The use that we have hitherto made of our biogenetic law will give the
reader an idea how far we may trust its guidance in phylogenetic
investigation. This differs considerably in the various systems of
organs; the reason is that heredity and variability have a very
different range in these systems. While some of them faithfully
preserve the original palingenetic development inherited from earlier
animal ancestors, others show little trace of this rigid heredity;
they are rather disposed to follow new and divergent CENOGENETIC lines
of development in consequence of adaptation. The organs of the first
kind represent the CONSERVATIVE element in the multicellular state of
the human frame, while the latter represent the PROGRESSIVE element.
The course of historic development is a result of the correlation of
the two tendencies, and they must be carefully distinguished.
There is perhaps no other system of organs in the human body in which
this is more necessary than in that of which we are now going to
consider the obscure development--the vascular system, or apparatus of
circulation. If we were to draw our conclusions as to the original
features in our earlier animal ancestors solely from the phenomena of
the development of this system in the embryo of man and the other
higher Vertebrates, we should be wholly misled. By a number of
important embryonic adaptations, the chief of which is the formation
of an extensive food-yelk, the original course of the development of
the vascular system has been so much falsified and curtailed in the
higher Vertebrates that little or nothing now remains in their
embryology of some of the principal phylogenetic features. We should
be quite unable to explain these if comparative anatomy and ontogeny
did not come to our assistance.
The vascular system in man and all the Craniotes is an elaborate
apparatus of cavities filled with juices or cell-containing fluids.
These "vessels" (vascula) play an important part in the nutrition of
the body. They partly conduct the nutritive red blood to the various
parts of the body (blood-vessels); partly absorb from the gut the
white chyle formed in digestion (chyle-vessels); and partly collect
the used-up juices and convey them away from the tissues (lymphatic
vessels). With the latter are connected the large cavities of the
body, especially the body-cavity, or coeloma. The lymphatic vessels
conduct both the colourless lymph and the white chyle into the venous
part of the circulation. The lymphatic glands act as producers of new
blood-cells, and with them is associated the spleen. The centre of
movement for the circulation of the fluids is the heart, a strong
muscular sac, which contracts regularly and is equipped with valves
like a pump. This constant and steady circulation of the blood makes
possible the complex metabolism of the higher animals.
But, however important the vascular system may be to the more advanced
and larger and highly-differentiated animals, it is not at all so
indispensable an element of animal life as is commonly supposed. The
older science of medicine regarded the blood as the real source of
life. Even in the still prevalent confused notions of heredity the
blood plays the chief part. People speak generally of full blood, half
blood, etc., and imagine that the hereditary transmission of certain
characters "lies in the blood." The incorrectness of these ideas is
clearly seen from the fact that in the act of generation the blood of
the parents is not directly transmitted to the offspring, nor does the
embryo possess blood in its early stages. We have already seen that
not only the differentiation of the four secondary germinal layers,
but also the first structures of the principal organs in the embryo of
all the Vertebrates, take place long before there is any trace of the
vascular system--the heart and the blood. In accordance with this
ontogenetic fact, we must regard the vascular system as one of the
latest organs from the phylogenetic point of view; just as we have
found the alimentary canal to be one of the earliest. In any case, the
vascular system is much later than the alimentary.
(FIGURE 2.358. Red blood-cells of various Vertebrates (equally
magnified). 1. of man, 2. camel, 3. dove, 4. proteus, 5.
water-salamander (Triton), 6. frog, 7. merlin (Cobitis), 8. lamprey
(Petromyzon). a surface-view, b edge-view. (From Wagner.)
FIGURE 2.359. Vascular tissues or endothelium (vasalium). A capillary
from the mesentery. a vascular cells, b their nuclei.)
The important nutritive fluid that circulates as blood and lymph in
the elaborate canals of our vascular system is not a clear, simple
fluid, but a very complex chemical juice with millions of cells
floating in it. These blood-cells are just as important in the
complicated life of the higher animal body as the circulation of money
is to the commerce of a civilised community. Just as the citizens meet
their needs most conveniently by means of a financial circulation, so
the various tissue-cells, the microscopic citizens of the
multicellular human body, have their food conveyed to them best by the
circulating cells in the blood. These blood cells (haemocytes) are of
two kinds in man and all the other Craniotes--red cells (rhodocytes or
erythrocytes) and colourless or lymph cells (leucocytes). The red
colour of the blood is caused by the great accumulation of the former,
the others circulate among them in much smaller quantity. When the
colourless cells increase at the expense of the red we get anaemia (or
chlorosis).
(FIGURE 2.360. Transverse section of the trunk of a chick-embryo,
forty-five hours old. (From Balfour.) A ectoderm (horny-plate), Mc
medullary tube, ch chorda, C entoderm (gut-gland layer), Pv primitive
segment (episomite), Wd prorenal duct, pp coeloma (secondary
body-cavity). So skin-fibre layer, Sp gut-fibre layer, v blood-vessels
in latter, ao primitive aortas, containing red blood-cells.)
