Full text of "Twelve lectures on comparative embryology, delivered before the Lowell Institute, in Boston, December and January, 1848-9"

See other formats

Louis

ctures on Comparative

FROM

t <fe TAYLOR

rs and Bookbinders,
ew Haven, Ct.

•i

a
uu
a

a
a

!§

IT
•O

ru
ru

TWELVE

LECTURES

COMPARATIVE EMBRYOLOGY,

DELIVERED BEFORE

THE LOWELL INSTITUTE, IN BOSTON,

DECEMBER AND JANUARY, 1848-9,

LOUISAGASSIZ,

PROFESSOR OF ZOOLOGY AND GEOLOGY IN THE LAWRENCE SCIENTIFIC SCHOOL,

CAMBRIDGE UNIVERSITY.

PHONOGRAPHIC REPORT, BY JAMES W. STONE, A, M., M. D.
President of the Boston Phonographic Reporting Association, and of the Buylston Medical Society,

ORIGINALLY REPORTED AND PUBLISHED IN THE BOSTON DAILY EVENING TRAVELLEK,

BOSTON:

HENRY FLANDERS & CO.,

REDDING & CO., GOULD, KENDALL & LINCOLN, JAMES MUNROE & CO-
NEW YORK : DEWITT £ DAVENPORT, TRIBUNE BUILDINGS,
PHILADELPHIA '. G. B. ZIEBER & CO.

1849.

PREFACE.

WE feel both pleasure and pride in being able to present to the public the
following Course of Lectures. It is the first enterprise of the kind in this city,
•and has therefore been attended with unusual trouble and expense.

•s

EMBRYOLOGY has but recently become the subject of scientific investigation.
Few persons have as yet entered upon it, and in this country it may be considered
as entirely new ; but it is destined to have a most important influence in the future
progress of Zoology, and greatly to modify the present classification of animals.
Prof. AGASSIZ has embodied in his Lectures all that has been hitherto done abroad,
and has added numerous observations of his own, made in this country, and in a
form at once highly scientific and so illustrated, as to be interesting to the common
reader. The application here made of Embryology to the improvement of the
classification of animals is peculiarly his own, as he has shown in his fourth
Lecture*

The point of the Lectures is to demonstrate that a natural method of classi-
fying the animal kingdom may be attained by a comparison of the changes which
are passed through by different animals in the course of their development from
the egg to the perfect state ; the changes they undergo being considered as a scale
to appreciate the relative position of the series.

The language has been retained almost precisely as delivered by the Professor,
because, although in many instances it wears a foreign idiom, yet it is peculiarly
expressive, and possesses a charm which would be lost in the attempt to reduce
it to Saxon phrases,

In proof of the fullness and accuracy of Dr. STONE'S phonographic report, and
also of the value of the phonographic system, we are enabled to state that
several gentlemen had the curiosity to compare a portion of manuscript which
the Professor had read, in one lecture, with the report of it ; when it was found
that every word appeared precisely as written, except that one word was missing,

which the Professor stated he had purposely omitted in reading.

o ;
Boston, January , 1849,

LECTURES ON COMPARATIVE PHYSIOLOGY.

A course of twelve Lectures on Comparative Physiology, now being
delivered by Prof. JEFFRIES WYMAN, before the Lowell Institute, will
also be reported in full and published in the Traveller, illustrated by
diagrams. The following is the Programme of this course :

LECT. I. — General Properties of Living Beings,
" II. — Locomotion — Skeleton.
" III. — Muscular Action.

IV. — Comparative Anatomy — Teeth,

V. — Digestion.

VI. — Absorption and Circulation.
" VII. — Respiration,

" VIII. — Nervous system — Nerves and Spinal Marrow*
« IX.— Brain.

X. — Senses — Touch, Taste.
XL — Smelling, Hearing.
« XI I.— Vision.

PROF. ACJASSIZ'.S

LECTURES ON EMBRYOLOGY.

DELIVERED BEFORE THE LOWELL INSTITUTE — 1848-9,

LECTURE I.

The time has past when it was possible to doubt
lhat there is order in Nature, when the existence
fcf a general system regulating the whole creation
could be questioned. However,, it has been oaly
step by step that man has acquired an insight into
this plan. Knowledge was to be gained before
this wonderful arrangement of nature could be
understood. And it was not at once fully under-
stood. Understanding has been acquired gradual-
ly, successively and With difficulties. However,
now we have sufficient data to be able to satisfy
ourselves that the various views which have been
brought forward respecting the order of nature
are not altogether fanciful, that they are not mere
artificial means to assist us in our investigations.
We can be satisfied that they correspond more or
less to nature. We have the positive hope that
they will one day correspond entirely to the nat-
ural phenomena, when we see how the investiga-
tions which are carried on in different directions
by different authors go on, converging gradually,
assisting each other, and harmonising subjects
which at first seamed entirely obscure, if not en-
tirely inaccessible.

The first attempts to an illustration of therelac
tions which exist among the natural phenomena —
which exist in particular in the animal kingdom-
were traced from external characters. It was
from external appearances that scientific men in
the beginning tried to combine animals,as it seem-
ed to them they resembledeach other most.

But the simple investigation of these external
characters was not sufficient. Mistakes were
made under the impression that the right thing
had been found. Animals, for instance, like the

whale, were placed among fishes ; though now it
is very well known that those animals have no re-
lation to each other — do not even belong to the
same class. Crocodiles and turtles were placed
among the viviparous quadrupeds, because they
have four legs. Barnacles were placed among
shells— among oysters snd clams— because they
had a solid external covering ; and other similar
mistakes were made, which have been successive-
ly corrected.

The corrections of these mistakes have been
made after a certain knowledge of the internal
tructure of animals had been obtained. And it
was found so satisfactory to derive information
from the investigation of their internal structure*
that soon comparative anatomy and the knowl-
edge of the internal structure of animals became
the real foundation of the classifications of the
animal kingdom.

It was the result of the brilliant investigations
of Cuvier, to show that a natural arrangement of
the animal kingdom could be based upon the struc-
ture of the beings which were to be classified. It
was from such data that arrangements could be
produced, according to which all the kinds of ani-
mals Which were brought together were found to
agree in the most essential peculiarities, even
when they had ilot been previously investigated
anatomically.

This is one of the promising results of those in-
vestigations of Cuvier which made internal struc
ture the foundation of the natural system. But
he found at the same time, that otherwise natural
groups had the same structure ; and that from a
knowledge of a few individuals, a great many

PROF. AGASSIZ?S

facts could be acquired. The knowledge of a few
fish, enabled him to compare the whole class of
fishes with reptiles ; a knowledge of a few rep-
tiles, enabled him to institute extensive compari-
sons between reptiles and birds ; and again, be-
tween these and mammalia ; and to find that all
these animals agree in certain respects. And how-
ever many have been examined since,— and three
or four times more have been examined than the
number which Cuvier had known when he laid
out his classification— however many have been
studied since, they have all been found to agree in
these essential particulars. So that it is now plain,
that structure ia the principle upon which animals
can be most satisfactorily classified. And as I shall
often have occasion to refer to this classification
let me at once, in a few words, indicate which great
divisions Cuvier introduced into his animal king-
dom.

All the animals which I have mentioned, Fishes,
Reptiles, Birds, and Mammalia, are combined to-
gether, because they have a series of backbones,
called vertebrae, by anatomists \ and hence the
name of vertebrated animals. They agree ia the
general structure of their brain ;• they agree in the
general arrangement of the fleshy parts, and in
the general arrangement of the organs of life —
as of the organs of respiration, the heart, the ali-
mentary canal, and so on.

Another grouptwhich was established on the same
principle, is that to which we may refer worms,
insects, crabs and lobsters — all animals whose bo-
dies are divided into a series of moveable rings, —
joints, which surround the body and enclose
the soft parts ; and which are provided with move-
able legs, and in some, even in addition to these
legs, also with wings. All these animals have a
most remarkable arrangement of the nervous sys-
tem ; there being a series of swellings of nervous
substance placed, one in each of the rings, and
connected together by double threads ; so that the
nervous system is all contained in one cavity, not
only the general arangement of parts,but this most
important organ of life is also different from that of
vertebrates.

The next great group is that of Mollusca, contain-
ing cuttle-fish, snails, slugs, clams, and oysters, —
all those animals which we generally call shell-fish
—those which are provided with hard structures —
the body being soft and generally surrounded by
a great quantity of mueosity j the nervous system
consisting simply of a circle surrounding the ali-
mentary tube, with a swelling above the intestine,
and another below, from which all the nervous

threads arise, which are diffused into all parts of
the body.

In these three groups of the animal kingdom,
all parts are in pairs, placed on two sides of the
longitudinal axis. In all of these there is an ante-
rior and posterior part ; two sides, a right side and
* left side •, and they have a back part and a lower
part ; they are, in fact, symmetrical.

But there is another group, in whicS there is a
different arrangement. The mouth is in the cen-
tre of a circular bodv j and from this mouth, the
organs are placed like rays, diverging in all direc
tions. Here we have no right, and no left side, no-
anterior and no posterior extremity. The body i»
star-shaped -, and the nervous system has the same
general structure, consisting of an horizontal ring
around the entrance of the alimentary tube, and
has no longer an upper and a lower swelling, as in
Mollasca.

There have been a few modifications made in the
details of this arrangement as proposed by Cuvier.
Some of the animals placed among the mollasca,
were found to belong to the group of articulata.— •
Barnacles are one of this group ; a very remarkable
family, from the numerous shells around the body.
Without knowing certainly, he had placed thern
among the mollusca; but on examination, it wa&
found that their nervous system consisted of swel-
lings, and that their bodies were divided into joints
— and an additional evidence was obtained from a
knowledge of their young, which were found to
resemble, in the earlier stage, much more the crus-
tacea than the mollusca ;. and indeed, that they
were Crustacea, and assumed this covering only ak
a later epoch.

However important these anatomical researches
have been,it is nevertheless my belief that in this line
of investigation we have gained all the important
information that we can gain ; and that we have to
run new tracks in order to improve our natural
method, — that we must even give up this funda-
mental principle, as the ruling principle, if we will
make further advance in this science. And my
reason is this : The minute investigations which are
now making in the anatomy of animals, are bring-
ing forward such differences between them, that
we have no principle by which we can appreciate
their value, And if we consider every difference in
structure as sufficient to separate animals, the time
would come when we should form as many groups
— as many divisions — as there would be smaller
groups in the animal kingdom, as it can be shown
that even genera differ anatomically among them-
selves.

If I am not entirely mistaken, these new investi-
gations, this new information, must be derived
from embryological data. It is to the study of
young animals— it is to the investigation of the
formation of the germ within the egg, that we
must appeal for a ruling principle to ascertain the
real, natural position of the subdivisions of the mi-
nor groups in the animal kingdom. I acknowledge
that the great divisions will always stand on the an-
atomical structure. But the subdivisions of the
classes cannot rest upon anatomical investigation j
and if I do not fail in my endeavors,! hope to show
it to you satisfactorily. This new step is a natural
consequence of the natural progress and state of
our science.
Investigations have recently been carried on,

LECTURES ON EMBRYOLOGY.

cuore particularly than before, upon the growth of
animals within the egg; and some facts have been
brought to light which have their bearing on Zoolo-
gy. Though how these facts have to be applied to
the study of classification, has not yet been traced.
Embryologieal investigations have been particular-
ly made with reference to Physiology— that is, with
reference to the mode of formation of the various
orgatve which exist in animals, and not with refer-
ence to ascertaining their natural relation among
themselves*

Another series of investigations which have mod-
ified considerably the views which were entertain-
ed of the structure of the animal kingdom, are
those microscopical researches upon the intimate
structure of the tissue of the mass of the body.
Of what does the flesh* the bone, the nerve, the
various masses of the body,consist ? and how have
they been gradually formed? has been the object
of various microscopical investigations. And
again, in this department facts have been brought
to light of whicii we can avail ourselves in inves-
tigating the natural relation of animals. On in-
troducing a series of Lectnres on Embryology, my
object is not to illustrate embryology in the same

sense, in the same manner, in which it has gen-
erally been traced.

I — EGGS OF FISHES.]

in the animal kingdom. My object is not merely
Embryology; it is Comparative Embryology. And
under Comparative Embryology, I mean the com-
parisons of those phenomena which have been
traced in the growth of the different animals, and
the different modifications which occur in individ-
dual species, throughout the different classes, in
their natural gradation, when full grown.

Let me, with a reference to a few diagrams,
show what I mean. Here are the various stages of
the growth of a fish. See here [A] the egg in the
earliest condition. Here is the first indication of
something different (Bi Next we see it still fur
ther advanced. There are afterwards successive
changes taking place, which go on to give rise to an
elongated mass, J Plate I, C D E] which swells and
elongates more and more till in the anterior por-
tion there is a greater swelling, which finally as-
sumes a more decided change, till there are indi-
cations of longitudinal lines, which grow more
prominent.

The transverse divisions are introduced until we
see a little fish is coming. [Laughter]. From this
time it undergoes another series of changes. IE
resembles more a fish. The head is now distinct.
The backbone appears here. It begins to be mov-
able and finally, [F] we have the form of the fish,.
with the mass of yolk under the abdom-en.

Now, embryology traces all these changes from
the first formation of an egg to the formation of
the germ within the egg; but the germ is not yet
formed. We have next to witness the formation
of the animal; and afterwards we trace io the prim-
itive egg, the successive changes of the first rudi-
ments—we trace its transformations. We have
first its formation in the egg. We trace afterward
its transformation through changes of different
forms. And it is important to distinguish between
these two orders of phenomena — the formation of
the germ, and the transformation of the animal
into different outlines. The one would be the sub-
ject of embryology proper; the other is called the
metamorphosis of an animal; and has been partic-
ularly studied among insects, where the new being
passes through very different and quite distinct
forms. For instance, in Butterflies it is first in the

; form of a caterpillar, as you see here:— [Plate II.
fig. A 3

[PLATE IE— BUTTERFLIES AND CATERPILLARS j

I shall not undertake to so back to the begin-
ning of animal life, to attempt to illustrate in what
manner individual life is produced, and how,
generation after generation, new sets of individ-
uals of each kind are made to succeed each other.
I shall simply take the germs as they occur in the
egg, to trace the changes they undergo ; and by
the knowledge of such changes, show that they
orm such series as agree with the natural series j

PROF, AG ASSIZE

Here it is older, [Plate II, fig. B.] It is what
we call pupa, but it is only an older caterpillar and
not yet fully grown. These are simply stages
in one and the same animal ; and we have been
misled by ideas which we had formed from what
the ancients called metamorphosis; we have
been allowed to let ourselves think that they were
one class of beings transforming themselves into
other beings ; but they are not. They are all one
thing in different stages.

As is the caterpillar, so- is the pupa ; and so is
the perfect animal, the butterfly. The animal re-
mains in the first condition for a certain time, and
changes his condition and remains in the second
condition a certain time, and finally arrives at its
last transformation. And before it can undergo
guch changes, is had to be formed. And in the
changes which the substance of the egg itself un-
dergoes, it is the substance of the egg which gives
rise to such a priaaative form, and then undergoes
metamorphosis.

[PLATE VII— EGGS OF

longitudinal line marked beneath, [Fig; C.J
here, [Fig. D] we have, after certain transforma-
tions, an animal with its blood vessels, growing
towards the perfeet form. And in this way we
have tbe various transformations or metamor-
phoses take place.

In other animals the metamorphoses are gradu-
al. We see, for ins-tance, the tadpole, from the
singular form first seen [Plate III. fig. A} passing
gradually into the form of a frog. But every
metamorphosis takes place gradually, not seem-
ingly from one animal to another, but by changes
of the same animal to others and other forms.

[PLATE III— FROGS 1

This is the egg ot a musquito, [Plate VII, fig. A ]
And there is the external mass [Fig. B,] making
its appearance. After some changes, it becomes
divided externally, and when the little worm which
is within the egg escapes, it is in the form of a
larva. It then undergoes transformations by which
it finally assumes its perfect form.

[PLATE VIII— EGGS OF RABBIT?.!

A transformation takes place in all animals.
This [Plate VIII, fig. A,] represents the egg of a
rabbit. It undergoes similar changes to those in
the fish. It gives rise to the prominent mass as
seen here, [Fig. B.] This spreads, and there i8 a

Now, it will be the knowledge of this metamor-
phosis of animals which I intend to make the
foundation of a natural system of Zoology. And
how that is to be done, I will explain by an exam-
ple, and refer to the class of reptiles ; as I find it is

LECTURES ON EMBRYOLOGY.

in that class in which we have the most matured
materials for such an investigation. I might have
selected a more worthy subject than frogs and
salamanders, and perhaps have alluded to the
higher animals. But let me say, there is nothing
unworthy of our attention in nature. And if we
can trace the action of the creative power in these
animals which we despise, let us consider that
they were made by Him, and if they were worth
making, they are worth considering by us.

The class of reptiles as it is now circumscribed,
is a very natural one, though it was not always so
in the works of natural history. There was a time
when crocodiles, lizards, turtles, were not ranked
among reptiles, but were placed among quadru-
peds, with all the higher animals — all the higher
mammalia— and when reptiles were to naturalists
only serpents and frogs ; and even then they divi-
ded those animals into two groups — the creeping
snakes in one, and the jumping batrachia in the
other.

Laurenti, an Austrian naturalist, was the first
who described these most carefully, bringing to-
gether frogs, lizards, turtle, salamanders, toads,
and combining in one natural division all the
principal animals which we now refer to it. —
But his classification was not much better on that
account. He placed in one and the same division,
salamanders, and lizards, and crocodiles, which we
now know to be widely different ; and he did not
place in that class another group of animals, which
we refer to it the Csecilia. I shall not enter into
too many details, for fear I should not finish what
I have to say this evening.
Linnaeus followed the same example. He brought
together turtles, crocodiles, lizards, snakes, frogs
and salamanders, but unfortunately left in the
same class some fishes, which he combined with
the reptiles, owing to some peculiarities of their
solid frame. Linnaeus also left the salamanders
with the lizards, because they had four legs.

Here is one of these animals [Plate IV, fig. F ]
Brongniart, the celebrated geologist of Paris, stud-
ied these animals, and happily threw great light
upon the subject, when he showed that reptiles
could be divided into four groups— the turtles
being one, the lizards another, the snakes a third
and the Batrachians, as he called the frogs and
salamanders, the fourth and last group. And in
this, for the first time, we see salamanders sep-
arated from lizards and brought into connexion
with frogs and toads. He had noticed that these
animals undergo similar changes— that they are
equally naked— that they have not the scales
which characterize higher reptiles, and he there-
fore brought them together, but he: left out an
animal which really belonged to that class. A
naked snake called Cascilia by naturalists, was left
out and included among the snakes.

I shall use the term Batrachia to designate all
those animals which are allied to frogs and sala-
manders. We have a great variety of these ani-

[PLATE IV— SALAMANDERS ]

mals. After the publication of the works of Brong-
niart, Oppel, Dumeril. etc., (who also introduced
new views on the subject) they were extensively
studied, so that in the museums these animals be-
came more numerous, and it became necessary to
introduce some subdivisions among them. Xow
let me show what sort of animals are referred to
this order of Batrachia. And in the first place we
have the type of frogs. [Plate III ] Animals
which have four fingers in the anterior leg, and
five behind. There is no tail to those belonging
to this group — we refer to the frog and the
treetoad. There is a web in the finger of the frog;
but in the tree toad there is a kind of web, and it
is floating. But in the toad the fingers are entirely
free.

In the salamanders there is a tail. There }ave
four fingers at the termination of the anterior ex-
tremity and five at the termination of the posterior
extremity. Without the tails, salamanders would
be compared with frogs and toads. If their body
was somewhat more contracted they would resem-
ble each other very strongly. And indeed, their in
ternal structure is similar. On account of the

PROF. AGASS1Z S

[PLATE V.]

presence or absence of a tail, these have been divi-
ded into two groups — without a tail and with the
tail. The tail is shorter and thicker and the whole
body is more contracted. [Plate V. fig. B.] Here are
gills which do not exist in any other of this group,
gills which exist in the whole life only with fishes;
but which here exist simultaneously with lungs in
the body. This is called [fig. B] Menobranchus
Maculatus; and this [fig. A] is called Menopoma
Alleghaniensis^

[PLATE VI.]

Here are three fingers forward and two back-
ward. [Plate VI. fig. A.] This is found in South-
ern Germany. Here [fig. B] is one with a very
minute fin. This is a species which occurs in Geor-
gia. And here is an animal [fig. C] which has
anterior legs but no posterior ones, and occurs in
our Southern Slates. There is another type which
is not figured, in which there is no tail, no legs,
and only a transient and temporary gill. It is the
Csecilia— the so called naked snake. The position
which is now assigned to these different an-
imals is as follows : As late as 1826, Fitzinger,
who has furnished an elaborate dissertation on
this class of reptiles, classes at the head of Batra-
chians the genus Ccecilia, still impressed with its

resemblance to the snake. He considered it as al-
lied to the snake and placed it at the head of Ba-
trachians, which are from their structure the low-
est type among reptiles. Next he placed the frogs
and toads, then the salamanders, and those ani-
mals next these, like salamanders [Plate V. fig. B.J
This was followed by all following investigators
of succeeding years. Cuvier, in his animal king-
dom, in 1829, however, made a step backward. He
replaced the Caecilia among snakes, though he
could not have overlooked the investigations of
naturalists who had shown that the want of ribs,
the peculiar articulation of the head with the
trunk, was much more closely allied to that of
frogs than to that of snakes ; and the want of mov-
able jaws, again, should have prevented him from
confounding the Cecilia with snakes.

He placed the frogs at the head, next the toads,
next the salamanders without external gills, and
finally the salamanders with external gills. I have
given these details on purpose to show that in
all these methods there is no principle ; and I refer
to the leading authors in the natural history of
reptiles in order that I may not be taxed with over-
rating the value of the principle which I am now
about to introduce, or of over-rating its influence —
its value. Wagler, who is also the author of a
system of Herpetology, places at the head, caecilia,
next frogs, then toads, next salamanders, and final-
ly, the proteus and menobranchus. Canino fol-
lowed in a similar track; so did Johannes Miller,
of Berlin, who modified it somewhat, placing the
naked snake lowest. Next this one, which has no
external gills, [Plate VI. fig. B.] and finally this
one [Plate V. fig. A.] And above these he places
those which have gills, and above the salamanders,
the frogs.

Tschudi, who has published a natural classifica-
tion entirely devoted to this subject — that of Batra-
chians— places Menobranchus lowest. Then he pla-
ces the naked snake between salamanders and frogs;
which he justifies simply from the structure of the
head, or at least, gives that as his reason for the
arrangement. Now you see that from want of
a principle, all these details differ in the various
authors. No one is ruled by anything but his im-
pression—his feeling about it. And I think that
we can substitute a principle, and we can show
that this principle has nothing arbitra^, and is
given to us by nature.

Let us trace the metamorphoses of frogs, and
there we have the key. What are the changes
which frogs and salamanders undergo ? In the be-
ginning,for instance, salamanders are animals with -
out legs at all, [Plate IV. fig. Aj with a long tail^
and large gills on the side of the head. A change
takes place. [Fig. B.] Another change occurs; the
gills remaining and growing larger, when an
anterior pair of legs appears, and in anoth-
er stage the gills are reduced 1 Figs. C. D ] when
the second pair of legs appears. [Fig. E J Here the
anterior pair has four fingers, but here [Fig. F] is a

LECTURES ON EMBRYOLOGY.

further change of the same animal,when it loses its
gills entirely, and the posterior pair of legs assumes
an additional finger, the animal having four fingers
forward and five backwards.

What changes does the frog have ? Hatched, he
is an animal without legs and without gills. [Plate
III. Fig. A] The salamander is hatched with
gills, but there is an epoch when it is without gills,
and without tail, and without head, and only a fis-
sure on the sides to indicate where the gills will be
formed, but not yet external gills. The frog has
not yet gills, and not yet a tail distinct from the
body. But next, the tail makes its appearance.
[Plate III. Fig. B,] when the head separates more
distinctly from the mass of the body, and the tail
grows longer, [Plate III, Fig. C] and here [Fig. D]
the tail grows still larger But in addition to that
we have a pair of anterior legs, and the gills have
disappeared. Then we have the same growth in
the posterior legs [Fig. E] coming out, though not
yet as large as they are here [Fig. F]. You see
that the size of the tail in proportion to the main
mass is reduced, and finally the tail disappears en-
tirely, and we have a frog, [Fig. G.j

Here, in these facts we have not only the history
of the transformation of salamanders and frogs,
but we have a natural system of batrachians, and
there is no longer any arbitrary arrangement in
our system possible. Every thing is indicated in
the metamorphoses of the animals.

Here we have a tailless, and gilless,and feetless
animal, [Plate III. Fig. A.] Suppose it grows no
longer it has the appearance of Csecilia. Next it
assumes gills — rudimentary gills, in the condition
in which we see the primary growth, with rudi-
mentary legs formed. This stage corresponds to
Siren, [C]. And here is a second pair of legs formed,
[Plate III. Fig. E,] answering to Proteus, [AJ. And
here we have it shortened, [Fig. F.J— the corres-
ponding animal in its iull formation. See Plate V.
Fig A.

Whether or not this one [Plate VI, Fig. B]. will be
lower in the scale than this [Fig. C ] we have yet to
determine. And all these American species will
be examined, which will throw so much more ad-
ditional light upon this metamorphosis, that there
will be no doubt in regard to the position of that
animal. The two posterior legs have only four
fingers, while the other has five fingers. The tail
is shortened, as we see successively, in the frogs
and toads. But of the three toads, which is to be
placed higher and which lower ? That menobran-
chus stands lower than menopoma is plain, as in
the former the existence of the web is a mere rudi-

mentary condition. The web fingers are observed in
all these early stages of growth, and those which
have distinct fingers, when fully grown, have them
webbed when young. Therefore, we shall see that
the frogs are not to be placed higher. And frogs
must be lowest, next treetoads and then toads the
highest, because their fingers are finally entirely
separated.

And in conclusion, I will say, that in studying the
metamorphoses of animals, we may find in the
transformations — -in the different formations
through which they pass, from the first formation
up to the full grown condition, a natural scale by
which we can measure and estimate the position
to ascribe to any animal belonging to this family.

And, undoubtedly, the various genera of this
family which I have mentioned, will find their
places as soon as all the different metamorphoses
of these different animals are known. At present,
we know only the transformations of frogs and of
salamanders, through the researches of European
Naturalists. The metamorphoses of the numer-
ous species of that family which occurs in the
United States not having been investigated.

But this agreement of transformation is most
remarkable. Nevertheless, we must acknowledge
that these perfect animals which occur in different
parts of the world in our day, are not copies from
metamorphoses from the different stages of the
growth of frogs ; but they are animals of a pecu-
liar kind, produced in various parts of the world,
showing proof that there is one and the same plan
ever producing the formation of this whole class,
as well in the developement of the young from the
beginning of their growth to their full grown stage,
as in the formation of the different animals which
inhabit different parts of the globe.

There is a, freedom in the developement of this
plan, a freedom in which we can see the action of
the intelligent Author of all these things.

We read here the intelligent action of the Creator
in the production of these animals ; and we read
more than the intelligent invention of his creation.
We read the omnipresence of his action, as his
action is developed on all parts of the globe, in the
United States, in Europe, in Japan, in South
America, and in all the portions of the globe. —
And when developed in that way in its actual con
dition, we see that every one of them, when repro-
ducing its species, passes through these differen
changes — the higher one, through more of the
changes ; the lower one, undergoing only the ear-
lier modifications. •

PROF. AGASSIZ'S

LECTURE II.

