The structural features that distinguish the vertebrates from the
invertebrates are so prominent that there was the greatest difficulty
in the earlier stages of classification in determining the affinity of
these two great groups. When scientists began to speak of the affinity
of the various animal groups in more than a figurative--in a
genealogical--sense, this question came at once to the front, and
seemed to constitute one of the chief obstacles to the carrying-out of
the evolutionary theory. Even earlier, when they had studied the
relations of the chief groups, without any idea of real genealogical
connection, they believed they had found here and there among the
invertebrates points of contact with the vertebrates: some of the
worms, especially, seemed to approach the vertebrates in structure,
such as the marine arrow-worm (Sagitta). But on closer study the
analogies proved untenable. When Darwin gave an impulse to the
construction of a real stem-history of the animal kingdom by his
reform of the theory of evolution, the solution of this problem was
found to be particularly difficult. When I made the first attempt in
my General Morphology (1866) to work out the theory and apply it to
classification, I found no problem of phylogeny that gave me so much
trouble as the linking of the vertebrates with the invertebrates.
But just at this time the true link was discovered, and at a point
where it was least expected. Towards the end of 1866 two works of the
Russian zoologist, Kowalevsky, who had lived for some time at Naples,
and studied the embryology of the lower animals, were issued in the
publications of the St. Petersburg Academy. A fortunate accident had
directed the attention of this able observer almost simultaneously to
the embryology of the lowest vertebrate, the Amphioxus, and that of an
invertebrate, the close affinity of which to the Amphioxus had been
least suspected, the Ascidia. To the extreme astonishment of all
zoologists who were interested in this important question, there
turned out to be the utmost resemblance in structure from the
commencement of development between these two very different
animals--the lowest vertebrate and the mis-shaped, sessile
invertebrate. With this undeniable identity of ontogenesis, which can
be demonstrated to an astounding extent, we had, in virtue of the
biogenetic law, discovered the long-sought genealogical link, and
definitely identified the invertebrate group that represents the
nearest blood-relatives of the vertebrates. The discovery was
confirmed by other zoologists, and there can no longer be any doubt
that of all the classes of invertebrates that of the Tunicates is most
closely related to the vertebrates, and of the Tunicates the nearest
are the Ascidiae. We cannot say that the vertebrates are descended
from the Ascidiae--and still less the reverse--but we can say that of
all the invertebrates it is the Tunicates, and, within this group, the
Ascidiae, that are the nearest blood-relatives of the ancient
stem-form of the vertebrates. We must assume as the common ancestral
group of both stems an extinct family of the extensive vermalia-stem,
the Prochordonia or Prochordata ("primitive chorda-animals").
In order to appreciate fully this remarkable fact, and especially to
secure the sound basis we seek for the genealogical tree of the
vertebrates, it is necessary to study thoroughly the embryology of
both these animals, and compare the individual development of the
Amphioxus step by step with that of the Ascidia. We begin with the
ontogeny of the Amphioxus.
From the concordant observations of Kowalevsky at Naples and Hatschek
at Messina, it follows, firstly, that the ovum-segmentation and
gastrulation of the Amphioxus are of the simplest character. They take
place in the same way as we find them in many of the lower animals of
different invertebrate stems, which we have already described as
original or primordial; the development of the Ascidia is of the same
type. Sexually mature specimens of the Amphioxus, which are found in
great quantities at Messina from April or May onwards, begin as a rule
to eject their sexual products in the evening; if you catch them about
the middle of a warm night and put them in a glass vessel with
seawater, they immediately eject through the mouth their accumulated
sexual products, in consequence of the disturbance. The males give out
masses of sperm, and the females discharge ova in such quantity that
many of them stick to the fibrils about their mouths. Both kinds of
cells pass first into the mantle-cavity after the opening of the
gonads, proceed through the gill-clefts into the branchial gut, and
are discharged from this through the mouth.