The lymph-cells (leucocytes), commonly called the "white corpuscles"
of the blood, are phylogenetically older and more widely distributed
in the animal world than the red. The great majority of the
Invertebrates that have acquired an independent vascular system have
only colourless lymph-cells in the circulating fluid. There is an
exception in the Nemertines (Figure 2.358) and some groups of
Annelids. When we examine the colourless blood of a cray-fish or a
snail (Figure 2.358) under a high power of the microscope, we find in
each drop numbers of mobile leucocytes, which behave just like
independent Amoebae (Figure 1.17). Like these unicellular Protozoa,
the colourless blood-cells creep slowly about, their unshapely
plasma-body constantly changing its form, and stretching out
finger-like processes first in one direction, then another. Like the
Amoebae, they take particles into their cell-body. On account of this
feature these amoeboid plastids are called "eating cells"
(phagocytes), and on account of their motions "travelling cells"
(planocytes). It has been shown by the discoveries of the last few
decades that these leucocytes are of the greatest physiological and
pathological consequence to the organism. They can absorb either solid
or dissolved particles from the wall of the gut, and convey them to
the blood in the chyle; they can absorb and remove unusable matter
from the tissues. When they pass in large quantities through the fine
pores of the capillaries and accumulate at irritated spots, they cause
inflammation. They can consume and destroy bacteria, the dreaded
vehicles of infectious diseases; but they can also transport these
injurious Monera to fresh regions, and so extend the sphere of
infection. It is probable that the sensitive and travelling leucocytes
of our invertebrate ancestors have powerfully co-operated for millions
of years in the phylogenesis of the advancing animal organisation.
The red blood-cells have a much more restricted sphere of distribution
and activity. But they also are very important in connection with
certain functions of the craniote-organism, especially the exchange of
gases or respiration. The cells of the dark red, carbonised or venous,
blood, which have absorbed carbonic acid from the animal tissues, give
this off in the respiratory organs; they receive instead of it fresh
oxygen, and thus bring about the bright red colour that distinguishes
oxydised or arterial blood. The red colouring matter of the blood
(haemoglobin) is regularly distributed in the pores of their
protoplasm. The red cells of most of the Vertebrates are elliptical
flat disks, and enclose a nucleus of the same shape; they differ a
good deal in size (Figure 2.358). The mammals are distinguished from
the other Vertebrates by the circular form of their biconcave red
cells and by the absence of a nucleus (Figure 1.1); only a few genera
still have the elliptic form inherited from the reptiles (Figure 1.2).
In the embryos of the mammals the red cells have a nucleus and the
power of increasing by cleavage (Figure 1.10).
The origin of the blood-cells and vessels in the embryo, and their
relation to the germinal layers and tissues, are among the most
difficult problems of ontogeny--those obscure questions on which the
most divergent opinions are still advanced by the most competent
scientists. In general, it is certain that the greater part of the
cells that compose the vessels and their contents come from the
mesoderm--in fact, from the gut-fibre layer; it was on this account
that Baer gave the name of "vascular layer" to this visceral layer of
the coeloma. But other important observers say that a part of these
cells come from other germinal layers, especially from the gut-gland
layer. It seems to be true that blood-cells may be formed from the
cells of the entoderm before the development of the mesoderm. If we
examine sections of chickens, the earliest and most familiar subjects
of embryology, we find at an early stage the "primitive-aortas" we
have already described (Figure 2.360 ao) in the ventral angle between
the episoma (Pv) and hyposoma (Sp). The thin wall of these first
vessels of the amniote embryo consists of flat cells (endothelia or
vascular epithelia); the fluid within already contains numbers of red
blood-cells; both have been developed from the gut-fibre layer. It is
the same with the vessels of the germinative area (Figure 2.361 v),
which lie on the entodermic membrane of the yelk-sac (c). These
features are seen still more clearly in the transverse section of the
duck-embryo in Figure 1.152. In this we see clearly how a number of
stellate cells proceed from the "vascular layer" and spread in all
directions in the "primary body-cavity"--i.e. in the spaces between
the germinal layers. A part of these travelling cells come together
and line the wall of the larger spaces, and thus form the first
vessels; others enter into the cavity, live in the fluid that fills
it, and multiply by cleavage--the first blood-cells.
But, besides these mesodermic cells of the "vascular layer" proper,
other travelling cells, of which the origin and purport are still
obscure, take part in the formation of blood in the meroblastic
Vertebrates (especially fishes). The chief of these are those that
Ruckert has most aptly denominated "merocytes." These "eating
yelk-cells" are found in large numbers in the food-yelk of the
Selachii, especially in the yelk-wall--the border zone of the germinal
disk in which the embryonic vascular net is first developed. The
nuclei of the merocytes become ten times as large as the ordinary
cell-nucleus, and are distinguished by their strong capacity for
taking colour, or their special richness in chromatin. Their
protoplasmic body resembles the stellate cells of osseous tissue
(astrocytes), and behaves just like a rhizopod (such as Gromia); it
sends out numbers of stellate processes all round, which ramify and
stretch into the surrounding food-yelk. These variable and very mobile
processes, the pseudopodia of the merocytes, serve both for locomotion
and for getting food; as in the real rhizopods, they surround the
solid particles of food (granules and plates of yelk), and accumulate
round their nucleus the food they have received and digested. Hence we
may regard them both as eating-cells (phagocytes) and travelling-cells
(planocytes). Their lively nucleus divides quickly and often
repeatedly, so that a number of new nuclei are formed in a short time;
as each fresh nucleus surrounds itself with a mantle of protoplasm, it
provides a new cell for the construction of the embryo. Their origin
is still much disputed.