The object of my first lecture was, to show that
after Comparative Anatomy had illustrated the
general relations of the animals throughout the
animal kingdom, it was possible to ascertain more
closely the nearer affinities of the different minor
groups, by tracing the relations which exist be-
tween full grown animals and the changes which
animals of the same family undergo during their
earlier stages of growth, from their first formation,
in the egg to the epoch when they are full grown.
In one instance, I think it has been possible for
me to show that the various forms which we ob-
serve in the class of reptiles, in that order of rep-
tiles which naturalists call Batracbians, really cor-
respond in their general character, though not in
the particular features of their proportions, to
those which the higher species of Batrachians
present up to the time when they have assumed
their higher form. This result shows that the
principle exists ; though its application in the dif-
ferent classes of the animal kingdom is at present
not possible in all its details.

But this result gives also evidence of another
important view ; that is, that there is really a plan
in the animal kingdom ; a plan which can be read
without any part of the view arising from us, but
being taken from nature. We read there the do-
ings of an Intelligence which created those things ;
and we can read even more than that, in this plan.
On dwelling upon another fact, in my Wednesday
Afternoon lecture, I showed that this plan is not
carried out in one locality, in a few types merely,;
but that it is worked out all over the surface of our
globe; and thai, therefore, the result of this in-
vestigation shows the Omnipresence of the Crea
tor in his creation.

It becomes now my duty to enter upon a special
illustration of the various classes of the animal
kingdom, in order to trace, if possible, similar re-
lations between them.

I shall begin with the great group of radiata, the
lowest in the animal kingdom. My reason for do-
ing, so is, that the animals belonging to this type
are the simplest; and perhaps it will be easier to
show here how the egg, with its simple elements,
can undergo such changes as to give rise to the
formation of an animal; and the changes not be-
ing so extensive as they are in higher animals, it
will be more readily understood how they are
brought about.

The type of radiated animals is divided into
three classes: the polypi, or polyps; the jelly-
fishes, or medusas; and the echinoderms, or star-
fishes and sea-urchins. These three classes differ
in their general structure, and their differences

have been made out by anatomical investigations.
They have general relations to each other, by which
they belong to the type — to the great group— of
radiata. Owing to their simpler structure, the po-
lypi stand lowest ; next come the medusae or jelly-
ashes ; and among radiata we place the echino*
lerms highest.

I shall begin with the echinoderms— though per-
haps the polypi, from their simpler structure, may
answer best the first purpose to which I alludedj
and be more easily understood, But there is an
objection to my taking up polypi first; it is the
fact that naturalists have not agreed as to the sub*
division of polypi into families, from the fact that
their structure being so simple, it is difficult to
estimate the value of the differences which they
present; and therefore, these differences have
not brought to light a clear gradation of the fami-
lies. And perhaps there is another difficulty with,
them to overcome — the fact that individual life is
not so distinct among polypi; that several individ-
uals remain combined together to lead a common
life ; and therefore, we should have to allude to
increased difficulties in the estimation of these
beings, when investigating the mode by which in*
dividual life is established, and by which individu-
als grow. Those difficulties will be easier under-
stood after we have traced the growth of animals
which are really individuals in the proper meaning
of the word; that is to say, which grow isolated—
which are detached from the parent early in life,
and grow separate.

The Medusae would perhaps answer next; but
so singular phenomena have been observed among
them that I fear to allude to them at once. We
observe, namely among the Medusae, the singular
circumstance of alternate generations ; that is, of a
progeny which do not resemble the parent — of a
second generation which differs from the first, a
second generation which returns to the form of the
grandparents; and so on successively. And this
singular order of succession of individuals of differ-
ent aspects, makes it difficult to understand their
different analogies— to understand the differences
by which the two generations differ. Therefore I
shall begin with the highest class — with the Echi-
noderms—where we have, in the successive gener^
ations, truly independent individuals, arising from
parents similar to their progeny. Moreover, the
Echinoderms have been extensively studied , they
have been the object of monographic investiga-
tions; their genera are well characterised and nat-
urally circumscribed. A great many of these have
been found in a fossil state, and these fossil remains
will compare with the living types. Such differen-

LECTURES ON EMBRYOLOGY

have been foundbetween the fossils and the
living ones, that we shall have an opportunity to
allude to another relation which exists between
.these different ^forms. Those which have existed
earliest upon our globe, in the ancient geological
epochs, do not indeed resemble those which live
now ; but they are related to the forms of the Ech-
inoderms of the present day in their earlier stages
of growth). And so the class of Echinoderms will
afford us the means of investigating all the differ-
ences which exist between the animals of that class
living now, as compared with their embryonic
changes, and also between the changes which the
representatives of the same class have undergone
from the earliest geological times, up to the time
when theorderof things which now prevails upon
this globe was introduced.

But yet very little was known of the embryology
of Echinoderms. Two singular investigations had
been made upon this subject, one by Mr. Thomp-
son, of Cork, who had ascertained that the Coma-
tula, a star fish with pinnate rays, of which you
have here a figure [Plate I,fig. B] produces youngs
like this [Plate I, fig. A], resting upon a slen-

PLATTC T— TIGS A AND B.

der stem, which during their growth cast this stem,
become free, and assume finally the appearance of
Fig. B.

Next, a Norwegian naturalist, Mr. Sars, traced
the changes which the egg of the Star-fish under-
goes. Here are the different figures which Sars
drew of the young of a small species of Star-fish
called Echinaster Sarsii, which occurs on the Nor
wegian coast. It is first a spheroidal mass, which
[PLATE II— SAKS YOUNG STAR-FISHES.]

is said to move free, like Infusoria, when upon
one of its surfaces three tubercles are first observ-
ed. [Plate K, fig Aj.

These tubercles soon become more extensive and
run together, forming a figure, similar to a Roman
T. [Fig.B].

Here it is in profile, [Fig. C] where the cross
of Fig. B. appears like two horns on the upper side.
This prominent part next assumes this figure [Fig.
D] and seen in profile, it is like the letter E. After
this the sphere is divided into five lobes, [Fig. F]
with a central one more prominent. Finally, that
figure would become more and more flat [Fig. G]
its prominent horns which had grown larger, are
afterwards reduced, and finally disappear entirely,
and an animal similar to a Star-fish is produced.

From these investigations, Sars concluded that
the young star-fish was originally a spherical being*
swimming free like the infusoria — that it soon as-
sumed a bilateral form, and that this was finally
changed to a star form. In this I think Sars has
been mistaken, in as far as the bilateral outlines of
the young as he represents it, is only the result of
a lateral flexion of the peduncle hanging under the
centre of the umbrella-shaped little animal.

But in order to show how a simple egg is trans-
formed into an animal so complicated as the star-
fish, it is now necessary for me to allude, first, to
the structure of Echinoderms in general. It would
be otherwise impossible for me to show how the
various parts are gradually developed, if I could
not refer to the complicated organization of the full
grown animal. These details would* indeed have
very little Interest if they were not described in
connexion with the complicated structure of the
perfect animal.

[PLATE III— GERMS OF STAR-FISHES.]

[PLATE IV— YOUNG STAR-FISHES.]

VERTICAL SECTIONS.

PROF. AGASS1Z7S

These figures [Plates III and IVj represent the
changes which I have observed in a species of
Star-fish from Boston harbor, from its first forma-
tion in the egg up to its perfect condition ; though
I have not been able to trace it to the full size to
which it grows on these shores. Sars has not
been able to ascertain the internal structure of the
Star-fish, because the species which he observed
was too opaque, and did not allow an investigation
of the internal parts. The species which I have
compared admitted of such an examination, hav-
ing more transparent parts, and by a peculiar pro-
cess of investigation it has been possible to ob-
serve the whole internal structure, the specimens
being pressed between two glass plates, when
placed under the microscope.

Before I allude to alj the details represented in
plate III & IV, let me show from these figures how
I conceive that the diagrams of Sars [Plate II]
though drawn from nature, give an erroneous im-
pression of the animal. It is simply that the pe-
duncle hanging from the centre of the discoid or
spherical body being laid flat upon a glass plate,
and perhaps pressed it on the glass, for the mi-
croscope is bent sideways, and thus it is seen as
in these figures. But when seen floating, it will be
noticed that this peduncle hangs downward, [Plate
III, fig. A, B, C, D].

As a class of animals the Echinoderms agree most
remarkably in their structure, though differing
most widely in their external forms. We have in
the first place elongated forms, somewhat like
worms, with 'a star- shaped extremitj', called Ho-
lothurise.

[PLATE V — HOLOTHUT?T^E ]

[PLATE VI— ECHINODERMATA J

Here are spherical or spheroidal forms of these
animals called Echini or Sea-Urchins, [Plate VI]
and finally star-shaped ones, called star-fishes,
and among which there are free ones, those which
rest on a stem, like lilies, [Plate VII. fig. A D ]
PLATE VH — STAK-FISHES — CRTNOIT>= 1

These various animals are so widely different that
it seems scarcely possible to find a fundamental
plan of structure and a uniform arrangement of
parts in all of them. Yet it is so. Conceive for a mo-
ment that the fundamental form is a spherical one.
K the sphere is extensively elongated, we have the
form of the Holothurise, Plate V. ; the spheroid
form itself may be more or less ovate [Plate
VI. I or angular; or if the corners of these be
drawn out, we have a real star-fish. In the cen-
tre of some of the circular ones there are plates
or prominent knobs on the summit, [Plate I. fig.
B.] which may form a kind of peduncle above. —
Now it is easy to conceive that these growing
longer will appear in the shape of a longer or
shorter stem upon which the animal will move,
[Plate VI. fig. A DJ balancing itself. So that from
these polypi-like forms up to the worm like forms
we have gradual transitions.

As the highest among the radiata the echino-
derms are more complicated in their structure, —
Their external coverings are already more distinct
than in any other. In the polypi the skin is close-
ly attached to the fleshy mass of the body. Here

LECTURES ON EMBRYOLOGY".

have an envelope which is entirely separated
from the internal organs, forming a covering, which
is either hard, leathery and strong, or a firm coat,
consisting of numerous calcareous plates united
together, or connected together in a movable
way. These external coverings are not like a
shell resting on the soft parts, but they form intrU
cated connections with all the different systems of
organs, although these be distinctly separated
from the external envelope. For this purpose they
are pierced by numerous holes of various kinds,
indeed the connexion of this external covering
with the internal frame is manifold. The mouth
again, which is always toward the centre of the
animal, is also only an opening in the middle of
the disk, and upon its edge are various movable
parts performing the functions either of teeth or
tentacles, by which the food is seized, In another
position, frequently opposite the mouth, there are
other apertures by which the ovaries discharge the
eggs, and little holes in which eye -like organs are
placed. The organs of respiration which admit
water from outside are either in the general cavity
of the body, or situated more externally, round the
mouth, or on the sides of the animal.

There is in these lower animals a closer connex-
ion between their inner cavity and the surrounding
media than in any of the higher classes. The
water rushes freely into the body through innu-
merable pores and fills its cavity, Some of these
tubes assume a very peculiar arrangement in
-achinoderms, and become simultaneously subser-
vient to locomotion. As this apparatus is one of
the first to appear.in the young, let me allude to
its structure as we observe it in the starfish. Here
are the different rays [Plate VIII fig, B] project-
ing from the centre. There is a sac projecting
in the main cavity of the body— a stomach — and
frr.m this stomach we have appendages projecting
into the rays, to which a kind of liver is annexed,
and filling for the most part the cavity of the rays.

IPLATE

In the figures [Plate IV, figs. E, F] in which the
starfish is cut vertically, the sacks extending from
the stomach, with the liver attached to them, are
ia tfeeir natural position The nervous sys-

tem forms a ring all around the walls of the open-
ing leading to the stomach ; and there are ner-
vous threads arising from this central ring to each
of the rays, and extending in five different direc-
tions to their extremity. And at the end of each
ray there is a colored dot protected by a hard
shield. This colored dot has been ascertained to
resemble the lowest form of eyes.

The solid frame which protects the whole ani-
mal consists of various little plates, [Plate VIII. ]

They are numerous on each side of the rays, and
there is another one at the end, and it is below this
last one that the eye is placed. Those solid plates
on the two sides unite with many others placed
transversely to form the lower surface, and alter-
nating with each other. Between these transverse
plates are the holes for the tubes mentioned be-
fore. At the end is the odd plate.

These tubes are seen here hanging down [Plate
IV. fig. E.j. They communicate inside with small
vesicles, to which minute tubes lead, communicat-
ing with larger tubes, which extend,along all the
rays, one for each ray, arising from a circular tube,
which surrounds the opening of the stomach. And
the whole apparatus communicates with another
tube, which penetrates from the dorsal surface
downwards, having its opening shut by a perforated
plate called the madreporicbody, which in starfish^
es is always seen in the angle between two of the
rays ; so that we have here an hydraulic apparatus
of a very complicated nature. Indeed, from the
upper surface of the starfish, where the little seive
through which the water penetrates is situated,
there is an uninterrupted communication to the
circular tube around the mouth, from which five
tubes branch out, one to each of the five rays ; and
from these, they open to the vesicles, and thence
penetrate into the tubes. But the water can enter
the vesicles through the external lower tubes, fill
the circular tubes, and pass out the other way
through the madreporic body. This apparatus is
subservient to various functions. In the first place^
the lower tubes serve as a walking apparatus ; the
animal being fixed and creeping by the contraction
of the tubes, and again water being introduced into
the vesicles upon which are spread numerous little
blood vessels, and the water acting upon these
blood vessels modifies the blood, and gives it the
peculiar character necessary to perform its func-
tions, constituting a peculiar kind of respiratory
system. The minute holes spread over the whole
surface of the body, serve simply to fill the general
cavity with water*

The heart is placed along the calcareous tube
whteh arises from the madreporic body, and the
blood vessels form circular rings around the en=
trance of the stomach, from which and to which
the radiating arteries and veins move.

Another apparatus which is very voluminous ift
starfishes is the ovary.

There is such an organ in each ray, concealed be-
tween the appendages of the stomach, which opes

PROF. AGASSIZ'S

upon the upper surface with little holes through
which the eggs escape. The ovary itself is a gra-
nular organ, of which several figures in various
stages of developement are here seen.

'PLATE IX— OVARIES OF

Such is the structure of the Echinoderms, the
stomach forming a simple cavity, without any oth-
er outlet except the mouth, In some the alimen-
tary tube is more complicated. In the Echini or
Sea-Urchins [Plate VI] there is an alimentary tube
forming several evolutions, and opening upwards.
The ovaries form more peculiar masses than in the
star-fishes. The mouth is also protected in most
Echini by a complicated set of jaws and teeth. In
Holothurise the whole system of organs assumes a
more bipartite arrangement.

In the process which gives rise to the formation
of new individuals, the first step consists in the ac-
cumulation of more or less consistent matter of a
somewhat opaque or yellowish appearance, and of
a granulated texture which divides soon into small
spherical masses. This takes place in the ovary. —
This mass, at first homogeneous, assumes soon the
aspect of little bunches, which soon grow more and
more isolated, and then assume around them a pe-
culiar membrane, and there are eggs. Eggs in their
simplest condition are microscopical spheres of a
homogeneous mass, called yolk, and surrounded
by a simple membrane, called the yolk membrane.
(Plate IX C.) However different in its aspect in
different animals, this mass is called yolk, through-
out the animal kingdom, from the fact that this
name has been applied to the part which corres-
ponds to this structure in the hen's esg.

The primitive egg is always microscopical, and
its contents homogeneous ; but this substance soon
becomes granular. It is so small as to escape the
observation of the naked eye. And there is anoth-
er little sphere formed within, which is called the
Kerminative vesicle, containing another little ves-
cicle, which is called the germinative dot. [Plate
IX fig. D.] Under a powerful microscope the gran-
ules of the yolk itself appear also like little cells.—
There are little spherical masses, and they contain
even in their turn other little dots.

Plate IX shows the various degrees of the growth
of such eggs, of which there are more or less de-
veloped ones in the same ovary ; assuming first
their regular form [Fig. AJ, and then a transparent
space appearing in the interior [Fig.B] ; next the

germinative vescicle becomes more distinct fii^'
C], and [Fig. DJ the germinative dot is now dis-
tinctly seen. The whole mass of yolk, which ha&
grown considerably, consists here of cells, which
have been formed by the expansion of its granules.
Through this growth of cells within cells, and of
granules growing into cells, there is finally a germ
formed, That which we call yolk in the beginning,
is finally a spherical germ, which will escape from-
its envelope. We have here [Fig. E} the ovary of
a star-fish, from which some germs have escaped,
and here is the figure of such a germ already
hatched, highly magnified [Fig. F]. The ovary of
sea urchins, have all the same structure, and
vary only in their size and proportions. Now a
curious observation which I have had an opportuni-
ty to make, is, that the eggs after they are laid are
taken up by the star-fish, and kept between its
tubes, below the mouth. The star-fish bends itself
around them, surrounds the eggs with its suckers,
and moves about with them. When the eggs had
been removed to some distance from the animal, it
went towards them and took them up again, and
moved off with them, showing that these animals^
so low in structure, and apparently deprived of all
instinct, really have so much instinct as to watch
over their young.

Now these eggs which are thus kept there, and
protected by the mother, will escape. These germs-
I have been able to trace from the lowest possible
condition, where they resemble ovarian eggs. At
no epoch did I see this new born animal living freey
and swimming like Infusoria, as is said to be the
case by Sars.

Soon, however, the external crust of the germ-
becomes more transparent, consisting of somewhat-
looser and larger granules, and the internal mass
assumes a color a little darker, so that two layers-
are distinct, between which there is another one,
which becomes also gradually more and more dis-
tinct. On one side of the germ there is now a pro-
tuberance forming, and the prominent portion
separates more ancUmore from the spherical mass?
[Plate IX, F] the difference in substance of Ms
layers growing more and more distinct. The promi-
nent portion, which is the lower part of the little
animal, becomes more and more elongated and as-
sumes more and more the form of a peduncle.
Often there are several grouped together, and at-
tached by this appendage to the empty egg casesl;
they would even form bunches remaining thus
attached till they are far advanced in their growth.
At this period, however, there is not yet any or*'
gan formed as you will notice on comparing Fig.
F of Plate IX. with those of Plate IV. p. 13. Onljr
changes of substance have taken place. But now
we begin to see little swellings in five points oc?
the sides ; the spherical portion of the germ ha»
also grown considerably, and has been flattened by
lateral dilatation.

The little animal has grown to a more hemis-
pherical shape ; and from that time there is an up-

LECTURES ON EMBRYOLOGY

per and lower surface to this umbrella-like disk \
[Plate HI, fig, C] and there is a tubular part and a
swollen portion to the peduncle* As soon as the
peripheric part of the umbrella begins to spread, we
observe five little tubercles forming underneath 5
and into these tubercles we see that the peculiar
aspect of the middle one extends. Soon there will
be other prominent swellings forming \ but two to
each of the former ones ; and next, two more, as
seen in Plate IV, fig. A, in which the peduncle is
represented from below projected upon the centre
of the disc. While this is going on, calcareous
nets are formed by the accumulation of crystals in
the cells of the germ, At first there are little iso-
lated crystals formed as nuclei in the cells ; and
then several close together will unite and form a
little irregular mass, and they will combine so as
to constitute a network of solid substance arrang-
ed very regularly. They aggregate first about the
prominent tubucles of the lower surface, corres-
ponding in position to the five primitive ones
[Plate IV, fig. B, page 13].

Now the points in which these calcareous de-
posites take place are symmetrically arranged
[Plate IV, fig. B, p. 13], Next, five alternating with
these arise in the intervening spaces, [Plate IV, B,
p.13] and another is formed in the centre of the disc.

All these networks are, however, not formed in
the same plane of the animal ; those arranged in
fives being deposited below, and the middle one
above the central mass of yolk in the periferic lay-
er of the germ.

At this periodfthe peripheric tubercles of the low-
er surface become colored in their centre and the ex-
ternal calcareous networks spread over them. The
red spots of the tubercles are now very conspicu-
ous. When examined under a high magnifying
power they appear like little heaps of colored
dots, and these are so many cells with colored nu-
clei. As peculiar organs, they answer to the rudi-
mentary eyes of the perfect star-fishes.

The calcareous nets which were at first only ten in
number, become now gradually more and more
numerous, marking out more and more distinctly
the rays of the little star-fish which are thus form-
ing, new being interposed in pairs between those
already existing, and small spines projecting from
the older ones. (Plate X., A.)

The tubercles of the lower surface, which alter-
nate with them, growing more prominent and
elongated, are finally transformed into suckers, as
I will call them, or the so called ambulacral tubes,
[Plate IV, fig. C.] With the addition of new cal-
careous nets they also become more numerous
and form finally rows of tentacles, D, E, F. Other
changes have also taken place. The cells within
the peduncle have undergone changes. Some
have become movable, and a kind of circula-
tion is going on in them. The internal space along
each ray has become more transparent ; the am-
bulacral tubes have become hollow, and from that
time there seems to be a communication between

the external water and the internal structure

What remains of the yolk is more distinctly cir-
cumscribed in the centre of the animal, extending*
as a star shaped disc into the rays, The radial
portion becomes finally distinct from the central
one, and we have at last an internal cavity, which
is the stomach, from which the coecal appendages
of the rays, with their liver-like organ, will be de-
veloped— [Plate IV, fig. E. p. 13]. The peduncle is
reduced to a mere vesicle ; a hole is formed in the
centre of the lower surface, the mouth, around
which A circular thread becomes visible, answer-
ing to the nervous system, and from which other
threads extend towards the extremity of the rays,-
being the radiating nerves which establish a con-
nection between the peripherical colored spots,
which are the eyes, and the central nervous sys-
tem which encircles the mouth. Before, the young
star-fish had thus assumed a life of about one line"
in diameter; it has now assumed the form and
structure of the perfect animal, To this growth
there is one point of peculiar interest — I mean the
correspondence between the development of the
calcareous net works [Pi. IV, fig. B, p. 13, and PI.
X, fig. A,] and the arrangement of the solid plates
in Crinoids— [PI. I, fig. A, p. 13, PI. VII, fig. A, I>
p. 14, and Plate X, fig. B.]

[PLATE X.J

But I see that the time has past, and I am obliged
to conclude. Let me only add a few remarks be-
fore I close. The mode of growth in the starfishes
as I have illustrated it, does not agree with obser-
vations which have been recently made by other
investigators. Von Baer, Johannes Muller, and
several other investigators, have traced the growth
of these animals recently. But they have traced
them at another epoch than the development
which I have observed here [PI. IIIp.13] ; and it is
now probable that in the Echinodewns, also, there
are two modes of reproduction during which the
growth of the germ is not identical, as in the ani-
mals reproducing by alternate generations. It was
during summer that the investigators just men-
tioned made their observations, and they found
that all their germs were surrounded with a most
remarkable external frame-work, whilst mine,
which are entirely destitute of such envelopes^
were observed growing during winter, at a season
when animals in general do not reproduce them-
selves.

However, it is remarkable how many of the low-

PROF. AGASS1Z S

er types produce their young during winter. But
on considering what may be the cause of their
egcs being deposited at this seaHon,we can suppose
it is owing to the fact, that during this epoch the
water is less changeable in its temperature and will
admit of a more uniform growth of animal life
than during the spring and summer. All animals
of low temperature or whose temperature is deep-
ly influenced by the surrounding medium, in op-
position to the higher organized ones, seem in-
deed to develope more naturally during the cold
period of the winter, when the possible Changes
are only slight, undulating about the freezing
point, from about the temperature of the greatest
density of water to that of the freezing point it-

self, that is between 32Q and 38&. The limits of va-
riation of the temperature of water being so very
slight under such circumstances, we can conceive
that these low animals are more likely to devel-
ope regularly than under the changing influences
of spring and summer; when along the shores the
influences are extremely variable and might kill
so delicate animals which have no means to main-
tain a temperature of their own.

In my next lecture, I shall compare these em-
bryonic changes with the perfect state of the vari-
ous Echinoderms of the present creation, and with
the perfect state of the numerous fossils of this
class which have been discovered by geologists in
the successive deposits of former ages.

LECTURE III.

I have shown, in one instance, the development
cf the star-fishes, as observed on these shores
during winter. It was mentioned, that from a
spherical form there was gradually a flattened disk,
a hanging peduncle, developed, out of which after-
wards arose a pentagonal form, which was finally
changed into a regular starfish, with the structure
of the full grown animals of that class. These
changes have been traced from the beginning of
the formation of the germ in the egg, when they
are protected by the mother who takes care of
them, carrying them about. At no period of this
development were the young star-fishes observed
swimming free. There can, however, scarcely be
any doubt that the young observed on the Norwe^-
gian shore were free. The observations of Sara
can the less be doubted in that points as similar
moving animals, which were afterwards ascertain-
ed to have been the star-fish and other Echino-
derms, have been discovered by the investigations
of Professor Johannes Muller, of Berlin. This
minute and leaded investigator described, several
years ago, a small animal as a new type in the ani-
mal kingdom, which he could not refer to any
class, nor to any family. It was a paradoxicon by
its form and its peculiarities, and he called it Plu-
teus Paradoxus. It is a transparent mass, support-
ed by several diverging sticks, surrounding an
internal cavity (PL XII, A) and moving free upon
the surface of the water* I have not dared to have
them shaded in my diagrams, in order to increase
the distinctness of the forms ; and only give these
slight outlines as they are figured by Muller. In
this condition, that animal is bi-lateral as seen from

above. At the two extremetieS of the longitudi-
nal axis, are two appendages 5 these appendages
are the stems which project laterally in Figs. A
and B, pi. XII., they being the anterior and poste-
rior ends of the longitudinal axis. Between thesej
you see one shorter pair on one side, and on the
other side another pair, which hang lower down*
[PI. XII., figs. A. B.] These two pairs of appen*
dages are indeed not equal in length. One pair on
one of the sides hangs lower down than the other
pair. Between those six supports, united by a gel*
atinous solid mass, there is an inner cavity, as
seen in the figures quoted. The side of the longer
ends has lateral projections, so that, in fact, there
are eight prominent sticks diverging from the sum-
mit of this curious being. No further structure
was observed in the first year by Mr. Muller. He
only ascertained they moved free in all directions^
sometimes rising forwards and sometimes revolv-
ing in different directions.

These movements were performed by vibratory
cilia, which are minute fringes extending all
around the edges of the frame, and which are also
grouped on the summit of the animaL These
fringes are microscopic. They form a swollen
edge round the whole of these dentations, extend-
ing all round the edge of these stems. (Plate XII,
fig. B.) What this being was, could not be ascer-
tained. It had been observed in the Northern
Sea, in thousands and thousands, and could not be
referred to its proper class. Whether it was to be
considered a medusa or a polyp, or whether it
was the germ of some other animal, could not fos
ascertained.

LECTURES ON EMBRYOLOGY.

[PLATE Xlf — SAKS' YOUNO OPFTTTTRA.]

The same observer afterwards found, that within
this curious frame there was forming a sack, with
an external opening hanging down between the
longer lateral sticks. (Plate XII, fig. A.) He
describes the opening, (Plate XII, fig. B), as a
mouth, the tube above as an oesophagus or ali-
mentary canal emptying into a sack, which be
calls a stomach. (Plate XII, fig. A.) The mouth
hangs here lowest, the tube above being an oeso-
phagus, and the sack in the centre a kind of sto-
mach. The changes which gradually take place
in this animal, and which are represented, Plate
XII, from A to F, were noticed, not by tracing one
and the same individual, but by comparing the dif-
ferences between those which were successively
fished up from time to time, as it was impossible
to trace for a long time the same animal, owing
to the fact of their dying away very rapidly. —
Comparing various individuals, Muller ascertained
that on the sides of the inner sac or stomach,
there were little processes or csecal appendages
arising (Plate XII, fig. B) from the side, which
grew out of two sides of that cavity which he con-
sidered as a stomach ; and these appendages grow-
ing more numerous, would form finally a bunch in
the centre, (Plate XII, fig C) consisting of about a
dozen of such rounded masses distinctly developed
from the sides of the stomach. Next there would
be (Plate XII, fig. D) a regular arrangement of the
growing protuberances arising from five definite
points, two and two, projecting more than the oth-

ers from each of these points, and from that time,
an indication of the starfish, forming within this
curious stage, is clearly noticed.

