In the embryo itself (fig. 144 A) the disproportion between head and trunk is less marked than before; the medullary plate dilates anteriorly to form a spatula-shaped cephalic enlargement; and three or four somites are established. In the lateral parts of the mesoblast of the head there may be seen on each side a tube-like structure (_hz_). Each of these is part of the heart, which arises as two independent tubes. The remains of the primitive streak (_pr_) are still present behind the medullary groove. In somewhat older embryos (fig. 144 B) with about eight somites, in which the trunk considerably exceeds the head in length, the first distinct traces of the folding-off of the head end of the embryo become apparent, and somewhat later a fold also appears at the hind end. In the formation of the hind end of the embryo the primitive streak gives rise to a tail swelling and to part of the ventral wall of the postanal gut. In the region of the head the rudiments of the heart (_h_) are far more definite. The medullary groove is still open for its whole length, but in the head it exhibits a series of well-marked dilatations. The foremost of these (_vh_) is the rudiment of the fore-brain, from the sides of which there project the two optic vesicles (_ab_); the next is the mid-brain (_mh_), and the last is the hind-brain (_hh_), which is again divided into smaller lobes by successive constrictions. The medullary groove behind the region of the somites dilates into an embryonic sinus rhomboidalis like that of the Bird. Traces of the amnion (_af_) are now apparent both in front of and behind the embryo. [FIG. 144. EMBRYO RABBITS OF ABOUT NINE DAYS FROM THE DORSAL SIDE. (From Kölliker.) A. magnified 22 times, and B. 21 times. _ap._ area pellucida; _rf._ medullary groove; _h´._ medullary plate in the region of the future fore-brain; _h´´._ medullary plate in the region of the future mid-brain; _vh._ fore-brain; _ab._ optic vesicle; _mh._ mid-brain; _hh._ and _h´´._ hind-brain; _uw._ mesoblastic somite; _stz._ vertebral zone; _pz._ lateral zone; _hz._ and _h._ heart; _ph._ pericardial section of body cavity; _vo._ vitelline vein; _af._ amnion fold.] The structure of the head and the formation of the heart at this age are illustrated in fig. 145. The widely-open medullary groove (_rf_) is shewn in the centre. Below it the hypoblast is thickened to form the notochord _dd´_; and at the sides are seen the two tubes, which, on the folding-in of the foregut, give rise to the unpaired heart. Each of these is formed of an outer muscular tube of splanchnic mesoblast (_ahh_), not quite closed towards the hypoblast, and an inner epithelioid layer (_ihh_); and is placed in a special section of the body cavity (_ph_), which afterwards forms the pericardial cavity. [FIG. 145. TRANSVERSE SECTION THROUGH THE HEAD OF A RABBIT OF THE SAME AGE AS FIG. 144 B. (From Kölliker.) B. is a more highly magnified representation of part of A.