A. A blastoderm before the formation of the medullary plate. _sc._ segmentation cavity; _es._ embryonic swelling. B. A somewhat older blastoderm in which the medullary groove has been established. _mg._ medullary groove. C. An embryo from the dorsal surface, as an opaque object, after the medullary groove has become posteriorly converted into a tube. _mg._ medullary groove: the reference line points very nearly to the junction between the open medullary groove with the medullary tube; _h._ cephalic plate; _ts._ tail swelling. D. Side view of a somewhat older embryo as a transparent object. _ch._ notochord; _op._ optic vesicle; _1.v.c._ 1st visceral cleft; _al._ alimentary tract; _so.s._ stalk connecting the yolk-sack with the embryo. E. Side view of an older embryo as a transparent object. _mp._ muscle-plates; _au.v._ auditory vesicle; _vc._ visceral cleft; _ht._ heart; _m._ mouth invagination; _an._ anal diverticulum; _al.v._ posterior vesicle of postanal gut. F. G. H. Older embryos as opaque objects.] The tissues of the body have now become fairly transparent, and there may be seen at the sides of the body seventeen mesoblastic somites. The notochord, which was formed long before the stage represented in figure 28 D, is now also distinctly visible. It extends from almost the extreme posterior to the anterior end of the embryo, and lies between the ventral wall of the spinal canal and the dorsal wall of the intestine. Round its posterior end the neural and alimentary tracts become continuous with each other. Anteriorly the termination of the notochord cannot be seen, it can only be traced into a mass of mesoblast at the base of the brain, which there separates the epiblast from the hypoblast. The alimentary canal (_al_) is completely closed anteriorly and posteriorly, though still widely open to the yolk-sack in the middle part of its course. In the region of the head it exhibits on each side a slight bulging outwards, the rudiment of the first visceral cleft. This is represented in the figure by two lines (1. _v.c._). The embryo represented in fig. 28 E is far larger than the one just described, but it has not been convenient to represent this increase of size in the figure. Accompanying this increase in size, the folding off from the yolk has considerably progressed, and the stalk which unites the embryo with the yolk is proportionately narrower and longer than before. The brain is now very distinctly divided into the three lobes, the rudiments of which appeared during the last stage. From the foremost of these the optic vesicles now present themselves as well-marked lateral outgrowths, towards which there has appeared an involution from the external skin (_op_) to form the lens.