In the trunk (fig. 108) the chief point to be noticed is the complete closure of the neural canal, though in the posterior part, where the open sinus rhomboidalis was situated at an earlier stage, there may still be seen a dilatation of the canal (fig. 108, _s.r_), on each side of which are the tail swellings; while the mesoblastic somites stop short somewhat in front of it. Underneath the neural canal may be seen the notochord (fig. 109, _ch_) extending into the head, as far as the base of the mid-brain. At the sides of the trunk are seen the mesoblastic somites (_p.v_), the outer edges of which mark the boundary between the vertebral and lateral plates. A fainter line can be seen marking off the part of the lateral plates which will become part of the body-wall, from that which pertains to the yolk-sack. [FIG. 109. AN EMBRYO CHICK OF ABOUT THIRTY-SIX HOURS VIEWED FROM BELOW AS A TRANSPARENT OBJECT. _FB._ the fore-brain or first cerebral vesicle, projecting from the sides of which are seen the optic vesicles _op_. A definite head is now constituted, the backward limit of the somatopleure fold being indicated by the faint line _S.O._ Around the head are seen the two limbs of the amniotic head-fold: one, the true amnion _a_, closely enveloping the head, the other, the false amnion _a´_, at some distance from it. The head is seen to project beyond the anterior limit of the pellucid area. The splanchnopleure fold extends as far back as _sp_. Along its diverging limbs are seen the conspicuous venous roots of the vitelline veins, uniting to form the heart _h_, already established by the coalescence of two lateral halves which, continuing forward as the bulbus arteriosus _b.a_, is lost in the substance of the head just in front of the somatopleure fold. _HB._ hind-brain; _MB._ mid-brain; _p.v._ and _v.pl._ mesoblastic somites; _ch._ front end of notochord; _mc._ posterior part of notochord; _e._ parietal mesoblast; _pl._ outline of area pellucida; _pv._ primitive streak.] [FIG. 110. TRANSVERSE SECTION THROUGH THE POSTERIOR PART OF THE HEAD OF AN EMBRYO CHICK OF THIRTY HOURS. _hb._ hind-brain; _vg._ vagus nerve; _ep._ epiblast; _ch._ notochord; _x._ thickening of hypoblast (possibly a rudiment of the subnotochordal rod); _al._ throat; _ht._ heart; _pp._ body cavity; _so._ somatic mesoblast; _sf._ splanchnic mesoblast; _hy._ hypoblast.] [FIG. 111. CHICK OF THE THIRD DAY (54 HOURS) VIEWED FROM UNDERNEATH AS A TRANSPARENT OBJECT. _a´._ the outer amniotic fold or false amnion. This is very conspicuous around the head, but may also be seen at the tail. _a._ the true amnion, very closely enveloping the head, and here seen only between the projections of the several cerebral vesicles. It may also be traced at the tail, _t_.