A regular star-fish has five beginning rays, en-
closed between those stems, developed from that
hollow organ which in the beginning is the simple
sack in its interior, with a wide opening on one
end, which gradually disappears in the new ani-
mal. At this epoch the young animal has no
opening at all ; what was was first considered as a
mouth is shut up (Plate XII. fig- C). After a cer-
tain time, however, upon one of the surfaces will
be found a new opening, (Plate XII fig. E). The
rays are advancing, growing longer by the ad-
dition of some new divisions in the mass ; and
growing larger and longer, the rays would become
soon very prominent, and suckers like those of the
little star fishes which I have described, would
come out, when a real mouth is seen in the centre,
and no indication as to what has become of the
curious tube first considered as an oesophagus,
(Fig. B). The surrounding transparent frame has
been reduced to a few processes, to a few append-
ages on the dorsal surface of the animal, (Fig. E).
They are afterwards still further reduced, only a
few remaining appended to the dorsal surface ; and
at last we have an animal entirely deprived of such
appendage, (Plate XII. fig. F). Out of such an
envelope will finally grow an ophiura, (Plate XVI.)

[PLATE XVI— OPHIURA. j

an animal in which there is no indication of dorsal
appendages, a regular star-fish, with slender cylin-
drical arms. It is easy to see how the central mass
is transformed into the star-fish, from the period
(Plate XII. fig. C) when the inner pouch has been
transformed into a spherical mass of globules.
But the whole series of changes can scarcely be
reconciled to what I have observed in the common
star-fish of these shores. Perhaps there is some
resemblance between the sac of yolk of the star-
fish with its peduncle below (PI III fig.A.p.13,} and
the so-called stomach oesophagus and mouth of
the ophiura, (Plate XII. A B). Perhaps instead
of being a stomach with an oesophagus and mouth,
this inner mass is to be considered as a yolk with
a peduncle. But whether it be so or not, the dif-
ference is nevertheless striking. The young
ophiura, when forming, is here (Plate XII ) sur-
rounded by a peculiar frame, of which there ia not
any indication in my star-fish, (PI IV. fig. C,p. 13).

PROF. AGASSIZ S

Now I have been able to trace the eggs of an
Oohinra which lived on this shore, and they, as well
as the voung star-fishes were free animals ; and
also were observed during winter

The nuestion is now, whether there are not
among Fchinoderms, as among other low animals,
—though the fact has not been traced by direct ob-
servations— phenomena similar to what has been
observed among Jelly-fishes, where alternate gen-
erations take place,— where animals of a peculiar
character are produced in one generation, from
which spring animals of another character, and
generation after generation alternately, the primi-
tive types are reproduced.

That these must be some phenomenon of that
kind I can scarcely doubt, when I see other ani-
mals indicating a similar change, which has been
also observed by Johannes Muller, during summer.

Here is a frame similar to that of the little ophi-
ura, containing within also a more opaque body,
with an opening below considered as a mouth, and
a connecting tube.

The external frame,also formed of a solid,gelati-
nous mass, in the interior of which there are calca-
rious nets,and on the edges of which there are again
vibratory cilia all around these stems (Plate XI, fig.
A). And there are several groups of these vibra-
tory cilia in the form of crescent-shaped epaulettes
on the four corners of the animal. Here is a figure
of the same, (Plate XI, fig. B,) seen from above.
We have in this being, the same arching appenda-
ges which were noticed in Plate XII •, but instead
of giving rise in their connection to a projecting
centre, they form a more rounded vault, from
which the elongated sticks hang down, diverging
somewhat. From the four corners, however, hang
down the four longest of these arms, and over the
arms of the corners are the fringed epaulettes; and
from one side two equally developed, between
which the mouth opens, an oesophagus and stom-
ach occurring in the centre, as in the young Ophiu-
ra. After the interior mass has undergone some
changes, you see, however, a very curious differ-
ence, which distinguishes at once this animal from
the other; a disc, which is observed upon that spher-
ical mass, namely, (Plate Xf, fig. B) the mouth— as
it is called by Muller — still hanging underneath, as
seen in another figure at the same stage of growth,
but viewed in profile (Plate XI, fig. C), where the
spherical mass with its lower tube is placed verti-
cally, and where that new disc formed upon iCs
surface is placed obliquely on one side of the up-
per portion. This disc has at this period five
somewhat prominent tubercles upon its surface, as
is seen here (Plate XI, figs. C and B), which will
become more developed (Plate XI, figure D)
The disc will grow larger over what was formerly
the main mass, the appendages will be somewhat
reduced in their lerrgth, and also in the develop-
ment of their vibrating cilia. And from that time
in addition to this, the five tubercles will be elong-
ated into five tubes or suckers, (Plate XI. fig. E.)

[PLATE XI — YOUNG SEA URCHINS j

and there will be spines coming out between them,
the oesophagus and mouth being reduced and finally
disappearing. At this period,the young animal con-
sists, therefore, of a circular disc upon a spheroidal
body,with elongated suckers coming out of its edge,
with spines between them ; but the suckers, instead
of being in pairs, as they are in the Ophiura, are
only five in number (Plate XI. fig. F.) ; the main
mass of the primitive sphere forms still a spherical
body under the shield, the shield itself bending over
the spherical mass (Plate XI. fig. F). This disap-
pears, however, more and more, and at last there
is a little flattened, sea urchin-like animal pro-
duced, with at first five suckers, next with ten,
(Plate XI. fig. G.) with large spines alternating
with them, the greater portion of the spherical
body remaining, nevertheless, soft, as there are not

LECTU'RES ON EMBRYOLOGY.

,yefc any ealetreous plates to support the isolated
spines, which rest only upon loose, calcareous nets
similar to those of the star-fish when first develop-
ed. With these changes in the main body, the ex-
ternal frame is gradually reduced, and finally en-
tirely lost. In this condition of the new animal,
when deprived of its transparent envelope (Plate
XL fig, G.) it is easy to recognise a young sea- ur-
chin, a young animal of which this figure, (Plate
XEII. fig B.)— represents the animal in its per-
fect condition.

[PLATE XIII— SEA URCHINS J

Muller has had an opportunity of tracing several
other larvse of the same kind, In one of them
there are appendages above as well as below, re-
sembling otherwise the first state of the Ophiura;
in another, similar in form to Plate XL, in which
there are, however, no crescent-shaped, vibrating
epaulettes ; in another still, there are two such cre-
scents only ; and in all of them there are hollow
spheres, with elongated tubes, and a seeming mouth
beneath; and all of them are transformed into
echini-like animals

All these observations leave no doubt as to the
fact that certain embryonic echinoderms, observed
during summer, are protected by an external, com-
plicated frame work, which has not been noticed
in those observed during winter. In addition to
this, t mav mention that this external envelope re-
sembles very much the transparent body of some
jelly fishes, the Beroe for instance, which are also
provided with vibratory cilia, arranged in a pecu-
liar manner, and which move freely in the water.
And if we compare this curious condition of the
young Echini, with what is known of the growth
of medusas, where a simple egg will divide into
several masses to give rise to several individuals
we cannot be surprised that there are in the Echini
also, similar phenomena; and that a body, entirely*
different from the animal in its full grown condi-
tion, is developed, to nurse, as it were, the perfect
animal, and not to acquire in itself any peculiar,
prominentj final form, These phenomena of alter-
nate generation I shall illustrate more fully in the
next lecture. I merely allude to them now, in
order to suggest the probability of alternate sum-
mer and winter generations in echinoderms, dif-
fering from each other in the same manner as
ordinary alternate generations are known to
differ.

The young Ophiura, the young Star-fish and the
young Echini, have not yet Been traced through
all their changes up to full grown animals. It was
the condition of their growth figured in these va-
rious diagrams, respecting which investigations
have been instituted; but the further investigation
was interrupted by the circumstances. But on
collecting along the shores small animals of those
species, and forming series of individuals from the
smallest size up to this perfect condition, the in-
vestigation of their growth and the changes which
they undergo, can be made out as completely as if
made upon one and the same animal during its
real growth. Indeed, most of the changes noticed
in the above described larvee have not been ob-
served upon one and the same individual, but by
comparing many individuals of the same kind in
their various stages of growth.

Now what are the changes which take place in
the further growth of the star-fishes, and of the
sea-urchins ? In the star-fishes, as they are fig*
ured here (Plate IV) the number of calcareous
plates is still very small. Only those which sur-
round the mouth, five in number, have acquired a
certain size, with those which protect the extremi-
ty of the rays, also five in number, and which are
also considerably developed ; small ones in addi-
tion, are successively forming in pairs in the in-
tervening spaces, (Plate IV, fig. C) between which
suckers come out. Gradually more and more are
developed, the animal pushing in this way the
primitive and terminal plates of the rays further
outwards,

It is therefore by the further intercalation of new
plates between the terminal and the oral ones, that
the rays are elongated, and they may grow to a
very prominent form, as we have here, [Plate XIII,
fig. A.] By varying proportions of their plates,
the rays, however, may grow to form very differ-
ent outlines of these animals, as may be ascertain-
ed by comparing their arrangement in different
genera of the family.

In the Echini the growth is more difficult
to understand. How is it that the circular
body can grow larger by the addition of new
plates ? In order to understand that, let me men-
tion the facts which I have been able to trace with
reference to their growth, If we have here an
Echinus of a certain size, we will observe that its
plates are arranged in two different kinds of rows,
[Plate XIII, fig. C.] You see these rows alternate
with each other, (Fig. B), and here again, (Fig. A),
very narrow rows, alternating with much broader
ones. The vertical rows of plates leave a circular
hole above, which is closed by plates of another
character. And below another holeswhich is closed
by plates of another character, but the sphere it-
self consists of plates, of two characters, narrower
ones, which have holes in themselves, and broader
ones,which have no holes, but upon which we ob-
serve more prominent tubercles, to which the
spines are attached— moveable spines, of wbick

PROP. AGASSIZ'S

there are as many large ones as there are large
plates. The first of these p'.ates,which are solidified
in Sea-Urchins, are those which surround the
rnouth, and which form the outline of that opening
which is closed by a membrane,and in the centre of
which we realk find the mouth in the perfect an-
imal. Above is formed the upper disc, consisting
chiefly of the plates, alternately larger and smaller,
through five of which the ovaries are discharged,
minute eyes being placed in the five others.

The new plates of the sphere are gradually form-
ed above the older ones, around the mouth, and
wherever additional plates are developed, they
arise higher and higher. In this way we see that
the young sea-urchins— the young Echini— are
growing larger by the addition of plates between
those of the upper disc and those which surround
the mouth.

But what are those plates of the upper disc ?
The five smaller contain the eyes and stand above
the rows with pierced plates ; the five larger ones
give passage to the ovaries and stand above the
rows with imperforated plates.

Now in star-fishes we have similar eyes, colored
dots, at the extremity of the rays, (Plate IV. fig. A)
The plates which protect them (Plate IV. fig. B)
correspond therefore to the smaller perforated
plates, of the upper disc of Echini, (Plate XVIII.)
and the ovarial plates correspond to the angles be
tween the rays of star-fishes.

(PLATE XVIII-SUPERIOR DlSC OF SEA URCHINS.)

It is therefore no exaggeration when we say that
a star fish is a sphere stretched into a pentagonal
shape, and in which the eyes are carried out into
the rays ; as there are holes opening between the
rays in their angles where the ovaries open. In
this, as well as in every other respect, the analogy
is most complete. Vice versa, we may say, that
Echini are swollen or spherical star-fishes, with
reduced rays ; and Holothuriae animals of the same
structure drawn out into a worm-like tubular form.
It is really of some importance to be able to trace
this comparison in detail, as it will now at once
enable us to show that the analogy of the various
embryonic forms with the perfect animals is made
out sufficiently to afford the means of appreciating

their relative positions in a natural system, lyy the
analogies which exist between the full grown ani"
mals of this class and the changes which they un-
dergo in their formation. Not only are the plates-
increasing and the body enlarging, but also its form
is assuming peculiar modifications. From these
pentagonal forms it is transformed into a regular
star, The Sea-urchin with a flat disc, as we have
it here (Plate XI fig. G.) is transformed into a
spherical body, seen here. (Plate XIII fig. B.)

[PLATE VIII— STAR FISHER 1

The star-fish is also gradually transformed from
its outlines in Plate IV, into the perfect animal,
(Plate VIII.) It now becomes an important point
to be able to ascertain to what peculiar forms of
Sea-urchin those embryos belong,as we have among
the living ones some with the flattened disks, oth-
ers with a spherical form, and others with more
prominent elongated forms. Let us see what sort
of living forms we have among Sea-urchins. There
are some in which large plates alternate with very
small ones (Plate XIII fig. A) which are called Ci-
daris,, There are others, in which the plates are
more numerous, in which the rows of holes are
broader (Fig. B.) and in which the spines are small,
Echinus. There are others in which we have plates
still more numerous, (Fig. C ) the body more coni-
cal, the rows of holes being still larger, and the
spines reduced almost to little heads, Holopneustes.
On the shores of the Northern Sea,where the above
described larvae of Sea-Urchins were observed,
there is no Echinoderm found belonging to the ge-
nus Cidaris. Nevertheless, you will notice that
that young Sea-urchin of Plate XI fig. G. has re-
markably large spines, equalling nearly the whole
diameter of the animal, although in its perfect con-
Tlition it will have proportionally small ones. From
that very fact we can conclude that the Cidaris
stands lower than the Echinus ; though it is usu-
ally considered a more elegant and higher form.—
This conclusion must be granted at once, when we
consider the great disproportion in the size of the
spines in Cidaris, and the large plates for the spines
resembling the embryonic form of the Echinus,
that the genus Cidaris ranks lower than Echinus.

In Holopneustes, (Plate XIII, fie. C,) in which
the rows of holes are wider still than in Echinus,

LECTURES ON EMBRYOLOGY.

approximating thus toHolothurise, and the body
more elongated. We have really a still higher de
gree of developement.

In the general classification of these animals, I
showed that the tubular form is the highest, as.
is seen among the Holothurice ( Plates XIV and V).

[PLATE XIV— HOLOTHURIA.]

( Jf LA IK V — iiuLOTH U K 1 A. J

I might have shown these animals to remind you
of what are the species on these shores. Here is
the common Five finger (Plate VIII, fig. A), and
he;e is the common Sea-urchin (Plate XQI, fig B)
a spherical body covered with spines, which may
assist us in comparing, better than simple dia-
grams, these animals with their embryonic states,
as illustrated before.

[PL ATI? I—

Lut us \io\v also compare those embryonic
\vitti the fossils of different geological epochs. How
the young Comatula (Plate I) casts off the stem, I
have already mentioned ; but if we consider its
embryonic form, it will compare most remarkably
v/ith the fossils figured here (Plate XV). In other
instances, however, the fossil Crinoids do not even
resemble the young of those of our present epoch,
but belong altogether to peculiar types, as figured
here (Plate XVII).

I have been able to bring here a natural speci
men of one of these lily-like animals, in a most

[PLATE XVII — FOSSIL CRINOIDS. |

OL OLC'.LC ur preservation, resting iu>uu it.-, h,< m,
which is composed of innumerable plates articu-
lating together. It is a Tentacriuus, from Wurt-
emberg, in Germany. The principal portion of the
animal, which is called its crown, divides into five
distinct rays, which are flattened down upon the
slab of stone upon which it rests, but so well pre-
served that every one of the ramifications can be
distingushed, and the connexion of these branches
upon the crown below are very distinct. (The
Prof, here showed a most splendid fossil, which ex-
cited great interest among the audience. Those in-
terested in this branch of natural history will find
the subject carefully investigated in Agassiz and
Gould's Principles of Zoology.) I doubt whether
there is another specimen so perfect as this, and I
would invite you after the lecture to pass by it and
observe it.

The number of joints which allow the animal to
move and expand is enormous. One hundred and
fifty thousand have been computed in one of them
by Dr. Buckland ; in others, the number of joints
are fewer (Plate XV.), the crown remaining more
closed and the rays not dividing so extensively

PROF. AGASSIZ S

These animals which were extremely numerous
in former geological ages, agree in the mode of
growth of their plates, with the young of that star-
fish called Comatula, as it has been observed by
Thompson. This diagram (Plate I, fig A.) seems
to represent a large animal but it is only highly
magnified, the natural size of it being only half an
inch long. Nevertheless we distinguish in it the
articulated stem with joints. We have the crown
above with its solid plates. We have the dividing
arm arising from it. We have the surrounding
tentacles contributing to seijze its prey and bring it
to the mouth, and the movable tentacles or suck-
ers along the inner side of the branched rays,
which this animal moves as the others use their
suckers. In addition to these, there are gradually
more tentacles coming out, and the body grows
larger, till it is freed from its stem in its perfect
condition. The star-fishes which do not rest upon
a stem and which uo not branch, resemble less
those fossils than the types of them in which the
rays are more numerous and in which the rays
branch (Plate I, fig. B.) But even in common star-
fishes in their earliest condition (Plates IV, and X,
fig. A-), we have an arrangement of the solid parts
which resemble more closely the arrangement of
the solid parts of Crinoids (Plate X, fig. B.) than
the arrangements of parts in the full grown star-
fish (Plate VIII, fig. A.) Compare the solid plate
in the young starfish with the solid plates of the
fullgrown animal. In the young we have a star in
in which five large plates seem to alternate with five
others, ten of them forming the principle mass of
the body.

[PLATE X— COMPARISON OF THE CALCAREOUS

NET WORKS OF STAR FISHES, WITH

THE SOLID PLATES OF GRINOIDS.]

Pentacrinus, here the five plates which surrop.nd
the mouth, and those alternating with them, will
form the five rays, and so on with successive little
plates in all the genera.

["PLATE VII— STAR FISHER— CRI>'OIDP.J

If we take the Pentacrinus (Plate XV, fig. A) we
observe above the stem a crown, in which five large
plates, forming the cup, alternate with five smaller
ones. In Apiocrinus, the larger plates constitute a
hollow cup and above them alternating with them,
there are others (Plate XV, fig. B) upon which the
branching arms rest. In Encrinus crown and arms
are not so widely separated and seem to form still
an undivided cavity, as in the genera of Plate
XVII. (Plate VII, fig. D). Everywhere the same
arrangement exists, so that on a diagram the same
drawing would answer for the crinoids and the
common star-fishes indiscriminately. Here
(Plate X, fig. A) is the central network of the
common star-fish, corresponding to the |stera of

In PUite X , tig. B, we have the corresponding
parts in a diagram of a crinoid, answering precise-
ly in position, number, and mode of growth, the
so^id frame of the starfishes. In all we find a plan
which is uniform, whether we observe such ani-
mals in which the young are provided with a
stem, or those in which the stem does not appear
at all. Even if we go back to the young echini,
which seem to differ so much from the starfishes,
we have an identical number of primitive suck-
ers, namely, five, (Plate XI, fig. F) which do not
give rise to a pentagonal body until the flattened
disc assumes a more spherical form (Fig. G). So
that there is a most intimate agreement between
the different growths of the embryo.

All these data upon the embryonic changes of
Echinoderms are very fragmentary, as I have al-
ready remarked ; nevertheless, with these incom-
plete series of observations, it can be shown, as I
think I have done, that these embryonic forms
agree intimately with those which occupy a higher
rank in the class, and that they resemble also the
form of those which existed in former geological
ages.

Would these data afford the means of now in-

LECTURES ON EMBRYOLOGY.

troducing a natural classification among these an-
imals ? 's a further question which lays in my
plan; as these embryonic investigations were trac-
ed from the beginning with reference to the classi-
fication of the animal kingdom in relation to the
order of all types, when compared with the chang-
es which embryos undergo.

Among Echinoderms the investigation of struc-
ture has already settled the classification to this
extent,that they have been divided into three fam-
ilies,Holothurians,the tubular ones. Plates XIV & V,
& V; Echini,the spherical ones, Plate XIII; and the
Asterians, (Plate VIII,) the star-shaped ones : but
from this general arrangement there is still a con-
siderable distance to the perfect fixation of the or-
der of succession of genera in all their details. The
various arrangements which have been proposed
have been influenced by the various states of our
knowledge. The improvements in the classifica-
tion of Echinoderms have been greatly advanced
by the knowledge of the Crinoids, which are
universally placed in the lowest rank among those
animals, from their resemblance to Polyps. When
their structure was ascertained. the knowledge thus
acquired, did not modify the position which was
assigned to them when not yet sufficiently known.
The knowledge of the change in the growth of one
Crinoid, the Comatula, has indeed influenced more
the classification than the knowledge of their struc-
ture. The free star-fishes are placed next to the
Echini and above all the Holothuriae. Among Ech-
ini we have some in which the mouth is central
and the alimentary canal ends on the margin ; and
there are others in which the alimentary canal
enda on the two extremities of the body, as seen
here, (Plate VI fig. B.) thus forming a transition to
the worm-like form, they indeed begin to be re-
lated to the Holothurite (Plate XIV) and will rank
higher.

[PLA.TE VI— SEA URCHINS ]

Structure and embryonic growth have satisfied us
thus far. But why should we not venture to go fur-

ther,and make use of the order of succession of these
types, in order to ascertain all their relations ? The
Crinoids which have been 'described as fossils, are
exceedingly numerous. Here are figured several
forms, to which I have not yet alluded. Plate
XVII, fig. C, is a genus called Caryoirinus. Here
is another, which occurs also in old strata, (Fig. B)
called Pentreraites; and here (Fig. D) one which oc-
curs in deposites of the coal period, called Echino-
crinus.

In Plate XVII, fig. C, we have a spherical body,
like an Echinus, with a stem as in Crinoids, but the
plates are not yet ranged in regular rows (Fig. C),
but alternate irregularly ; there are not yet rows
for the pores distinctly circumscribed, but only at
irregular intervals, and few of them. This form,
as also the Sphoronites are the most primitive Cri-
noids, and they correspond somewhat in structure
to the earliest condition which we observe in Echi-
ni, and which we observe also in the youngest
stage of the star fish.

Here is one (Plate XVII, fig B) in which we have
a mere star' fish-like form ; the sphere is in its full
condition of development ; and here we have one
which would seem to be a common sea-urchin,
(Fig. D.) But on comparing both (Plate XIII, fig.
C) they are found widely different. In Echinus
(Plate XIII, fig. C) there are two rows of perfora-
ted and two of imperforated plates, while in Echi-
nocrinus (Plate XVII, fig. D) there are four rows
of imperforated plates, and the animal is really a
crinoid. and not a sea-urchin. This (Fig. D) has a
stem : that (Plate XIII, fig. C) has not. The Cri-
noids are found in ancient geological strata— in the
middle geological ages are those of (Plate VII).
Free Star -fishes begin later in the geological for-
mations. The Comatula or free Crinoids are again
later (Plate I, tig A). The Echini appear long af-
ter the families of Crinoids and free star-fishes
have been introduced upon our globe. We have
not yet one of the spherical Echinoderms before
the deposition of the red stone or the Marchalkalk
of Germany. And those spherical Echini or Cida-
ris are the earliest ones, (Plate VI, figs. D and E.)
Next we have such as have a central mouth, and
in which the alimentary canal ends laterally.

And at a later epoch those which have an elong-
ated body (Plate VI. fig. B.) The first epoch in
which elongated Echini appear is in the chalk de-
posit.

When there was not yet one free starfish, there
were only Crinoids on earth. And what sort of
Crinoids had we ? Not such as already resembled
common starfishes (Plate VII.), but which resem-
bled the lowest stage of growth of these animals,
when they are still without arms (Plate XI. fig. E.)
with irregular arrangement of their plates (Plate
XVII. fig. C.) Next we have such which assume
the.shapeof the star-fish, (Plate XVII. fig. B.) but
are stilll Crinoids resting on stems with few irregu-
lar plates, but in which holes are arranged in a re-
gular star above. And next we have Echinocrinus,

PROF. AGASSIZ'S

(Plate XVII. fig. D.) that is, a crinoidal echinoderm
aping the sea-urchins by its spherical form and by
the regular arrangement of its plates and by the fact
that there are zones of holes,alternating with zones
of plates without holes. But that they are not
echini is shown by the fact,that they rest on a stem,
and that in each row of imperforated plates there
are four sets of plates instead of two, as in Echini.
Here crinoids are perfectly developed into the
form of higher types, but under the general char-
acter of the lowest group of these animals ; those
forms, however, become more and more individu-
alized in later periods. And here are other Cri-
noids, (Plate XV & VII) from which free star fishes
branch off during the subsequent geological times.
But what is most curious, is the fact, that among
the Echini, the oldest are the Cidaris{ Plate VI, D)
spherical bodies somewhat flattened, with large
plates, narrow rows of holes, and remarkably large
spines in proportion to their proper size, {Plate VII,
E) but precisely as we have them in the youngest
condition of the true Echinus. (Plate XI, G).—
The Cidaris are numerous before any true Echinus
occurs. Next, those are developed and become
gradually more and more numerous, and they are

soon succeeded by others of a more oblong form
and those greatly elongated Echinoderms which
we call Holothurise, occur only in the present pe-
riod (Plates XIV and V.)

So that by all the facts to which I have briefly
alluded, I can come to the conclusion that the class
of Echinoderms presents, notwithstanding the im-
perfect condition of our information upon this
point, the most perfect agreement between the va-
rious embryonic forms observed and the different
permanent forms of the animals of that class in
their full grown condition ; that these embryonic
forms agree also with the different structures of
the fossil types through all the geological ages; and
that these again in their order of succession, agree
with the different appearances of the full ^rown
living animals, or more precisely with their grada-
tion as derived from a knowledge of their internal
structure.

These various relations, so complicated, and nev-
ertheless so permanent in every respect, show the
same thought throughout the whole— that struc-
ture, development and order of succession in time,
are regulated by one and the same unique princi-
ple.

LECTURE IV.

The result thus far obtained in the lectures
which I have delivered, can be expressed as fol-
lows : There is a gradation of types in the class
of Echinoderms, and indeed in every class of the
animal kingdom, which, in its general outlines,
can be satisfactorily ascertained by anatomical
investigation ; but it is possible to arrive at a
more precise illustration of this gradation by em-
bryological data. The gradation of structure in
the animal kingdom does not only agree with the
general outlines of the embryonic changes. The
most special comparison of these metamorphoses
with full grown animals of the same type, leads to
the fullest agreement between both, and hence
to the establishment of a more definite progressive
series than can be obtained by the investigation
of the internal structure. These phases of the in-
dividual development are the new foundations
upon which I intend to rebuild the system of zool-
ogy. These metamorphoses correspond, indeed,
in a double sense, to the natural series established
in the animal kingdom; first, by the correspond-
ence of the external forms, and secondly, by the
successive changes of structure ; so that we are

here guided by the double evidence upon which
the progress in zoology has, up to this time, gen-
erally been based.

Their natural series again correspond with the
order of succession of animals in former geologi-
cal ages ; so. that it is equally true to say thai the
oldest animals of any class correspond to their
lower types in the present day, as to institute a
comparison with the embryonic changes, and to
say that the most ancient animals correspond with
the earlier stages of growth of the types which live
in the present period. In whatever point of view
we consider the animal kingdom,we find its natural
series agree with each other : its embryonic phases
of growth correspond to its order of succession in
time; and its structural'gradation, both to the em-
bryonic development and the geological succes-
sion, corresponds to its structure ; and if the inves-
tigations had been sufficiently matured upon this
point, I might add that all these series agree also
in a general way with the geographical distribu-
tion of animals upon the surface of our globe ; but
this is a point upon which I am not yet prepared
to give full and satisfactory evidence, and which

LECTURES ON EMBRYOLOGY.

So much for the views referring to embryology
in its bearing upon zoological classification.

