The Evolution of Man, V.2

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Produced by Sue Asscher

THE EVOLUTION OF MAN

A POPULAR SCIENTIFIC STUDY

BY

ERNST HAECKEL

VOLUME 2.

HUMAN STEM-HISTORY, OR PHYLOGENY.

TRANSLATED FROM THE FIFTH (ENLARGED) EDITION BY JOSEPH MCCABE.

[ISSUED FOR THE RATIONALIST PRESS ASSOCIATION, LIMITED.]

WATTS & CO.,
17, JOHNSON'S COURT, FLEET STREET, LONDON, E.C.
1911.

CONTENTS OF VOLUME 2.

LIST OF ILLUSTRATIONS.

INDEX.

CHAPTER 2.16. STRUCTURE OF THE LANCELET AND THE SEA-SQUIRT.

CHAPTER 2.17. EMBRYOLOGY OF THE LANCELET AND THE SEA-SQUIRT.

CHAPTER 2.18. DURATION OF THE HISTORY OF OUR STEM.

CHAPTER 2.19. OUR PROTIST ANCESTORS.

CHAPTER 2.20. OUR WORM-LIKE ANCESTORS.

CHAPTER 2.21. OUR FISH-LIKE ANCESTORS.

CHAPTER 2.22. OUR FIVE-TOED ANCESTORS.

CHAPTER 2.23. OUR APE ANCESTORS.

CHAPTER 2.24. EVOLUTION OF THE NERVOUS SYSTEM.

CHAPTER 2.25. EVOLUTION OF THE SENSE-ORGANS.

CHAPTER 2.26. EVOLUTION OF THE ORGANS OF MOVEMENT.

CHAPTER 2.27. EVOLUTION OF THE ALIMENTARY SYSTEM.

CHAPTER 2.28. EVOLUTION OF THE VASCULAR SYSTEM.

CHAPTER 2.29. EVOLUTION OF THE SEXUAL ORGANS.

CHAPTER 2.30. RESULTS OF ANTHROPOGENY.

LIST OF ILLUSTRATIONS.

FIGURE 2.210. THE LANCELET.

FIGURE 2.211. SECTION OF THE HEAD OF THE LANCELET.

FIGURE 2.212. SECTION OF AN AMPHIOXUS-LARVA.

FIGURE 2.213. DIAGRAM OF PRECEDING.

FIGURE 2.214. SECTION OF A YOUNG AMPHIOXUS.

FIGURE 2.215. DIAGRAM OF A YOUNG AMPHIOXUS.

FIGURE 2.216. TRANSVERSE SECTION OF LANCELET.

FIGURE 2.217. SECTION THROUGH THE MIDDLE OF THE LANCELET.

FIGURE 2.218. SECTION OF A PRIMITIVE-FISH EMBRYO.

FIGURE 2.219. SECTION OF THE HEAD OF THE LANCELET.

FIGURES 2.220 AND 2.221. ORGANISATION OF AN ASCIDIA.

FIGURES 2.222 TO 2.224. SECTIONS OF YOUNG AMPHIOXUS-LARVAE.

FIGURE 2.225. AN APPENDICARIA.

FIGURE 2.226. Chroococcus minor.

FIGURE 2.227. Aphanocapsa primordialis.

FIGURE 2.228. PROTAMOEBA.

FIGURE 2.229. ORIGINAL OVUM-CLEAVAGE.

FIGURE 2.230. MORULA.

FIGURES 2.231 AND 2.232. Magosphaera planula.

FIGURE 2.233. MODERN GASTRAEADS.

FIGURES 2.234 AND 2.235. Prophysema primordiale.

FIGURES 2.236 AND 2.237. Ascula of Gastrophysema.

FIGURE 2.238. Olynthus.

FIGURE 2.239. Aphanostomum Langii.

FIGURES 2.240 AND 2.241. A TURBELLARIAN.

FIGURES 2.242 AND 2.243. Chaetonotus.

FIGURE 2.244. A NEMERTINE WORM.

FIGURE 2.245. AN ENTEROPNEUST.