(FIGURE 2.361. Merocytes of a shark-embryo, rhizopod-like yelk-cells
underneath the embryonic cavity (B). (From Ruckert.) z two embryonic
cells, k nuclei of the merocytes, which wander about in the yelk and
eat small yelk-plates (d), k smaller, more superficial, lighter
nuclei, k apostrophe a deeper nucleus, in the act of cleavage, k
asterisk chromatin-filled border-nucleus, freed from the surrounding
yelk in order to show the numerous pseudopodia of the protoplasmic
cell-body.)
Half of the twelve stems of the animal world have no blood-vessels.
They make their first appearance in the Vermalia. Their earliest
source is the primary body-cavity, the simple space between the two
primary germinal layers, which is either a relic of the
segmentation-cavity, or is a subsequent formation. Amoeboid
planocytes, which migrate from the entoderm and reach this
fluid-filled primary cavity, live and multiply there, and form the
first colourless blood-cells. We find the vascular system in this very
simple form to-day in the Bryozoa, Rotatoria, Nematoda, and other
lower Vermalia.
The first step in the improvement of this primitive vascular system is
the formation of larger canals or blood-conducting tubes. The spaces
filled with blood, the relics of the primary body-cavity, receive a
special wall. "Blood-vessels" of this kind (in the narrower sense) are
found among the higher worms in various forms, sometimes very simple,
at other times very complex. The form that was probably the incipient
structure of the elaborate vascular system of the Vertebrates (and of
the Articulates) is found in two primordial principal vessels--a
dorsal vessel in the middle line of the dorsal wall of the gut, and a
ventral vessel that runs from front to rear in the middle line of its
ventral wall. From the dorsal vessel is evolved the aorta (or
principal artery), from the ventral vessel the principal or
subintestinal vein. The two vessels are connected in front and behind
by a loop that runs round the gut. The blood contained in the two
tubes is propelled by their peristaltic contractions.
(FIGURE 2.362. Vascular system of an Annelid (Saenuris), foremost
section. d dorsal vessel, v ventral vessel, c transverse connection of
two (enlarged in shape of heart). The arrows indicate the direction of
the flow of blood. (From Gegenbaur.)
The earliest Vermalia in which we first find this independent vascular
system are the Nemertina (Figure 2.244). As a rule, they have three
parallel longitudinal vessels connected by loops, a single dorsal
vessel above the gut and a pair of lateral vessels to the right and
left. In some of the Nemertina the blood is already coloured, and the
red colouring matter is real haemoglobin, connected with elliptical
discoid cells, as in the Vertebrates. The further evolution of this
rudimentary vascular system can be gathered from the class of the
Annelids in which we find it at various stages of development. First,
a number of transverse connections are formed between the dorsal and
ventral vessels, which pass round the gut ring-wise (Figure 2.362).
Other vessels grow into the body-wall and ramify in order to convey
blood to it. In addition to the two large vessels of the middle plane
there are often two lateral vessels, one to the right and one to the
left; as, for instance, in the leech. There are four of these parallel
longitudinal vessels in the Enteropneusts (Balanoglossus, Figure
2.245). In these important Vermalia the foremost section of the gut
has already been converted into a gill-crate, and the vascular arches
that rise in the wall of this from the ventral to the dorsal vessel
have become branchial vessels.
We have a further important advance in the Tunicates, which we have
recognised as the nearest blood-relatives of our early vertebrate
ancestors. Here we find for the first time a real heart--i.e. a
central organ of circulation, driving the blood into the vessels by
the regular contractions of its muscular wall, it is of a very
rudimentary character, a spindle-shaped tube, passing at both ends
into a principal vessel (Figure 2.221). By its original position
behind the gill-crate, on ventral side of the Tunicates (sometimes
more, sometimes less, forward), the head shows clearly that it has
been formed by the local enlargement of a section of the ventral
vessel. We have already noticed the remarkable alternation of the
direction of the blood stream, the heart driving it first from one
end, then from the other (Chapter 2.16). This is very instructive,
because in most of the worms (even the Enteropneust) the blood in the
dorsal vessel travels from back to front, but in the Vertebrates in
the opposite direction. As the Ascidia-heart alternates steadily from
one direction to the other, it shows us permanently, in a sense, the
phylogenetic transition from the earlier forward direction of the
dorsal current (in the worms) to the new backward direction (in the
Vertebrates).
