. Quain's elements of anatomy . orgranular yolk substance. more upon the prhnitive streak, which they, along with the meso-blastic columns on either side of them, partly extrude and partlyenclose ; and the ridges, rising and approaching one another, unitetogether along the dorsal line, first at a limited space and then morecompletely till at last they form a closed medullary tube, wider anteriorlyin its cephalic part, the whole thus giving rise to the primitive form ofthe brain and spinal marrow. Below the medullary tube there is formed about the same time thecellular column named chorda dorsa
. Quain's elements of anatomy . orgranular yolk substance. more upon the prhnitive streak, which they, along with the meso-blastic columns on either side of them, partly extrude and partlyenclose ; and the ridges, rising and approaching one another, unitetogether along the dorsal line, first at a limited space and then morecompletely till at last they form a closed medullary tube, wider anteriorlyin its cephalic part, the whole thus giving rise to the primitive form ofthe brain and spinal marrow. Below the medullary tube there is formed about the same time thecellular column named chorda dorsalis or notochord, which occupies theplace of the centres of the future bodies of the vertebrje and basis of thecranium, and by a somewhat later process the rudiments of the vertebraethemselves in their centra or bodies, which enclose the notochord, andtheir neural arches which surround the medullary canal, together withthe muscular plates, which are the source of the volmitary muscles, cometo be developed from the middle 750 THE FECUNDATED OVUM. The formation of blood-vessels and blood and the simultaneous deve-lopment of the heart follow in another part of this layer, and in a some-what later stage there take place the inflection and other changes of thewhole blastodermic layers Avhich mould the body of the embryo into asemblance of its later form, give the distinction of head, trunk, and limbs,and lay the foundation of the alimentary canal, its accompanying glandular Fig. 638. A -r
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