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EVOLUTION OF THE F02TUS. 179
meter, including, of course, its membraneous envelop
ments.
In the uterus, the growth of the new being is rapid.
Still, in the human subject, up to the seventh day, nothing
is visible to the naked eye. On the tenth day, there
may be perceived a semi-transparent, grayish flake.'
On the twelfth there is a ve icle, nearly of the size of a
pea, filled with fluid, in the middle of which swims an
opaque spot, presenting the first appearance of an em
bryo, which may be clearly seen as an or oblong curved
body, according as it is viewed, and plainly visible to the
naked on eye the fourteenth day. The entire weight
of the embryo and its two investing membranes, waters,
etc., is now about one grain.
Fig. 58 represents the ger- Fig. 58.
minal process of the hen's
egg, after twenty-four hours'
incubation, a. Vitellus, or
yolk. 6. Area pclucida, or
germinal spot. c. Vascular
space. This is the natural
size, the yoke being flat
tened.
The increiise from
the first is astonish
ingly rapid, when we
consider its original
minuteness. On the
twenty-first day the
embryo resembles an
ant, or a lettuce-seed ; its length is four or five lines,
and it weighs three or four grains. Many of its parts
dow begin to show themselves, especially the cartilagi-