Quain's elements of anatomy . rther from themargin of the blastoderm is effected by the rapid extension of the latter beyond THE BLASTODERM AND OVUM. 761 the aperture of invagiaation, and thus the blastopore and its prolongation as aprimitive trace and groove are brought more fully within the embryonic area,while the medullary groove stands in the same relation as in the lower will be seen, however, that the first indication of the primitive trace even inthe amniota arises marginally, and it is probable therefore that it essentiallycoiTesponds in position with that of the lower vert
Quain's elements of anatomy . rther from themargin of the blastoderm is effected by the rapid extension of the latter beyond THE BLASTODERM AND OVUM. 761 the aperture of invagiaation, and thus the blastopore and its prolongation as aprimitive trace and groove are brought more fully within the embryonic area,while the medullary groove stands in the same relation as in the lower will be seen, however, that the first indication of the primitive trace even inthe amniota arises marginally, and it is probable therefore that it essentiallycoiTesponds in position with that of the lower vertebrates. From this it appears that the first traces of embryonic formation in theseseveral groups of animals stand in a constant and determinate relation to theblastopore, or if we do not go so far as to admit the occurrence of actual invagi-nation, to the place of continuity between the upper and lower primary layers ofthe blastoderm, and to the seat of primary differentiation of the principal paxt ofthe mesoblast. Fig. Fig. 655.—ClAGEAMMATIC LONGITUDINAL SECTIONS OP ElASMOBRANCH EmBRYO AND Blastoderm. (From Balfour.) A, younger stage with two primary layers ; B, more advanced stage with three layersand invagination at the hinder end of the embiyo ; C, still more advanced ; the embryoraised from the blastoderiQ with neural and primitive alimentary canals and neurentericcommunication between them. ep, epiblast; m,, mesoblast ; cc, epiblast continuous with hy^Doblast; nc, neural canal;ch, notochord ; al, alimentary cavity ; sg, segmentation cavity ; n, nuclei of the yolk. It may be proper to make mention here cf the remarkable observation by Beneden of the existence of a small aperture or deficiency in the externallayer of cells on that side of the rabbits ovum where the inner cells or segmentalspheres (foi-ming the yolk-rest) adhere to the outer, which appeared to himto have the effect of establishing continuity between the outer and inner layersof the blastoderm
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