FIGURE 2.246. SECTION OF THE BRANCHIAL GUT.

FIGURE 2.247. THE MARINE LAMPREY.

FIGURE 2.248. FOSSIL PRIMITIVE FISH.

FIGURE 2.249. EMBRYO OF A SHARK.

FIGURE 2.250. MAN-EATING SHARK.

FIGURE 2.251. FOSSIL ANGEL-SHARK.

FIGURE 2.252. TOOTH OF A GIGANTIC SHARK.

FIGURES 2.253 TO 2.255. CROSSOPTERYGII.

FIGURE 2.256. FOSSIL DIPNEUST.

FIGURE 2.257. THE AUSTRALIAN DIPNEUST.

FIGURES 2.258 AND 2.259. YOUNG CERATODUS.

FIGURE 2.260. FOSSIL AMPHIBIAN.

FIGURE 2.261. LARVA OF THE SPOTTED SALAMANDER.

FIGURE 2.262. LARVA OF COMMON FROG.

FIGURE 2.263. FOSSIL MAILED AMPHIBIAN.

FIGURE 2.264. THE NEW ZEALAND LIZARD.

FIGURE 2.265. Homoeosaurus pulchellus.

FIGURE 2.266. SKULL OF A PERMIAN LIZARD.

FIGURE 2.267. SKULL OF A THEROMORPHUM.

FIGURE 2.268. LOWER JAW OF A PRIMITIVE MAMMAL.

FIGURES 2.269 AND 2.270. THE ORNITHORHYNCUS.

FIGURE 2.271. LOWER JAW OF A PROMAMMAL.

FIGURE 2.272. THE CRAB-EATING OPOSSUM.

FIGURE 2.273. FOETAL MEMBRANES OF THE HUMAN EMBRYO.

FIGURE 2.274. SKULL OF A FOSSIL LEMUR.

FIGURE 2.275. THE SLENDER LORI.

FIGURE 2.276. THE WHITE-NOSED APE.

FIGURE 2.277. THE DRILL-BABOON.

FIGURES 2.278 TO 2.282. SKELETONS OF MAN AND THE ANTHROPOID APES.

FIGURE 2.283. SKULL OF THE JAVA APE-MAN.

FIGURE 2.284. SECTION OF THE HUMAN SKIN.

FIGURE 2.285. EPIDERMIC CELLS.

FIGURE 2.286. RUDIMENTARY LACHRYMAL GLANDS.

FIGURE 2.287. THE FEMALE BREAST.

FIGURE 2.288. MAMMARY GLAND OF A NEW-BORN INFANT.

FIGURE 2.289. EMBRYO OF A BEAR.

FIGURE 2.290. HUMAN EMBRYO.

FIGURE 2.291. CENTRAL MARROW OF A HUMAN EMBRYO.

FIGURES 2.292 AND 2.293. THE HUMAN BRAIN.

FIGURES 2.294 TO 2.296. CENTRAL MARROW OF HUMAN EMBRYO.

FIGURE 2.297. HEAD OF A CHICK EMBRYO.

FIGURE 2.298. BRAIN OF THREE CRANIOTE EMBRYOS.

FIGURE 2.299. BRAIN OF A SHARK.

FIGURE 2.300. BRAIN AND SPINAL CORD OF A FROG.

FIGURE 2.301. BRAIN OF AN OX-EMBRYO.

FIGURES 2.302 AND 2.303. BRAIN OF A HUMAN EMBRYO.

FIGURE 2.304. BRAIN OF THE RABBIT.

FIGURE 2.305. HEAD OF A SHARK.

FIGURES 2.306 TO 2.310. HEADS OF CHICK-EMBRYOS.

FIGURE 2.311. SECTION OF MOUTH OF HUMAN EMBRYO.

FIGURE 2.312. DIAGRAM OF MOUTH-NOSE CAVITY.

FIGURES 2.313 AND 2.314. HEADS OF HUMAN EMBRYOS.

FIGURES 2.315 AND 2.316. FACE OF HUMAN EMBRYO.

