62 
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 in- 
visible knife had been drawn round it, and thus appears 
divided into two hemispheres (Fig. 13, 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 brick field. She takes the rough plastic ma- 
terial 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. 
Next, the mass of organic bricks, or ‘cells’ as they are 
technically called, thus formed, acquires an orderly arrange- 
ment, 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. 14, 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 ap- 
pears a solid cellular cord, the so-called ‘notochord’ One 
end of the inclosed cavity dilates to form the head (Fig.14, B), 
the other remains narrow, and eventually becomes the tail ; 
the side walls of the body are fashioned out of the downward 
