rosities, like extra digits, absence of horns or tail, etc., run in
families and are produced almost as certainly as color or form. Others
are associated with too close breeding, the powers of symmetrical
development being interfered with, just as in other cases a sexual
incompatibility is developed, near relatives failing to breed with each
other. Mere arrest of development of a part may arise from accidental
disease of the embryo; hence vital organs are left out, or portions of
organs, like the dividing walls of the heart, are omitted. Sometimes an
older fetus is inclosed in the body of another, each having started
independently from a separate ovum, but the one having become embedded
in the semifluid mass of the other and having developed there
simultaneously with it, but not so largely nor perfectly. In many cases
of redundance of parts the extra part or member has manifestly developed
from the same ovum and nutrient center with the normal member to which
it remains adherent, just as a new tail will grow out in a newt when the
former has been cut off. In the early embryo, with its great powers of
development, this factor can operate to far greater purpose than in the
adult animal. Its influence is seen in the fact pointed out by St.
Hilaire that such redundant parts are nearly always connected with the
corresponding portions in the normal fetus. Thus superfluous legs or
digits are attached to the normal ones, double heads or tails are
connected to a common neck or rump, and double bodies are attached to
each other by corresponding points, navel to navel, breast to breast,
back to back. All this suggests the development of extra parts from the
same primary layer of the impregnated and developing ovum. The effect of
disturbing conditions in giving such wrong directions to the
developmental forces is well shown in the experiments of St. Hilaire and
Valentine in varnishing, shaking, and otherwise breaking up the natural
connections in eggs, and thereby determining the formation of
monstrosities at will. So, in the mammal, blows and other injuries that
detach the fetal membranes from the walls of the womb or that modify
their circulation by inducing inflammation are at times followed by the
development of a monster. The excitement, mental and physical, attendant
on fright occasionally acts in a similar way, acting probably through
the same channels.

The monstrous forms liable to interfere with parturition are such as,
f