In normal development such distribution comprises, first, all that we designate as right and left; second, all that we designate fore and hind, with reference to the ends of the long axis of the body, or to what has been called fore and hind symmetry. In the fully developed animal this last is less easily recognized than the first, but in the embryo it can be traced without much difficulty.
"1st, in the nervous canal, enlarged at either end, the foremost enlargement for the brain, permanent; the hindmost, transitory, except in birds, where it forms the enlargement enclosing the rhomboidal sinus.
"2d, the plates of the vertebral column first appear midway, and are then developed in opposite directions, extending it backwards and forwards.
"3d, in the intestinal canal, with its allantoidian sac, which is respiratory, antagonizing the pulmonary sac, also respiratory.
"4th, in the limbs, which throughout all their earlier stages assume positions strictly antagonistic.
"If on one and the same yolk two nervous axes are formed on opposite sides of the yolk, each will distribute the matter under its influence without coming in collision with that of the other, except near the umbilicus, so that two nearly perfect embryos will be formed.
"If the two nervous axes are formed near to each other, so that the particles under their influence come in contact, then at the line of contact a series of intermediate organs or limbs, as the case may be, under the influence of the two axes, will be formed, as in the specimen described in this paper.
"Furthermore, if the axes are inclined so as to form a V-shaped figure, the two separated ends will have a head more or less complete; but as the two converge, the organs become more or less fused, and at length the hindmost are reduced to the normal type, the intermediate ones having become obsolete."
893. Double foetus; in spirit.
Two heads and spinal columns; one pair, each, of upper and lower extremities, and a third imperfectly developed,