There is, however, another field in which the
animal kingdom has been represented as developed
according to the gradation of its structure : I mean
the order of succession of extinct species in geolo-
gical times, It has been long and generally aa-
sertedj especially by the physio-philosophers, that
the lower animals were first introduced upon our
globe, and formed alone the population of the
earliest periods in past time; that Polypi existed
before Mollusks ; these before Articulata, and that
Vertebrata were the last to make their appear-
ance. But the discoveries in fossil Ichthyology,
which it has been my good fortune to describe in
my researches upon fossil fishes, have shown that
vertebrated animals, fishes, have existed in the
oldest epochs, and that such an order of succes-
sion, as mentioned before, did not agree with the
plan of creation. Indeed, that representatives of
all the four great divisions of the animal kingdom,
Articulata, Mollusca and Radiata, occur simulta-
neously with fishes, in all the lowest geological
formations, was soon ascertained by the investi-
gations of paleontologists, and the fact of any reg-
ular succession was afterwards altogether denied.
However, the simultaneous occurrence of the four
great types does not yet indicate the want of reg-
ularity in the development of the various classes
of the animal kingdom, taken isolately. Several
eminent paleontologists, Leopold Von Buch, Count
Von Murster, Sir R. Murchison, d'Orbigny, Prof.
James Hall, and many others, have shown that the
types of different classes which characterize the
different geological ages, follow each other in an
order which agrees with their zoological gradation
as ascertained by structural evidence. The great
difference between this fact and the views enter-
tained before, consists in the knowledge of the in-
dependent gradation of the different classes, which
in the lower types arise all simultaneously, to un-
dergo their metamorphoses simultaneously,through
all geological periods, whilst among Vertebrates,
the Fishes were found to occur earlier than Rep-
tiles, and these earlier than Birds and Mammalia,
which made their appearance last. It was in that
way shown that there is a progressive succession
of classes among Vertebrata, ending with the cre-
ation of Man ; whilst Polypi and Echinoderms
among Radiata; Acephala, Gasteropoda and Ceph-
alopoda among Mollusks; Worms, Insects and
Crustacea among Articulata, existed simultane-
ously during all great periods, and presented each
a development of its own.

However, another step had to be made to show
a reaPagreement between the earlier types of an-
imals and the gradual development of the animal
kingdom, which has been the last progress in our
science of fossils: namely, to show that these ear-
lier types are embryonic in their character— that
is to say, that they are not only lower in their
structure when compared with the animals now

living upon the surface of our globe, but that they
actually correspond to the changes which embryos
of the same classes undergo during their growth.
This was first discovered among fishes, which I
have shown to present, in their earlier types, char-
acters which agree in many respects with the
changes which young fishes undergo within the
egg. Without entering i nto all the details of these
researches, I will concluc by saying, it can now be
generally maintained tha earlier animals corres-
pond not only to lower vpes of their respective
classes, but that their chief peculiarities have ref-
erence to the modifications which are successively
introduced during the embryonic life of their cor-
responding representatives in the present creation,
To carry out these results in detail must now be,
for years to came, the task of paleontological in-
vestigations.

But the other connections mentioned above, I
consider as established, and I claim these views as
the results of my own investigation, though much
has already been said upon the natural and suc-
cessive development of the animal kingdom, and
upon the propriety of introducing a classification
based upon embryology, The views to which I
allude are indeed not the same as those which I
advocate ; and in order to avoid mistakes in this
respect, I will now dwell for a moment upon this-
point, with the hope, perhaps, to show that these
views are incorrect, and must be given up, though
they pretend to lead to a natural arrangement of
the animal kingdom. The first notion of progres-
sive development of the animal kingdom, of an
agreement between the order of succession of types
and their structural gradation, has been brought
forward by that school of philosophers who in
Germany take the name of nature-philosophers,
(physio-philosophers.) But with them the idea of
a gradual development of the animal kingdom,
was by no means the result of investigations — was
not the expression of facts, but was an a priori
conception, in which they made their view of the
animal kingdom the foundation for a particular
classification, seeming also to agree with the little
that was known of geological succession of types.
Dr. Martin Barry, a distinguished physiologist
in London, has however proposed principles for
classification of the animal kingdom, which de-
serve more particular notice, as he presents them
as the results of his extensive investigations in
embryology, and he has put his view upon the
subject in the following words. Dr. Barry is one
of the ablest investigators in this department, one
of those who have most extensively studied the
egg and its developments in the mammalia. To
him and to Dr. Bischoff we are indebted for the
mosf elaborate investigations upon this subject ;
but I am not aware that Dr. Barry has traced the
metamorphoses of animals in other classes. His
views are substantially expressed in the following
statements : " There is no appreciable difference
in the germs of all animals. There is a fundamen-

PROF. AGASS1Z S

tal unity in all of them." This is a result which
is btjyond all doubt, which is beyond all contro-
versy. The eggs in the whole animal kingdom
are identical in structure. However, this funda-
mental unity must be restricted in one sense. —
They are identical in structure for our senses, but
we cannot consider them as identical in a higher
point of view4 as from each kind of egg there will
never arise but one kin of animal ; there is an
essential, though not a i. iiterial difference in the
egg from the beginning but in their material
structure the eggs of all animals are identical. —
The first position must therefore be granted; but
with the restriction upon which I insist, that though
identical in structure, there is something which
presides over the individual growth, from the be-
ginning .even of the formation of the egg, and
makes each one give rise only to one sort of ani-
mals,, It could, then, just as well be said, that the
eggs, though apparently uniform, are essentially
different in different species. But Dr, Barry states
that the class, or the characters of the class, be*
come manifest in the egg in the germ, before the
order can be distinguished. That is to say, that
the first change which takes place in the embryo,
is to bring forth in the new animal what charac-
terises it as belonging to one particular class. —
For instance, that a young rabbit would first as-
sume the peculiarities by which -it is referred to
the class of Mammaliat Next, the order becomes
manifest; but the family is not yet shov/n. The
young rabbit would be distinguished as belonging
to the gnawing animals* Next the family (here
the family of Hares) becomes manifest; but.the
genus not yet known. Next the genus (Lepus)
obvious; but not the species. Next the species,
(Rabbit) distinct; but the variety unpronounced. —
Next the variety (white, grey, black rabbit) ob-
vious; but the sexual differences,scarcely apparent.
Next the sexual character obvious; but the indi-
vidual character not noticed. Next the individual
character developed in its most special form. This
is very logical, but not in accordance with nature 5
we may frame such a system in our closets, but it
does not answer our observations.

Let us remember what we saw in the egg, with
which I began illustrating the growth of frogs*—
Was it the character by which the frog is found
to belong to the class of reptiles, which was first
apparent ? By no means. .It appeared first, under
the form and with the structure of a fish, and not
under the form and with the characters of a reptile.
The lowest form of vertebrated animals was first
developed in the earlier changes of the egg, before
the class to which that animal belonged could be
recognized. Not only would the first form under
which the young Batrachian appears, exclude the
class 10 which it will belong afterwards, but even
the internal structure of the tadpole differs from
that of the reptiles They have no lungs, no inter-
nal saria! respiratory organ, nor even a rudiment
«?f it*, and also no nostrils communicating from

outside with this innner hollow sac. What did we
find among the starfishes? among the echini?—
Did we recognize there the hard plates or the
rows of regular plates which mark that class, of
the rows of suckers 1 By no means. Forms
which would lead us to mistake them for Polypi
or Medusa were first noticed, and not the indica-
tions of their class ; thus showing that there is no
such thing as an earlier development of those
characters which indicate the respective class of
the animals under observation in the progress of
embryonic growth.

Next, it is said that the orders are manifest, but
not the genus. But let us take as a test the em-
bryo of a very well known animal among mam-=
malia. To what order does the cat belong? To
the Carnivora and to the family of Digitigrades. —
What are now the characters of carnivora ? Sharp-
pointed, canine teeth, with chisel-like incisors and
various molars, the principal one of which is a
sharp-cutting tooth. The claws again, are strong,
curved nails, adapted for their peculiar mode of
seising their prey. Now, the young cat is already
far advanced in its development before it has any
teeth at all, and its paw is a real fm> with undivided
fingers, and without nails in the earlier stage of
growth. We have at first, therefore, not one of
those characters which distinguish the order of
Carnivora and the family of Digitigrades ; and
nevertheless such an imaginary order of succes =
sion in the development of parts is made the fun-
damental principle of a system which is given as
natural, though the whole is merely a logical par-
tition of principles.

The genus next should be shown. What are the
characteristics of the genus, cats ? To have four
molars in the upper jaw, and three in the lower*
But before the cat has all its teeth, the genus can
be recognised, by its protractile and retractile
claws. The species indeed, is ascertained, is well
characterized, by its peculiar form, before we can re-
fer it to the genus, according to its soological char
acteristic. But it is said that the variety becomes
next obvious. The cat, however, may have already
assumed a peculiar variety of color 5 it may be a
grey or a white, it may be of any color before the
teeth, the characteristic of the genus, are fully de*
veloped. And as for its individual character, the
young kitten is playful, and shows its character
long before its peculiar genus is marked out; and
in short, every thing takes place in the reverse or«
der from what it is supposed in this system. Nev-
ertheless, such views are considered as suited to
express the real gradation in the animal kingdom^
from the simple reason that the whole statement
seems natural and logical.

A renewed examination of the metamorphoses
of the frog will lead to the same conclusions. At
first we do not observe changes indicating the
class to which that animal belongs, but such char-
acters as would rather indicate the class of fishes ;
nor are the characters of the order of batrachians

LECTURES ON EMBRYOLOGY.

developed before the 3'oung animal assumes forms
related to genera to which it can never be refer-
red. Indeed, the tadpole has all the peculiar ap-
pearances of batrachians with permanent gills, be-
fore a frog can be recognized ; it resembles suc-
cessively Menobranchus, Triton, and Menopoma,
before it loses its tail : and as for toads, they have
webbed feet, that is to say, they resemble another
genus, the frogs, before their fingers are entirely
separated, though the species can be recognized in
the distribution of colors long before.

(PLATE III— FROGS )

Professor Mi lne<Ed wards, of the Jardin des
Plantes,has proposed similar views, and indeed ex-
pressed in nearly the same words, his conviction
about the gradation of the animal kingdom ; but
not with reference to the development of zoologi-
cal characters, but with reference to the changes
W hich the animals undergo in their structure. He

has referred his views more particularly to the
structure and the development of the functions of
animal life; and from this circumstance his views
agree better to nature, when he says that those
organs are first developed which are more impor-
tant to life. However, strictly speaking, it is not
absolutely true. It is the nervous system which
we may consider as the organ most important to
life; and it is not the nervous system which becomes
first apparent in the embryonic changes. The sys •
terns by which the body grows are developed be-
fore those by which it lives a higher life come into
play; so that, though in a general way, the organs
most important to existence are really developed
first, it cannot strictly be said that they are the
higher organs which are developed first; and that
the special differences which characterise families
and genera should be engrafted as it were upon a
fundamental plan.

My aim is an entirely different one, as you may
have perceived from my first lecture. It is to show
that in the real changes which animals undergo
during their embryonic growth, in those external
transformations as well as in those structural mod-
ifications within the body, we have a natural scale
to measure the degree or the gradation of those
full grown animals which correspond in their ex-
ternal form and in their structure, to those various
degrees in the metamorphoses, and therefore to
make the metamorphoses of animals, as illustrated
by embryonic changes, a real foundation for zoolo-
gical classification.

Let me only mention that on the whole, the high-
er families of the various classes of the animal king-
dom are distributed over the warmer parts of the
present surface of our globe, and that the lower
families are rather numerous in the milder and
colder regions. Thus among mammalia,the Mon-
keys are strictfy circumscribed within the limits of
the growth of Palm trees ; the large carnivorous
beasts prevail in the tropical regions; whilst the
sheep, goats, and oxen are natives of the temper-
ate zone ; among reptiles.the crocodiles occur only
in the warmest countries; whilst the lower Batra-
chians, those with external gills or permanent tail,
extend even far north. There are, nevertheless,
inferior families which are also strictly tropical;
such for instance as the Pachyderms, and to some
extent, the Edentata ; but this fact has doubtless
reference to the early introduction of these fami-
lies in the plan of the creation, during a period
when the surface of our globe was warmer than it
is iu our days; so that the location of their modern
representatives in the torrid zone, can be consid-
ered as merely determined by the peculiar adapta-
tion of their general plan of structure for warmer
climates, rather than related to the gradation of
the types, according to the present condition of the
distribution of heat upon our globe. The induce-
ment for their present location is not their higher
structure, but their relation to earlier types.
But now, I proceed to illustrate the history of

PROF, AGASSIZ S

another class, that of Medusae; ihe next among ra>-
diata, whose embryology we have to investigate.
But it is out of the question to understand the
changes which Medusae undergo,without knowing
their structure, and this structure is not only very
complicated, but it has been little studied and is
still obscure. I stand, therefore, with a very diffi
cuit task before me, and I ask your indulgence
upon this point.

Let me begin by pointing out a few diagrams,
and saying a few words upon the figures before
you.

(PLATE XIX— YOUNG MEDUSAE.)

Here (Plate XIX, fig's A B G) are outlines of a
family which has been described by Sars, the dis
tinguished Norwegian naturalist, as a peculiar
polypus, under the name of Scyphistoma. Here
(Fig. I) are other figures, which have been also
described by Sars as polypes, under the name of
Strobila. Here (Fig. J) is another free animal, de-
scribed under the rame of Ephyra. And here
(Fig. M) is another, found on the shores of the At-
lantic, both in Europe and in the United States, in
the temperate zone, which belongs to the genus
medusas. As to the class to which these various
animals belong, I may mention that the two ge-
nera, Strobila and Scyphistoma, were referred to
polypi, and the other two (Fig. J M) to jelly-fish-
es, or Medusoe. Now, gentlemen, it has been as-
certained within a few years, both by Sars and
Yon Siebold, that all these figures are the various

stages of growth of one and the same animal.—
We have here (Plate XIX) the metamorphoses of
one and the same animal — changes which take
place in the growth of an egg. This (A) is an
egg,as it is laid by a Medusae. Here (Plate XX, A)
(PLATE XX.— POLYPI— CORYN^E, SYNCORYN^E,

PODOCORYNuE )

we have a still more extraordinary structure
(syncoryna). You see these stems terminated by a
rosy colored head, from which tentacles, half a
dozen or more, arise, and out of these various bo-
dies, a little tubercle here, a more prominent one
there, and another bell-shaped here, with tentacles
around its opening. Here is another form (Fig,
B) called Podocoryna, by Sars, from which va-
rious kinds of buds arise, which do not resemble
the primitive stem; also much larger buds, which
differ still more, and which are at a certain time
freed and grow into other animals. Indeed, stems
of polypi, from which arise buds of medusae or
jelly-fishes, budding from polypi-like stems, be-
coming free and growing into a regular, simple,
isolated jelly-fish, like this (Plate XIX, N) ; this is
the case here, (Plate XX) a bud which grows into
a jelly-fish. It is, however, out of the question,
that in its different stages of growth, an animal
could belong to various classes, or that an animal
of one class could give rise by budding to animals
of another class. Therefore, it is perfectly obvious,
from the nature of these well authenticated facts,
that there has been a want of understanding of
these phenomena when they were first described ;
and it was not until a few years ago, when Steer-
strupp found out the key to this astonishing com-
plication, by ascertaining that there is an alter-
nation in the mode of reproduction of many ani-
mals, which takes place in different ways in the
animal kingdom. In some, there are eggs laid,
which eggs give rise to animals different from
their parents, and these in their turn give rise to
eggs, from which arise animals similar to their
grandparents and different from their parents. —
In other cases, animals lay eggs which go to form
individuals different from themselves, and these
individuals, by buddinfir,or transverse division, pro-
duce forms which are freed, grow, and then re-
semble the parent, by a complicated process of
metamorphoses.

However, though Steerstrupp, for the first time,
brought out these conclusions distinctly, he

LECTURES ON EMBRYOLOGY.

somewhat anticipated by Sars, by Sir John Daly-
'ell, find by a French naturalist. Da Jardin; though
they did not carry out their investigations to the
same purpose, yet they led the way in the same
track. How these changes take place, will be I
suppose better understood if I begin by giving an
outline of the structure of these animals, which it
has been possible for me to examine more com-
pletely than it had been done before; availing my-
self of several small species which liv-e in Boston
harbor. The large animals are not those which
are best suited to such investigations; when large
their bulk prevents their being isxami&ed under the
microscope. But let the animal be small enough
to be placed entire under the microscope and you
get a general view of the structure ; and by apply-
ing a higher power to the various parts, you can
trace the details in such a way as to ascertain
most completely their organisation. Such was the
process by which I was enabled to discover in
these minute medusas, even the nervous system,
which had been only suspected, but not traced in
its distribution. And let me add, that beside their
physiological interest, these animals are wonder-
ful in their aspect, and present the most attractive
.sight which can be witnessed. Their transparent,
•delicate bodies swimming freely in the water and
moving regularly by the contraction of their
whole mass— the elegance of their outline and the
diversity of the appendages which hang down from
their globular body— or the suckers which rise from
the centre,, and constitute other appendages from
the middle of the sphere— all these contribute to
make these animals wonderfully beautiful. An in-
creased interest is felt when seeing at 5rst scarcely
•an outline, so transparent are they, and discovering
afterwards by Che simplest process of examination,
consisting in modifying the light which passes from
the mirror of the microscope through their body,
-all the differences of structure so easily overlook-
ed at first sight.

And again, they belong to a class of which so
many are transparent, or pliosphorescent,that there
are endless inducements to investigate these ani-
mals. Here are various figures (Plates XXI, 'II,
III, 'IV, 'V, 'VI, 'VII), all representing Medusa.

Many of these figures are of a hemispherical
form, as plate XXI ; and this form (Plate XXVII,
fig. B). In the margin of this form (Plate
XXI, tig- A) you see we have two kinds of appen

[PLATE XXI— MEDUSA.]

[PLATE XXII— MEDUSA.]

(PLATK XXIII— MBI>C«A.J

dages, and you see (tig. B) that there is a central
cavity, and that there are four bunches of a pecu-
liar character here, the ovaries, (fig. C), and that
the lower surface presents various rays diverging
towards the edge. In another form, Beroe, (Plate
XXII) we have a tubular hody with vertical rows
of vibrating cilia., and a wide opening below the
internal cavity, which is more complicated than
that of the odier types. Here is (Plate XXVI) an-
other, Agalcnopsis, which is still more complicated,
from the diversity of all the appendages which
hang from the main stock ; and here is another,
which is, if possible, still more complicated, and
has a very large vesicle above and numerous ten-
tacles hanging below. This animal (Plate XXIII)
is known to the sailors by the name of Portugese
Man-of-war. Naturalists call it Physalia. Others
are flat, circular, or oval, with several rows of sim-
ple appendages, as Velella, plate XXIV, and Por-
pita, plate XXV.

[PLATE XXIV— MEDUSA.]

Esciischoit, who fius studied iliese
animals more extensively than any one else, has
divided them into three groups— Ctenophora, Dis-
cophora, and Physophora, Those which have
these vesicles, by which they are suspended in the
water, are called Physophora. They are all con-
sidered as simple animals, though their form is
extremely complicated. Here is an enlarged fig-

PROF. AGASS1Z S

[PLATE XXV-MEDUSA ]

ure (Plate XXVI) of one of these animals, with
all various appendages — tentacles, suckers, groups
of eggs, and all sorts of vesicles— forming one
elongated body, with fringes. It is the Agalmopsis
of Sars. Let us now see what is the structure of
these animals. The internal cavity communicates
with the exterior by a broad open mouth, as yon
see in this sketch, or by bunches of tentacles which
terminate in little suckers, as you observe in this
figure (Plate XXVII, fig. B) — numerous suckers
hanging down from the central appendages and
forming as many mouths, as many openings com-
municating with the central cavity, as there are
such appendages. In another case (Plate XXVII
fig. A) we have simple little openings, or pores, up-
on the surface of the larger appendages, all direct-
ed inwards, uniting and combining to form larger
stems— finally combining into fewer tubes and
emptying into a main cavity, and from that main
cavity branching off again into numerous tubes,
and dividing over the margin of the disc Those
ramifications from the central cavity towards the
surface can be easily seen by holding the light in a
certain angle before these animals in their living
condition. And by injecting colored water, you
may fill them in all directions, and see that there
is, as in plate XXVII, fig. A, a net work of vessels
ramified around the animal.

There are others, [Plate XIX., fig M.] in which
there are main stems, which divide into some few
more towards the margin, or unite again into at
circular canal all around the edge.

There are even some in which the central cavity
IPlate XXVIILj is very small, having only a little
sack at the summit of a long proboseis,whicb i&fihe

mouth ; the little sack next divides into four tabes,
which then extend towards the edge, where they
unite again to form a circular tube. Liquids are
constantly circulated in these cavities. The food
is digested within that cavity and then circulated
through the tubes, and in those which have only
minute pores as oral apertures, the food can con-
sist only of microscopic animals, or of decomposed
organic matters— in others which have a larger
mouth, larger animals are introduced In the
small species of Boston harbor, |Plate XXVII. fig.
C.j which was first described by Dr. Gould in his
Report upon Invertebrate Animals of Massachu-
setts, and which will be exceedingly common in a
a few weeks, I have seen this pioboscis hanging
down and stretched three times the length wnich
you see here; and after it had swallowed some-
thing, and the food had been digested, the globules
arising from the digestion would be circulated
through the tubes and would be seen under the mi-
croscope most plainly, diverging towards the mar-
gin of the sphere, there moving into the circular
tubes, or perhaps even moving down into the ap-
pendages— those hanging arms which are hollow —
and again trace back their course into the circular
tubes -T some of the globules would disappear wheu
absorbed by the surface, but the remainder is cir-
culated forwards and backwards— to and fro— in
those tubes before disappearing entirely.

Such a structure can be considered the lowest
condition of a system of circulation, which is at
the same time a modification of the alimentary
tube, where the stomach divides, and where the
divided stomach again unites into vessels— into
common vessels, which branch in their turn. Here
we have ihe tubes uniting and then branching off
again, [Plate XXVII. fig A.J but in Fig. C there is
a distinct mouth and proboscis.

The mass which forms the body in medusa i&
transparent and cellular. And then there are
distinct muscular fibres of two kinds, circular
ones around the whole disc, and radiating ones,
which form distinct bundles diverging from the
centre towards the periphery; in those medu-
sa which have four diverging alimentary tubes-
the main radiating muscular bundles alternate with
the tubes (Plate XXVII, fig. C.) All these muscular
bundles and the circular fibres contract alternate-
ly, so that the body can be shortened or flattened
in various ways, and thus, through the agency of
these muscles, the animal moves in all directions,
upwards, sideways and downwards, at will. That
these animals moved by contraction, had long been
observed ; but the existence of regularly arranged
muscular fibres in the class of medusse, was still
doubtfal When Ehrenberg published his investi-
gations upon the structure of the medusae of the
Northern Seas, though he concluded that there
must be muscular fibres, he could not discover a
regular, complete, muscular system. However, in
these small medusse, the muscular fibres ar®

LECTURES ON EMBRYOLOGY.

[PLATE XXVII-

enough to be seen in the living animal, under a
power of a few hundred diameters.

Beside this, there is around the upper part of
the alimentary tube, a linear circle of another sub-
stance, from'which radiate four threads, following
the direction of the alimentary tubes, and ex-
tending towards the periphery, which reach there
the spherical, colored bodies, now generally con-
sidered as eye specks, and uniting with each other,
form a circular thread all around the margin of
the disc. This apparatus I consider to be the
nervous system. Its position is the same as in the
other radiated animals, a circle around the alimen-
tary tube, with diverging rays, ending in the small
colored organs which since discovery (Plate XIX,
fig. M) have been considered as Ehrenberg's eye
specks, similar to those which I have already no-
ticed, at the end of the rays of star-fishes, and
upon the plates of Echinoderms. The fact of these
threads going to those spots (Plate XXVII, fig. C),
leaves no doubt, in my mind, that it is a complete
radiating, nervous system, similar to that of star-
fisnes. So that the structure of medusae, though
peculiar in itself, by the remarkable mode of dis-
tribution of its inner cavity, which does not con-
stitute an alimentary canal proper, resembles al-
most entirely the structure of Echinoderms, and
constitutes one of the main classes among Radiata
as Echinoderms do. The position of these ani-
mals was mentioned. They swim free, the mouth
downwards, the sphere upwards ; and this is al

ways the position which the Echinoderms assume,
The Echini, Sea Urchins, walk about, the mouth
downwards. Star-fishes walk about, the mouth
downwards. The Crinoids, however, stand up-
right, the mouth upwards, and this is the position
which the animals of the lowest class assume.

In all Polypi, the main body stands upon a stem,
the mouth upwards ; and we have also among Me-
dusae [Plate XIX, figs. G & I] a similar condition
during one period of growth.

When the young animals are fixed by the lower
portion of their body, the tentacles, or appendages,
which every where hang downwards, stand here
upwards; so that you see how remarkably the lower
types among Echinoderms resemble in this respect
the Polypi in their constant position, and how in
youth, Medusae, in that respect also agree with
Polypi. There is a constant recurrence of charac-
ters from one of these classes to another. They are
interwoven in a most remarkable manner.

All Jelly-fishes are generally considered as simple
animals; but I am satisfied that there are, on the
contrary, highly complicated ones among them.
The Physophora differ indeed widely from the
other Medusa, by their diversified appendages, as
is shown by the structures figured on this diagram.
[Plates XXIII and XXVI J I am prepared to show
that these are compound animals, composed of
groups of individuals of different kinds ; indeed,
compound animals as we find them among Polypi.
[PLATE XXVIII— HYDRA-CAMPANULART A ]

In order to show that this is the case, let me il
lustrate in detail the metamorphoses of Medusae.
Let me also refer you to some Polypi, [Plate
XXVIII] in which you see how individuals are
combined together, forming a compound stick.
Though all these individuals are of different ages
and have been found successively, they form living
colonies, as it were, of successive generations, uni-
ted by material connections, -which remain for life
—the new individuals not separating during life.
In others, the successive buds may be more or less
different, and nevertheless remain united in one
common colony, or as it %vere form a community
of individuals closely united, though differing in
age, size, form, and even in sexes. Such is the
case at least in the Campanularia, figured Plate
XXVIII. But there are also among Polypi simple
ones, like this little Hydra, I Plate XXIX].

When alive the Medusa lays eggs, and the em-
bryos are hatched, these germs swim freely, and
then become attached. And the point by which
they become attached grows longer [PL XIX, B], in
proportion as the mass above grows larger. The

PROF. AGASSIZ S

[PLATE XXIX— A FRESH WATER POLYPUS, WITH
A SIMPLE CAVITY AND A MOVEABLE STEM.]

vibratory cicil'a, by which they first moved, are
finally cast. There is a depression forming upon
the summits, and then two little horn-like appen-
dages grow out. [Fig. C.] They grow larger. [Fig.
D.] The tentacles grow longer, the depressions
still deeper, and then there is finally a central cav-
ity with four distinct tentacles. [Fig. E.] Then
there will be a little Hydra like animal, with eight
tentacles, a cential cavity, and a peduncle by
•which it is attached. [Fig. F ]

This is the first development of the germ of the
common medusse, the jelly fishes of this shore,
which are known in Boston harbor under the
name of sun-fishes. When it is grown somewhat
larger, a contraction takes place under the rows of
those tentacles, which have become more numer-
ous. In this stage of growth buds may also be
found. (Fig. H) New individuals may thus
arise from buds on the sides of this simple stem,
and these new individuals may grow to a consid-
siderable size with the parent stalk before they
separate. But at last they will separate, and grow
by themselves and form new sticks. So that we
have here two modes of reproduction among me-
dusae; in the first place, from eggs, which grow
into polyp-like animals, (Plate XIX, fig. A— F) and
secondly, by buds which will produce new individ-
uals, (fig. H.) The bad being separated from the
main body, will even form new colonies, and so
on, (Fig. H.) At first these buds differ somewhat
from the parent stock, but soon assume the same
character, differing slightly when they are finally
freed.

