It will be convenient to take the Fowl as a type for the general development of the Sauropsida. The embryo occupies a fairly constant position with reference to the egg-shell. Its long axis is placed at right angles to that of the egg, and the broad end of the egg is on the left side of the embryo. The general history of the embryo has already been traced up to the formation of the first formed mesoblastic somites (fig. 107). This stage is usually reached at about the close of the first day. After this stage the embryo rapidly grows in length, and becomes, especially in front and to the sides, more and more definitely folded off from the yolk-sack. The general appearance of the embryo between the 30th and 40th hours of incubation is shewn in fig. 108 from the upper surface, and in fig. 109 from the lower. The outlines of the embryo are far bolder than during the earlier stages. Fig. 109 shews the nature of the folding, by which the embryo is constricted off from the yolk-sack. The folds are complicated by the fact that the mesoblast has already become split into two layers--a splanchnic layer adjoining the hypoblast and a somatic layer adjoining the epiblast--and that the body cavity between these two layers has already become pretty wide in the lateral parts of the body of the embryo and the area pellucida. The fold by which the embryo is constricted off from the yolk-sack is in consequence a double one, formed of two limbs or laminæ, an inner limb constituted by the splanchnopleure, and an outer limb by the somatopleure. The relation of these two limbs is shewn in the diagrammatic longitudinal section (fig. 105), and in the surface view (fig. 109) the splanchnic limb being shewn at _sf_ and the somatic at _so_. Between the two limbs, and closely adjoining the splanchnopleure, is seen the heart (_ht_). At the stage figured the head is well marked off from the trunk, but the first separation between the two regions was effected at an earlier period, on the appearance of the foremost somite (fig. 107). Very shortly after the cephalic region is established, and before the closure of the medullary folds, the anterior part of the neural canal becomes enlarged to form the first cerebral vesicle, from which two lateral diverticula--rudiments of the optic lobes--are almost at once given off (fig. 108, _op.v_). By the stage figured the cephalic part of the neural canal has become distinctly differentiated into a fore- (_f.b_), a mid- (_m.b_) and a hind-brain (_h.b_); and the hind-brain is often subdivided into successive lobes. In the region of the hind-brain two shallow epiblastic invaginations form the rudiments of the auditory pits (_au.p_). [FIG. 107. DORSAL VIEW OF THE HARDENED BLASTODERM OF A CHICK WITH FIVE MESOBLASTIC SOMITES. THE MEDULLARY FOLDS HAVE MET FOR PART OF THEIR EXTENT, BUT HAVE NOT UNITED. _a.pr._ anterior part of the primitive streak; _p.pr._ posterior part of the primitive streak.]