In the embryo of which this is a drawing the head-fold of the amnion reached a little farther backward than the reference _u_, but its limit cannot be distinctly seen through the body of the embryo. _C.H._ cerebral hemisphere; _F.B._ vesicle of the third ventricle; _M.B._ mid-brain; _H.B._ hind-brain; _Op._ eye; _Ot._ auditory vesicle. _OfV._ vitelline veins forming the venous roots of the heart. The trunk on the right hand (left trunk when the embryo is viewed in its natural position from above) receives a large branch, shewn by dotted lines, coming from the anterior portion of the sinus terminalis. _Ht._ the heart, now completely twisted on itself. _Ao._ the bulbus arteriosus, the three aortic arches being dimly seen stretching from it across the throat, and uniting into the aorta, still more dimly seen as a curved dark line running along the body. The other curved dark line by its side, ending near the reference _y_, is the notochord _ch_. About opposite the line of reference _x_ the aorta divides into two trunks, which running in the line of the somewhat opaque somites on either side, are not clearly seen. Their branches however, _Of.a_, the vitelline arteries, are conspicuous and are seen to curve round the commencing side-folds. _Pv._ mesoblastic somites. _x_ is placed at the "point of divergence" of the splanchnopleure folds. The blind foregut begins here and extends about up to near _y_, the more transparent space marked by that letter is however mainly due to the presence there of investing mass at the base of the brain. _x_ marks the hind limit of the splanchnopleure folds. The limit of the more transparent somatopleure folds cannot be seen. It will be of course understood that all the body of the embryo above the level of the reference _x_, is seen through the portion of the yolk-sack (vascular and pellucid area), which has been removed with the embryo from the egg, as well as through the double amniotic fold. The view being from below, whatever is described in the natural position as being to the right appears here to the left, and _vice versâ_.]