The ova are simply round cells. They are only 1/250 of an inch in
diameter, and thus are only half the size of the mammal ova, and have
no distinctive features. The clear protoplasm of the mature ovum is
made so turbid by the numbers of dark granules of food-yelk or
deutoplasm scattered in it that it is difficult to follow the process
of fecundation and the behaviour of the two nuclei during it (Chapter
1.7). The active elements of the male sperm, the cone-shaped
spermatozoa, are similar to those of most other animals (cf. Figure
1.20). Fecundation takes place when these lively ciliated cells of the
sperm approach the ovum, and seek to penetrate into the yelk-matter or
the cellular substance of the ovum with their head-part--the thicker
part of the cell that encloses the nucleus. Only one spermatozoon can
bore its way into the yelk at one pole of the ovum-axis; its head or
nucleus coalesces with the female nucleus, which remains after the
extrusion of the directive bodies from the germinal vesicle. Thus is
formed the "stem-nucleus," or the nucleus of the "stem-cell" (cytula,
Figure 1.2). This now undergoes total segmentation, dividing into two,
four, eight, sixteen, thirty-two cells, and so on. In this way we get
the spherical, mulberry-shaped body, which we call the morula.
The segmentation of the Amphioxus is not entirely regular, as was
supposed after the first observations of Kowalevsky (1866). It is not
completely equal, but a little unequal. As Hatschek afterwards found
(1879), the segmentation-cells only remain equal up to the
morula-stage, the spherical body of which consists of thirty-two
cells. Then, as always happens in unequal segmentation, the more
sluggish vegetal cells are outstripped in the cleavage. At the lower
or vegetal pole of the ovum a crown of eight large entodermic cells
remains for a long time unchanged, while the other cells divide, owing
to the formation of a series of horizontal circles, into an increasing
number of crowns of sixteen cells each. Afterwards the
segmentation-cells get more or less irregularly displaced, while the
segmentation-cavity enlarges in the centre of the morula; in the end
the former all lie on the surface of the latter, so that the foetus
attains the familiar blastula shape and forms a hollow ball, the wall
of which consists of a single stratum of cells (Figure 1.38 A to C).
This layer is the blastoderm, the simple epithelium from the cells of
which all the tissues of the body proceed.
These important early embryonic processes take place so quickly in the
Amphioxus that four or five hours after fecundation, or about
midnight, the spherical blastula is completed. A pit-like depression
is then formed at the vegetal pole of it, and in consequence of this
the hollow sphere doubles on itself (Figure 1.38 D). This pit becomes
deeper and deeper (Figure 1.38 E and F); at last the invagination (or
doubling) is complete, and the inner or folded part of the
blastula-wall lies on the inside of the outer wall. We thus get a
hollow hemisphere, the thin wall of which is made up of two layers of
cells (Figure 1.38 E). From hemispherical the body soon becomes almost
spherical once more, and then oval, the internal cavity enlarging
considerably and its mouth growing narrower (Figure 2.213). The form
which the Amphioxus-embryo has thus reached is a real "cup-larva" or
gastrula, of the original simple type that we have previously
described as the "bell-gastrula" or archigastrula (Figures 1.29 to
1.35).