(FIGURE 2.363. Head of a fish-embryo, with rudimentary vascular
system, from the left. dc Cuvier's duct (juncture of the anterior and
posterior principal veins), sv venous sinus (enlarged end of Cuvier's
duct), a auricle, v ventricle, abr trunk of branchial artery, s
gill-clefts (arterial arches between), ad aorta, c carotid artery, n
nasal pit. (From Gegenbaur.)
FIGURE 2.364. The five arterial arches of the Craniotes (1 to 5) in
their original disposition, a arterial cone or bulb, a double
apostrophe aorta-trunk, c carotid artery (foremost continuation of the
roots of the aorta). (From Rathke.)
FIGURE 2.365. The five arterial arches of the birds; the lighter parts
of the structure disappear; only the shaded parts remain. Letters as
in Figure 2.364. s subclavian arteries, p pulmonary artery, p
apostrophe branches of same, c apostrophe outer carotid, c double
apostrophe inner carotid. (From Rathke.)
FIGURE 2.366. The five arterial arches of mammals; letters as in
Figure 2.365. v vertebral artery, b Botall's duct (open in the embryo,
closed afterwards). (From Rathke.))
As the new direction became permanent in the earlier Prochordonia,
which gave rise to the Vertebrate stem, the two vessels that proceed
from either end of the tubular heart acquired a fixed function. The
foremost section of the ventral vessel henceforth always conveys blood
from the heart, and so acts as an artery; the hind section of the same
vessel brings the blood from the body to the heart, and so becomes a
vein. In view of their relation to the two sections of the gut, we may
call the latter the intestinal vein and the former the branchial
artery. The blood contained in both vessels, and also in the heart, is
venous or carbonised blood--i.e. rich in carbonic acid; on the other
hand, the blood that passes from the gills into the dorsal vessel is
provided with fresh oxygen--arterial or oxydised blood. The finest
branches of the arteries and veins pass into each other in the tissues
by means of a network of very fine, ventral, hair-like vessels, or
capillaries (Figure 2.359).
When we turn from the Tunicates to the closely-related Amphioxus we
are astonished at first to find an apparent retrogression in the
formation of the vascular system. As we have seen, the Amphioxus has
no real heart; its colourless blood is driven along in its vascular
system by the principal vessel itself, which contracts regularly in
its whole length (cf. Figure 2.210). A dorsal vessel that lies above
the gut (aorta) receives the arterial blood from the gills and drives
it into the body. Returning from here, the venous blood gathers in a
ventral vessel under the gut (intestinal vein), and goes back to the
gills. A number of branchial vascular arches, which effect respiration
and rise in the wall of the branchial gut from belly to back, absorb
oxygen from the water and give off carbonic acid; they connect the
ventral with the dorsal vessel. As the same section of the ventral
vessel, which also forms the heart in the Craniotes, has developed in
the Ascidia into a simple tubular heart, we may regard the absence of
this in the Amphioxus as a result of degeneration, a return in this
case to the earlier form of the vascular system, as we find it in many
of the worms. We may assume that the Acrania that really belong to our
ancestral series did not share this retrogression, but inherited the
one-chambered heart of the Prochordonia, and transmitted it directly
to the earliest Craniotes (cf. the ideal Primitive Vertebrate,
Prospondylus, Figures 1.98 to 1.102).
(FIGURES 2.367 TO 2.370. Metamorphosis of the five arterial arches in
the human embryo (diagram from Rathke). la arterial cone, 1, 2, 3, 4,
5 first to fifth pair of arteries, ad trunk of aorta, aw roots of
aorta. In Figure 2.367 only three, in Figure 2.368 all five, of the
aortic arches are given (the dotted ones only are developed). In
Figure 2.369 the first two pairs have disappeared again. In Figure
2.370 the permanent trunks of the artery are shown; the dotted parts
disappear, s subclavian artery, v vertebral, ax axillary, c carotid (c
apostrophe outer, c double apostrophe inner carotid), p pulmonary.)
The further phylogenetic evolution of the vascular system is revealed
to us by the comparative anatomy of the Craniotes. At the lowest stage
of this group, in the Cyclostomes, we find for the first time the
differentiation of the vasorium into two sections: a system of
blood-vessels proper, which convey the RED blood about the body, and a
system of lymphatic vessels, which absorb the colourless lymph from
the tissues and convey it to the blood. The lymphatics that absorb
from the gut and pour into the blood-stream the milky food-fluid
formed by digestion are distinguished by the special name of
"chyle-vessels." While the chyle is white on account of its high
proportion of fatty particles, the lymph proper is colourless. Both
chyle and lymph contain the colourless amoeboid cells (leucocytes,
Figure 1.12) that we also find distributed in the blood as colourless
blood-cells (or "white corpuscles"); but the blood also contains a
much larger quantity of red cells, and these give its characteristic
colour to the blood of the Craniotes (rhodocytes, Figure 2.358). The
distinction between lymph, chyle, and blood-vessels which is found in
all the Craniotes may be regarded as an outcome of division of labour
between various sections of our originally simple vascular system. In
the Gnathostomes the spleen makes its first appearance, an organ rich
in blood, the chief function of which is the extensive formation of
new colourless and red cells. It is not found in the Acrania and
Cyclostomes, or any of the Invertebrates. It has been transmitted from
the earliest fishes to all the Craniotes.
