pharyngeal and oral cavities. Meanwhile, and indeed rather
earlier, two longitudinal parallel ridges on the top or along the
back of the disk have grown up and united in the middle line to
form the second barrel of the body—the neural canal—of small
and uniform width in the lower three-fourths or spinal region,
but expanding into a wide chamber for the brain. This division
into neural (dorsal) and haemal (ventral) canals underlies all
vertebrate development. Imperfect closure along either of
those embryonic lines of junction may produce various degrees of
monstrosity. The simplest and commonest form, hardly to be
reckoned in the present category, is harelip with or without cleft
palate, which results from defective closure of the ventral
laminae at their extreme upper end. Another simple form,
but of much more serious import, is a gap left in the neural
canal at its lower end; usually the arches of the lumbar vertebrae
are deficient, and the fluid that surrounds the spinal cord bulges
out in its membranes, producing a soft tumour under the skin
at the lower part of the back. This is the condition known
as hydrorhachis, depending on the osseous defect known as spina
bifida. More rarely the gap in the arches of the vertebrae is in
the region of the neck. If it extend all along the back, it will
probably involve the skull also. Deficiency of the crown of the
head, and in the spine as well, may be not always traceable to
want of formative power to close the canal in the middle line;
an over-distended condition of the central canal of the cord and
brain may prevent the closure of the bones, and ultimately lead
to the disruption of the nervous organs themselves; and injuries
to the mother, with inflammation set up in the foetus and its
appendages, may be the more remote cause. But it is by defect
in the middle line that the mischief manifests itself, and it is
in that anatomical category that the malformations are included.
The osseous deficiency at the crown of the head is usually
accompanied by want of the scalp, as well as of the brain and
membranes. The bones of the face may be well developed
and the features regular, except that the eyeballs bulge forward
under the closed lids; but there is an abrupt horizontal line above
the orbits where the bones cease, the skin of the brow joining on
to a spongy kind of tissue that occupies the sides and floor of
the cranium. This is the commonest form of an anencephalous
or brainless monster. There are generally mere traces of the
brain, although, in some rare and curious instances, the hemispheres
are developed in an exposed position on the back of
the neck. The cranial nerves are usually perfect, with the
exception sometimes of the optic (and retina). Vegetative
existence is not impossible, and a brainless monster has been
known to survive sixty-five days. The child is usually a very
large one.
Closely allied, as we have seen, to the anencephalous condition is the condition of congenital hydrocephalus. The nervous system at its beginning is a neural canal, not only as regards its bony covering, but in its interior; a wide space lined by ciliated epithelium and filled with fluid extends along the axis of the spinal cord, and expands into a series of chambers in the brain. As development proceeds the walls thicken at the expense of the internal spaces, the original tubular or chambered plan of the central nervous system is departed from, and those organs assume the practically solid form in which we familiarly know them. If, however, the spaces persist in their embryonic proportions notwithstanding the thickening of the nervous substance forming their walls, there results an enormous brain which is more than half occupied inside with fluid, contained in spaces that correspond on the whole to the ventricles of the brain as normally bounded. A hydrocephalic foetus may survive its birth, and will be more apt to be affected in its nutrition than in its intelligence. In many cases the hydrocephalic condition does not come on till after the child is born.
Returning to the ventral middle line, there may be defects of closure below the lips and palate, as in the breast-bone (fissure of the sternum), at the navel (the last point to close in any case), and along the middle line of the abdomen generally. The commonest point for a gap in the middle line of the belly is at its lower part, an inch or two above the pubes. At that point in the embryo there issues the allantois, a balloon-like expansion from the ventral cavity, which carries on its outer surface bloodvessels from the embryo to interdigitate with those of the mother on the uterine surface. Having served its temporary purpose of carrying the bloodvessels across a space, the balloon-like allantois collapses, and rolls up into the rounded stem-like umbilical cord through most of its extent; but a portion of the sac within the body of the foetus is retained as the permanent urinary bladder. That economical adaptation of a portion of a vesicular organ, originally formed for purposes of communication between the embryo and the mother, appears to entail sometimes a defect in the wall of the abdomen just above the pubes, and a defect in the anterior wall of the bladder itself. This is the distressing congenital condition of fissure of the urinary bladder, in which its interior is exposed through an opening in the skin; the pubic bones are separated by an interval, and the reproductive organs are ill-formed; the urachus is wanting, and the umbilicus is always placed exactly at the upper end of the gap in the skin. A monstrosity recalling the cloacal arrangement of the bird is met with as a more extreme defect in the same parts.
Hermaphroditism.—Although this anomalous condition does not fall under defective closure in the middle line, it may be said to be due to a similar failure of purpose, or to an uncertainty in the nisus formativus at a corresponding stage of development. Strictly speaking, a hermaphrodite is a creature containing ovaries and testes—the essential organs of each sex. Evidence accumulates, however, that at least in all the higher vertebrates, including man, the sex is predetermined in the fertilized ovum, and it is more than doubtful if true hermaphroditism occurs. On the other hand, if there be no such double sex in the essential organs (as in the majority of so-called hermaphrodites) there is a great deal of doubling and ambiguity entailed in the secondary or external organs and parts of generation. Those parts which are rudimentary or obsolete in the male but highly developed in the female, and those parts which are rudimentary in the female but highly developed in the male tend in the hermaphrodite to be developed equally, and all of them badly. Amongst human beings the greater number of so-called hermaphrodites are really females, in which there is an abnormal development of the clitoris, but it also happens that true males may be born with a small clitoris-like penis, with hypospadia—that is to say, with imperfect urethra, open on the ventral side, and with undescended testes. Failure of the development of the testes or ovary, or their removal in the adult condition induces an ambiguous condition of the body in which the secondary sexual characters approach those of the other sex. Experimental removal of the ovaries or testes, followed by implantation of organs of the other sex, has produced an inversion of the secondary sexual characters.
Cyclops, Siren, &c.—The same feebleness of the formative energy which gives rise to some at least of the cases of defective closure in the middle line, and to the cases of ambiguous sex, leads also to imperfect separation of symmetrical parts. The most remarkable case of the kind is the cyclops monster. At a point corresponding to the root of the nose there is found a single orbital cavity, sometimes of small size and with no eyeball in it, at other times of the usual size of the orbit and containing an eyeball more or less complete. In still other cases, which indicate the nature of the anomaly, the orbital cavity extends for some distance on each side of the middle line, and contains two eyeballs lying close together. The usual nose is wanting but above the single orbital cavity there is often a nasal process on the forehead, with which nasal bones may be articulated, and cartilages joined to the latter; these form the framework of a short fleshy protuberance like a small proboscis. The lower jaw is sometimes wanting in cyclopeans; the cheek-bones are apt to be small, and the mouth a small round hole, or altogether absent; the rest of the body may be well developed. The key to the cyclopean condition is found in the state of the brain. The olfactory nerves or lobes are frequently absent; the brain is very imperfectly divided into hemispheres, and appears as a