As in all the other animals that form an archigastrula, the whole body
is nothing but a simple gastric sac or stomach; its internal cavity is
the primitive gut (progaster or archenteron, Figure 1.38 g, 1.35 d),
and its aperture the primitive mouth (prostoma or blastoporus, o). The
wall is at once gut-wall and body-wall. It is composed of two simple
cell-layers, the familiar primary germinal layers. The inner layer or
the invaginated part of the blastoderm, which immediately encloses the
gut-cavity is the entoderm, the inner or vegetal germ-layer, from
which develop the wall of the alimentary canal and all its appendages,
the coelom-pouches, etc. (Figures 1.35 and 1.36 i). The outer stratum
of cells, or the non-invaginated part of the blastoderm, is the
ectoderm, the outer or animal germ-layer, which provides the outer
skin (epidermis) and the nervous system (e). The cells of the entoderm
are much larger, darker, and more fatty than those of the ectoderm,
which are clearer and less rich in fatty particles. Hence before and
during invagination there is an increasing differentiation of the
inner from the outer layer. The animal cells of the outer layer soon
develop vibratory hairs; the vegetal cells of the inner layer do so
much later. A thread-like process grows out of each cell, and effects
continuous vibratory movements. By the vibrations of these slender
hairs the gastrula of the Amphioxus swims about in the sea, when it
has pierced the thin ovolemma, like the gastrula of many other animals
(Figure 1.36). As in many other lower animals, the cells have only one
whip-like hair each, and so are called flagellate (whip) cells (in
contrast with the ciliated cells, which have a number of short lashes
or cilia).
In the further course of its rapid development the roundish
bell-gastrula becomes elongated, and begins to flatten on one side,
parallel to the long axis. The flattened side is the subsequent dorsal
side; the opposite or ventral side remains curved. The latter grows
more quickly than the former, with the result that the primitive mouth
is forced to the dorsal side (Figure 1.39). In the middle of the
dorsal surface a shallow longitudinal groove or furrow is formed
(Figure 1.79), and the edges of the body rise up on each side of this
groove in the shape of two parallel swellings. This groove is, of
course, the dorsal furrow, and the swellings are the dorsal or
medullary swellings; they form the first structure of the central
nervous system, the medullary tube. The medullary swellings now rise
higher; the groove between them becomes deeper and deeper. The edges
of the parallel swellings curve towards each other, and at last unite,
and the medullary tube is formed (Figures 1.83 m and 1.84 m). Hence
the formation of a medullary tube out of the outer skin takes place in
the naked dorsal surface of the free-swimming larva of the Amphioxus
in just the same way as we have found in the embryo of man and the
higher animals within the foetal membranes.
Simultaneously with the construction of the medullary tube we have in
the Amphioxus-embryo the formation of the chorda, the coelom-pouches,
and the mesoderm proceeding from their wall. These processes also take
place with characteristic simplicity and clearness, so that they are
very instructive to compare with the vermalia on the one hand and with
the higher vertebrates on the other. While the medullary groove is
sinking in the middle line of the flat dorsal side of the oval embryo,
and its parallel edges unite to form the ectodermic neural tube, the
single chorda is formed directly underneath them, and on each side of
this a parallel longitudinal fold, from the dorsal wall of the
primitive gut. These longitudinal folds of the entoderm proceed from
the primitive mouth, or from its lower and hinder edge. Here we see at
an early stage a couple of large entodermic cells, which are
distinguished from all the others by their great size, round form, and
fine-grained protoplasm; they are the two promesoblasts, or polar
cells of the mesoderm (Figure 1.83 p). They indicate the original
starting-point of the two coelom-pouches, which grow from this spot
between the inner and outer germinal layers, sever themselves from the
primitive gut, and provide the cellular material for the middle layer.
Immediately after their formation the two coelom-pouches of the
Amphioxus are divided into several parts by longitudinal and
transverse folds. Each of the primary pouches is divided into an upper
dorsal and a lower ventral section by a couple of lateral longitudinal
folds (Figure 1.82). But these are again divided by several parallel
transverse folds into a number of successive sacs, the primitive
segments or somites (formerly called by the unsuitable name of
"primitive vertebrae"). They have a different future above and below.
The upper or dorsal segments, the episomites, lose their cavity later
on, and form with their cells the muscular plates of the trunk. The
lower or ventral segments, the hyposomites, corresponding to the
lateral plates of the craniote-embryo, fuse together in the upper part
owing to the disappearance of their lateral walls, and thus form the
later body-cavity (metacoel); in the lower part they remain separate,
and afterwards form the segmental gonads.