The heart also, the central organ of circulation in all the Craniotes,
shows an advance in structure in the Cyclostomes. The simple,
spindle-shaped heart-tube, found in the same form in the embryo of all
the Craniotes, is divided into two sections or chambers in the
Cyclostomes, and these are separated by a pair of valves. The hind
section, the auricle, receives the venous blood from the body and
passes it on to the anterior section, the ventricle. From this it is
driven through the trunk of the branchial artery (the foremost section
of the ventral vessel or principal vein) into the gills.
In the Selachii an arterial cone is developed from the foremost end of
the ventricle, as a special division, cut off by valves. It passes
into the enlarged base of the trunk of the branchial artery (Figure
2.363 abr). On each side 5 to 7 arteries proceed from it. These rise
between the gill-clefts (s) on the gill-arches, surround the gullet,
and unite above into a common trunk-aorta, the continuation of which
over the gut corresponds to the dorsal vessel of the worms. As the
curved arteries on the gill-arches spread into a network of
respiratory capillaries, they contain venous blood in their lower part
(as arches of the branchial artery) and arterial blood in the upper
part (as arches of the aorta). The junctures of the various aortic
arches on the right and left are called the roots of the aorta. Of an
originally large number of aortic arches there remain at first six,
then (owing to degeneration of the fifth arch) only five, pairs; and
from these five pairs (Figure 2.364) the chief parts of the arterial
system develop in all the higher Vertebrates.
(FIGURE 2.371. Heart of a rabbit-embryo, from behind, a vitelline
veins, b auricles of the heart, c atrium, d ventricle, e arterial
bulb, f base of the three pairs of arterial arches. (From Bischoff.)
FIGURE 2.372. Heart of the same embryo (Figure 2.371), from the front.
v vitelline veins, a auricle, ca auricular canal, l left ventricle, r
right ventricle, ta arterial bulb. (From Bischoff.))
The appearance of the lungs and the atmospheric respiration connected
therewith, which we first meet in the Dipneusts, is the next important
step in vascular evolution. In the Dipneusts the auricle of the heart
is divided by an incomplete partition into two halves. Only the right
auricle now receives the venous blood from the veins of the body. The
left auricle receives the arterial blood from the pulmonary veins. The
two auricles have a common opening into the simple ventricle, where
the two kinds of blood mix, and are driven through the arterial cone
or bulb into the arterial arches. From the last arterial arches the
pulmonary arteries arise (Figure 2.365 p). These force a part of the
mixed blood into the lungs, the other part of it going through the
aorta into the body.
From the Dipneusts upwards we now trace a progressive development of
the vascular system, which ends finally with the loss of branchial
respiration and a complete separation of the two halves of the
circulation. In the Amphibia the partition between the two auricles is
complete. In their earlier stages, as tadpoles (Figure 2.262), they
have still the branchial respiration and the circulation of the
fishes, and their heart contains venous blood alone. Afterwards the
lungs and pulmonary vessels are developed, and henceforth the
ventricle of the heart contains mixed blood. In the reptiles the
ventricle and its arterial cone begin to divide into two halves by a
longitudinal partition, and this partition becomes complete in the
higher reptiles and birds on the one hand, and the stem-forms of the
mammals on the other. Henceforth, the right half of the heart contains
only venous, and the left half only arterial, blood, as we find in all
birds and mammals. The right auricle receives its carbonised or venous
blood from the veins of the body, and the right ventricle drives it
through the pulmonary arteries into the lungs. From here the blood
returns, as oxydised or arterial blood, through the pulmonary veins to
the left auricle, and is forced by the left ventricle into the
arteries of the body. Between the pulmonary arteries and veins is the
capillary system of the small or pulmonary circulation. Between the
body-arteries and veins is the capillary system of the large or
body-circulation. It is only in the two highest classes of
Vertebrates--the birds and mammals--that we find a complete division
of the circulations. Moreover, this complete separation has been
developed quite independently in the two classes, as the dissimilar
formation of the aortas shows of itself. In the birds the RIGHT half
of the fourth arterial arch has become the permanent arch (Figure
2.365). In the mammals this has been developed from the LEFT half of
the same fourth arch (Figure 2.366).
(FIGURE 2.373. Heart and head of a dog-embryo, from the front, a fore
brain, b eyes, c middle brain, d primitive lower jaw, e primitive
upper jaw, f gill-arches, g right auricle, h left auricle, i left
ventricle, k right ventricle. (From Bischoff.)
