Quain's elements of anatomy . Fig. 645.—Embryonic area from the ovum op a rabbit of seven days. Kolliker.) jpT, first rudiment of the primitive streak. Fig. 646.—Embryonic area from the ovum of a rabbit of eight days. \ Kolliker.) arg, border of the embryonic area ; pr, primitive streak with groove. f. (From (From besides the fluid remains of the original yolk-substance, there is theinner mass of granular cells or segmental spheres, which by their fmtherdevelopment and extension come to produce one or more deep or internallayers, which gradually spreading over the interior give a bilaminar ort
Quain's elements of anatomy . Fig. 645.—Embryonic area from the ovum op a rabbit of seven days. Kolliker.) jpT, first rudiment of the primitive streak. Fig. 646.—Embryonic area from the ovum of a rabbit of eight days. \ Kolliker.) arg, border of the embryonic area ; pr, primitive streak with groove. f. (From (From besides the fluid remains of the original yolk-substance, there is theinner mass of granular cells or segmental spheres, which by their fmtherdevelopment and extension come to produce one or more deep or internallayers, which gradually spreading over the interior give a bilaminar ortrilaminar structure to an increasing area of the blastoderm. Fig. Fig. ^Embryonic area, with outline of part op the vascular area, from arabbits ovum of seven days -f. (From Kolliker.)00, vascular area; acj, embryonic area ; pr, primitive streak and groove ; rf, medullarygroove. It is in the central part of the blastoderm, at the fifth day in therabbit, and when the inner layer has advanced over about half of theinterior, that a discoid thickening or opacity occurs which gives rise to 3 c 2 756 RELATION OF BLASTODEEM TO DEVELOPMENT. the enibryonal area {tache mibryonnaire of Coste) ; and it is now ascer-tained by the observations of Hensen, Ivolliker and others, that the firstappearance of the primitive streak and its groove and the commencementof the medullary canal and vertebral rudiments take place in a manneressentially the same as in birds. But the same or even a greater degreeof doubt exists in mammals as in birds as to the mode of origin of themesoblast, .and there arises also in them the fuither question, viz., inhow far the external ve
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