In the middle, between the two lateral coelom-folds of the primitive
gut, a single central organ detaches from this at an early stage in
the middle line of its dorsal wall. This is the dorsal chorda (Figures
1.83 and 1.84 ch). This axial rod, which is the first foundation of
the later vertebral column in all the vertebrates, and is the only
representative of it in the Amphioxus, originates from the entoderm.
In consequence of these important folding-processes in the primitive
gut, the simple entodermic tube divides into four different
sections:--
1. underneath, at the ventral side, the permanent alimentary canal or
permanent gut;
2. above, at the dorsal side, the axial rod or chorda; and
3. the two coelom-sacs, which immediately sub-divide into two
structures:--
3A. above, on the dorsal side, the episomites, the double row of
primitive or muscular segments; and
3B. below, on each side of the gut, the hyposomites, the two lateral
plates that give rise to the sex-glands, and the cavities of which
partly unite to form the body-cavity. At the same time, the neural or
medullary tube is formed above the chorda, on the dorsal surface, by
the closing of the parallel medullary swellings.
All these processes, which outline the typical structure of the
vertebrate, take place with astonishing rapidity in the embryo of the
Amphioxus; in the afternoon of the first day, or twenty-four hours
after fertilisation, the young vertebrate, the typical embryo, is
formed; it then has, as a rule, six to eight somites.
The chief occurrence on the second day of development is the
construction of the two permanent openings of the gut--the mouth and
anus. In the earlier stages the alimentary tube is found to be
entirely closed, after the closing of the primitive mouth; it only
communicates behind by the neurenteric canal with the medullary tube.
The permanent mouth is a secondary formation, at the opposite end.
Here, at the end of the second day, we find a pit-like depression in
the outer skin, which penetrates inwards into the closed gut. The anus
is formed behind in the same way a few hours later (in the vicinity of
the additional gastrula-mouth). In man and the higher vertebrates also
the mouth and anus are formed, as we have seen, as flat pits in the
outer skin; they then penetrate inwards, gradually becoming connected
with the blind ends of the closed gut-tube. During the second day the
Amphioxus-embryo undergoes few other changes. The number of primitive
segments increases, and generally amounts to fourteen, some
forty-eight to fifty hours after impregnation.
Almost simultaneously with the formation of the mouth the first
gill-cleft breaks through in the fore section of the Amphioxus-embryo
(generally forty hours after the commencement of development). It now
begins to nourish itself independently, as the food material stored up
in the ovum is completely used up. The further development of the free
larvae takes place very slowly, and extends over several months. The
body becomes much longer, and is compressed at the sides, the head-end
being broadened in a sort of triangle. Two rudimentary sense-organs
are developed in it. Inside we find the first blood-vessels, an upper
or dorsal vessel, corresponding to the aorta, between the gut and the
dorsal cord, and a lower or ventral vessel, corresponding to the
subintestinal vein, at the lower border of the gut. Now, the gills or
respiratory organs also are formed at the fore-end of the alimentary
canal. The whole of the anterior or respiratory section of the gut is
converted into a gill-crate, which is pierced trellis-wise by numbers
of branchial-holes, as in the ascidia. This is done by the foremost
part of the gut-wall joining star-wise with the outer skin, and the
formation of clefts at the point of connection, piercing the wall and
leading into the gut from without. At first there are very few of
these branchial clefts; but there are soon a number of them--first in
one, then in two, rows. The foremost gill-cleft is the oldest. In the
end we have a sort of lattice work of fine gill-clefts, supported on a
number of stiff branchial rods; these are connected in pairs by
transverse rods.