FIGURE 2.374. Heart of the same dog-embryo, from behind. a
inosculation of the vitelline veins, b left auricle, c right auricle,
d auricle, e auricular canal, f left ventricle, g right ventricle, h
arterial bulb, (From Bischoff)
FIGURE 2.375. Heart of a human embryo, four weeks old; 1. front view,
2. back view, 3. opened, and upper half of the atrium removed. a
apostrophe left auricle, a double apostrophe right auricle, v
apostrophe left ventricle, v double apostrophe right ventricle, ao
arterial bulb, c superior vena cava (cd right, cs left), s rudiment of
the interventricular wall. (From Kolliker.)
FIGURE 2.376. Heart of a human embryo, six weeks old, front view. r
right ventricle, t left ventricle, s furrow between ventricles, ta
arterial bulb, af furrow on its surface; to right and left are the two
large auricles. (From Ecker.)
FIGURE 2.377. Heart of a human embryo, eight weeks old, back view. a
apostrophe left auricle, a double apostrophe right auricle, v
apostrophe left ventricle, v double apostrophe right ventricle, cd
apostrophe right superior vena cava, ci inferior vena cava. (From
Kolliker.))
If we compare the fully-developed arterial system of the various
classes of Craniotes, it shows a good deal of variety, yet it always
proceeds from the same fundamental type. Its development is just the
same in man as in the other mammals; in particular, the modification
of the six pairs of arterial arches is the same in both (Figures 2.367
to 2.370). At first there is only a single pair of arches, which lie
on the inner surface of the first pair of gill-arches. Behind this
there then develop a second and third pair of arches (lying on the
inner side of the second and third gill-arches, Figure 2.367).
Finally, we get a fourth, fifth, and sixth pair. Of the six primitive
arterial arches of the Amniotes three soon pass away (the first,
second, and fifth); of the remaining three, the third gives the
carotids, the fourth the aortas, and the sixth (number 5 in Figures
2.364 and 2.368) the pulmonary arteries.
The human heart also develops in just the same way as that of the
other mammals (Figure 2.378). We have already seen the first rudiments
of its embryology, which in the main corresponds to its phylogeny
(Figures 1.201 and 1.202). We saw that the palingenetic form of the
heart is a spindle-shaped thickening of the gut-fibre layer in the
ventral wall of the head-gut. The structure is then hollowed out,
forms a simple tube, detaches from its place of origin, and henceforth
lies freely in the cardiac cavity. Presently the tube bends into the
shape of an S, and turns spirally on an imaginary axis in such a way
that the hind part comes to lie on the dorsal surface of the fore
part. The united vitelline veins open into the posterior end. From the
anterior end spring the aortic arches.
(FIGURE 2.378. Heart of the adult man, fully developed, front view,
natural position. a right auricle (underneath it the right ventricle),
b left auricle (under it the left ventricle), C superior vena cava, V
pulmonary veins, P pulmonary artery, d Botalli's duct, A aorta. (From
Meyer.))
This first structure of the human heart, enclosing a very simple
cavity, corresponds to the tunicate-heart, and is a reproduction of
that of the Prochordonia, but it now divides into two, and
subsequently into three, compartments; this reminds us for a time of
the heart of the Cyclostomes and fishes. The spiral turning and
bending of the heart increases, and at the same time two transverse
constrictions appear, dividing it externally into three sections
(Figures 2.371 and 2.372). The foremost section, which is turned
towards the ventral side, and from which the aortic arches rise,
reproduces the arterial bulb of the Selachii. The middle section is a
simple ventricle, and the hindmost, the section turned towards the
dorsal side, into which the vitelline veins inosculate, is a simple
auricle (or atrium). The latter forms, like the simple atrium of the
fish-heart, a pair of lateral dilatations, the auricles (Figure 2.371
b); and the constriction between the atrium and ventricle is called
the auricular canal (Figure 2.372 ca). The heart of the human embryo
is now a complete fish-heart.
(FIGURE 2.379. Transverse section of the back of the head of a
chick-embryo, forty hours old. (From Kolliker.) m medulla oblongata,
ph pharyngeal cavity (head-gut), h horny plate, h apostrophe thicker
part of it, from which the auscultory pits afterwards develop, hp
skin-fibre plate, hh cervical cavity (head-coelom or cardiocoel), hzp
cardiac plate (the outermost mesodermic wall of the heart), connected
by the ventral mesocardium (uhg) with the gut-fibre layer or visceral
coelom-layer (dfp apostrophe), Ent entoderm, ihh inner (entodermic?)
wall of the heart; the two endothelial cardiac tubes are still
separated by the cenogenetic septum (s) of the Amniotes, g vessels.)