There are animals in which the successive buds
differ much more. There are in this (Plate XXVIII)
Campanularia, as it is called, buds which give rise
to animals with large tentacles, and there are oth°
ers with shorter tentacles, and there are even
others of a differently pe; so that the various buds
which grow from one stock may differ widely
and yet be buds of one and the same stock.— -
Here, in the young Medusas (Plate XIX) we see
that only one kind of buds arise — but there has
been still another mode of reproduction and multi«
plication observed in the same animal (Plate XIX,
fig. I). The stem, on growing longer and higher,
(Fig. G ) will begin to divide by transverse contrac-
tions into articulations. There are at first, simple
folds noticed in the skin, scarcely deepened to any
extent, but gradually growing deeper and deeper,
so that at last it seems as if a pile of discs were
heaped upon each other, (Plate XIX, fig. I,) the
lower part of which is a simple stem, as in Fig. G,
and the upper part, still consisting of a row of ap-
pendages as they have grown upon the summit of
this little Polyp and Serrate (Figs H and G). Next,
the edges of the discs begin to be fringed, (Fig. I,)
the cut growing deeper and deeper, these serra-
tures assume a regular form, and the contraction
growing successively deeper and deeper, those ser-
rated discs, almost separated from each other,form
a pile of loose discs simply connected by a central
axis. And as soon as the Polyp has divided into
this series of discs, the upper tentacles, that is to
say, the tentacles of the primivite Polyp, with the
upper disc, die away. What formed first the prin-
cipal part of the growing animal, dies away, ex-
cept the basal attachment,which remains; and next,
in the remaining pile, the uppermost disc frees it-
self from the pile and begins to swim. But the
moment it is free it assumes an inverted position,
(Fig. K) ; those fringes which were upwards, now
are turned downwards. The inner surface, which
was first upward, is now downward also. In this
way, a series of these serrated discs (Fig. L) are
successively freed from a primitively undivided
stem, by gradual transverse articulations, to form
as many independent individuals (Fig. T), which
after all can be traced to one single egg.

There are finally quite a number of individuals
formed, which have arisen simply by transverse
division, and by the successive modifications
which each of these discs has undergone. And,
after freeing themselves, the Ephyrss, as they are
called, (Fig. J M) will undergo such changes as to
assume those structural peculiarities which char-
acterise the perfect Medusae. The tube will be-
come hollow. The cavity will enlarge, and that
will have its tubes, branching into the disc by va-
rious canals, (Fig. M.) Those canals will circulate
fluid around the disc, and finally the complicated
structure of Medusae (Plate XIX. Fig. M.) is pro-
duced by the addition of fringes on the edge ; and
the growth of processes on the side of the stomach
which give rise to the egg, the eggs always hang-

LECTURES ON EMBRYOLOGY,

ing from the sides of the stomach, being, indeed,
simple pouches from the stomach. I ought to
have mentioned before, that the eggs in Medusae
are universally formed in connection with the ali-
mentary tube, and that in some of them, as the
small species of Boston harbor above described,
they are simply diverticula of the digesiive cavity,
formed in coecal appendages of the same, to be-
come free, independent eggs afterwards. , Their
position varies even most remarkably along the
alimentary tubes, in some, that before mentioned,
being developed along the central proboscis ; in
others, the Stomobrachium, being formed in four
bunches along the four tubes diverging from the
central cavity. Their mode of formation in such
positions has nothing more to astonish us, since
we know, from the investigations of Sars, that
there are Medusas, the Cythers, in which new indi
viduals are developed from buds arising from tLe
stomach. At a certain epoch the whole genera-
tion produced, arises by transverse division of the
stem derived from the eggs of the Medusae, pro-
ducing a number of connected individuals, from
the sides of the primitive stem (Plate XIX. Fig. H) ;
there are also often found buds growing upon the
lower portion, but invariably, at some period, the
perfect Mudusge will produce eggs.

In some Polypi we have also eggs arising from
the sides, like buds as in Hydra. [PI. XXIX.J We
have here, [Plate XX] from Polyp, Syncoryna and
Podocoryna buds arising which differ entirely from
the main stock, but which are successively freed
from it, and which give rise to animals which are
metamorphosed into real Medusae. Instead of be
ing considered as Polypi, those beings should no
longer.be considered as perfect animals — should
no longer be arranged in our systems by them-
selves, any more than Ephyra, the larva of Medu
S83 [Plate XIX fig. J.J; any more than Strobila [Fig.
I.J or Scyphistoma [Fig. E.]. They are only to
be considered as the stages of growth of Medusae ;
in some of which the regular Polyp divides into
many buds, forming as many Medusae [Plate XX
fig. B ], or in others, of which simple Polypi give
also rise by budding to regular Medusae, there being
simultaneously other modifications of the process-
es of budding introduced, by which the animal is
finally brought to its higher metamorphosis, [Fig.
A.J; the budding being [Plate XX fig. B.] the step
by which the higher metamorphosis is introduced.
The free individuals , which differ so much from
the parent stock, being finally cast off.

In Medusse proper the budding does not intro
duce the higher metamorphosis; this taking place
only in the individuals formed by transverse divi
sion.

Now, let us for a moment compare such a being
as Agalmopsis (Plate XXVI) with the dividing
stock of Strobila (Plate XIX, fig. I). We see at
once that their position is inverted. Here (Plate
XXVI) the fringes hang downwards, but here
(Plate XIX, fig. I) they are upright. To institute

a close comparison, we must therefore consider
them in the same position, and the resemblance
will be striking, especially towards the narrow end.
But when we know that in Polypi buds of various
aspects can arise from one stem, and remain con-
nected with the cavity of the main stem, as it is
here shown in Campanularia (Plate XXVIII)— the
connecting axis being the main body with a con-
tinuous cavity which extends into the branches
— we have no reason to wonder at a similar growth
in animals like Strobila (Plate XIX, figs. G and H)
where there is also a similar connection between
the bud and the main cavity of the body.

And now in Agalmopsis (Plate XXVI) instead
of considering those various appendages as organs
of a simple animal, let us for a moment inquire if
we could not consider them as buds of various
kinds remaining around one stock, and forming a
community of heterogeneous individuals, living a
common life, in the same manner as in polypi,
where we have observed individuals, though some-
what heterogeneous, living also a common life.
And if this comparison can be carried out, we
have established that Agalmopsis must be consid-
ered as a community of distinct individuals.

Now, what are, in the first place, those largest
bottle-shaped appendages ? They are considered
as suckers. But they ajre suckers which pump
food, which digest it in each of these bottles. —
There is a cavity in which the food is digested;
and the result of this digestion is circulated
through the main tube. It is a condition identical
with the condition of the polypi, in which a new
bud arises to remain connected with the main bo-
dy, to have, however, a cavity of its own in which
to digest food, and then circulate it with the main
mass. Here (Plate XXVI) is another kind of
suckers, but performing the same function. They
are similar individuals in a lo-ver degree of
growth.

At first these bottle-shaped open suckers are small,
simple appendages from the main tube,which grow
larger and finally assume a more individualist life,
so that we would have eating individuals upon a
common stem, which provide the whole communi-
ty with food. They are the mouths, the eating in-
dividuals— other appendages which seize upon
the prey and which bring it to the suckers, may be
considered as compound stems. Of these apporates
here is one highly magnified : you have first, the
bottle-shaped apporates with their various modifi-
cations. Here we have the nettling organs, which
are,when highly magnified, also bottle-shaped, and
from which threads hang down. They are another
kind of individuals, suspended by their peduncles
and from which fringes hang down — ^but not sim-
ple individuals. They are individuals which bud
in their turn, so as to form groups of individuals —
groups of catching individuals.

Then there are other buds, which remain hollow
cavities, and are considered as vesicles to suspend
the animals. It is the swimming apparatus of the

PROF. AGASSIZ S

body; but this form resembles so much that of the
suckers, that they must be considered simply as a
modification of them

And if the suckers are buds, these must be closed
buds. Then there are still other buds, which re-
main closed, and which gradually swell and sink.
They do not assume so much individuality as to
open outside, arid to peform other functions. Ad-
mitting simply the fluid within, and pushing it out
again into the common cavity.

Such buds are imperfectly developed individ-
uals, performing the function of respiration.

They are individuals to breathe, as there are in-
dividuals to seize the prey; as there are individuals
to digest, living upon one common stock. There
are other individuals which bud also, and they are
ovaries. Here, Fig. XXVI, apparently an organ,
but nevertheless, arising like the other buds— re-
productive individuals, and of these there are
even two kinds — such as assume the form of
bunches of grape?, and others which assume the
form of those small Medusas here, (of Plate XX.
Fig. B) and which occur, especially in the lower
portion of the animal — swim away freely, and re-
produce free individuals.

Now if it was not for these cases — such buds
which may reproduce the whole colony — such a
conclusion as I am about to present would seem
untrue. But, when there are some among these
various buds which actually present the structure
of medusae, we must conclude that the s -called

Physophoridse are compound animals, in which
the various functions of the body of medusae are
distributed to different individuals in a most diver-
sified manner, they being, however, not organs of
one animal, but of a community of individuals.each
performing special functions ; the whole exempli-
fying what a well regulated Society should be.

There is the most remarkable resemblance be-
tween, the mode of association of individuals in the
compound animals which throw out buds, connec-
ted with the primitive stock, and the plants which
produce successively buds of different kinds. In-
deed the branching of trees from buds compares in
all its features with the budding of compound ani-
mals, and the similarity is closer in proportion as
there are more buds of different kinds produced,
which through life are confined to particular pur-
poses ; for instance, plants which produce similar
buds, growing into branches, identical with the
main stalk, will compare with the simpler forms of
compound animals, in which all the buds produce
individuals similar to the primitive stem. Plants,
on the contrary, which produce at various periods
leafy buds, and flowering buds, in which the male
and female flowers may even be separated, will
compare more closely with compound animals,
consisting of heterogenous buds which remain gen-
erally united for life, and from which only from
time to time eggs, or peculiar buds, are detached,
like seeds, to produce new individuals and new
communities.

LECTURE V.

After illustrating the structure and embryonic
development of the Jelly-fishes, I did not draw
any conclusion in my last lecture as to the natural
classification of these animals ; because I wanted
first to examine more closely the class of Polypi,
in order to trace, if possible, defined limits between
these two classes. Indeed, there is great difficulty in
ascertaining the proper limits of the class of Poly-
pi as a natural division of the animal kingdom,
owing to their low position in the series. Their
structure is so simple, that they are apparently re-
lated to all the lower types of other divisions. And
indeed we find that animals of very different types
have been referred to the class of Polypi. TheVe
have been articulated animals brought in connec-

tion with them. There have been Mollusca re-
ferred to that group. And even at the present
moment, after anatomical investigations have
thrown so much light upon this subject, I incline
to admit that the class of Polypi, as it is now cir-
cumscribed, is by no means a natural one ; and in-
tend this evening to show that entire groups, con-
sidered by all naturalists at the present moment as
Polypi, will have to be removed from that class,
and that other types, which are referred to other
classes, will have to be combined with this class. —
It will be perhaps best to begin this illustration by
pointing out the various forms which are thus
combined at the present moment as one class, un-
der the name of Polypi.

LECTURES ON EMBRYOLOGY.

XXXIV— VEBETILLHM.J

[PLATE XXXVI-RKTEPORE

Vv e nave ucre Umgrauia (<me ot wuicia, Vtretil-
lum, is given in Plate XXXIV) of the principal
groups of this class; and indeed there is scarcely
one family of Polypes of which these diagrams do
not represent some species. The Corals are among
those which have from the beginning been consid=
ered as a type belonging to the class of Polypi. —
And various species are represented here ; among
them are stems, branching and supporting soft lit-
tle animals, which come out like flowers.

The variety of these beings is such that indeed
they rival, by their glorious colors and variety of
form, the most brilliant fiowers of the dry land. —
Such as this Actinia are common on these shores,
and have also universally been considered as Poly-
pi ever since these beings have been combined
into one class, and have been separated from the
vegetable kingdom,

Jt, would carry me too far if I were to give now
the full history of the knowledge successively ac
quired upon these animals, and to refer io those
views of these beings which were entertained by
naturalists at the time when some w^re supposed
lo be simple mineral concretions, and others were
•considered as marine flowering plants ; the ani-
mals upon the stems being mistaken for flowers,
and the stems compared to the stems of plants.

But after it was ascertained that there were con-
tractions taking place in the soft parts, that there
was an internal cavity into which food was intro-
duced and digested, no doubt could remain as to
the animal nature ef these beings; and all small
animals whose upper opening is surrounded by
tentacles, and which are grouped together upon
a common stem, were at once referred to that class.
And some simple animals, like the Actinia, were
also referred to the same class, being considered
as isolated forms of the same character. But we
gee upon the following Plate (Plate XXXVI)
one of these coral like stems, (Retepora) with mi-
nute openings, in which numerous animals are
contained, whose structure has been investigated
by MM. Audouin and Milne-Edwards, and has
been found to differ so materially from that of
Polypi, that this type, of which there are various
forms, is now generally considered as belonging
(G the great division of Mollusca, although they

compound animal. All the investigations
which have followed since this suggestion was
first made, have only gone to confirm the view,
that these porous animals do not belong to the
class of Polypi, but to a higher type, and indeed
resemble in some respects even the oysters, the
clams, and still more the compound ascidiag, in
whose vicinity they will in all probability be placed
forever, showing that compound animals may be-
long to ali great groups of the animal kingdom,
and even occur as anomalies among mammalia,
in the shape of twins.

[PLATE XXXI— ALCYONIUM AND RKNILLA ]

Oilier Uia.Mra.ms repreaeiu various oiuer types.
Here, (PI. 30) for instance, the beautiful Tubulariaa
are seen forming most beautiful flower-like animals
uniting in bouquets upon the old logs and swim-
ming lumber which are fastened in the water.

Two species of this kind are very common
around the city of Boston. One (Plate XXX, fig.
G) with a larger crown, occurs in great abundance
upon the logs in Craigie's bathing house ; another
smaller species is found almost everywhere upon
old logs, The larger is about two or three inches
high, and the crown, when fully expanded, about
one inch in diameter.

This diagram, (Plate XXXI, fig. A) represents
another still undescribed species, with compound
stems, from Boston harbor, belonging to the fam-
ily of Alcyonium, in which every one of the indi-
viduals terminates with a star of eight fringed ap-
pendages or tentacles (fig, B). The most curious,
however, is this one (fig. C), a Eenilla, which t
collected in Charleston, S. C.— an animal with a
soft body of a hollow stem, sticking in the wet
sand, with a large disc, spreading above which*
seen from below, shows lateral dilations, front
which, upon ths upper surface, arise a great

PROF. AGASSIZ7S

PLATE XXX

isolated luue flower-like Pol v pus \tig. U), ot which
one is figured (fig. D) upon a larger scale, showing
that the tentacles, eight in number, are also fringed
like those Aleyonimn, being regularly arranged in
three pairs upon the two sides of the elongated
mouth, a seventh and eighth tentacle being in the
prolongation of the oral aperture. This animal is
of a beautiful purplish color, emitting in the dark
a most wonderful, soft, greenish-golden phosphor-
escent light.

There is another type of Polypi very common
on the shores of Massachusetts and farther South,
the Actinia, of which one species (Actinia Margi-
aata, plate XX, DV is found upon logs along the
wharves in Boston harbor and upon the rocks at
Nahant, in great numbers. They are isolated ani-
mals, growing to a comparative!}' larger size than
the other Polypi; remarkable for their extraordi-
nary contractility, the body assuming constantly
new forms and new positions ; now entirely drawn
out in the shape of an elongated tube with a circle
of tentacles around the free extremity, (Fig. D)
then the tentacles rising and falling, or shutting
in and expanding ; next shortened and contracted
with the tentacles closed (Fig. E) ; or the external
envelope entirely shut over the inner part, pssum-
ing then a hemispheric shape, like round tubercles
sticking to the ground by their fleshy base. The
variations of color are as numerous as the changes
of form; upon the same spot may be seen brown
ones, and others dark brown or blackish, yellow-
is-hs purple, salmoa, rose-colored and more or less

mottled, the tentacles presenting siternatiocs of
dark and lighter rings, or at least having their
tips differently colored than the lower part.

That Yelella and Porpita, now generally ar-
ranged among Jelly fishes,will have to be removed
from the class of Aealephss and placed side by
side with the Actinia, will not escape the attention
of those who are familiar with these animals.*

Recently, the Polypi have again been extensive--
ly investigated by Prof Milne -Edwards, whose
name is always to be mentioned when speaking of
the lower animals, as scarcely any one has dene
more than he has in their investigation. Ehren-
burg has also largely contributed to our knowl-
edge of the Polypi, But no one has done more to
illustrate their natural history than Mr. James
Dana, of New Haven, Conn., who accompanied the
exploring expedition under Capt, Wilkes, and who*
has published the most elaborate work upon this
subject which has ever issued from the press. A
work, indeed, which will remain a standard of au-
thority in this department for many years to come.

The embryonic growth of these animals has
been sin-died almost exclusively by Naturalist©
living in countries which have been wanting ire
facilities for investigation, and are deprived of
privileges which Naturalists have enjoyed in other
parts of the world, where the animal kingdom is
more luxuriantly developed.

It is on the shores of Norway and Sweden that
the most important investigations upon the em-
bryonic growth of these animals have been made,-
There, where the observer is neither attracted by
the variety of animals, nor by the possibility of
discovering easily new species, the interest of the
subject has drawn them into a deeper and more
thorough channel of investigation, which has en-
dowed science with a more extensive acquaintance
with all the difference of structure which is shown-
by the animals of those shores. And, indeedj far
from considering it an advantage to bs placed
upon a shore where new treasures are thrown:
abundantly into the hands of investigators, I think
it is, on the contrary, an unhappy inducement for
observers to devote their whole attention to the
multiplication of specific distinctions, without al^
lowing time for the more important and more ex-
tensive investigation of the physiological phenom-
ena attending the life — attending the development
of those beings.

The structure of the Polypi can bs best exem-
plified in the Actinia (Plate XX, fig. !>) as they
are among the largest, and as they are now more-
extensively illustrated than any other type of the
class has been before. And what I have to say
of these animals will be scarcely more than a
repetition of what Dr. Jeffries Wynnan has pub-
lished in the work of Mr, Dana, already mention-
ed ; some few observations only, the result of my
own investigations, having been added to his,,
since the publication of that work. The body is
of father large sise for a Polype, measuring

LECTURES ON EMBRYOLOGY.

IPLAXE XX — POLYPI ACTINIAE, CORYNJE, SYN-

CORYN^E, PODOCORAN.E.]

one to several inches in length when fully ex-
panded ; it consists of a membranous sac, as in all
Polypi, with numerous tentacles round the upper
extremity, and contains within, another sac, open-
ing above between the several rows of tentacles,

[PLATE XXXII— POLYPI — ACTINIA.]

In this drawing (Plate XXXII, fig. B) you notice
the whole structure in a vertical section of the an-
imal, in which the relations between the different
parts and their interior cavities are at once seen.
You notice the external walls of the animal, and
the rows of tentacles forming the upper outline.
And from the centre, where there is a large open-
ing which must be considered as the mouth, hangs
down a thin sac, suspended within the cavity form-
ed by the external arms and the surrounding thick
envelop of the bodjr.

This sac is a stomach; it is maintained in its po-
sition by internal nuliating membranes, extending
all around the mouth and stomach and uniting with
the external envelop so as to divide the interven-
ing space into many chambers. There are also
shorter folds which penetrate from the external
walls towards the centre, so that the space between
the stomach and the lateral walls is not one con-
tinuous cavity, nor uniformly divided into equal
chambers, but it is a cavity divided and subdivided
anto wider and narrower spaces by partitions which
extend either entirely across the cavity surround-
ing the stomach, or only partly into it, thus form-
ing imperfect chambers; all the them, however,
remaining connected by the open space which is
left free of divisions below the stomach. Here is
a diagram [Plate XXXII fig. A.] in which the ani-
mal is represented as divided horizontally, and in
this horizontal section you see the cavity of the
stomach forming one great whole in the centre
and the partitions which extend from the external

walls towards the centre either reaching the walls
of the stomach or not, from the intervening septa,
But these are not all equal. There are some of the
partitions which reach half way towards the stom-
ach— others whieh reach two-thirds of the way —
and others still which reach most of the distance.
Below [Plate XXXII fig. B.] we see them as they
present themselves upon a vertical cut. From the
external surface something of those partitions is
already seen. The vertical striae noticed [Plate
XX fig D ] are the external points of attachment
of the fleshy partitions upon the external envelop
of the whole body, and they extend high up into
the margin from which the tentacles arise, And
indeed on close examination it will be seen that
one tentacle arises always between two partitions ;
so that a tentacle is, as it were, a radiating prolon-
gation of the main cavity of the body, extending
like the finger of a glove from each of the divided
spaces upwards. You see this [Plate XXXII fig.
BJ where the tentacles show plainly their connec-
tion with the main cavity, and where the divisions
are as numerous as the tentacles.

These partitions are muscular fibres, and by their
contractions they can shorten the animal. Sup-
pose these vertical partitions to be at once contrac-
ted, the animal, instead of forming a vertical cylin-
der, becomes depressed. (Plate XX fig. E J And
as there are muscular fibres around the whole
body, the tentacles cae be drawn in, and the upper
fibres, contracting more and more, may entirely
conceal the tentacles and form such hemi spheri-
cal bodies as are observed in these diagrams. —
[Plate XX fig. G.J

Between these partitions, by very careful investi-
gation, small holes can be discovered, arranged in
vertical series {fig, D). The use of these tubes is
not yet fully ascertained. I shall have an oppor-
tunity to refer to them again.

But I would mention, further, that the mouth
(Plate XX , fig. F) is not a simple circular hole on
the summit of the animal, but presents lateral
folds upon a longitudinal fissure. At first sight,
when seen from above, the inner membrane of the
Actinia stretched between the tentacles seems to
form a circular mouth (Plate XXXIII, fig. A); but
on close examination, it will be noticed that it is

[PLATE XXXIII. -POLYPI— YOUNG ACTINIA.]

PROF. AGASSIZ7S

really a longitudinal fissure with lateral folds. All
the tentacles terminate with a hole ; they also con-
stitute muscular tubes, with longitudinal and cir-
cular fibres, by the contraction of which they are
alternately drawn in and out. The stomach, like
the tentacles, empties into the main cavity of the
body (Plnte XXXII, fig. B), so that when the Acti-
niae swallows its food, the results of the digestion
are thrown into this common cavity, and there
circulates by the agency of vibrating cilia between
the partitions and in the hollow tentacles, until
absorbed by the surfaces in contact. You see, also,
in that diagram, that water can be introduced into
the inner cavity through the mouth and the stom-
ach, as well as through every tentacle, and also
thrown out through stomach and mouth, and
through every tentacle. The body is thus swollen
by the water pumped through the suckers, or by
that swallowed through the mouth. When the ani-
mal re-opens its mouth to throw out water, the un-
digested remains of the food are also expelled.

When the animal comes out from its contracted
position, we see the suckers gradually expanding,
(Plate XX, fig. E) and these numerous tentacles
pumping water, and the animal successively swell-
ing into its various movable changing forms. The
existence of eyes in Polypi has been mentioned by
Mr. Quartrefages. I have observed them in a new
species of Lucernaria discovered upon the beach
at Chelsea. In addition to these structures there
is hanging from the partitions of the main cavity,
[Plate XXXII. fig, B.} below the stomach, a series
of bunches of eggs — ovaries, below those lower
muscular partitions. All Polypi seem to have a
structure similar to this. Those which do not re-
semble these in structure, are the types which I
consider not to belong strictly to the class of Poly-
pi. When the eggs of Actiniae are matured, they
are let out through the moath. I have had an op-
portunity to see this myself. These bunches of
eggs are freed in the main cavity of the body, and
then through the lower opening of the stomach
pressed into that cavity and finally discharged from
the mouth, as represented in. this figure. fPlate
XXXIII. fig. A.]

They are sometimes entangled in the cavities of
the tentacles, and have even been reported by Sir
John Dalyell to be discharged from the tentacles.
The young egg of the Actinia presents the struc-
ture which we observe universally throughout the
whole animal kingdom. They consist of a mass of
yolk substance, enclosed in a special membrane
{Plate XXXIII, fig. B}. Within is a germinative
vesicle, and in the centre a germinative dot (fig. C).
These yolks will grow (fig. D), the germinative ve-
sicles and dots will disappear, and the germ being
formed in the shape of spheroidal bodies, with a
darker mass in the centre, will be hatched, and form
a more elongated body, (fig. E>— the yolk being
more distinctly separated from the animal layer
proper, which is the external crust of the germ. —
Above, a depression is formed; the lower part

is attached upon the soil, and around the sppor
depression, (Plate XXXIII, fis F.) there are little
protuberances formed, (Fig. G.) the central depres-
sion growing deeper, and the mouth is finally pro-
duced, surrounded by tentacles. (Fig. H,) Bat the
most remarkable feature which I have observed in
this development, is that the young Actinia differ
from the old ones, in having at first only a few ex-
ternal tentacles \ and these few are arranged in a
very peculiar manner* Suppose this to be the
first indication of the mouth ; there will soon be
surrounding tentacles, (Plate XXXIII., fig. H.) at
first only five, though ia the full grown animal
there will be hundreds. Next there will be others,
coming out between the first ones, so that soon
ten are formed. Then there are everywhere in the
intervening spaces more coming out, so that twen-
ty will occur ; and in this way the number is grad-
ually increased. But the position of the primitive
five ones has a relation to the longitudinal form of
the mouth; one of the five primitive ones being-
always in the same diameter of the animal as the
longitudinal fissure of the mouth. (Plate XXXIIL
fig. A.) But the other four are in pairs.

After I had made this observation, I asked Mr
Dana whether he had observed such a symmetry
in the arrangement of the tentacles. He stated
that he had ; and that in addition, one of the tenta-
cles was sometimes different in color from the oth-
ers. What this means I shall soon show when
comparing the Polypi with the other radiated an-
imals.

But now there are other Polypi whose embryol-
ogy has been extensively studied. I mean the Co-
rynse (Plate XX., fig. C.) Syncorynae (Fig. A.) and
Podocorynse, (Fig. B.) upon which Loven. Sars
Steerstrup, R. Wagner, and others, hare made
most remarkable observations. And also the Cam-
panularia Tubularia, upon which we are indebted
to Loven and Von Bereden, and others, for exten-
sive information. The Coryne, and alike types are
so closely related to the Tubularise, that the resem-
blance has been particularly noticed. And this close
resemblance alluded to as a sufficient ground to-
leave the club-shaped Polypi with Medusa like buds
among Polypi, notwithstanding the great differ-
ence which has been noticed, both in their struc-
ture and mode of development.

Here we have the Podocorynse (Plate XX, fig. B),
and here (Fig. C) the Syncorynse, which are small
PolypL The existence in Boston harbor of simi-
lar Polypi of the genus Corynse, first described
from the Northern shores of Europe, I have as-
certained last year, and indeed there is a vast field
to explore on these shores, as during a cruise on
the South Shoals with Capt. Davis, in 1847, 1 have
ascertained the existence of not less than seven-
teen species of this family, among which there are
types of new genera, which I shall describe oc
another occasion. From the upper part of the
stem of these Corynoid Polypes there are hanging
down several little bell-shaped bodies, of a quad-

LECTURES ON EMBRYOLOGY.

rangular form. The outline of these bell shaped
bodies being, when seen from below, as in figures
A and B. The angles are prominent, and from
them there are colored specks rising, similar to the
eye specks of common Medusas. A membrane is
stretched across over the central cavity, leaving, j
however, an opening below ; and from the cor-
ners are produced short tentacles, which, in the ;
progress of time, grow longer and more moveable.
In the interior there is a sucker-like projection,
first with a single margin, which will be fringed
afterwards. From these details it is plain that
these buds, when fully developed, resemble most
remarkably the small Medusa, (Plate XXVIII, fig.
C) to which I have before referred.