(FIGURES 2.222 TO 2.224. Transverse sections of young Amphioxus-larvae
(diagrammatic, from Ralph.) (Cf. also Figure 2.216.) In Figure 2.222
there is free communication from without with the gut-cavity (D)
through the gill-clefts (K). In Figure 2.223 the lateral folds of the
body-wall, or the gill-covers, which grow downwards, are formed. In
Figure 2.224 these lateral folds have united underneath and joined
their edges in the middle line of the ventral side (R seam). The
respiratory water now passes from the gut-cavity (D) into the
mantle-cavity (A). The letters have the same meaning throughout: N
medullary tube, Ch chorda, M lateral muscles, Lh body-cavity, G part
of the body-cavity in which the sexual organs are subsequently formed.
D gut-cavity, clothed with the gut-gland layer (a). A mantle-cavity, K
gill-clefts, b = E epidermis, E1 the same as visceral epithelium of
the mantle-cavity, E2 as parietal epithelium of the mantle-cavity.)
At an early stage of embryonic development the structure of the
Amphioxus-larva is substantially the same as the ideal picture we have
previously formed of the "Primitive Vertebrate" (Figures 1.98 to
1.102). But the body afterwards undergoes various modifications,
especially in the fore-part. These modifications do not concern us, as
they depend on special adaptations, and do not affect the hereditary
vertebrate type. When the free-swimming Amphioxus-larva is three
months old, it abandons its pelagic habits and changes into the young
animal that lives in the sand. In spite of its smallness (one-eighth
of an inch), it has substantially the same structure as the adult. As
regards the remaining organs of the Amphioxus, we need only mention
that the gonads or sexual glands are developed very late, immediately
out of the inner cell-layer of the body-cavity. Although we can find
afterwards no continuation of the body-cavity (Figure 2.216 U) in the
lateral walls of the mantle-cavity, in the gill-covers or mantle-folds
(Figure 2.224 U), there is one present in the beginning (Figure 2.224
Lh). The sexual cells are formed below, at the bottom of this
continuation (Figure 2.224 S). For the rest, the subsequent
development into the adult Amphioxus of the larva we have followed is
so simple that we need not go further into it here.
We may now turn to the embryology of the Ascidia, an animal that seems
to stand so much lower and to be so much more simply organised,
remaining for the greater part of its life attached to the bottom of
the sea like a shapeless lump. It was a fortunate accident that
Kowalevsky first examined just those larger specimens of the Ascidiae
that show most clearly the relationship of the vertebrates to the
invertebrates, and the larvae of which behave exactly like those of
the Amphioxus in the first stages of development. This resemblance is
so close in the main features that we have only to repeat what we have
already said of the ontogenesis of the Amphioxus.
The ovum of the larger Ascidia (Phallusia, Cynthia, etc.) is a simple
round cell of 1/250 to 1/125 of an inch in diameter. In the thick
fine-grained yelk we find a clear round germinal vesicle of about
1/750 of an inch in diameter, and this encloses a small embryonic spot
or nucleolus. Inside the membrane that surrounds the ovum, the
stem-cell of the Ascidia, after fecundation, passes through just the
same metamorphoses as the stem-cell of the Amphioxus. It undergoes
total segmentation; it divides into two, four, eight, sixteen,
thirty-two cells, and so on. By continued total cleavage the morula,
or mulberry-shaped cluster of cells, is formed. Fluid gathers inside
it, and thus we get once more a globular vesicle (the blastula); the
wall of this is a single stratum of cells, the blastoderm. A real
gastrula (a simple bell-gastrula) is formed from the blastula by
invagination, in the same way as in the amphioxus.
Up to this there is no definite ground in the embryology of the
Ascidiae for bringing them into close relationship with the
Vertebrates; the same gastrula is formed in the same way in many other
animals of different stems. But we now find an embryonic process that
is peculiar to the Vertebrates, and that proves irrefragably the
affinity of the Ascidiae to the Vertebrates. From the epidermis of the
gastrula a medullary tube is formed on the dorsal side, and, between
this and the primitive gut, a chorda; these are the organs that are
otherwise only found in Vertebrates. The formation of these very
important organs takes place in the Ascidia-gastrula in precisely the
same way as in that of the Amphioxus. In the Ascidia (as in the other
case) the oval gastrula is first flattened on one side--the subsequent
dorsal side. A groove or furrow (the medullary groove) is sunk in the
middle line of the flat surface, and two parallel longitudinal
swellings arise on either side from the skin layer. These medullary
swellings join together over the furrow, and form a tube; in this
case, again, the neural or medullary tube is at first open in front,
and connected with the primitive gut behind by the neurenteric canal.