FIGURE 2.317. THE HUMAN EYE.

FIGURE 2.318. EYE OF THE CHICK EMBRYO.

FIGURE 2.319. SECTION OF EYE OF A HUMAN EMBRYO.

FIGURE 2.320. THE HUMAN EAR.

FIGURE 2.321. THE BONY LABYRINTH.

FIGURE 2.322. DEVELOPMENT OF THE LABYRINTH.

FIGURE 2.323. PRIMITIVE SKULL OF HUMAN EMBRYO.

FIGURE 2.324. RUDIMENTARY MUSCLES OF THE EAR.

FIGURES 2.325 AND 2.326. THE HUMAN SKELETON.

FIGURE 2.327. THE HUMAN VERTEBRAL COLUMN.

FIGURE 2.328. PIECE OF THE DORSAL CORD.

FIGURES 2.329 AND 2.330. DORSAL VERTEBRAE.

FIGURE 2.331. INTERVERTEBRAL DISK.

FIGURE 2.332. HUMAN SKULL.

FIGURE 2.333. SKULL OF NEW-BORN CHILD.

FIGURE 2.334. HEAD-SKELETON OF A PRIMITIVE FISH.

FIGURE 2.335. SKULLS OF NINE PRIMATES.

FIGURES 2.336 TO 2.338. EVOLUTION OF THE FIN.

FIGURE 2.339. SKELETON OF THE FORE-LEG OF AN AMPHIBIAN.

FIGURE 2.340. SKELETON OF GORILLA'S HAND.

FIGURE 2.341. SKELETON OF HUMAN HAND.

FIGURE 2.342. SKELETON OF HAND OF SIX MAMMALS.

FIGURES 2.343 TO 2.345. ARM AND HAND OF THREE ANTHROPOIDS.

FIGURE 2.346. SECTION OF FISH'S TAIL.

FIGURE 2.347. HUMAN SKELETON.

FIGURE 2.348. SKELETON OF THE GIANT GORILLA.

FIGURE 2.349. THE HUMAN STOMACH.

FIGURE 2.350. SECTION OF THE HEAD OF A RABBIT-EMBRYO.

FIGURE 2.351. SHARK'S TEETH.

FIGURE 2.352. GUT OF A HUMAN EMBRYO.

FIGURES 2.353 AND 2.354. GUT OF A DOG EMBRYO.

FIGURES 2.355 AND 2.356. SECTIONS OF HEAD OF LAMPREY.

FIGURE 2.357. VISCERA OF A HUMAN EMBRYO.

FIGURE 2.358. RED BLOOD-CELLS.

FIGURE 2.359. VASCULAR TISSUE.

FIGURE 2.360. SECTION OF TRUNK OF A CHICK-EMBRYO.

FIGURE 2.361. MEROCYTES.

FIGURE 2.362. VASCULAR SYSTEM OF AN ANNELID.

FIGURE 2.363. HEAD OF A FISH-EMBRYO.

FIGURES 2.364 TO 2.370. THE FIVE ARTERIAL ARCHES.

FIGURES 2.371 AND 2.372. HEART OF A RABBIT-EMBRYO.

FIGURES 2.373 AND 2.374. HEART OF A DOG-EMBRYO.

FIGURES 2.375 TO 2.377. HEART OF A HUMAN EMBRYO.

FIGURE 2.378. HEART OF ADULT MAN.

FIGURE 2.379. SECTION OF HEAD OF A CHICK-EMBRYO.

FIGURE 2.380. SECTION OF A HUMAN EMBRYO.

FIGURES 2.381 AND 2.382. SECTIONS OF A CHICK-EMBRYO.

FIGURE 2.383. EMBRYOS OF SAGITTA.

FIGURE 2.384. KIDNEYS OF BDELLOSTOMA.

FIGURE 2.385. SECTION OF EMBRYONIC SHIELD.

FIGURES 2.386 AND 2.387. PRIMITIVE KIDNEYS.

FIGURE 2.388. PIG-EMBRYO.

FIGURE 2.389. HUMAN EMBRYO.

