Here, when subjected to the required conditions, this
minute and apparently insignificant particle of living matter becomes
animated by a new and mysterious activity. The germinal vesicle and
spot cease to be discernible (their precise fate being one of the yet
unsolved problems of embryology), but the yelk becomes
circumferentially indented, as if an invisible knife had been drawn
round it, and thus appears divided into two hemispheres (Fig. 12, C).
By the repetition of this process in various planes, these hemispheres
become subdivided, so that four segments are produced (D); and these,
in like manner, divide and subdivide again, until the whole yelk is
converted into a mass of granules, each of which consists of a minute
spheroid of yelk-substance, inclosing a central particle, the so-called
'nucleus' (F). Nature, by this process, has attained much the same
result as that at which a human artificer arrives by his operations in
a brickfield. She takes the rough plastic material of the yelk and
breaks it up into well-shaped tolerably even-sized masses, handy for
building up into any part of the living edifice.
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