Further, in the Ascidia-larva also the two permanent apertures of the
alimentary canal only appear later, as independent and new formations.
The permanent mouth does not develop from the primitive mouth of the
gastrula; this primitive mouth closes up, and the later anus is formed
near it by invagination from without, on the hinder end of the body,
opposite to the aperture of the medullary tube.
During these important processes, that take place in just the same way
in the Amphioxus, a tail-like projection grows out of the posterior
end of the larva-body, and the larva folds itself up within the round
ovolemma in such a way that the dorsal side is curved and the tail is
forced on to the ventral side. In this tail is developed--starting
from the primitive gut--a cylindrical string of cells, the fore end of
which pushes into the body of the larva, between the alimentary canal
and the neural canal, and is no other than the chorda dorsalis. This
important organ had hitherto been found only in the Vertebrates, not a
single trace of it being discoverable in the Invertebrates. At first
the chorda only consists of a single row of large entodermic cells. It
is afterwards composed of several rows of cells. In the Ascidia-larva,
also, the chorda develops from the dorsal middle part of the primitive
gut, while the two coelom-pouches detach themselves from it on both
sides. The simple body-cavity is formed by the coalescence of the two.
When the Ascidia-larva has attained this stage of development it
begins to move about in the ovolemma. This causes the membrane to
burst. The larva emerges from it, and swims about in the sea by means
of its oar-like tail. These free-swimming larvae of the Ascidia have
been known for a long time. They were first observed by Darwin during
his voyage round the world in 1833. They resemble tadpoles in outward
appearance, and use their tails as oars, as the tadpoles do. However,
this lively and highly-developed condition does not last long. At
first there is a progressive development; the foremost part of the
medullary tube enlarges into a brain, and inside this two single
sense-organs are developed, a dorsal auditory vesicle and a ventral
eye. Then a heart is formed on the ventral side of the animal, or the
lower wall of the gut, in the same simple form and at the same spot at
which the heart is developed in man and all the other vertebrates. In
the lower muscular wall of the gut we find a weal-like thickening, a
solid, spindle-shaped string of cells, which becomes hollow in the
centre; it begins to contract in different directions, now forward and
now backward, as is the case with the adult Ascidia. In this way the
sanguineous fluid accumulated in the hollow muscular tube is driven in
alternate directions into the blood-vessels, which develop at both
ends of the cardiac tube. One principal vessel runs along the dorsal
side of the gut, another along its ventral side. The former
corresponds to the aorta and the dorsal vessel in the worms. The other
corresponds to the subintestinal vein and the ventral vessel of the
worms.
With the formation of these organs the progressive development of the
Ascidia comes to an end, and degeneration sets in. The free-swimming
larva sinks to the floor of the sea, abandons its locomotive habits,
and attaches itself to stones, marine plants, mussel-shells, corals,
and other objects; this is done with the part of the body that was
foremost in movement. The attachment is effected by a number of
out-growths, usually three, which can be seen even in the
free-swimming larva. The tail is lost, as there is no further use for
it. It undergoes a fatty degeneration, and disappears with the chorda
dorsalis. The tailless body changes into an unshapely tube, and, by
the atrophy of some parts and the modification of others, gradually
assumes the appearance we have already described.
(FIGURE 2.225. An Appendicaria (Copelata), seen from the left. m
mouth, k branchial gut, o gullet, v stomach, a anus, n brain (ganglion
above the gullet), g auditory vesicle, f ciliated groove under the
gills, h heart, t testicles, e ovary, c chorda, s tail.)