FIGURES 2.390 TO 2.392. RUDIMENTARY KIDNEYS AND SEXUAL ORGANS.

FIGURES 2.393 AND 2.394. URINARY AND SEXUAL ORGANS OF SALAMANDER.

FIGURE 2.395. PRIMITIVE KIDNEYS OF HUMAN EMBRYO.

FIGURES 2.396 TO 2.398. URINARY ORGANS OF OX-EMBRYOS.

FIGURE 2.399. SEXUAL ORGANS OF WATER-MOLE.

FIGURES 2.400 AND 2.401. ORIGINAL POSITION OF SEXUAL GLANDS.

FIGURE 2.402. UROGENITAL SYSTEM OF HUMAN EMBRYO.

FIGURE 2.403. SECTION OF OVARY.

FIGURES 2.404 TO 2.406. GRAAFIAN FOLLICLES.

FIGURE 2.407. A RIPE GRAAFIAN FOLLICLE.

FIGURE 2.408. THE HUMAN OVUM.

CHAPTER 2.16. STRUCTURE OF THE LANCELET AND THE SEA-SQUIRT.

In turning from the embryology to the phylogeny of man--from the
development of the individual to that of the species--we must bear in
mind the direct causal connection that exists between these two main
branches of the science of human evolution. This important causal
nexus finds its simplest expression in "the fundamental law of organic
development," the content and purport of which we have fully
considered in the first chapter. According to this biogenetic law,
ontogeny is a brief and condensed recapitulation of phylogeny. If this
compendious reproduction were complete in all cases, it would be very
easy to construct the whole story of evolution on an embryonic basis.
When we wanted to know the ancestors of any higher organism, and,
therefore, of man--to know from what forms the race as a whole has
been evolved we should merely have to follow the series of forms in
the development of the individual from the ovum; we could then regard
each of the successive forms as the representative of an extinct
ancestral form. However, this direct application of ontogenetic facts
to phylogenetic ideas is possible, without limitations, only in a very
small section of the animal kingdom. There are, it is true, still a
number of lower invertebrates (for instance, some of the Zoophyta and
Vermalia) in which we are justified in recognising at once each
embryonic form as the historical reproduction, or silhouette, as it
were, of an extinct ancestor. But in the great majority of the
animals, and in the case of man, this is impossible, because the
embryonic forms themselves have been modified through the change of
the conditions of existence, and have lost their original character to
some extent. During the immeasurable course of organic history, the
many millions of years during which life was developing on our planet,
secondary changes of the embryonic forms have taken place in most
animals. The young of animals (not only detached larvae, but also the
embryos enclosed in the womb) may be modified by the influence of the
environment, just as well as the mature organisms are by adaptation to
the conditions of life; even species are altered during the embryonic
development. Moreover, it is an advantage for all higher organisms
(and the advantage is greater the more advanced they are) to curtail
and simplify the original course of development, and thus to
obliterate the traces of their ancestors. The higher the individual
organism is in the animal kingdom, the less completely does it
reproduce in its embryonic development the series of its ancestors,
for reasons that are as yet only partly known to us. The fact is
easily proved by comparing the different developments of higher and
lower animals in any single stem.

In order to appreciate this important feature, we have distributed the
embryological phenomena in two groups, palingenetic and cenogenetic.
Under palingenesis we count those facts of embryology that we can
directly regard as a faithful synopsis of the corresponding
stem-history. By cenogenesis we understand those embryonic processes
which we cannot directly correlate with corresponding evolutionary
processes, but must regard as modifications or falsifications of them.
With this careful discrimination between palingenetic and cenogenetic
phenomena, our biogenetic law assumes the following more precise
shape:--The rapid and brief development of the individual (ontogeny)
is a condensed synopsis of the long and slow history of the stem
(phylogeny): this synopsis is the more faithful and complete in
proportion as the original features have been preserved by heredity,
and modifications have not been introduced by adaptation.

