds, in the region of the mid-brain. Anteriorly the folding-off of the embryo proceeds so far that the head becomes quite free, and a considerable portion of the throat, ending blindly in front, becomes established. In the course of this folding the, at first widely separated, halves of the heart are brought together, coalesce on the ventral side of the throat, and so give rise to a median undivided heart. The fold at the tail end of the embryo progresses considerably, and during its advance the allantois is formed in the same way as in Birds. The somites increase in number to about twelve. The amniotic folds nearly meet above the embryo. [FIG. 146. ADVANCED EMBRYO OF A RABBIT (ABOUT TWELVE DAYS)[82]. _mb_. mid-brain; _th._ thalamencephalon; _ce._ cerebral hemisphere; _op._ eye; _iv.v._ fourth ventricle; _mx._ maxillary process; _md._ mandibular arch; _hy._ hyoid arch; _fl._ fore-limb; _hl._ hind-limb; _um._ umbilical stalk.] [82] This figure was drawn for me by my pupil, Mr Weldon. The later stages in the development proceed in the main in the same manner as in the Bird. The cranial flexure soon becomes very marked, the mid-brain forming the end of the long axis of the embryo (fig. 146). The sense organs have the usual development. Under the fore-brain appears an epiblastic involution giving rise both to the mouth and to the pituitary body. Behind the mouth are three well-marked pairs of visceral arches. The first of these is the mandibular arch (fig. 146, _md_), which meets its fellow in the middle line, and forms the posterior boundary of the mouth. It sends forward on each side a superior maxillary process (_mx_) which partially forms the anterior margin of the mouth. Behind the mandibular arch are present a well-developed hyoid (_hy_) and a first branchial arch (not shewn in fig. 146). There are four clefts, as in other Amniota, but the fourth is not bounded behind by a definite arch. Only the first of these clefts persists as the tympanic cavity and Eustachian tube.