Indeed, they are finally freed from the stem
upon which they grow, and move as independent
animals.

The structure of these small animals is indeed
very simple ; as they have only four straight tubes
branching in four directions from their summit.—
The investigators of these phenomena have been
unwilling to refer them to the class of Medusae,
but have considered them as closely allied to Tu-
bularise, and belonging therefore to the class oj
Polypi. They have compared the Medusa-like
buds of Coryne, Syncoryneand Podocoryne, (Plate
XX,) to the crown of the Tubularias, (Plate
XXX, fig. A.) and you see that the comparison is
very close. You see that the hollow tube within
the Medusa-like bud (Plate XX, fig. A.) will com-
pare to the hollow cavity with fringes hanging be
low the tentacles of Tubularise. (Plate XXX, fig.
A.). Then you see the tentacles above spreading
around the bunches of eggs and arising from the
upper cavity, as the main cavity of the little Me-
dusa-like buds surrounds its inner hollow tube,
from which the eggs are developed in them, form-
ing also special bunches, exterior to the inferior
or anterior part of the alimentary canal, so that
the resemblances between these bell-shaped bulbs
(Plate XX, fig. F) and the crown of Tubularise
(Plate XXX, fig. A) is as close as it can be. The
conclusion derived by Steerstrup from these facts
is that the genera Syncoryne, Coryne and Podoco-
ryne, (Plate XX, figs. A, B, C) should no longer be
considered as genera by themselves, but only as
the nurses of animals of a higher order, the little
Medusa-like animal, but that they nevertheless
should remain with the Polypi near the Tubula-
rias. Steerstrup insists upon this point, when he
says : " The more perfect forms, however, notwith-
standing their resemblance to Medusae, must still
occupy the systematic place of the clariform Po-
lypes, or Coryne, as animals closely allied to Tubu-
larise, Sertularia, &c. &c.

Let us now examine the Tubularias and also the
Campanularise, as they have been carefully studied,
and then we shall be prepared for an opinion upon
these conclusions. We have here [Plate XXVIII]
a stem of the Campanularise, which has branches
of various kinds. How these branches grow must
be examined more fully.

[PLATE XXVIII — CAMPANULARISE. j

In a growing stem — the first origin of the stem,
we shall examine afterwards— there is in the inte-
rior a cavity, which cavity expands above and
forms a kind of stomach ; the moveable part of
the animal forming tentacles around, and the
mouth being therefore above. And from the side
of such a Polype there will be, after a certain time,
a bud, forming a simple sac, communicating with
the main cavity, and the changes which have pro-
duced the main stem will be repeated here so as to
give rise to another Polype of the same structure as
the terminal one, with a open communication with
its main cavity ; and after by repeated budding,
numerous branches, all alike, have been found as
they are figured in this diagram, [Plate XXVIIIj
Where you see seven buds all alike,some new buds
forming in the axis between the main stem and
the first buds. And these new buds differ from the
former, inasmuch as the bud will not terminate
with a new Polype,similar to those of the first buds,
but will remain closed, and while it is still closed
there may be buds arising on its side in which eggs
are developed.

Loven, who described these phenomena more
extensively, represents these axilary buds as giv-
ing rise, bv budding, to new branches, remaining
longer shut in a common cavity, and indeed being
branches similar to the external one; with the
only differences that the terminating animais have
smaller tentacles, and are of a slightly different
shape; communicating with the main cavity, and
giving finally rise to free moving individuals ;
whilst there are below simpler sacs, of the same
order, but still less developed. Plate XXXV rep-
resents the various stages of this growth.

Now these sacs are something like buds ; but
they are, in fact, eggs, which, In the beginning,
are simple buds, or diverticula from the common
cavity, so that we can consider the whole as buds,
which throw out new buds, from which eggs are
developed, in the shape of pouches. And that
these are eggs, can be proved by the characters
which distinguish eggs. (PI. XXXV. fig. D.) They
may have a germinative vesicle, and agerminative
dot ; and there a new animal is formed, which
will escape as soon as the upper buds, which are
now full grown, have removed the closing opercu-
lum : so that, by a process of budding — of bud-
ding egg-like buds — there is a new generation,
formed, which does not remain upon the primitive
stem, but is freed; and when freed, the germs
arising from the eggs are elongated, and little
cylindrical animals, which swim free, appear; and

PROF. AGASSIZ'S

[Pi. ATM XXXV — BtrnrxNO 01-- CAMPANULA.!^.]

after having continued free fora certain time, they
become attached, and then the whole mass is de-
pressed and enlarged into a disc-like body, the
centre of which is somewhat prominent; rises
then more and more, and begins to be transformed
into a little stem ; and this little stem will open
«•. above, and form a termination, like that of the
commoa buds of Campanulariae (Plate XXVIII.)
that is to say, an animal with an internal cavity —
We have thus again a beginning of one of those
complicated stems, which, by the multiplication of
their buds, form communities of animals, of two
kinds, viz: such as are individuals similar to
the animal at the end of the main stem, and
others from which a free generation is produced,
and which, after remaining free for a certain time,
go on to repeat the same process of branching and
budding.

In the Tubulariae (Plate XXX, fig. A) we have a
similar growth. One of these bunches of eggs,
when examined in its immature condition — in its
earliest formation, (Plate XXX, fig. F) — is simply
a branch with lateral buds, and the digestive cav-
ity communicates freely with all these little buds.
But their interior mass assumes gradually a more
rounded form, and is successively enclosed in the
external mass, which will enlarge, and then there
will be finally isolated eggs developed, in the form
of bunches, when upon the summit of every one
of them a distinct animal envelop is formed,
which extends downwards upon the yolk— as the

internal mass can be considered as a yolk— and
after it has grown so as to appear like a cup, with
tentacle-like appendages, the little animal is freed,
and has a structure like the young Medusa, as it
is figured from a Campanularia, in Plate XXXV,
figures T, P and Q. The whole process of budding
in this animal is shown in figures A, B, C, &c.,
(Plate XXXV)— first, the changes which regular
common buds undergo in their development, and
next, (Fig. E to G), the changes of the eggs prop-
er, with their animal envelop surrounding the
yolk, and finally dividing into tentacle-like appen-
dages below. The internal cavity being formed
by the changes which the remnant of the yolk
undergoes. The young animals which are derived
in this way in Tubularia; and Campanulariae, from
egg bunches, are so similar to the free buds from
Corynae, Podocorynae and Syncorynae,( Plate XX,
figures A, B and C), that their analogy cannot be
mistaken. This resemblance can even be recog-
nized in stages of growth not further advanced
than these, (Plate XXXV, figs. T, P, Q). Some
Medusas occurring on these shores— for instance
the genus Stomabrachium — have a very close re-
semblance to those germs of the Campanulariae
(Plate XXVIII), and Medusae, with only four arms
and four tubes diverging from the central cavity,
with fringes all round : and I should not be sur-
prised at all, if Stomabrachium was finally found
to be the free Medusae-like generation of Campa-
nularia;. But now as the affinity between all these
Polypi (Plate XX, XXVIII and XXXV) and the
Tubulariae (Plate XXX) is very clearly shown,
and as on that account these animals are all con-
sidered as belonging to the class of Polypi, though
they give rise to animals so closely allied to Medu-
sae, the question arises how far Tubulariae itself
can be considered as strictly belonging to the class
of Polypi, or whether it would not be more nat-
ural to view it as a type of Medusae, furnished
with a permanent stem.

The only objection to this is, that true Medusae
are not formed in the same way as Medusae like
free buds of Coryne, Podocoryne and Syncoryne
(Plate XX.) These have arisen from buds grow-
ing upon Polypi-like stems, though they are final-
ly Medusae-like animals; whilst true Medusae are
multiplied by transverse division of Polypi-like
stems, which can have no influence upon our ap-
preciation of their real structure; so that the ques-
tion properly is, whether there can be real Medusae
with a stem, or not. We have, therefore, in this
stage of the investigation, before deciding one way
or anotherj to compare the true Medusae (Plate
XXXVII and Plate XXVIII, fig. C) with those
Polypi, the Tubularia, (Plate XXX), when it will
be seen that their structure agrees in every respect
but that one, that the Tubularia, with its crown,
rests upon a stem, whilst Medusae proper are en-
tirely free. The great difference there seems to be
in the forms of these animals is more apparent
than real, the cavity which hangs below the ten-

LECTURES ON EMBRYOLOGY.

tacies corresponding to the central alimentary
tube of the Medusae, which is only drawn in be-
tween the gelatinous walls of the disc, though it
remains equally free as in Tubularia. The upper
cavity of Tubularia answers to the disc of Medusas
proper with its cavities ; and in both the ovaries
are outside of the alimentary cavity, as well as of
the main cavity of the body Indeed, the agree-
ment is perfect in every respect, and we must
come to the conclusion that from their structure
Medusas and Tubularia must belong to the same
class, Tubularia being Medusa with a stem, and
bearing the same relation to free Medusas, as cri-
noids bear to free starflshes. And so we have in
the class of Medusas attached types, as well as in
that of Echinoderms, and in that of Polypi.

In a more general point of view, we may, how-
ever, compare further, all radiated animals, when
we shall find that they really constitute a natural,
well circumscribed group in the animal kingdom
agreeing in all important points of their structure
being strictly constructed upon the same plan, al-
though the three classes which we refer to this
great department differ in the manner in which
the plan is carried out. In the first place, I may
mention that besides Polypi, Medusae and Echi-
noderms, the other classes which were referred to
the type of Radiata, have been removed from it,
or are to be removed from this connection. The
intestinal worms indeed are truly articulated ani-
mals in tbeir fundamental plan of structure, and
have to be connected with the worms proper,
while the Infusoria, Polygastrica and Rotatoria
are very heterogeneous classes, the latter of which
has to be united with the Crustacea and the so-
called Polygastrica, to be divided off according to
their various structures, some being germs of
aquatic plants, and others the first stages of growth
of various worms, as I have ascertained by direct
observation. As for the classes of Polypi, Medusae
and Echinoderms, if we bring together the dia-
grams (Plate XXXII) representing an Actinia in
a vertical section, with that of Plate XXXYII,
which represents a similar section of a Medusa,

[PLATE XXXVII— MEDUSA.!

vert the Polype and place it with the mouth
downwards, as it is naturally in free Medusae,
we could see at once that in the Polypus we

XXXVTU—

and other illustrations of Echinoderms exhibited
in a former lecture, and the vertical section of an
Echinarachnius, we shall have the elements
(Plate XXXVIII, fig. E) of a closer comparison be-
tween the three classes. If we were indeed to in-

have lUc sumo -i.-n-^iU arrdnjfcitient as in the
dusae. There being a separate alimentary cavity
and a common cavity of the body only in Medu-
sae, (Plate XXXVII) the anterior part of the ali-
mentary cavity hangs down with the mouth freely
from the walls of the body. This part of the ali-
mentary canal answers to the cavity of Actinia
(Plate XXXII. f\s B) which is called stomach, and
from the upper part of the Actinia, in its inverted
position, arise those partitions which end in ten-
tacles answering to the disc of Medusae, with its
cavity, branching into similar tentacles.

We have also again a common cavity in Medu-
sas (Plate XXXVII), as well as in Actiniae, only
more circumscribed, and branching off into tubes
which communicates in similar mannerwith the ten-
tacles, so that the general arrangement is perfectly
identical. The difference is, however, this— that in
Medusae the tubes arising from the central cavity
are circumscribed, while in the Actinire (Plate
XXXII, fig. B) they are only partitions communi-
cating all together. And in the Medusae (Plate
XXXVII) there is a distinct nervous system. I
suspect that in Polypes we should find the nervous
system in the same position as a ring round the
mouth, if it is at all distinct in those animals ; that
however eye-like specks have been noticed, even
in these lowest animals, I have already mentioned.
As for the ovaries of the Medusas (Plate XXXVII),

PROF. AGASSIZ S

they arise externally from the lower or central ca
rities of the alimentary canal, and are surrounded
by the disc, which contains the main cavity of the
body, and from the periphery of which the tenta-
cles hang down, so that here the ovaries are out
side of the stomach, and outside of the main cav-
ity, as in Tubularie, — and not within the common
cavity, as in Polypi.

Now in order to insist more strongly upon the
fundamental differences which exist between Polypi
and Medusae, even if we include Tubulariae among
the latter, let me once more call your attention to
the Tubularia (Plate XXX, fig. A). We have here
a mouth, with the anterior alimentary cavity,
•which will assume all possible shapes, as we see in
these various diagrams, hanging outside of the
common cavity, and not within it, as in Polypi.
We see those bunches of eggs, arising below the
tentacles, between the tentacles and the anterior
alimentary cavity, also outside of the alimentary
cavity, the central cavity extending above, so that
the analogy is perfect in every respect.

And as we have in the Corynas, Syncorynas and
Podocorynas buds, which, though growing from
Polype-like animals, will produce real Medusae,
their close resemblance to Tubularite will only be
an additional evidence that these must be referred
to the class of Jelly-fishes, and that the club-
shaped Polypi, in their perfect condition, are also
Medusae, and that their earlier stages of growth
are only nurses to produce real Medusas by alter-
nate generation. The Tubularias themselves will
have, however, to be considered as the lowest
type of Medusae, preserving something of the Po-
lype structure, as they are for life provided with a
stem, from which the crown hangs down. And
from this stem would arise buds similar to the ter-
minal animal (Plate XXX, fig. G) which would
remain connected with the. stem, thus forming
branched compound Medusas. And if this ground
be correct, not only Tubularia, but also Campanu-
laria and Sertularia shall be united with Corynas,
Syncorynas and Podocorynae in the class of Medu-
sas. Thus circumscribed, the class of Medusas
would present the most remarkable parallelism
with the class of Echinoderms and that of Polypi,
in both of which there are free types and such
as rest upon attached stems, a parallelism
upon which Oker has already insisted, in a general
way, is his classification of the animal kingdom.

To investigate further this subject, there is a rich
field in this vicinity, where animals, Tubularia,
Companularia and Sertularia, occur all around the
shores of Massachusetts.

Again, if my conjecture of the necessity of com-
bining these Tubularia with Medusa is correct, I
venture to foretell, that among those small species
of this class, which are found on this shore, we
have the Medusa-like form of the Coryna, in the
little Oceania of Dr. Gould's Report, whose struc-
ture is Mustrated in Plate XXVIII, fig. C., as I have

ascertained by dredging, that Coryna occurs in
Boston and harbor.

That Coryna has been found so seldom is be-
cause it lives in deep water, and is not discovered
unless by dredging.

I should not be surprised at all to find also
Stomabrachium, as the Medusa-form of Campan^
ularia, which occur all over the shores of this con-
tinent, and that Bongainvillia could be the Medusa
of Tubularia, if they produce at all a free genera-
tion, seems to be probable, when we consider the
form of its crown. (Plate XXX., fig. A.)

As for the gradation of types in the class of Me-
dusa, we should consider the Tubularias as the
lowest, for the reasons already stated. Next we
should place the free compound Medusae, the Phy-
sophoras of Eschscholtz, which correspond to the
next stage of growth of Medusa, known under the
name of Strobila.

[PLATE XXVI— MEDUSA [

Next we should place the free Medusae or Disco-
phora of Eschscholtz, and highest the Ctenophoras,
as by their comb-like rows of fringes, which may
be considered as a lower form of Ambulacra, they

LECTURES ON EMBRYOLOGY

come nearest to the Echinoderms. This arrange
ment, which is natural in itself, would show the
most admirable agreement between classification
&nd the phases of embryonic growth in this class,
and also because they come nearest to the first stage
of growth of the common Medusa. (Plate XIV
and XIX ]

[SEE PLA.TES XIV LECTURE 3, AND XIX LEG

TURK 4 1

The analogy again between Medusa3 and Echi-
noderms is too easily ascertained to be ever mista-
ken by any one who attempts to compare them in
the same close manner. The chief difference here
consists in the more developed inner structure of
Echinoderms, whose organs are more diversified
and isolated, and in the harder coverings which
protect the soft parts, besides the addition of some
special apparatus which do not occur in the two
lower classes of Radiata.

The improvements which I anticipate in the class
of Polypi are fewer, after removing the Retepora
and alied types, to the great groups of Mollusea
and the Tubulariae to the class of Medusa. We
shall only introduce the Porpite and Velella in the
vicinity of Actinia,and then, as Mr. Dana has done,

divide the Polypi proper in Actinoids and Alegon-
oids. the former division embracing those with,
simple tentacles, as Actinic, (Plate XX fig. D.) the
latter those ;vith fringed tantacles as Alegorium
and Renilla. {Plate XXXI.)

All the stone corals proper belong to the type of
Actinia, and upon a close comparison of the struc-
ture of this animal with the ancient fossil Cyntho-
phyllum-like Polypi of palaeofoic rocks, some fur-
ther hints may be derived as to the order of sue
cession of Polypi in geological times, which is at
present very little understood. How the calcare-
ous stem is formed in Polyps, can be perhaps no-
where better studied than in the little Alcyonium
(Pate XXXI, figs. A, B,) of Boston harbor, where
calcareous nets and spicules are deposited in regu-
lar groups below and within the base of the tenta-
cles, and at the opposite extremities of the animal,
between which the muscular fibres are attached. —
There is, moreover, a peculiarity in the structure
of Polypi, which can be easily observed in the
Ranella. (Plate XXXI, fig. K) In this Polype the
mouth has an elongated form, and there is one
tentacle in advance and one behind this opening,
in the longitudinal diameter of that fissure.

Under tue form of radiated animals we have, in-
deed, through the classes of Echinoderms, Medusae
and Polypi, every where indications of a bilateral
symmetry, concealed under the more prominent
outlines of a radiated arrangement of the parts. —
We have really among Radiata the first indica-
tions of the general bilateral symmetry which pre-
vails universally throughout the animal kingdom,
even in the class of Polypi. (Plate XXXIII, fig.
A.) In Actinia, the lowest condition, this bilateral
symmetry is noticed in the longitudinal direction
of the mouth, (Plate XX, fig. F) and in the ar-
rangement of the first formed tentacles, of which
one is seen always in the same diameter with the
mouth, whilst the other tentacles are placed in
two pairs on each side, (Plate XXXIII, fig. H)
which is peculiar in such species. We have also
indications of a bilateral arrangement in those
Medu^ie in which the body is compressed laterally
and more or less oblong, as in Beroe, Cestum, £c.
where one diameter is much longer than the other.
We have it still further in the division of the ten-
tacles hanging down from the mouth in the com-
mon Medusa?, in which there is frequently one
tentacle more developed than the others. That
Echinoderms are regularly bilateral under their
spherical forms, I have already shown, fifteen
years ago, when I first ascertained that the Ma-
dreporia bodies lie always symmetrically between
two of their rays in the longitudinal axis, which is
parallel to the direction of the alimentary canal,
as It extends towards the elongated extremities of
the higher types of that class.

Another peculiar arrangement which is common
to the Radiata, is the existence of water tubes, es-
tablishing a permanent connexion between the
surrounding element and the internal cavity of
the body. In the Medusa? (Plate XXVII,.figs. A

PROF. AGASSIZ7S

and B), I have already shown the structure by
which the water is introduced into the cavity. In
the Echinoderms is figured this arrangement in the
star-fishes (Plate XXXVIII, fig. D).

Through these almost microscopic tubes the
main cavity is constantly filled with water, which
escapes freely from the star-fishes when they are
taken out of the water. They should not be mis-
taken for ambulaesral tubes, which are placed in re-
gular rows— whilst the water tubes are scattered
almost over the whole surface of the animal, but
only seen when fully expanded in the living ani-
mal. In the Actinia, the water system is plainly
developed (Plate XX, fig. D), in the forms of mi
nute pores arranged in vertical series.

From the above statements it can be concluded,
that there is the strictest agreement between all
Kadiata in the general plan of their structure ; and
this analogy can even be traced in the embryonic
growth— all the Radiata beginning by the formation
of a distinct layer round the yolk in the form of a
spherical crust, from which the more animated
parts are derived, whilst the alimentary cavity is
formed by the modification of the central mass of
yolk. In addition to this regular mode of repro-
duction, the Polypi and Medusae are also multiplied
by buds, and some of the Medusas by a peculiar
modification of the alternate generation— new ind1
viduals being formed by the transverse division of
a primitively simple stem. Whether anything like
an alternate generation takes place in the class of
Echinoderms, remains still doubtful ; but I cannot
help thinking that the Pedicellarice are the last in-
dications of a kind of budding, giving rise to very
low organisms, which can only be compared to the
peculiar beak-like buds of some of the Sertularise.
This uniformity of structure and growth calls for
an additional remark. Ever since the natural and
physical sciences of graphical representations have
been introduced, progress has been made much
more rapidly than before.

As soon as Humboldt had drawn his isothermal
lines, investigations in all parts of the globe
were at once called for. And so it was in chemis-
try, when the formulae were introduced to re-
present chemical composition, by which an insight
into the constitution of numerous bodies could be
obtained at one single look. Now in the animal
kingdom nothing has yet been done to represent
by symbols either structures or natural affinities;
only the teeth of Mammalia are noticed in a regular
system. Something, however, has been done, and
is extensively introduced in Botany, to represent
the arrangement of the leaves of plants and the
parts of the flowers, by formulas. But to represent
structures — to represent affinities by symbols is an
attempt which has not yet been made, and which I
think could now be satisfactorily introduced. Only
general symbols for the main groups of the animal
kingdom, representing their fundamental erabry-
ological character, have been Introduced into the
text book which I have published in connection

with Dr. Gould, where a star was used to represent
the Radiata, where Mollusca when represented by
an inverted Greek W, Articulata by a W, and
Vertebrata by the figure 8, these diagrams having
reference to the peculiar mode of development and
of the germ. That the Radiata is best represented
by a circle, is shown by what I have said of the
first formation of the germ, wLich surrouuds the
yolk entirely from beginning, and forms, as ifi
were, an animal crust round the yolk, so that we
could have, instead of a star to represent Radiata,
any general simple circular outlines with a dot in
the centre, to remember the analogy of their gen-
eral structure with that of the eggs, with the low-
estcondition of all animals.

[PLATE XXXIX ]

But when we would like to represent special
classes, either,. Polypi, Medusas or Echinoderms, I
would propose that instead of a dot, we should
have for the Polypi a longitudinal line across the
circle, (Fig. B.) indicating the first apperance of a
bilateral arrangement under the form of a sphe-
rical circle. To represent the Madusae, I would
propose a circle with a cross within, (Fig. C,) to
indicate that in these animals there is a radia-
tion of branching tubes from the central cavity.
And to represent Echinoderms, I would have a
star in the circle, (Fig. D) corresponding to the
form which is the most characteristic of that class.
So that the three classes of Radiata would be
represented by their peculiar figures, and by the
addition of a single letter to these symbols, we
might at once represent either of their families —
for instance, having the diagram of Echinoderms,
an additionrl C would represent Crinoids, E would
Indicate Echini, and A would represent Asterid»
(Fig. E).

And how important this would be, is at once ob-
vious, if we look at geological works, where the
lists of fossils, simply mentioned by their names, do
not convey any idea to the reader. But if, instead
of Saccocoma, shortly we append the figure of
Echinoderms, and add aC, we should know at first
sight that this is a fossil of the class of Echino-
derms belonging to the family of Crinoids, and the
symbol itself would at once remind us of the pecu-
liar structure of these animals. Those great fig-
ures being used to indicate the families, an addi-
tional small letter might indicate minor divisions,
and so on ; so that these symbols would show all
the affinity of any given animal, and form in real-
ity a complete picture of the various relations
which exist among all animals.

In my next Lecture, I shall enter into the depart-
ment of articulated animals.

LECTURES ON EMBRYOLOGY.

LECTURE VI

L r/ow proceed to examine the great group of the
animal kingdom, which Naturalists have desig-
nated under the name cf Articulata, There ani-
mat3 are remaikable for one peculiar feamre of
their structure ; the body consisting of a series of
joints rnoveable upon each other, to which are fre-
quently added rnoveable appendages, sometimes
subdivided into joints, which are rnoveable also.
This is the common character of all Articulata,
nnd upon Plates IV, V, VI, VII, IX, X and XI you
see various forms of this great type.

[PLATE TV— R.\T-TAILKT> WORMS 1

VI— L'»"JSIER ]

The Articulata have been divided into three
classes : Crustacea, as crabs, lobsters and a'll the
animals like them; Insects, as butterflies, beetles,
flies; and Worms, the worms which live free in
the water or in the soil, and also the parasitic and
intestinal worms.

These three classes differ in their structure as
well as rs their general form, and they have been
placed in our systematic works ia an order which
deserves particularly to attract our attention.

The Crustacea are placed highest in the series of
Articulata, and the Worms lowest^ and between
them, the Insects, so numerous and so exceeding-
ly diversified. In the opinion of Naturalists, this
•order of succession agrees with the complication
in structure of these animals. And they insist
upon this order as really indicating the natural
gradation among them; the Crustacea being con-
ilJered highest, owing to the perfect development
•of a heart and a regular circulation, and also
owing to the concentration of the nervous system
and the combination of its elements. The want of
a regular circulation in the Insects has been the
reason why they have been placed in the second
rank. The Worms, from the uniformity and num-
ber of their rings, to which are attached feet-like
appendages almost as numerous as the rings them-
selves, have been considered as the lowest.

Now in this order of succession-, to which Natu-
ralists have specially devoted their attention,
which they have investigated with particular refer-
ence to a natural classification, I think we have i

another instance of a mistaken view of the
ject, derived from a mistaken estimation of an-
atomical characters. I am prepared to show that
Crustacea are not the highest-; that Insects should
be placed at the head of Articulata; and that they
are in every respect the highest. And after the
grounds upon which I intend to place them high-
est have been illustrated,, I expect it will be found
that the anatomical structure agrees here again
with the order which the metamorphoses actually
indicate; and that it was a mistaken view of the
complicated structure of the Crustacea which in-
fluenced Anatomists, and induced them to place
Crustacea highest.

Before, however, I can go through this compar-
son, I must illustrate in detail the different classes
of this great group ; otherwise my comparisons and
my grounds would scarcely be intelligible,

I shall devote this evening to the illustration of
that one class which I consider as highest among

PROF. A6ASSIZ7S

(PLATE VITI— SCORPION.^

Articulata— that of Insects. And before begin-
ning this investigation, I will simply mention that
the group of Articulata, as it is now circumscribed,
has not always been considered as containing only
three classes. A great number of divisions and
other arrangements were, at various periods, at-
tempted by Naturalists. The &piuers, for instance,
were considered as one entirely distinct class,
placed between Crustacea aad Inseers, though I
am of opinion that they are better united with the
Insects, owing to their structure, as well as their
natural development.

Among Articulata, groups have been introduced,
which were formerly placed ia other great divi-
sions. For instance, the Barnacles were long con-
sidered as Shells, from their external coverings,
which are really shells j but their anatomical struc-
ture has proved a relation between them and Artic-
ulate animals, and really a close relation to Crusta-
cea proper — so close a relation to Crabs and Lob-
sters, that,et the present time, no Anatomist doubts
that the Barnacles must be placed in one and the
same class with them ; though perhaps among
Zoologists, there may be some who still think that
the external form should be taken into considera-
tion, and not overruled by the internal structure;
but such doubta deserve scarcely any longer no-
lice.