In order to distinguish correctly between palingenetic and cenogenetic
phenomena in embryology, and deduce sound conclusions in connection
with stem-history, we must especially make a comparative study of the
former. In doing this it is best to employ the methods that have long
been used by geologists for the purpose of establishing the succession
of the sedimentary rocks in the crust of the earth. This solid crust,
which encloses the glowing central mass like a thin shell, is composed
of different kinds of rocks: there are, firstly, the volcanic rocks
which were formed directly by the cooling at the surface of the molten
mass of the earth; secondly, there are the sedimentary rocks, that
have been made out of the former by the action of water, and have been
laid in successive strata at the bottom of the sea. Each of these
sedimentary strata was at first a soft layer of mud; but in the course
of thousands of years it condensed into a solid, hard mass of stone
(sandstone, limestone, marl, etc.), and at the same time permanently
preserved the solid and imperishable bodies that had chanced to fall
into the soft mud. Among these bodies, which were either fossilised or
left characteristic impressions of their forms in the soft slime, we
have especially the more solid parts of the animals and plants that
lived and died during the deposit of the slimy strata.

Hence each of the sedimentary strata has its characteristic fossils,
the remains of the animals and plants that lived during that
particular period of the earth's history. When we make a comparative
study of these strata, we can survey the whole series of such periods.
All geologists are now agreed that we can demonstrate a definite
historical succession in the strata, and that the lowest of them were
deposited in very remote, and the uppermost in comparatively recent,
times. However, there is no part of the earth where we find the series
of strata in its entirety, or even approximately complete. The
succession of strata and of corresponding historical periods generally
given in geology is an ideal construction, formed by piecing together
the various partial discoveries of the succession of strata that have
been made at different points of the earth's surface (cf. Chapter
2.18).

We must act in this way in constructing the phylogeny of man. We must
try to piece together a fairly complete picture of the series of our
ancestors from the various phylogenetic fragments that we find in the
different groups of the animal kingdom. We shall see that we are
really in a position to form an approximate picture of the evolution
of man and the mammals by a proper comparison of the embryology of
very different animals--a picture that we could never have framed from
the ontogeny of the mammals alone. As a result of the above-mentioned
cenogenetic processes--those of disturbed and curtailed
heredity--whole series of lower stages have dropped out in the
embryonic development of man and the other mammals especially from the
earliest periods, or been falsified by modification. But we find these
lower stages in their original purity in the lower vertebrates and
their invertebrate ancestors. Especially in the lowest of all the
vertebrates, the lancelet or Amphioxus, we have the oldest stem-forms
completely preserved in the embryonic development. We also find
important evidence in the fishes, which stand between the lower and
higher vertebrates, and throw further light on the course of evolution
in certain periods. Next to the fishes come the amphibia, from the
embryology of which we can also draw instructive conclusions. They
represent the transition to the higher vertebrates, in which the
middle and older stages of ancestral development have been either
distorted or curtailed, but in which we find the more recent stages of
the phylogenetic process well preserved in ontogeny. We are thus in a
position to form a fairly complete idea of the past development of
man's ancestors within the vertebrate stem by putting together and
comparing the embryological developments of the various groups of
vertebrates. And when we go below the lowest vertebrates and compare
their embryology with that of their invertebrate relatives, we can
follow the genealogical tree of our animal ancestors much farther,
down to the very lowest groups of animals.

In entering the obscure paths of this phylogenetic labyrinth, clinging
to the Ariadne-thread of the biogenetic law and guided by the light of
comparative anatomy, we will first, in accordance with the methods we
have adopted, discover and arrange those fragments from the manifold
embryonic developments of very different animals from which the
stem-history of man can be composed. I would call attention
particularly to the fact that we can employ this method with the same
confidence and right as the geologist. No geologist has ever had
ocular proof that the vast rocks that compose our Carboniferous or
Jurassic or Cretaceous strata were really deposited in water. Yet no
one doubts the fact. Further, no geologist has ever learned by direct
observation that these various sedimentary formations were deposited
in a certain order; yet all are agreed as to this order. This is
because the nature and origin of these rocks cannot be rationally
understood unless we assume that they were so deposited. These
hypotheses are universally received as safe and indispensable
"geological theories," because they alone give a rational explanation
of the strata.

