STRUCTURE AND GROWTH OF DOMESTICATED ANIMALS. 653 
and quartering, and then the process of self-division goes on only 
in one-half, viz., in the upper half of the yolk, the lower half 
undergoing no change, so that you have at first only two spheres, 
one below and one above, then two in the upper part, then four in 
the upper part, then eight in the upper part, then sixteen in the 
upper part, the lower part remaining in its primitive condition, and 
the whole upper part finally being transformed into a body similar 
to what we have as a whole in the mammal, resting as it were on 
a cup of unaltered, unchanged yolk in the lower part. In the fish, 
it is this mulberry-like, segmented portion of the yolk which is 
changed into the germ, while the other half takes no part in the 
formation of the germ, but only feeds it, being in fact absorbed into 
it. The egg is actually a live being, only it is a live being which 
struggles into its structure by its own activity; and in the for- 
mation of the organs it afterward possesses, the process of growth 
is not one of enlargement simply, but involves such changes as to 
transform a uniform mass into a variety of systems built of differ- 
ent tissues and endowed with special functions. In the chicken, 
two parallel swellings first arise along the middle line of the back, 
leaving a shallow furrow between themselves ; and the white disk, 
spoken of above as a white speck, enlarges and spreads so as to 
cover the whole surface of the yolk visible from above. If you 
look at this furrow in a section it will be something like an arch, 
open above. Gradually this furrow grows wider at one end, with 
indentations right and left, and then the margins of the disk spread, 
and, folding downward, enclose more and more of the yolk, and 
the sides of the furrow thicken, so that represented in profile it 
will be no longer a shallow furrow, but something like a channel 
or tube. 
At this stage the whole mass has still about the same consis- 
tency everywhere. It is like soft jelly and a little pulpy, but pres- 
ently the two edges of the furrow come more closely together, and 
finally touch. Meanwhile the margins of the new being rise in a 
fold and enclose the central parts, forming a sac around the germ, 
own as the amnios. The natural result of the closing of the 
upturned edges of the germ is the formation of a cavity, enclosed 
between these edges. That cavity now fills with a transparent fluid, 
and as it fills there appears something a little more substantial 
upon its sides and below it; the walls protecting the cavity be- 
Come less transparent or even slightly opaque; then the cavity 
