THE BEGINNINGS OF THINGS (_continued_)
Without entering into the description of the development of the whole
circulatory system, we may just mention briefly the origin of the heart
itself, which begins at a very early stage by the appearance of a small
body of cells, which come to arrange themselves in a tubular form
enclosed in the mesoderm. The two halves of this tube are at first quite
separate from each other, but gradually come together and finally unite
into a single tube with walls. The folds of these ultimately form the
heart muscle. The organ, at this stage of its development, does not lie
within the region of the chest cavity, as it afterwards does, but more
anteriorly in the region of the neck. The simple tubular arrangement,
however, is quite a passing phase, and as the tube increases in length
it becomes bent upon itself, somewhat in the form of the letter S. One
end of it now enlarges and forms a pouch on each side, these forming the
two auricles, right and left. From these auricles a partition grows
vertically, and when complete, cuts them off from each other, except
that a communication is left in the upper part (_the foramen ovale_)
which closes up after birth. This partition allows of the blood from one
side of the heart passing to the other. Another partition eventually
divides off that portion which has formed the auricles from the
remaining portion which develops into the ventricles, which in their
turn become again divided by a still further partition. In this way the
heart which, in the first place, was a simple tube, grows ultimately
into an organ with four distinct chambers, two auricles and two
ventricles, the only difference from this and the adult heart being the
communication which exists through the partition separating the two
auricles.
The development of the organ of hearing is somewhat complicated. The
first part to appear is a portion of the inner ear, which shows itself
as a round, hollowing of the ectoderm. This depression becomes deeper
and sinks further in, while its floor becomes thicker, and finally the
whole assumes the shape of a closed cavity. An outgrowth from this gives
rise to the _cochlea_. The cavity becomes divided into two portions, in
one of which the _semicircular canals_ arise. Around the whole, the
embryonic tissue has been forming into a strong protective covering,
some of which finally becomes cartilage, and some bone. The middle
portion of the ear is the remains of a cleft in the side of the embryo.
This cleft becomes changed into a canal by the closing of its edges, the
upper part ultimately forming the _tympanic cavity_, and the rest of it
remaining as the _Eustachian canal_. This canal opens into the pharynx.
In the cavity there are subsequently developed three small bones which
play an important part in the process of hearing. After the birth of the
embryo, air reaches the tympanic cavity, which then enlarges. One of the
walls of this cavity persists as the tympanic membrane or drum. Finally
the outer ear, that portion which is popularly spoken of as _the_ ear,
is formed from the upper portion of the same cleft which gave rise to
the tube of the tympanum.
We have referred in the preceding description to the origin of some
embryonic structures from a cleft in the early embryo itself. As a
matter of fact, no less than four of these clefts, or fissures, appear
in the region of the neck on each side, and are of the very greatest
interest and importance in connection with embryology. They are termed
the "branchial clefts," and are seen in the embryos of all vertebrates.
In the human embryo there are four. They are situated on each side of
the pharynx, and they correspond to the gill slits in lower vertebrates.
Amongst other structures which arise from the important layer of
ectoderm are the teeth. Of these there are during life two sets, a
temporary and a permanent. The temporary teeth, though they do not make
their appearance till after the birth of the embryo, still are partly
developed during embryonic life, lying embedded in the tissues until the
familiar process known as "cutting the teeth" takes place. This is, of
course, merely the time of their external appearance. The first stage in
the development, however, is a thickening of the epithelium of the gums
in a direction which is to correspond with the line where the teeth will
eventually pierce. This thickening is called the "dental ridge." This
grows downwards into the underlying tissue in flask-shaped growths. From
the neck of each of these flasks there is a small projection which
indicates where the permanent teeth will ultimately be. This first stage
is termed that of the _enamel germ_. This becomes surrounded by cells
which ultimately form a _dental sac_. Next, tissue from below grows
into the flask, and the further growth of this gives rise to the _enamel
organ_. Finally, enamel itself and dentine are developed, and the
embryonic tooth remains covered under the gums until it cuts them.
So far we have considered merely the mode of development of the most
important organs of the body, but we have said nothing of the most
important supporting structure, namely, the skeleton. The earliest
appearance of anything in the shape of a skeleton is the structure known
as the notochord, a structure of immense importance and interest in the
embryology of all vertebrate animals, in which it is a temporary thing
only. The first appearance of this notochord in lower animals is the
earliest indication of the vertebrate type, because it is found that in
the higher vertebrates it is the forerunner of the bony spinal column
and the skull. It appears first as a groove underneath the medullary
groove, of which we have already spoken, and its two lips unite to form
a cavity, as did those of the medullary groove. In this case, however,
the groove becomes a solid rod, then termed the notochord, and it lies
immediately under the medullary groove itself, which, as we have seen,
gives rise to the central nervous system. In the course of development,
masses of cells come to arrange themselves on each side of the
notochord, which they eventually include, and at the same time they grow
upwards and around the spinal cord which is thus enclosed. Later on
these surrounding portions become cartilage, and, still later, bone; the
notochord meanwhile gradually disappearing where the bony spinal column
appears. This primitive vertebrate structure therefore, of the
notochord, has the all-important function of coming to enclose, and
thus protect, the spinal cord and nervous system.
As regards the other bones of the body, all that need be said here is
that they are preceded by the structure which we know as cartilage, and
in the bones of the limbs at two or three different points this
cartilage begins to be transformed into bone. These points are known as
centres of ossification.