Our evolutionary hypotheses can claim the same value, for the same
reasons. In formulating them we are acting on the same inductive and
deductive methods, and with almost equal confidence, as the geologist.
We hold them to be correct, and claim the status of "biological
theories" for them, because we cannot understand the nature and origin
of man and the other organisms without them, and because they alone
satisfy our demand for a knowledge of causes. And just as the
geological hypotheses that were ridiculed as dreams at the beginning
of the nineteenth century are now universally admitted, so our
phylogenetic hypotheses, which are still regarded as fantastic in
certain quarters, will sooner or later be generally received. It is
true that, as will soon appear, our task is not so simple as that of
the geologist. It is just as much more difficult and complex as man's
organisation is more elaborate than the structure of the rocks.

When we approach this task, we find an auxiliary of the utmost
importance in the comparative anatomy and embryology of two lower
animal-forms. One of these animals is the lancelet (Amphioxus), the
other the sea-squirt (Ascidia). Both of these animals are very
instructive. Both are at the border between the two chief divisions of
the animal kingdom--the vertebrates and invertebrates. The vertebrates
comprise the already mentioned classes, from the Amphioxus to man
(acrania, lampreys, fishes, dipneusts, amphibia, reptiles, birds, and
mammals). Following the example of Lamarck, it is usual to put all the
other animals together under the head of invertebrates. But, as I have
often mentioned already, the group is composed of a number of very
different stems. Of these we have no interest just now in the
echinoderms, molluscs, and articulates, as they are independent
branches of the animal-tree, and have nothing to do with the
vertebrates. On the other hand, we are greatly concerned with a very
interesting group that has only recently been carefully studied, and
that has a most important relation to the ancestral tree of the
vertebrates. This is the stem of the Tunicates. One member of this
group, the sea-squirt, very closely approaches the lowest vertebrate,
the Amphioxus, in its essential internal structure and embryonic
development. Until 1866 no one had any idea of the close connection of
these apparently very different animals; it was a very fortunate
accident that the embryology of these related forms was discovered
just at the time when the question of the descent of the vertebrates
from the invertebrates came to the front. In order to understand it
properly, we must first consider these remarkable animals in their
fully-developed forms and compare their anatomy.

We begin with the lancelet--after man the most important and
interesting of all animals. Man is at the highest summit, the lancelet
at the lowest root, of the vertebrate stem.

It lives on the flat, sandy parts of the Mediterranean coast, partly
buried in the sand, and is apparently found in a number of seas.* (*
See the ample monograph by Arthur Willey, Amphioxus and the Ancestry
of the Vertebrates; Boston, 1894.) It has been found in the North Sea
(on the British and Scandinavian coasts and in Heligoland), and at
various places on the Mediterranean (for instance, at Nice, Naples,
and Messina). It is also found on the coast of Brazil and in the most
distant parts of the Pacific Ocean (the coast of Peru, Borneo, China,
Australia, etc.). Recently eight to ten species of the amphioxus have
been determined, distributed in two or three genera.

(FIGURE 2.210. The lancelet (Amphioxus lanceolatus), twice natural
size, left view. The long axis is vertical; the mouth-end is above,
the tail-end below; a mouth, surrounded by threads of beard; b anus, c
gill-opening (porus branchialis), d gill-crate, e stomach, f liver, g
small intestine, h branchial cavity, i chorda (axial rod), underneath
it the aorta; k aortic arches, l trunk of the branchial artery, m
swellings on its branches, n vena cava, o visceral vein.

FIGURE 2.211. Transverse section of the head of the Amphioxus. (From
Boveri.) Above the branchial gut (kd) is the chorda, above this the
neural tube (in which we can distinguish the inner grey and the outer
white matter); above again is the dorsal fin (fh). To the right and
left above (in the episoma) are the thick muscular plates (m); below
(in the hyposoma) the gonads (g). ao aorta (here double), c corium, ec
endostyl, f fascie, gl glomerulus of the kidneys, k branchial vessel,
ld partition between the coeloma (sc) and atrium (p), mt transverse
ventral muscle, n renal canals, of upper and uf lower canals in the
mantle-folds, p peribranchial cavity, (atrium), sc coeloma (subchordal
body-cavity), si principal (or subintestinal) vein, sk perichorda
(skeletal layer).)

