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.
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.
During the latter half of the second day, and during the third day, great progress is made in the folding off of the embryo. Both the head- and tail-ends of the embryo become quite distinct, and the side-folds make such considerable progress that the embryo is only connected with the yolk by a broad stalk. This stalk is double, and consists of an inner splanchnic stalk, continuous with the walls of the alimentary canal, and an outer somatic stalk, continuous with the body-walls of the embryo. The somatic stalk is very much wider than the splanchnic. (Compare [fig. 121] E and F, which may be taken as diagrammatic longitudinal and transverse sections of the embryo on the third day.) A change also takes place in the position of the embryo. Up to the third day it is placed symmetrically, on the yolk, with its ventral face downwards. During this day it turns so as partially to lie on its left side. This rotation affects first the head ([fig. 111]), but in the course of the fourth day gradually extends to the rest of the body ([fig. 118]). Coincidently with this change in position the whole embryo undergoes a ventral and somewhat spiral flexure.
During the latter part of the second day and during the third day important changes take place in the head. One of these is the cranial flexure. This, which must not be confounded with the curvature of the body just referred to, commences by the bending downwards of the front part of the head round a point which may be considered as the extreme end either of the notochord or of the alimentary canal.
The cranial flexure progresses rapidly, the front-brain being more and more folded down till, at the end of the third day, it is no longer the first vesicle or fore-brain, but the second cerebral vesicle or mid-brain, which occupies the extreme front of the long axis of the embryo. In fact a straight line through the long axis of the embryo would now pass through the mid-brain instead of, as at the beginning of the second day, through the fore-brain, so completely has the front end of the neural canal been folded over the end of the notochord. The commencement of this cranial flexure gives the body of an embryo of the third day somewhat the appearance of a chemist’s retort, the head of the embryo corresponding to the bulb. On the fourth day the flexure is still greater than on the third, but on the fifth and succeeding days it becomes less obvious.
The anterior part of the fore-brain has now become greatly dilated, and may be distinguished from the posterior part as the unpaired rudiment of the cerebral hemispheres. It soon bulges out laterally into two lobes, which do not however become separated by a median partition till a much later period.