As I mentioned in the last lecture, intestinal
worms were placed among Radiata, but they are
•proved to be Artieulata, since the nervoas system

has been lately discovered by Bfr. Blancnard m a] "I
the principal types of intestinal worms, and foand
to agree, but with some modifications in its general
arrangement, with that of Articulata. The Infuso-
ria were also formerly arranged among the Radi-
atu, bat now their stracture is more extensively
known, they should be scattered and arranged1
among various classes, according to their inner
organisation and mode of growth— some belong-
ing to the Worms, and being only the young, or
embryonic condition of worms of Planaria, for in-
stance ; others belonging to the vegetable kingdom,
and being also embryonic conditions of various
Alggcr; and others still, belonging to the Crustacea,
as for instance the Sotifera. It is remarkable time
the extensive investigations made upon the Infu-
soria, the object of which was to illustrate the uni-
form structure of these animals as a class, go to
show that the class ought to be broken up as a na-
tural group, and distributed among various other
classes.

How much remains to be done among tke small
organized beings,whieh hare to be investigated by
the microscope, will be at once understood when I
mention that, for instance, the egg of the Mosquito-
like animals whose embryonic changes are repre-
sented in Piate VII, figs. A, and E., was first consid-
ered as an Alga.and described as a species of Gloc-
onema, before it was found to be a Musquito-like
insect.

[PLATE VII.— EGGS OF

The great class of Insects is particularly remark-
able for the metamorphoses which these animals
undergo. And you may at once perceive how dif-
ficult it must be to trace all the changes of these
animals when I mention, that the perfect being—-
the perfect insect may be an aerial animal, provided
with wings, and flying about; when in another con-
dition, it is Quietly buried in the soil, immovable,
not taking any food :• or, in another condition, it is
an aq-uatic worm, swimming freely in Ihe water.

Under such circumstances, unless there ia an op-
portunity to trace all these successive changes, you
see how mistakes, as gross as the one to which I
have alluded, may be made. Naturalists are now
aware of the possibility of such mistakes, and do-
not consider an investigation as perfect, as long a»
the direct connection between the facts in any giv-
en case has not been ascertained by continuous*
observations. Articulata undergoing such exten-
sive changes, must, therefore, be studied in many

LECTURES ON EMBRYOLOGY.

more points of view, under more manifold aspects,
than any other animals. And we have here to in-
vestigate external changes, as well as internal mod-
ifications of structures ; changes of habits, as well as
changes of forms; indeed all the successive trans-
formations through which these animals gradually
pass from their formation in the egg to their perfect
condition.

The embryology of Insects proper has not been
so extensively and so fully studied as the embryol-
ogy of other classes. There is generally a great
difficulty in examining the eggs of insects, owing
to the opaque condition of the yolk-substance, the
softness and transparency of the primitive germ,
and the thickness of the horny envelope which
surrounds the e^rg. You see under what difficult
circumstances the observer is placed, to have to
break up this hard crust without injuring the soft
and delicate germ— which isi, besides, exceedingly
small, — and then to distinguish the various forms of
their transparent body, resting upon a dark,opaque
centre ; — circumstances the most difficult for micro-
scopic investigation which can be found. And so
we have only a few species whose embryonic
growth has been satisfactorily examined.

Professor Kolliker of Zurich, has made those
investigations, and I introduce here, (Plate VII.)
the diagrams which he has published of one of
those series, in order to show how peculiar the
mode of growth of insects is, and how different it
is from the changes which other animals undergo
within the egg.

After tracing those changes which take place
within the egg, I shall proceed to allude to the
changes which the Worm undergoes to form a per-
fect Insect. The egg itself consists universally
among all insects, of a yolk of opaque substance,
enclosed in a hard envelope. When the eggs are
laid, there is no germinative vesicle, no germina-
tive dot, seen withia. The eggs have really un-
dergone extensive changes before they are laid,
and when laid, the envelope which surrounds
them is already thick and opaque. In order to as-
certain whether the egg has primitively the same
structure as that of other animals throughout the
animal kingdom, it is necessary to trace the for-
mation of their substance back to the ovary, and
examine the young egg, when the germinative
vesicle, with the germinative dot, surrounded by
a transparent mass of yolk, enclosed in a mem-
brane, will be observed, as in all animals ; afad it is
only shortly before the egg is laid that a thicker
envelope is formed by the addition of layers of
more consistent matter, which are successively de-
posited in the oviduct around the yolk membrane,
to protect more effectually the eggs, which in so
many insects have to pass the winter in that con-
dition, before the caterpillar or worm is hatched.
However, in the investigation of the formation of
the egg and its envelopes, there remains much to
be done in the class of Insects.

It is a peculiarity with the eggs of insects that

they remain a long time after they are laid, before
undergoing their regular transformation ; at least,
this is the general impression. That, however,
regular transformations begin in the winter, and
go on during the cold season in this well-protected
cuirass, has recently been ascertained by a gen-
tleman of this city, Mr. Waldo I. Burnett, who
is at present in vestigating successfully this diffi-
cult subject; so that the changes taking place in
the eggs of various insects are likely to be soon
supplied.
[PLATE X.— INSECTS WITH THEIR LARV^S AND

The form of the eggs of Insects is exceedingly
variable. There are eirgs, for instance, which are
attached to a long stem, (Plate X, fig. B) from
which they hang down. That stem, however, be-
longs not to the egg proper, but is only a part of
its external covering. The layers of protecting
substance around the egs, are extended beyond the
growth of the egg itself; and through these stems
the eggs are attached to leaves of trees, resem-
bling little fungi or cryptogamic plants, for which
they have been sometimes mistaken. The first
thing which takes place in the egg after the ger-
minative dot and germinative vesicle are gone —
after the yolk has become opaque, is the forma-
tion of a transparent layer of substance all around
the yolk, as seen in Plate VII, fig. A, which repre-
sents the young animal, or germ, in its earliest con-
dition. As soon as this animal coating has grown

PROF. AGASSIZ'S

sufficiently thick to assume definite outlines, a
broad open space is noticed on one side of the
germ, through which the yolk is very extensively
seen From further changes, it will be ascertained
that the continuous mass represents the ventral
portion of the animal, and that the free opening is
on the dorsal side of the germ. At this earliest
stage^ome few changes oi substance have already
taken place. The animal layer, when first formed
and examined under the microscope, is seen to
consist of small cells, which have little dots with-
in. At first, there is only one layer of such cells ;
then, a second layer is formed, probably derived from
the substance of the yolk itself. Then there are
three or four such layers, the cells being probably
multiplied and increased in number by the burst-
ing of the primitive cells, and by the growing into
cells of their minute inner dots.

This seems the more probable, as with the in-
crease of layers,the cells becoming more numerous,
are also found to be smaller ; so that, when there
are four or five such layers, the cells are so minute
as to require a higher power of the microscope to
examine them ; showing that these cells increase
by evolution from the primitive ones. The ap-
pearance of a thick animal layer around the yolk,
as the first indication of the srerm, with a large
open space opposite the main bulk of the embryo,
is a peculiar feature of the mode of formation of in-
sects, by which they differ widely from other ani-
mals. Here, (Plate VII, fig. B) the opening is to-
wards one end of the egg, at which end we also no-
tice upon one side of the germ, the first indication
of a transverse division, marking out the head. —
Next, (Plate VII, fig. C,) there will be some con-
tractions taking place upon the longitudinal axis of
the body, dividing the germ into several joints.
The first change which takes place in the germ of
an articulated animal is, therefore, an indication of
the type to which it belongs. It is really an artic-
ulated animal before any further indications of a
structure are introduced. The first division which
takes place goes to indicate the position of the
head. At this period, (Plate VII. fig. B), the yolk
mass is already reduced to a smaller space. Next
the transverse divisions appear, those of the head
growing more complicated as represented in Plate
VII, fiirs. C, G, H. And then, there is a well defined
outline formed below the yolk, (Fig. D) extending
to the anterior divisions of the germ, and towards
its upper side, going to form the alimentary canal.
The mass of the yolk is still more reduced, the
membrane which now encloses it from below hav-
ing folded itself upwards, so as to assume the shape
of a little boat, (Fig. E ) and parcels of yolk re-
maining scattered on the sides. At this period we
can already observe that the folds on the outside
of the body will be transformed into joints. There
is a head at the upper end of the germ, and at its
lower side there are indications of legs (Fig. E).
A wonderful arrangement is now plain, which
was first discovered among Articulata by Herold,

in Spiders, and afterwards confirmed by Rathke in
Crawfishes, namely, that in articulated animals the
folding of the germ takes place in such a manner
as to have the navel upon the back, that is to say,
the opening by which the mass of yolk communi-
cates with the alimentary cavity has a position
strictly opposite to what is observed in other ani-
mals. The germ, indeed, folds itself around the
yolk, leaving a broad opening on that side of the
animal which, in its final structure, will be the
back. (PlateVII,Fig. D.) The side opposite the na-
vel being the one from whence the feet come out,
and that where the opening is observed, being the
sideifrom which the wings will be developed. The
membrane which was developed below the yolk
has now folded itself more extensively upward, and
forms an elongated open channel, which finally
grows into a closed tube, the alimentary canal, aa
it is seen in the animal more fully developed (Fig.
F), where there are some parts of the yolk remain-
ing in the joints. Before the yolk has entirely
disappeared, there is a pair of rudimentary feet
developed in the anterior part of the embryo,
which will disappear before this embryonic ani-
mal has the proper form of the larva to which it
gives rise. There are also at the posterior extrem-
ity indications of false feet forming, and all along
the various joints of the body, which have been
successively marked. These are, however, not feet
proper, but only stiff hairs.

From the facts stated above, it is plain that in
the class of Insects, after a complete investigation
of the growth of the egg of one species, (and indeed
of several species) it has been ascertained that the
germ is not developed above the yolk, but below,
as we have observed it in Kadiata There is not, as
in Radiata, a cavity formed below, extending with-
in the bodv to the stomach and the mouth ; but we
have in this case a germ which is forming below
the yolk. Of course, such an egg could be re-
versed, and it might be said that there is no differ-
ence between the germ of Radiata and Insects — that
we may just as well turn the egg of Insects so as
to have the germ in the same apparent position in
both cases. But if we turn in such a manner an
egg of an insect, with its germ, we shall find the
feet growing out of the upper side, and we shall
find the opposite, or lower side, giving rise to a
pair of wings. This would only show a re-
versed position of the whole; as we may place
the fee^ of an Insect upwards, and the wings
downwards, and have only an inverted Insect.
But by thus changing the external position
of the animal, the legs remaining opposite the
wings, whether the navel be primitively open
between the wings or above the animal, or vice
versa, we shall not change the relation of ita
parts, in their growth. And so you see, that the
articulated animals grow in a position the reverse
to that of the Radiata, and undergo successive
changes, which at a very early period give rise to
those moveable joints which characterize Articu-

LECTURES ON EMBRYOLOGY.

lata in general, and are seen in the lowest forms, as
well as in the Lobsters (Plate VI), or Scorpions
(Plate VIII), or any of -the insects.

That this mode of growth is not peculiar to in-
sects alone, but is characteristic of Articulata at
large, follows, from the beautiful investigations of
the embryonic growth of Crustacea and Spiders,
which have been traced by many Naturalists, but
above all by Herold, Pvathke, Ercil, &c.

That the same mode of growth is also observed
in Crustacea and Spiders, can be satisfactorily as-
certained by a glance at plate III, where in a
Shrimp the germ is seen developing below the
yolk.

[PLATE ITI— YOUNG SHRIMPS.]

Tne details of tueae metamorphoses I s>hall illus-
trate thereafter. I mention it now, only in or-
der to add, that this mode of growth is not pecu
liar to insects alone, but that it is characteristic of
most Articulata to have this inverted mode of
growth from their earliest embryonic condition. —
They grow, as it were, in opposition to all other
animals. And it is a fact in no small degree re-
markable, that among such animals there should
be such a number of Parasites. Articulata are,
however, the only type in the animal kingdom" in

h.ch parasitism is the prevailing rule, though
there are other Parasites which belong to other
classes.

The metamorphoses of Insects which take
place after the little Larva (as Entomologists call
the earlier condition of the animal) is born, have
been extensively studied. This* little Worm (Plate
VII, fig. F) is like the primitive form of the com-
mon Mosquito, of which we see (Plate IX, figs. B>
B, C) all the different changes which the animal
undergoes before it is changed into its perfect
state. Figs. D, E, F represent the same successive
changes from the Horsefly (CEstrus) ; figs. G, H, I,
those of the common Flea ; figs. J, K, L, M, those
of the Cochineal. In plate X, the figs. A, B. C
represent the egg, larva and perfect Hemerobius ;
figs. D, E. F the metamorphoses of a Moth, of the
genus Geometra ; figs. G, H, I those of Phryganea,
and figs. J, K. L those of an Ephemera; plate XI
represents Beetles; figs. A, B, C the metamorpho-
ses of a Dermestes, whose larva is hairy and col-
ored, like that of a Butterfly ; and figs. D, E, F,
that of a Cetonia, in which the larva is a Maggot.
The Naturalists of the last century have studied,

more carefully and more extensively the metamor-
phoses of insects than the Entomologists of the
present day. It is to works long since almost for-
gotten among entomologists, that we must resort
to find extensive, minute, and correct information
upon the metamorphoses of Insects in their vari-
ous stages of growth, bwammerdam, in his Bible of
Nature, full of interesting details, has given a great
variety of metamorphoses. So have the investiga-
tions by Degeer, Geoffroy, and Rosel, done more
in this department than all modern investigatiors
put together.

[PLATE IX — METAMORPHOSES or THE MOSQUI-
TO, HORSEFLY, FLEA AND COCHINEAL.

The title of Rosel's work, which he styles
"Amusements with Insects "(Instktfn Belustigunyen)
shows how much he must have enjoyed his re-
searches. He has, perhaps, illustrated the meta-
morphoses of insects more fully than they have
been examined before or since. In our modern
times, Entomologists have devoted almost all their
attention to the study of genera and species, of the
external forms of families and specific distinctions,
and have in this way, endowed Entomology with
treasures of detail, but have made very few refer-
ences to the study of metamorphoses, which would
however, render this minute knowledge of details
much more valuable; for if the changes which take
place in various families were brought under rules,
these details would at once be made useful in the
comparison of extensive series. But, for the pres-
ent, we have only to hope for a general comparison
between the modifications of parts as they occur

PROF. AGASSIZ S

in the larva state, with those of perfect insects. I
would, however, except from this criticism some
few modern authors, who have followed the glori-
ous tracks of the great Entomologists of the past
century. Eminent among such exceptional works
containing more than descriptive details, stands
the remarkable report of Dr. Harris upon the In-
sects of Massachusetts injurious to Vegetation, in
which the author has given most valuable inform-
ation upon the metamorphoses of insects living in
this State. Also, Professor Audouir has given
many beautifully illustrated facts about the insects
injurious to grape vines. Ratzenburg has made
similar investigations on insects injurious to the
forest trees in Germany. To these works we shall
have constantly to refer when studying the meta-
morphoses of articulated animals.

The larvae differ from each other, not only in
form but also in structure, and in the successive
changes which they undergo. There are larvae
which arise from the egg almost under the same
form as the perfect insect, and in their metamor-
phoses undergo only slight changes of form ; per-
haps changing the length of their legs, or modify-
ing the apparent number of rings which they had
when coming out of the egg. There are others
which are born widely different from the perfect
insect, which will remain in that form for a certain
time, and then change into an Animal entirely dif-
ferent in its outline — to remain in that condition
again for a longer or a shorter period, and then to
undergo the last transformation. Insects which
undergo such complete changes in form, are called
insects with perfect metamorphoses. Those into
which changes are introduced gradually, and in
which the differences in various periods of life are
not so great, are called insects with imperfect met-
amorphoses, or half metamorphoses. We have
insects in which the young are born under nearly
the same form as the perfect insect. I would men-
tion the Grasshoppers, for instance, in which the
young have the same forms except the wings,
which are wanting. The greatest differences are
noticed among Butterflies (Plate II, fig, C), where

[PLATE II— CATERPILLAR PUPA & BUTTERFLY.]
A

the Caterpillar is first seen (Fig. A), next the Pupa
(Fig. B),and lastly the perfect animal (Fig. C); also
in the Beetles (Plate XI, Fig. D). where the form

represented by figure E, is first seen; next
the Pupa (Fig. F), and then the perfect condition
(fig. D). Fig. A. represents another Beetle in
which the larva (Fig. B) is similar to the Caterpil-
lars. In most insects, the larvae, when colorless, are
called Maggots, or Worms. In the Ephemera (Plate
X, fig. L), we have the same form of the body as
is seen in the perfect insect ; but on the sides of
the larva there are aquatic respiratory organs, gill?,
(Fig, L,) which do no longer exist in the perfect
insect (Fig. J). Such cases indicate the extensive
differences of structure which may exist among
larvae of the same class.
[PLATE XI— BEETLES WITH THEIR LARVJE AND

Some (Plate X; nave aquatic Dreaming organs,
and others aerial ones— a difference which in oth-
er departments of the animal kingdom is consider-
ed sufficient to divide some of them into different
classes. Fishes and Reptiles are not left in the
same classes, because the respiration of the one
takes place by gills, and in the others, by lungs. —
You will notice in this figure, (Plate X, fig, L) and
in Plate XI, fig A, considerable differences : In the
one there are gills, and in the other lung-like or-
gans for the same function.

In others we see still different combinations. In
the Phryganea, for instance, (Plate X, fig. H) there
are legs only upon the anterior rings, and there aro
stiff hairs upon the other rings; whilst in the
Caterpillar (Plate II, fig. A) there are legs upon
the anterior part of the body; others on the mid-
dle joints ; and still others, behind. The larva of the
Horse-fly (Plate IX, fig F) has no legs at all, only
stiff hairs. In the Mosquito (Plate IX, fig. C) the
larva is aquatic, provided with gilis. The pupa
(Fig. B) assumes another form, but remains aquat-
ic, and finally, the animal appears with legs in a
very different form (Fig. A) and with a pair of long
wings and various appendages in addition.

Now, it is important— I insist upon this point —
not only to trace the changes which the larvse un-

LECTURES ON EMBRYOLOGY.

dergo in their metamorphoses, but also to investi-
gate the changes in their structure, which are
brought about during their metamorphoses ; and
happily we have upon these points most admira-
ble investigations by Dr. Herold, though upon
only one species, the white Butterfly which feeds
apon the cabbage. It is remarkable, however,
how few investigations have been made upon these
animals at large, when we take all points of view
into consideration ; and we find ourselves reduced,
for illustration to one single well studied exam-
ple. Prof. Kerold in his admirable work begins,
unfortunately the investigation only with the full
grown Caterpillar, which he goes on comparing
with the pupa, and then with the perfect insect.

Now with reference to these differences between
the larvae— before I allude to peculiar differences
of structure— let me make another general remark.
There are groups of insects in which considerable
•differences occur among the larvae even in their
structure, wben the perfect insects constitute nat-
ural families, and are identical in structure. Again,
there are others, the Butterflies for instance, in
which thelarvas agree as perfectly as the full grown
insects, having alia distinct head (Plate II, fig. A),
with powerful jaws, and a slight indication of eyes.
Then, we find upon the three anterior rings there
are three pairs of legs provided with horny claws,
next two rings without legs at all, then, rings with
feet of an entire different structure, resembling
suckers, then two rings without legs, and a pair of
legs upon the last ring. And this arrangement of
parts is uniform through all Butterflies. It occurs
in the Diurnal as well as in the Sphinx and Noc-
turnal Moth. The larva of Butterfly is never an
aquatic animal, but is always an air-breathing crea-
ture,but there are many aerial insects whose larvae
are entirely aquatic.

Another difference is, that these insects in their
lower condition have powerful jaws, by which they
chew their food, moving their jaws from right to
left and from left to right, on the two sides ; while
the perfect animal is very different in having no
longer jaws to chew the food, but suckers to take
food from the nectar of flowers. And the change
in the mode of living is so great, that the Caterpil-
ler will consume ten times his own weight of food
in a given time, while the perfect animal will not
consume more than one tenth of his weight during
all the remainder of his life, as a perfect insect

This fact has great importance in connection
with one question about which Naturalists have
had much discussion, viz: whether the insects
which chew their food should be considered as
higher than those which suck their food by suck-
ers. The Insects provided with powerful jaws—
the Beetles, the Wasps, the Bumblebees, Dragon-
flies—all these insects, which have powerful jaws,
are generally considered higher in their structure,
because so many of them are carnivorous, and
' stand in our systems as at the head of insects ;
whilst the sucking insects are placed in a lower

range. That the former are placed higher, arises
from no other reason, I think, than the fact that
there are so many of them which live upon ani-
mal food, or which are properly carnivorous : and
as we are accustomed from our intimate acquaint-
ance with mammifera to consider Carnivora higher
than Herbivora, we are naturally misled to con-
sider all carnivorous animals, for thj simple reas-
on, that they are carnivorous, as higher than the
herbivorous ones. But such impressions can have
no value in the estimation of the characters of an-
imals of another department. The larvae of many
sucking insects have equally powerful jaws as the
carnivorous, which are made into another appara-
tus of an entirely different structure, introduced in
the last transformation of the insect.

(PLATE V— ARTICULATA— TRILOBITE.]

My impression is, therefore, that oc this account
we should rather incline for an inverse view of the
subject, and an inverse arrangement of the insects,
and consider the sucking insects as higher than
the chewing Insects. And I would place the But-
terflies highest, for the reason that they undergo
such extensive metamorphoses — passing through
so many changes in which the structure grows
successively more perfect. That they should be
placed highest amongst the sucking insects will be
obvious, when we consider that they are aerial
worms from the beginning — while other insects,
with the sucking apparatus, as Flies and Mosqui-
toes, constitute a family in which there are many
aquatic worms, and we know from other depart-
ments, that aquatic animals provided with gill-
like apparatus are universally lower in structure
than those which breathe air. But such an uni-
formity in larvee as we have among Caterpillars
is not noticed in other insects. You can of
course compare the larva of Dermestes (Plate XI.,
fig B ) with a Caterpillar, (Plate II., rtir. A.) But,
of the external appearances, the appendages of the
skin agree ; the arrangement of the feet will be
found different.

The aquatic insects have their larvae still more
different, being provided with gills, so that the ex-
ternal form in its earlier condition, is far from uni-
form in the families which reckon aquatic types.
Among the hymenopterous insects, Bees, Wasps,
&c«, we have some in which the larvae assume the
form of Maggots and Worms, and others in which
the larvae assume the form of the higher insects.
For instance, in Tenthredo, the larva assumes the
form of a Caterpillar. (Plate II., fig. A) But in-
stead of having only four pairs of suctorial feet,
they have seven. And this is at once an indica-

PROF. AGASSIS S

tton that they do cot belong to the family of Lep-
idoptera.

I see the time will not allow me to go through
the whole o? this extensive subject -T so that I shall
call your attention again in my next lecture to the
transformation of structure which takes place in
these animals. Let me only make one remark
more with reference to the relative position of the
various families of the numerous order of insects,
and to the relative value of their distinguishing
character. Why should we be led to arrange the
insects and articulated animals in a natural order,
by other considerations than those derived from
their own mode of growth ? For, if we find that in
insects the earliest period of life is that of the car-
nivorous animals, let that be the lower condition
for articulated animals. And if we see that they
successively undergo changes, in which, growing
to our eyes to more perfect animals, they finally

assume the structure of seeking insects, than let
us consider the condition of sucking insects the
higher condition. And let us no longer transfer
our impressions from one department into the other.
The same difficulties occur really in all other
classes. Because the Carnivora among Mammalia,,
come so near to the Monkey, and thus approach to
the affinity which raises the Monkey next in rank
to man, it is no legitimate consequence, that the
Birds of Prey should be the highest. Nor does it
prove that the carnivorous fishes should rank high-
er than the others : and still less, does it follow, that
the chewing insect should take the highest rank,
especially when we see that the ehewiag condition
is the lowest embryonic condition of their life*
And tet us, in future, arrange insects according to
the rale of insects, and not according to tie struc-
ture of other animals.

LECTURE VII.

Before entering upon the proper subject of this
evening's lecture, I have to mention a few facts
which I have ascertained upon the growth of some
Polypi (or rather Medusae, if Tubularise have to be
considered as Medusae) which I consider so highly
valuable as to deserve really to call our attention
for a few moments. I have received from Mr.
Hawks, of the Navy Yard in Charlestown, a bunch
of Polypi, taken from the bottom of a ship which
has been lying for three months and a half in the
harbor. When she was launched, on the 14th of
September last, she had, of course, none upon her.
She had been lying in the water from the 14th of
September to the 28th of December, when she was
taken into the dry dock. During this time, the
bottom of this ship has been covered with the most
astonishing, the most luxuriant growth of Polypi
which can be imagined. Thousands and thousands
of Polypi stems, as long as five, or six, or seven
inches, forming the most beautiful flower garden,
upon the bottom of the vessel. And not only have
all these Polypi grown to this size, but they have
branches, and these branches— these secondary
branches— have given rise to branches of a third
order, in this short time. Now, the question is, how
can these innumerable stems have grown upon
this vessel ? They could not have been attached
to it accidentally, as Tubularise, in their ordinary
growth, are always attached, and when freed, fall
to the bottom, without having the means to move

about. The uniformity of their growth, shows thai
they have grown upon the vessel from a uniform
starting point, not from a certain number of
stems which had accidentally become attached to
the vessel ; all of which must be supposed in the
same condition, in the same state of growth, when
they became attached, and that they have grown
upon this vessel naturally, uniformly, up to the
present day, or rather up to last week. But to be
attached there, in such a manner, not accidental-
ly, thev must have been free ; and it is just a point
to which I alluded in a former lecture, whether
Tubularise had or not, a free generation, alter-
nating with their fixed growth. A free generation
among them is not known ; yet I inferred from
some data, that the affinity of Tubularise with Me-
dusse was very close, and I ventured even to predict
that some one of the small free Medusae of Boston
harbor might be their free form— that a free gen-
eration might be found.

Now, the circumstances above stated, show that
there must be a free generation of Tubularise,which,
by the 14th of September, or some time later, were
swimming in Boston harbor in countless num-
bers, and attached themselves to the keel of that
vessel, and grew there to form these innumerable
stems. Whether this growth is immediately de-
rived from the germs, which are produced in the
bunches, which are known to exist in Tubularise,
or whether it is only another generation, derived

LECTURES ON EMBRYOLOGY.

ta&fc free one, is still a point which only di-
rect investigation can ascertain. I incline to sup-
pose, that the Medusa-like germs which are devel-
oped from the bunches of eggs, hanging below the
outer tentacles, are the intermediate, free genera-
tion which grows to lay moveable eggs, similar to
those of Campanularia^ and that these eggs, and
not the soft free buds, grow into Tubularise. How-
over, so much, at least, can already be inferred
with precision: that Tubulari« must have some
free generation, — a generation which is about to
attach itself in tke latter part of September, and to
produce a luxuriant growth of common branching
Tubularise.

Now, how rapid this growth must have been, and
how rapidly the branches must have succeeded, an
illustration of the details will show. Each single,
isolated stem, from five to seven inches long, ter-
minates with a crown, having its tentacles and
bunches of eggs, like the most perfect Tubularia I
have seen. The terminating Polyp has bunches of
•eggs, and all these eggs have already their yolk,
with its envelope— their germinative vesicle, with
its geriainative dot. The lateral branches, per-
Qaps five or six, in \ arious stems, growiag from
different parts of the stem {but the lower always,
in every case, being longer than the upper ones J
were terminated also with regular crowns ^ but
Chose smaller and simpler individuals, the number
of their tentacles being fewer, were found to be
without any eggs. They had not grown to the
formation, to the development of organs of repro-
duction. The tertiary branches, sometimes as
many as five or six upon one of tke secondary,
were found to terminate also with a small Polypi
but like the secondary, to be without eggs. Hun-
dreds of these branches, compared together, show-
ed no difference. They were so alike as to indi-
cate, distinctly, that they were the growth of one
epoch ;— that they had been attached to the vessel
<at one time, and had grown under identical cir-
cumstances. That stems already formed, could
not be attached to tke vessel, is shown by the cir-
cumstance that the loose branches sink to the bot-
tom, and have no means of transportation from
one place to another, Thus, the being which was
•fixed, must have been a free animal. You remem-
ber, perhaps, what I have said in a former lecture
upon the embryonic growth of Tubularise. I
showed the formation in the bunches of eggs of
little Medusa-like beings, with four or more arms,
—four prominent ones, and others alternating with
shem, less developed, which became free, bat
whose £nal development had not been observed,
I now suspect that these Medusa-like buds would
grow into Medusa-like animals, and that these
Medusa-like animals would lay eggs, and that
these eggs, like those of Campanula^, being first
free, would then become attached — grow to a disk-
like surface, rise from the centre to a stem-like
growth, and then pass through the same meta-
morphose? whicb have been observed ia tae Cam-

panularias. At all events, here is one fact it
history of this animal ascertained, which wa
known before — the fact of its rapid growth, o. J1*
rapid branching, and of the existence of a free
generation, though not ascertained by investiga-
tion, so strongly indicated by circumstantial evi-
dence as to be almost a positive fact, in the opin-
ion of one who has been accustomed to compare
these phenomena and to refer them to a common
type.