Johannes Muller classed the lancelet with the fishes, although he
pointed out that the differences between this simple vertebrate and
the lowest fishes are much greater than between the fishes and the
amphibia. But this was far from expressing the real significance of
the animal. We may confidently lay down the following principle: The
Amphioxus differs more from the fishes than the fishes do from man and
the other vertebrates. As a matter of fact, it is so different from
all the other vertebrates in its whole organisation that the laws of
logical classification compel us to distinguish two divisions of this
stem: 1, the Acrania (Amphioxus and its extinct relatives); and 2, the
Craniota (man and the other vertebrates). The first and lower division
comprises the vertebrates that have no vertebrae or skull (cranium).
Of these the only living representatives are the Amphioxus and
Paramphioxus, though there must have been a number of different
species at an early period of the earth's history.

Opposed to the Acrania is the second division of the vertebrates,
which comprises all the other members of the stem, from the fishes up
to man. All these vertebrates have a head quite distinct from the
trunk, with a skull (cranium) and brain; all have a centralised heart,
fully-formed kidneys, etc. Hence they are called the Craniota. These
Craniotes are, however, without a skull in their earlier period. As we
already know from embryology, even man, like every other mammal,
passes in the earlier course of his development through the important
stage which we call the chordula; at this lower stage the animal has
neither vertebrae nor skull nor limbs (Figures 1.83 to 1.86). And even
after the formation of the primitive vertebrae has begun, the
segmented foetus of the amniotes still has for a long time the simple
form of a lyre-shaped disk or a sandal, without limbs or extremities.
When we compare this embryonic condition, the sandal-shaped foetus,
with the developed lancelet, we may say that the amphioxus is, in a
certain sense, a permanent sandal-embryo, or a permanent embryonic
form of the Acrania; it never rises above a low grade of development
which we have long since passed.

The fully-developed lancelet (Figure 2.210) is about two inches long,
is colourless or of a light red tint, and has the shape of a narrow
lancet-formed leaf. The body is pointed at both ends, but much
compressed at the sides. There is no trace of limbs. The outer skin is
very thin and delicate, naked, transparent, and composed of two
different layers, a simple external stratum of cells, the epidermis,
and a thin underlying cutis-layer. Along the middle line of the back
runs a narrow fin-fringe which expands behind into an oval tail-fin,
and is continued below in a short anus-fin. The fin-fringe is
supported by a number of square elastic fin-plates.

In the middle of the body we find a thin string of cartilage, which
goes the whole length of the body from front to back, and is pointed
at both ends (Figure 2.210 i). This straight, cylindrical rod
(somewhat compressed for a time) is the axial rod or the chorda
dorsalis; in the lancelet this is the only trace of a vertebral
column. The chorda develops no further, but retains its original
simplicity throughout life. It is enclosed by a firm membrane, the
chorda-sheath or perichorda. The real features of this and of its
dependent formations are best seen in the transverse section of the
Amphioxus (Figure 2.211). The perichorda forms a cylindrical tube
immediately over the chorda, and the central nervous system, the
medullary tube, is enclosed in it. This important psychic organ also
remains in its simplest shape throughout life, as a cylindrical tube,
terminating with almost equal plainness at either end, and enclosing a
narrow canal in its thick wall. However, the fore end is a little
rounder, and contains a small, almost imperceptible bulbous swelling
of the canal. This must be regarded as the beginning of a rudimentary
brain. At the foremost end of it there is a small black pigment-spot,
a rudimentary eye; and a narrow canal leads to a superficial
sense-organ. In the vicinity of this optic spot we find at the left
side a small ciliated depression, the single olfactory organ. There is
no organ of hearing. This defective development of the higher
sense-organs is probably, in the main, not an original feature, but a
result of degeneration.

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