Among the living Tunicates there is a very interesting group of small
animals that remain throughout life at the stage of development of the
tailed, free Ascidia-larva, and swim about briskly in the sea by means
of their broad oar-tail. These are the remarkable Copelata
(Appendicaria and Vexillaria, Figure 2.225). They are the only living
Vertebrates that have throughout life a chorda dorsalis and a neural
string above it; the latter must be regarded as the prolongation of
the cerebral ganglion and the equivalent of the medullary tube. Their
branchial gut also opens directly outwards by a pair of branchial
clefts. These instructive Copelata, comparable to permanent
Ascidia-larvae, come next to the extinct Prochordonia, those ancient
worms which we must regard as the common ancestors of the Tunicates
and Vertebrates. The chorda of the Appendicaria is a long, cylindrical
string (Figure 2.225 c), and serves as an attachment for the muscles
that work the flat oar-tail.
Among the various modifications which the Ascidia-larva undergoes
after its establishment at the sea-floor, the most interesting (after
the loss of the axial rod) is the atrophy of one of its chief organs,
the medullary tube. In the Amphioxus the spinal marrow continues to
develop, but in the Ascidia the tube soon shrinks into a small and
insignificant nervous ganglion that lies above the mouth and the
gill-crate, and is in accord with the extremely slight mental power of
the animal. This insignificant relic of the medullary tube seems to be
quite beyond comparison with the nervous centre of the vertebrate, yet
it started from the same structure as the spinal cord of the
Amphioxus. The sense-organs that had been developed in the fore part
of the neural tube are also lost; no trace of which can be found in
the adult Ascidia. On the other hand, the alimentary canal becomes a
most extensive organ. It divides presently into two sections--a wide
fore or branchial gut that serves for respiration, and a narrower hind
or hepatic gut that accomplishes digestion. The branchial or head-gut
of the Ascidia is small at first, and opens directly outwards only by
a couple of lateral ducts or gill-clefts--a permanent arrangement in
the Copelata. The gill-clefts are developed in the same way as in the
Amphioxus. As their number greatly increases we get a large
gill-crate, pierced like lattice work. In the middle line of its
ventral side we find the hypobranchial groove. The mantle or
cloaca-cavity (the atrium) that surrounds the gill-crate is also
formed in the same way in the Ascidia as in the Amphioxus. The
ejection-opening of this peribranchial cavity corresponds to the
branchial pore of the Amphioxus. In the adult Ascidia the branchial
gut and the heart on its ventral side are almost the only organs that
recall the original affinity with the vertebrates.
The further development of the Ascidia in detail has no particular
interest for us, and we will not go into it. The chief result that we
obtain from its embryology is the complete agreement with that of the
Amphioxus in the earliest and most important embryonic stages. They do
not begin to diverge until after the medullary tube and alimentary
canal, and the axial rod with the muscles between the two, have been
formed. The Amphioxus continues to advance, and resembles the
embryonic forms of the higher vertebrates; the Ascidia degenerates
more and more, and at last, in its adult condition, has the appearance
of a very imperfect invertebrate.
If we now look back on all the remarkable features we have encountered
in the structure and the embryonic development of the Amphioxus and
the Ascidia, and compare them with the features of man's embryonic
development which we have previously studied, it will be clear that I
have not exaggerated the importance of these very interesting animals.
It is evident that the Amphioxus from the vertebrate side and the
Ascidia from the invertebrate form the bridge by which we can span the
deep gulf that separates the two great divisions of the animal
kingdom. The radical agreement of the lancelet and the sea-squirt in
the first and most important stages of development shows something
more than their close anatomic affinity and their proximity in
classification; it shows also their real blood-relationship and their
common origin from one and the same stem-form. In this way, it throws
considerable light on the oldest roots of man's genealogical tree.