In perfect harmony with its phylogeny, the embryonic development of
the human heart shows a gradual transition from the fish-heart,
through the amphibian and reptile, to the mammal form, The most
important point in the transition is the formation of a longitudinal
partition--incomplete at first, but afterwards complete--which
separates all three divisions of the heart into right (venous) and
left (arterial) halves (cf. Figures 2.373 to 2.378). The atrium is
separated into a right and left half, each of which absorbs the
corresponding auricle; into the right auricle open the body-veins
(upper and lower vena cava, Figures 2.375 c and 2.377 c); the left
auricle receives the pulmonary veins. In the same way a superficial
interventricular furrow is soon seen in the ventricle (Figure 2.376
s). This is the external sign of the internal partition by which the
ventricle is divided into two--a right venous and left arterial
ventricle. Finally a longitudinal partition is formed in the third
section of the primitive fish-like heart, the arterial bulb,
externally indicated by a longitudinal furrow (Figure 2.376 af). The
cavity of the bulb is divided into two lateral halves, the
pulmonary-artery bulb, that opens into the right ventricle, and the
aorta-bulb, that opens into the left ventricle. When all the
partitions are complete, the small (pulmonary) circulation is
distinguished from the large (body) circulation; the motive centre of
the former is the right half, and that of the latter the left half, of
the heart.
The heart of all the Vertebrates belongs originally to the hyposoma of
the head, and we accordingly find it in the embryo of man and all the
other Amniotes right in front on the under-side of the head; just as
in the fishes it remains permanently in front of the gullet. It
afterwards descends into the trunk, with the advance in the
development of the neck and breast, and at last reaches the breast,
between the two lungs. At first it lies symmetrically in the middle
plane of the body, so that its long axis corresponds with that of the
body. In most of the mammals it remains permanently in this position.
But in the apes the axis begins to be oblique, and the apex of the
heart to move towards the left side. The displacement is greatest in
the anthropoid apes--chimpanzee, gorilla, and orang--which resemble
man in this.
As the heart of all Vertebrates is originally, in the light of
phylogeny, only a local enlargement of the middle principal vein, it
is in perfect accord with the biogenetic law that its first structure
in the embryo is a simple spindle-shaped tube in the ventral wall of
the head-gut. A thin membrane, standing vertically in the middle
plane, the mesocardium, connects the ventral wall of the head-gut with
the lower head-wall. As the cardiac tube extends and detaches from the
gut-wall, it divides the mesocardium into an upper (dorsal) and lower
(ventral) plate (usually called the mesocardium anterius and posterius
in man, Figure 2.379 uhg). The mesocardium divides two lateral
cavities, Remak's "neck-cavities" (Figure 2.379 hh). These cavities
afterwards join and form the simple pericardial cavity, and are
therefore called by Kolliker the "primitive pericardial cavities."
(FIGURE 2.380. Frontal section of a human embryo, one-twelfth of an
inch long in the neck, magnified forty times; "invented" by Wilhelm
His. Seen from ventral side. mb mouth-fissure, surrounded by the
branchial processes, ab bulbus of aorta, hm middle part of ventricle,
hl left lateral part of same, ho auricle, d diaphragm, vc superior
vena cava, vu umbilical vein, vo vitelline space, lb liver, lg hepatic
duct.)
The double cervical cavity of the Amniotes is very interesting, both
from the anatomical and the evolutionary point of view; it corresponds
to a part of the hyposomites of the head of the lower
Vertebrates--that part of the ventral coelom-pouches which comes next
to Van Wijhe's "visceral cavities" below. Each of the cavities still
communicates freely behind with the two coelom-pouches of the trunk;
and, just as these afterwards coalesce into a simple body-cavity (the
ventral mesentery disappearing), we find the same thing happening in
the head. This simple primary pericardial cavity has been well called
by Gegenbaur the "head-coeloma," and by Hertwig the "pericardial
breast-cavity." As it now encloses the heart, it may also be called
cardiocoel.
The cardiocoel, or head-coelom, is often disproportionately large in
the Amniotes, the simple cardiac tube growing considerably and lying
in several folds. This causes the ventral wall of the amniote embryo,
between the head and the navel, to be pushed outwards as in rupture
(cf. Figure 1.180 h). A transverse fold of the ventral wall, which
receives all the vein-trunks that open into the heart, grows up from
below between the pericardium and the stomach, and forms a transverse
partition, which is the first structure of the primary diaphragm
(Figure 2.380 d). This important muscular partition, which completely
separates the thoracic and abdominal cavities in the mammals alone, is
still very imperfect here; the two cavities still communicate for a
time by two narrow canals. These canals, which belong to the dorsal
part of the head-coelom, and which we may call briefly pleural ducts,
receive the two pulmonary sacs, which develop from the hind end of the
ventral wall of the head-gut; they thus become the two pleural
cavities.
The diaphragm makes its first appearance in the class of the Amphibia
(in the salamanders) as an insignificant muscular transverse fold of
the ventral wall, which rises from the fore end of the transverse
abdominal muscle, and grows between the pericardium and the liver. In
the reptiles (tortoises and crocodiles) a later dorsal part is joined
to this earlier ventral part of the rudimentary diaphragm, a pair of
subvertebral muscles rising from the vertebral column and being added
as "columns" to the transverse partition. But it was probably in the
Permian sauro-mammals that the two originally separate parts were
united, and the diaphragm became a complete partition between the
thoracic and abdominal cavities in the mammals; as it considerably
enlarges the chest-cavity when it contracts, it becomes an important
respiratory muscle. The ontogeny of the diaphragm in man and the other
mammals reproduces this phylogenetic process to-day, in accordance
with the biogenetic law; in all the mammals the diaphragm is formed by
the secondary conjunction of the two originally separate structures,
the earlier ventral part and the later dorsal part.
