ing 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.

[Illustration: FIG. 13.--Earliest rudiment of the Dog. B. Rudiment
further advanced, showing the foundations of the head, tail, and
vertebral column. C. The very young puppy, with attached ends of the
yelk-sac and allantois, and invested in the amnion.]

Next, the mass of organic bricks, or 'cells' as they are technically
called, thus formed, acquires an orderly arrangement, becoming converted
into a hollow spheroid with double walls. Then, upon one side of this
spheroid, appears a thickening, and, by and bye, in the centre of the
area of thickening, a straight shallow groove (Fig. 13, A) marks the
central line of the edifice which is to be raised, or, in other words,
indicates the position of the middle line of the body of the future
dog. The substance bounding the groove on each side next rises up into
a fold, the rudiment of the side wall of that long cavity, which will
eventually lodge the spinal marrow and the brain; and in the floor of
this chamber appears a solid cellular cord, the so-called 'notochord.'
One end of the inclosed cavity dilates to form the head (Fig. 13, B),
the other remains narrow, and eventually becomes the tail; the side
walls of the body are fashioned out of the downward continuation of the
walls of the groove; and from them, by and bye, grow out little buds
which, by degrees, assume the shape of limbs. Watching the fashioning
process stage by stage, one is forcibly reminded of the mod