tetrical works which deal with it in a very
exhaustive manner.

As soon as the ovum is impregnated by the male element, it begins a
process of symmetrical division. The first division produces two cells
out of the single one which first existed. By the next division, four
segments are produced; then eight, sixteen, etc. While this process
is going on, the ovum becomes adherent to the internal wall of the uterus,
and is soon enveloped by its mucous membrane, which grows up about and
incloses it.

The Primitive Trace.--When the process of segmentation has advanced
to a certain point, the cells are aggregated together in a compact layer
at the surface. Soon a straight line appears upon this layer, which
is called the _primitive trace_. This delicate line becomes the basis
for the spinal column; and upon and about it the whole individual is
developed by an intricate process of folding, dividing, and
reduplication of the layer of cells. One end of the line becomes the
head, and the other becomes the tail. Even man has a caudal appendage
at an early stage of his existence. After a further lapse of time, little
excrescences, buds, or "pads," appear in the proper positions to
represent the arms and legs. After further development the ends split
up into fingers and toes, and by the continued development of the parts,
perfect arms and legs are formed.

Curious Relation to Lower Animals.--It is a very remarkable fact that
in the lower animals we have numerous examples in which the permanent
condition of the individual is the same as some one of the stages through
which man passes in the process of development. The same author
previously quoted makes the following interesting statements:--

"The webbed feet of the seal and ornithorhynchus typify the period when
the hands and feet of the human embryo are as yet only partly subdivided
into fingers and toes. Indeed, it is not uncommon for the 'web' to
persist to some extent between the toes of adults; and occasionally
children are born with two or more fingers or toes united to their tips.

"With the seal and the walrus, the limbs are protruded but little beyond
the wrist and ankle. With the ordinary quadrupeds, the knee and elbow
are visible. The cats, the lemurs, and the monkeys form a series in
which the limbs are successively freed from the trunk, and in the
highest apes they are capable of nearly the same movements as the human
arm and leg, which, in their development, passe