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.
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