Fig. 108. Embryo of the Chick between 30 and 36 hours viewed from above as an opaque object. (Chromic acid preparation.)
f.b. front-brain; m.b. mid-brain; h.b. hind-brain; op.v. optic vesicle; au.p. auditory pit; o.f. vitelline vein; p.v. mesoblastic somite; m.f. line of junction of the medullary folds above the medullary canal; s.r. sinus rhomboidalis; t. tail-fold; p.r. remains of primitive groove (not satisfactorily represented); a.p. area pellucida.
The line to the side between p.v. and m.f. represents the true length of the embryo.
The fiddle-shaped outline indicates the margin of the pellucid area. The head, which reaches as far back as o.f., is distinctly marked off; but neither the somatopleuric nor splanchnopleuric folds are shewn in the figure; the latter diverge at the level of o.f., the former considerably nearer the front, somewhere between the lines m.b. and h.b. The optic vesicles op.v. are seen bulging out beneath the superficial epiblast. The heart lying underneath the opaque body cannot be seen. The tail-fold t. is just indicated; no distinct lateral folds are as yet visible in the region midway between head and tail. At m.f. the line of junction between the medullary folds is still visible, being lost forwards over the cerebral vesicles, while behind may be seen the remains of the sinus rhomboidalis, s.r.
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. 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.
A section through the posterior part of the head of an embryo of 30 hours is represented in [fig. 110]. The enlarged part of the neural tube, forming the hind-brain, is shewn at (hb). It is still connected with the epidermis, and at its dorsal border an outgrowth on each side forming the root of the vagus nerve is present (vg). The notochord (ch) is seen below the brain, and below this again the crescentic foregut (al). The commencing heart (ht), formed at this stage of two distinct tubes, is attached to the ventral side of the foregut.
On the dorsal side of the foregut immediately below the notochord is seen a small body (x) formed as a thickening of the hypoblast. This may possibly be a rudiment of the subnotochordal rod of the Ichthyopsida.
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.
