THE EARLY BLASTOCYST IS BILATERALLY SYMMETRICAL AND ITS AXIS OF SYMMETRY IS ALIGNED WITH THE ANIMAL-VEGETAL AXIS OF THE ZYGOTE IN THE MOUSE

At least one polar body, almost invariably the second, persists intact to the early blastocyst stage in nearly two-thirds of mouse conceptus es of the PO strain, The distribution in early blastocysts of these su rviving polar bodies was highly non-random, Most not only lay in the m idregion of the embryonic-abembryonic axis but, on discovering that ea rly blastocysts are bilaterally rather than radially symmetrical about this axis, were found to align with the bilateral axis, Cell marking experiments failed to detect movement of polar bodies relative to the surface of the conceptus during either cleavage or blastulation, That the distribution of degenerating polar bodies and their presumed debri s was similar to intact ones also argued against their motility, as di d the finding that at all stages second polar bodies were attached to conceptuses by a thin, extensible, weakly elastic 'tether', Although t he transfer of small fluorochromes between them was rarely observed be yond second cleavage, the second polar body and conceptus could remain coupled ionically up to the blastocyst stage, It is concluded that th e second polar body normally remains attached to the conceptus through persistence of the intercellular bridge formed during its abstriction , and therefore provides an enduring marker of the animal pole of the zygote, Hence, according to the distribution of polar bodies, the axis of bilateral symmetry of the early blastocysts is normally aligned wi th the animal-vegetal axis of the zygote and its embryonic-abembryonic axis is orthogonal to it, Such relationships suggest that, at least i n undisturbed development, specification of the axes of the blastocyst depends on spatial patterning of the zygote.