Sometimes the blending of the two diaphragmatic structures, and
consequently the severance of the one pleural duct from the abdominal
cavity, is not completed in man. This leads to a diaphragmatic rupture
(hernia diaphragmatica). The two cavities then remain in communication
by an open pleural duct, and loops of the intestine may penetrate by
this "rupture opening" into the chest-cavity. This is one of those
fatal mis-growths that show the great part that blind chance has in
organic development.
(FIGURE 2.381. Transverse section of the head of a chick-embryo,
thirty-six hours old. Underneath the medullary tube the two primitive
aortas (pa) can be seen in the head-plates (s) at each side of the
chorda. Underneath the gullet (d) we see the aorta-end of the heart
(ae), hh cervical cavity or head coelom, hk top of heart, ks
head-sheath, amniotic fold, h horny plate. (From Remak.)
(FIGURE 2.382. Transverse section of the cardiac region of the same
chick-embryo (behind the preceding). In the cervical cavity (hh) the
heart (h) is still connected by a mesocard (hg) with the gut-fibre
layer (pf). d gut-gland layer, up provertebral plates, jb rudimentary
auditory vesicle in the horny plate, hp first rise of the amniotic
fold. (From Remak.))
Thus the thoracic cavity of the mammals, with its important contents,
the heart and lungs, belongs originally to the HEAD-PART of the
vertebrate body, and its inclusion in the trunk is secondary. This
instructive and very interesting fact is entirely proved by the
concordant evidence of comparative anatomy and ontogeny. The lungs are
outgrowths of the head-gut; the heart develops from its inner wall.
The pleural sacs that enclose the lungs are dorsal parts of the
head-coelom, originating from the pleuroducts; the pericardium in
which the heart afterwards lies is also double originally, being
formed from ventral halves of the head-coelom, which only combine at a
later stage. When the lung of the air-breathing Vertebrates issues
from the head-cavity and enters the trunk-cavity, it follows the
example of the floating bladder of the fishes, which also originates
from the pharyngeal wall in the shape of a small pouch-like
out-growth, but soon grows so large that, in order to find room, it
has to pass far behind into the trunk-cavity. To put it more
precisely, the lung of the quadrupeds retains this hereditary
growth-process of the fishes; for the hydrostatic floating bladder of
the latter is the air-filled organ from which the air-breathing organ
of the former has been evolved.
There is an interesting cenogenetic phenomenon in the formation of the
heart of the higher Vertebrates that deserves special notice. In its
earliest form the heart is DOUBLE, as recent observation has shown, in
all the Amniotes, and the simple spindle-shaped cardiac tube, which we
took as our starting-point, is only formed at a later stage, when the
two lateral tubes move backwards, touch each other, and at last
combine in the middle line. In man, as in the rabbit, the two
embryonic hearts are still far apart at the stage when there are
already eight primitive segments (Figure 1.134 h). So also the two
coelom-pouches of the head in which they lie are still separated by a
broad space. It is not until the permanent body of the embryo develops
and detaches from the embryonic vesicle that the separate lateral
structures join together, and finally combine in the middle line. As
the median partition between the right and left cardiocoel disappears,
the two cervical cavities freely communicate (Figure 2.381), and form,
on the ventral side of the amniote head, a horseshoe-shaped arch, the
points of which advance backwards into the pleuro-ducts or pleural
cavities, and from there into the two peritoneal sacs of the trunk.
But even after the conjunction of the cervical cavities (Figure 2.381)
the two cardiac tubes remain separate at first; and even after they
have united a delicate partition in the middle of the simple
endothelial tube (Figures 2.379 s and 2.382 h) indicates the original
separation. This CENOGENETIC "primary cardiac septum" presently
disappears, and has no relation to the subsequent permanent partition
between the halves of the heart, which, as a heritage from the
reptiles, has a great PALINGENETIC importance.
Thorough opponents of the biogenetic law have laid great stress on
these and similar cenogenetic phenomena, and endeavoured to urge them
as striking disproofs of the law. As in every other instance, careful,
discriminating, comparative-morphological examination converts these
supposed disproofs of evolution into strong arguments in its favour.
In his excellent work, On the structure of the Heart in the Amphibia
(1886), Carl Rabl has shown how easily these curious cenogenetic facts
can be explained by the secondary adaptation of the embryonic
structure to the great extension of the food-yelk.
The embryology of all the other parts of the vascular system also
gives us abundant and valuable data for the purposes of phylogeny. But
as one needs a thorough knowledge of the intricate structure of the
whole vascular system in man and the other Vertebrates in order to
follow this with profit, we cannot go into it further here. Moreover,
many important features in the ontogeny of the vascular system are
still very obscure and controverted. The characters of the embryonic
circulation of the Amniotes, which we have previously considered
(Chapter 1.15), are late acquisitions and entirely cenogenetic. (Cf.