The blastodermic vesicle still continues to grow rapidly; the circular patch of hypoblast also grows all round its edge, and so extends further and further round the inside of the vesicle; and in the embryonic area a third layer of cells—the mesoblast—appears between the epiblast and hypoblast. The appearance of the blastodermic vesicle on the seventh day is shown in Fig. 94. The central white spot is the embryonic area; this is now somewhat pear-shaped, and consists of all three buyers of cells mesoblast, and hypoblast. The rest of the upper half of the vesicle consists of two layers, epiblast and hypoblast, the slight constric tion round the middle of the vesicle, seen in the right-hand figure, marking the limit to which the hypoblast has extended; and, finally, the lower half of the vesicle, below the constriction, consists of epiblast alone. The whole vesicle • is still invested by the zona pellucida, which is not shown in the figure.' Formation of the Embryo.
During the seventh day a narrow opaque patch—the primitive streak —appears in posterior half of the embryonic area; and in front of this streak there is formed, on the eighth day, a shallow median groove—the medullary groove (Fig. 95), bounded by two folds—the medullary folds.
This groove rapidly deepens; the two folds bounding it bend over towards one another, meet and fuse, thereby converting the groove into a closed tubular canal—the neural canal—which is the rudiment of the central nervous system, one of the very earliest systems to appear in the embryo, and which becomes differentiated into the brain in front and spinal cord behind. From the mode of its formation—at first an open groove—it is clearly lined by, and indeed formed from, the epiblast, the most superficial of the three layers of the embryonic area.
At the sides of this medullary or neural canal, about the ninth day, the mesoblast: becomes divided into a number of somewhat cubical masses arranged in a linear series on either side of the middle line; these masses (Fig. 96) are the protovertebrte, and the transverse lines between them mark the division of the body into segments or somites.
We thus see that the formation of the embryo commences in the embryonic area of the blastodermic vesicle; and, further, that if the vesicle be placed with the embryonic area upwards, as in the right-hand figure of Fig. 94, then the dorsal surface of the embryo, indicated by the central nervous system, will be directed upwards; and the ventral surface downwards—i.e., towards the cavity of the blastodermic vesicle. The
head end is indicated in Fig. 96, by the dilatations of the neural canal forming the lobes of the brain, and notably by the two large lateral out growths from its front end which form the optic vesicles.
Yolk sac, or Umbilical Vesicle.
Not only does the development of the embryo commence in the em bryonic area, it is also confined to it. About the ninth day the embryo begins to be marked off by a slight constriction from the rest of the blas todermic vesicle. This constriction commences first and is most marked at the anterior end of the embryo, where it receives the name of the head fold; it is more prominent at the posterior end or tail-fold than at the sides. Its effect, well shown in Fig. 98, 2, is gradually to pinch off the embryonic portion from the rest of the blastodermic vesicle, which forms then a thin-walled sac filled with fluid, and is connected with the embryo by a stalk, which, at first short and wide (Fig. 98, 2), becomes, as the constriction deepens (Fig.. 98, 4, 5), longer and narrower.
The portion of the blastodermic vesicle which is separated in this way from the embryonic portion, and takes no direct part in the formation of the embryo, is spoken of as the yolk-sac or umbilical vesicle, and the stalk connecting it with the embryo may be called the yolk-stalk.
Alimentary Canal.
From the mode of formation of this umbilical vesicle, and from an examination of Fig. 98, it is clear that there will be formed underneath the embryo a cavity closed in front and behind by the head and tail folds, but opening freely in the middle portion of its length through the yolk stalk into the cavity of the umbilical vesicle—i.e., into the cavity of the original blastodermic vesicle, of which it is really a portion. The cavity, which it is also clear from the figure is lined by the hypoblast, or lower most of the three layers of the embryonic area, is the rudiment of the alimentary canal of the embryo. From the mode of its development it is clear that there is at first neither mouth nor anus, that the alimentary canal, indeed, has at this stage no communication whatever with the exterior. Its communication through the yolk-stalk with the cavity of the yolk-sac is at first (Fig. 98, 2) a very wide one, but as the constriction separating the embryo from the yolk-sac gets more and more marked, the yolk-stalk necessarily becomes narrower and narrower (cf. Fig. 98, 3, 4, 5), until ultimately its cavity becomes obliterated, and the alimentary canal becomes a completely closed tube.