In my last lecture, the first upon articulated an-
imals, I began by illustrating, in a general man-
ner, the character of the great and numerous type
of Articulata^ how they are subdivided into three
classes — the Worms, Insects and Crustacea — or
in the order which I would prefer, Worms, Crus-
tacea and Insects ; then further, I alluded to the pe-
culiar characteristics of insects, to their extensive
metamorphoses ; and then more fully illustrated
the embryonic growth of these animals, as as-
certained by the investigations of Professor Kolli-
ker; and finally investigated the different meta-
morphoses in different families of Insects,

We now proceed to the investigation of the
changes of structure which these insects undergo
during their metamorphoses. We have examined
the general changes of form which these animals
undergo in various families. We have now to ex-
am iae the changes in the internal structure, which
take place in the larvse of Insects, till they acquire
their perfect development. And in tracing these
changes, we shall acquire an invaluable key to ap-
preciate the relative value of the differences which
exist between all insects — between articulated ani-
mals at large.

If it is true that Insects are the feighest among
articulated animals, even if they should occupy a
second rank, a thorough acquaintance with all the
changes of structure which they undergo during
growth, must give us a key to appreciate the real
value of these differences, their relative order of
succession in a scale — in a gradation of structural
differences,

The value of these comparisons must be so ob-
vious, that I need not apologise for dwelling more
extensively upon these topics than I would other-
wise, i repeat it— that the facts which we are
now about to examine will famish (if there is one)
the key for estimating the value of characters in
one of the greatest types of the animal kingdom.

In Plate XII are diagrams representing the ner-
vous system of a White Butterfly. (which is exceed-
ingly common in Europe) living upon cabbage,
in its various stages of growth, as figured by
Herold,.in his remarkable work upon the metamor-
phoses of that animal. In Plate XIII are diagrams
representing the changes which take place in the
digestive apparatus of the same animal ; and here
<in Plate XIV, figs. A and B) are represented lon-
gitudinal sections of a Moth <Fig. A) and its Cater-
pillar form (Fig. B) from Prof. Newport's research-
es, to show the different systems of organs in their

PROF. &GAS81Z8

relative position within, and also the changes
which thev undergo during their growth, as well
as in their proportional development. To these
diagrams I shall mostly refer daring this illustra-
tion. But in such a comparison of structural dif-
ferences, the external arrangement of parts is as
mportant as I e internal differences,

We have examined the forms of the various sta-
ges of growth in Insects. We have not examined
the differences in the arrangement of the external
parts. Let us begin the comparison with these.

[See Plate IX, Lecture 6 J

In the various Caterpillars or Maggots— in the
various larvae of Insects which you see figured in
Plates IX, X and XI, and Plate II of the first lec-
ture, there is one form which is characteristic
in all — which occurs universally in all. It is
the greater uniformity of rings when compared
to each other. The rings of the anterior part
of the body, (Plate X, fig. H, or Plate XI, fig. B)

>rSee Plate X. Lecture 6.]

though here provided with legs, resemble the
rings in the middle portion of the body ; however,
they resemble these more closely than the anterior
rings resemble the posterior ones ; but as a whole,
considered in its general arrangement, the various
rings of the larvae are more uniform than in the
perfect insect, which arises from them ; and they
are naturally more uniform, but they are not
grouped together in any particular way. There
are no differences in the rings, indicating more
circumscribed parts of the body. Scarcely is the
head more defined from, the other rings by its co-
lor. But, between the so-called chest of Insects
and the abdominal region, there is no separation
(see Plates IX, X and XI) as we notice it in the
perfect insect.

There is always in the perfect insect, between
the head and the chest, and the posterior part of
the body, a strong division, as we see in these fig-
ures. (Plate XI, figs. D and A) where the head is
more distinct; a certain number of rings consti-
tute another region behind the head, the so-called
thorax, or chest ; and behind this, there is a third
one — the abdomen. Xow, such a division of rings
into distinct divisions— into a head, thorax and
abdomen— is not yet introduced into the condition
of the larva, though it is indicated by the appen-
dages; though not universally, but very general-
ly, there are among the anterior rings some which
have appendages more developed than the others,
which will correspond with the rings which form
the chest, and then the other rings behind will
correspond with the rings which form or constitute
the abdomen.

But now, compare the proportional size between
those rings in a perfect insect— Grasshopper for
instance as in Plate XV.

Here (Fig. A) is the head. This middle region,
here separated into its constituent rings, (Figs. B,
C, D) will correspond with the ehest; and here,

[PLATE XV—

posteriorly, a portion ot tue body, scarcely larger
than the head and thorax together, though com-
posed of twice as many rings, corresponding to the
abdomen. In the imperfect larva (Plate XI, fig.
E) we have precisely the reversed proportions in
the size of the rings of the different regions of the
body, or what will finally constitute these differ-
ent regions. The posterior rings in this case are
reduced considerably in the perfect condition, but
the rings giving rise to the thorax are enlarged,
and closely united in fewer joints, so that there is
a real reduction of rings, and a real reduction of
the moveable parts, inasmuch as the three rings of
the chest, which in the earlier stages are equally
moveable upon each other, now are united togeth-
er, and form only one mass. The reduction, there-
fore, of the number of rings or their closer com-
bination, or the reduction in size of the posterior
ones, with a proportional increase of the anterior
ones, when they acquire a higher development,
are stages of growth which indicate a progress — a
really progressive development.

From these first superficial investigations, we
learn one important fact in Entomology— that elon-
gated species, in any given type, consisting of well
divided, uniformly moveable rings, must be con"
sidered as lower than those in which the rings
combine or unite together, and divide into distinct
regions. So that the Caterpillars give us the first
hint towards a classification, namely, that Insects,
or Articulata at large, stand higher or lower, inas-
much as the rings are more or less numerous or
reduced, uniformly moveable or combined, uncon-
nected, or united into distinct regions.
Plate IV, Lecture 6.1

LECTURES ON EMBRYOLOGY.

And if we test with this first result the proposed
modifications in the general classification of Ar-
ticulata, we will find that on this ground Worms
(Plate IV) will stand lowest, Crustacea (Plate VI)
come next, and Insects highest.

[See Plate VI, Lecture 6 ]

Let us now examine the changes which take
place in the nervous system of the Caterpillar
when full grown, (the changes during the growth
of the Caterpillar itself have not yet been investi-
gated) till it is transformed into a perfect Butter-
fly. We have at first, a nervous system, consisting
of a series of equally developed and almost equally
distinct swellings (Plate XII, fig. A)— in the head
two large ones ; next, one small oce ; at about an
equal distance, a second ; a third, nearly equally
distant: a fourth, somewhat more distant; a fifth,
tixth, seventh, eighth, ninth, tenth, eleventh, al-
most uniformly equally distant; and then a twelfth,
which is nearer the eleventh, making, with the
head, thirteen. Now, precisely the same number
of nervous swellings which we observe, consti-
tute the number of rings existing in the Caterpil-
lar.

Uniformly throughout the family of Lepidoptera,
that is to say, among Butterflies and Moths, the
body consists of thirteen successive rings : and in
the lowest condition of these animals — in their
caterpillar state — the nervous system has as many
nervous swellings,— one for each ring, almost
equally distant from each other, and sending off
threads to the parts around in each ring. The
general structure and position of the nervous sys-
tem is as follows :— The swellings are throughout
united by double threads, which towards the poste-
rior part of the body come so near together as to
seem a continuous, thick cord; but properly speak-
ing, they consist uniformly of double threads. And
in the position of these threads, there are some im-
portant points. The anterior ones are above the
alimentary canal; the others are below; so that
the thread which unites the anterior ones with the
second,constitute a sort of collar around the alimen-
tary tube (PI XIV). Bat all the swellings are united
by double threads, even where the threads come
near together and seem to be one continuous cord.
I insist upon this point, because it shows the uni-
formity of structure of the nervous system in all
articulated animals, and illustrates it, even in the
structure of the nervous system which has recent-
ly been discovered in Intestinal Worms. When
discovered, it was supposed that Intestinal Worms
had a nervous system so different from Articulata
as not to belong to that group. The nervous sys-
tem in Worms forms a sort of collar, with swellings
around the anterior part of the alimentary canal,
from which arise a double row of swellings, con-
nected by simple threads, extending backwards.—
This arrangement is indeed not very different from
that of the higher Articulata : let only swellings,
with their double threads, be disconnected, and
we have the arrangement of Worms ; and let the

two chains of Worms be united in one, and we
have the arrangement of Insects.

As soon as the Caterpillar undergoes the first
change towards forming the Pupa — towards be-
coming immoveable, before it casts its skin for the
last time— we see (Plate XII, fig. B) that the third
and fourth swellings are brought nearer together ;
and also the first and second are brought nearer
together; the others remaining in the same relative
position and in the same proportional distances
apart.

But as soon as, for the last time, the Caterpillar
has lost its skin and assumed that peculiar form of
Pupa in which it is motionless, then the nervous
system in its longitudinal extension assumes this
winding form [Plate XII, fig. C.J It brings the
swellings nearer together, the first of them being
at this time entirely united with those at the head-
[PLATE XII — N"ET*VF.S OF BFTTFT!FT.TFS ]

In the following stage, (Fig. D ) the 2d; 3d, 4th and
5th swellings are brought nearer the head,whils
the 6th, and 7th, disappear entirely during the pu-
pa state, and with them disappear also the lateral
threads which arose from them in an earlier con-
dition. The second, third, fourth and fifth swellings
remain now for some time at the same distance,
but are gradually combined in one single and more
connected mass. The sixth and seventh, disap-
pear. The eighth, ninth, tenth and eleventh re-
main at equal distances. And if we compare this
condition with the perfect insect, we can see that
these few anterior swellings, though arising from
five distinct ganglia, will send the nerves to the
parts answering to the chest. A region liehind,
with the long medial thread without lateral nerves,
is the region where the separation between the
chest and abdomen will take place. Before the
Pupa passes into the state of the perfect insect,
the approach of the swellings number two, three,
four and five is still increased. So that there are
now only three regions of distribution of the ner-
vous centres : the head with one large mass; next,
the chest with separted, though approximated

PROF. AGASSIZ S

swellings; next, a great spacewithout lateral nerves;
and then, a space with swellings at equal distances,
corresponding to the abdomen. Remember now
the arrangement of rings and legs in the Caterpil-
lar and in the Grasshopper, [Plate XV], You will
see that the arrangement of the external parts
agrees with that of the nervous system. The
head consists of one undivided mass [Plate XV.
fig. A ] There are three pairs of horny claws in the
Caterpillars (Plates IX, X, and XI,) and three rings
to the chest in the Insects proper, (Plate XV, figs.
B, C, D) receiving nerves from the concentrated
swellings of the anterior part of the body. Then,
there is a region from which no nerves are deriv-
ed ; and a region from which four pair of sucker-
like legs are produced, answering to the region in
which these four swellings remain equally distant;
and then another region, of two rings without; and
another, last, with suctorial legs, which corres-
ponds to the large terminal nervous swelling.

It is a question which it is not possible to solve
now, and which it will be very difficult to solve, if
it can be solved at all, whether the larger terminal
swelling of nervous matter consisted originally of
one nervous mass; and whether the anterior ce-
phalic ganglion consisted also, primitively, of one
nervous mass. That it consists of two now, is
shown here, [Plate XI. fig. A] by the entire disap-
pearance of the first small ganglion. But there
may be other changes in the structure of the ner-
vous system, taking place previous to the full
growth of the Caterpillar. And this remains for
the present undecided. But, so much is shown as
to prove that the nervous system is equally dis-
tributed in the solid rings, and they will gradually
combine in such a manner as to present arrange-
ments answering to the changes which take place
in the external form. There is one mass more,
properly belonging to the head, another mass more
concentrated, belonging to the chest, and another
mass remaining stationary and belonging to the
abdomen.

We now can, with these facts, arrive at another
general conclusion, viz. : that wherever among ar-
ticulated animals, among Insects, we find the ner-
vous system constituted of equally distributed ner-
vous swellings, such animals are lower than those
in which several swellings unite together to form
few masses. Now, in this respect, what do we ob-
serve in the different classes compared together ?
I now no longer compare the same animal in
its different stages of growth, but different classes
of Articulata with each other. What do we observe
in comparing Insects with Worms,and Worms with
Crustacea 1 All worms have equal rings and very
numerous joints ; and joints which are never com-
bined so as to form regions distinct from each
other. There is never a distinct thorax or abdo-
men in any Worm. So that, from what we have
learned, we know that the lower position assigned,
for many and all sorts of good reasons, to worms,
is the - -T position which they must preserve ;

and where a nervous system has been observed
among them, it agrees with the condition of that
system in Caterpillars, rather than with that of the
later metamorphoses. The question remains be-
tween Crustacea and Insects. What is the condi-
tion of the nervous system in Crustacea1? The
nervous system occurs in various conditions there.
In the lower Crustacea,the swellings being scatter-
ed all along the body, one to each ring — a condi-
tion which we observe in the earlier stages of
growth in the Caterpillar. Next, we have other
Crustacea in which the nervous swellings contract
and combine together, nearer and nearer. But in
them, strange to say, there is only one point of
concentration. And then there are Crustacea, as
the Crabs, in which the nervous system is con-
tracted into one single, central mass. And the
question is, what shall we consider superior? — an
arrangement which gives rise to several distinct
centres, and corresponds to distinct regions of the
body, (as in Insects, Plate XII., fig. F. and Plate
XIV, fig. A) a head,with a central nervous swelling
of a peculiar kind ; a chest, with a nervous mass of
a peculiar kind, sending its thread to the legs of
that region ; and another posterior combination of
nervous swellings, corresponding to the other re-
gion, called abdomen, and sending nerves to its
part?

It seems to me that we cannot remain doubtful.
We cannot fully derive this conclusion from direct
investigations, as we have not, in any instance, a
case to settle it by direct comparison ; but we may
say,that in Crustacea we have concentrated unifor-
mity; while in Insects in their perfect condition,
we have concentrated diversity. And, if we are al-
lowed to compare the one with the other, I would
incline to the opinion that concentrated diversity,
with prevailing influences over peculiar functions
of the life of the different centres, is a condition of
structure which stands higher than concentrated
uniformity ,in which we have only one centre. We
have all the primitive diversity reduced to one
centre, which does not acquire any distinct influ-
ence upon different parts.

The alimentary canal undergoes corresponding
metamorphoses. Here is the straight tube (Plate
XIII, fig. A) of the digestive canal of a Caterpillar.
It is very wide in comparison to its length, and ca-
pable of digesting an immense mass of food, com-
paratively to the size of the animal. In its earlier
condition, it is provided with an apparatus which
disappears afterward. There are considerable sali-
vary glands in the anterior portion of the alimen-
tary canal, which disappear in the pupa state and
do not exist in the perfect insect. These figures
(Plate XIII) must impress you as very singular. —
No animal has more curious organs than this In-
sect. The liver, or hepatic glands, and the salivary
glands are massive organs in other animals. Here,
they are slender tubes, and form little winding
branches on the sides of the alimentary tube. In-
eed, all glandular organs in Insects have such a

LECTURES ON EMBRYOLOGY.

[PLATE XIII— ALIMENTARY CANAL OF BUTTER-
FLIES ]

general arrangement — they are all tubular, thread-
like, and very long.

The next glandular apparatus here (Plate XIII.
fig. A.,) is the gland seen on each side, behind the
salivary tubes, the silk glands, which are much
larger in the Caterpillar than in the perfect in-
sect. These silk glands still exist in the perfect
insect, but they are much larger in the Caterpillar
than in the Pupa, and again larger in the Pupa
than in the perfect insect. You are aware that the
Caterpillar draws its silk from its mouth, winds it
regularly around its body, to protect it during its
second stage of metamorphosis. The third gland-
ular apparatus, a kind of liver, consisting of three
pairs of hepatic tubes, emptying in the posterior
part of the wide tube of the Caterpillar, but about
its middle in the perfect insect. This condition
of the glands, which we find among all
the Insects, is far from the structure
of those massive glandular organs which
occur in other animals. The lower portion of the
alimentary canal is scarcely at all contracted, in
the Caterpillar, as you will observe in this figure
(Plate XIII., fig. A ) Before entering the papa
state (Piate XIII., fig. B ,) at a period when the
insect is more perfect, the cesophagus has become
narrower and longer; and the colon has also
become more elongated and narrower, and in the
pupa state you see how the digestive tubes appear.

(Plate XIII., figrue C.) The animal has now
ceased to take food, and the salivary glands dis-
appear entirely. (Plate XIII., fig. D ) Next, the
colon grows more slender, to be transformed into
a narrow cylindric tube. When the Pupa is ready
to be transformed (Plate XIII., fig. E ,) into a But-
terfly, there is a new pouch formed between the
cesophagus and stomach, a pouch which secretes
the honey. It is a sack, to produce the sweet fluids
which so many insects are capable of secreting, or
at least of preparing. This pouch (Plate XIII.,
fig. E ,) has grown to a somewhat large size, and
the posterior part of the alimentary canal has been
elongated very considerably, in proportion as the
middle part or the stomach proper has been re-
duced. And finally, in the Butterfly, it is fully
developed, but we see no longer any salivary
glands. (Plate XIII., fig. F.) The posterior part
of the alimentary canal is now long and slender,
and the hepatic duct of the liver nearly as large
and as complicated as in the beginning.

Here again, we see that in proportion as the ali-
mentary tube is a uniform tube — or in proportion
as there are cavities of different diameters devel-
oping along its longitudinal diameter— we have
another scale to determine the relative rank of an-
imals in which this organization is observed. —

[PLATE XIV.— LONGITUDINAL SECTION OF
SPHINX LIGUSTRI.]

This is, perhaps, better seen in another diagram of
a Moth, where we see the cesophagus passing
through the anterior nervous ring, and extending
in the perfect insect PI. XIV. (Fig. A) through the
chest, where the wings are cut off and the legs
also. The large thorax answers to that part of
the Caterpillar (Figure B,) where the horny
legs are seen, and the ganglionic portion of the
nervous system is seen all along below the alimen-
tary canal. And in the Caterpillar you see how
intimately and uniformly the nervous swellings
follow each other, (Plate XIV, fig. B) and how the
alimentary canal is a uniform tube, whilst in the
perfect insect, alimentary canal and nervous sys-
tem have undergone remarkable concentrations
(Fig. A).

Another apparatus is very simple among Insects.
It is one of those functions which is not so high-
ly developed as in other Articulata, but which,

PROF. AGASSIZ'S

nevertheless, exists. There is a circulation in In-
sects which is only more generally overlooked.—
The heart is a more elongated tube than in Crus-
tacea, but it exists in all insects. It exists more
developed in their larval condition, which shows
that having a large heart in articulated animals, is
not characteristic of a higher structure ; and how
a great bulk of blood can be concentrated upon
one point in Articulata, without assigning them a
character of great eminence, is distincly shown,
when we consider that in Worms, which undoubt-
edly stand below the other two classes, there are
as many as six, eight or more hearts, and in which
the bulk of the blood is proportionally much great-
er than in Crustacea or in Insects ; so that, the im-
portance ascribed to the circulation of Crustacea,
when this class was placed above Insects, I think
vanishes before the consideration of the value of
these characteristics, as noticed throughout the
metamorphoses of Insects.

A few words upon the subject of mastication
and upon the chewing orders, will further show
that Insects have to stand higher than other articu-
lated animals. The chewing apparatus in Insects
is a very complicated apparatus, so complicated
that it is scarcely possible to give a correct idea of
the arrangement of these parts, unless a person
has become familiar with the objects themselves.

I must, however, attempt to convey some idea
of this apparatus. On the two sides of the head
in those insects which are generally considered the
highest, there are two large moveable pieces, mov-
ing from right to left on the right side, and from
left to right on the left side, in opposite directions
horizontally. Ttuese parts are called mandibles. —
Below these, is another pair of similar organs,
moving also horizontally, which are often ser-
rated, and to which are frequently added articu-
lated appendages: these are called the maxillae. —
These constitute two pairs of strong forcep-like
jaws, very different, it seems, from any part in the
whole insect.

In the diagram here, Jaws of Insects (Plate XVI.
ngs. A, B) your see the whole apparatus, first from
a Beetle and a Grasshopper, (fig. C). Seen from
above (fig. A) there is u kind of lip in sight, cov-
ering the mandibles, and below, are the maxillse ;
and below (tig. B) there is another kind of lip,keep-
ing these in their respective positions. To the
lower lip are also frequently appended articulated
tentacles — the palpi. Fig. C represents the maxillae
of a Grasshopper seen in profile.

Now, each of these parts being taken asunder,
we will have a strong mandible above ; and some-
what below and inward, the maxillae ; and farther
below, we have the lower lip. So that, between two
horizontal continuous plates, called lips, there are
moving forceps, the upper, called mandibles, and
the lower maxillae. Then we have maxillary pal-
pi. And to the lower lip there is another pair of
palpi attached — the labial palpi.

This is the structure of the jaws in all chewing

[PLATE XVI— JAWS OF INSECTS.]

insects. The Caterpillars have also such maxillae
as the perfect chewing insects, though not so com-
plicated, to be sure, as in the most perfect Beetles,
but nevertheless constructed in the same way.with
a horny, powerful jaw, by which they chew the
large quantity of food which they devour. Now
this condition is changed in the Caterpillar during
the pupa condition, when we have no longer such
enlarged jaws; but a long sucker [Plate XVI fig. D]
consisting, however, of the same parts as in the
chewing insects, only those parts which were mov-
ing horizontally have become elongated, and with
their margin have united, and instead of now mov-
ing in that way, remain closed together, and form,
a tube, a real sucker, through which, by the assist-
ance of the tongue, they actually pump liquid food
into their stomachs. (The Professor here repre-
sented, by means of his fingers, the jaws of the
chewing insect, and the manner in which, by uni-
ting, they can be transformed into a sucker.)

Let the tube now be contractile and retractile,be-
tween the upper and lower lip, and you have pow-
erful jaws transformed into a narrow tube. It is
a transformation which takes place with the other
successive and progressive changes, so that we are
entitled to consider such changes as also a pro-
gress, if I am not mistaken ; and to consider the
condition of the insect in which he chews food, as
the lower one, as it is the condition of the Larva;
and the condition in which he sucks, to be the
higher condition of the insect. And therefore, in
principles derived from the study of Insects, and
not from the study of other animals, judging of
Insects by notions gained from that class, we shall
consider those which suck their food, in which the
jaws are elongated, those which pass through vari-
ous metamorphoses, higher than those in which
the jaws are placed horizontally— sharp cutting
jaws,which devour large quantities of food. But this

LECTURES ON EMBRYOLOGY.

condition of jaws, I say*, is of higher structure than
Chat which is observed in Crustacea; and affords
an additional evidence than Insects should stand
above Crustacea, To show this to be the case, let
me first answer a question. What are these jaws
in Insects ? By most difficult and extensive com-
parison, it has been ascertained that the iaws are
simply modified legs, and that there are all possi-
ble transitions to be observed in the various fami-
lies, between their ordinary legs and that peculiar
kind of moving appendages which perform, the
function of jaws, but which are so exceedingly dif-
ferent, owing to the great eminence in form to
which they arrive.

Now in Crustacea, the changes which take place
between the appendages functioning as legs, and
those functioning as jaws, are so slight as scarcely
"Jo present any difficulty in ascertaining their com-
mon nature ; the differences are much less plain in
Insects, with their different sorts of jaws. You
scarcely can find the combining thread, showing
that in Insects there is one, and an uniform modi-
fication of appendages in legs and jaws. But com-
pare, on the ether hand,various appendages of the
Crustacea, and it strikes tus at once that they are
the same thing, slightly modified.

Before I illustrate this point, let me remark, that
on looking at this diagram, there is scarcely any
one who would suspect that these figures represent
any thing more than the various claws which are
observed on the side of the lobster. And so it is ;
but nevertheless, some are :jaws, others claws, oth-
ers tins; the jaws being somewhat modified legs; so
that those parts are only a little diversified among
each other. We have something left of uniformity;
while we rise in Insects to the greatest possible di-
versity, and even a diversity which presents an
analogy with the character of concentration, ob-
served ia the various arrangements of their nerv-

ous system, compared with that of Crustacea, So
that here we have another, and perhaps one of the
best, indications, that Insects stand higher than
Crustacea, notwithstanding we have the anatomi-
cal evidence to the contrary, which has been relied
on. Now if upon these data we should attempt a
classification of the class of Insects, let me in a
very few words make it clear.

Insects have generally been divided into chew*
ing and sucking Insects, and then into other fami-
lies. Spiders have been separated as a class, and
also all Apterous Insects.

Now, the Millipedes rank lowest, as among In-
sects they represeat the form of Caterpillars or
Worms, Next the Spiders, in which the concen-
tration takes place, in which the head and thorax
are distinct from the abdomen, bat in which head
and thorax are not separate, as in other insects
indicating some analogy to Crustacea. Then, we
would have those in which head, chest and abdo-
men are separated, but among them, place those
in which there are chewing jaws lower ; and high-
est the sucking insects. And curious it is, among
those which chew their food, that we have the less
perfect metamorphoses and many which are
aquatic in their larval condition; also among
them the forms are less perfect, inasmuch as in
Neuropterous insects the parts of the thorax are
only partially united, and the number of joints
remain greater even in the perfect insect i while in
the sucking insect, the parts of the thorax unites in
one mass, distinct from the head. In the Butter-
fly, we have, the evidence in the earliest lar-
val condition, that the Worm is an aerial animal,
rising above the other insects. And with these
data I think I have shown that I am not wrong in
considering the Insects as highest, if we judg«
upon entomological grounds and not upon other
evidence.

LECTURE VIII,

i hoped to introduce another subject, which is
connected with the history and metamorphoses of
Insects ; but my time is so short that I scarcely
dare to mention it, only as connected with these
investigations. I mean, the singular peculiarities
of many insects who live in large communitiesscon-
sisting of individuals of different kinds, combined
in various numeric proportions, among which
there are not only Male and Female, but also an-
other kind of individuals, differing from them,
called Neuters. In those communities, individ-
uals live in various combinations ; there being, for

instance,m a bee hive, one Female, a Queen, as she
is called, a few hundred Males, and thousands of
Neuters, living together in one community. The
proportions are somewhat different in other spe-
cies—the Wasps, Bumblebees, &c. &c.

These facts, which are well known to Entomol-
ogists, and all those who have become acquainted
with the growth and education of Bees, show that
the ideas which are generally entertained about
the specific distinctions and the characteristics of
species, are not altogether correct. «

It is not throughout the Animal Kingdom that

?ROF. AGASSIZ'S

species consist of individuals of two kinds ; and to
know those two kinds, is not sufficient to form a
toTect idea of the species. There are species in
which individuals of various kinds are combined
together, and in which the combination, in pro-
portion to the numbers which are consJant, con-
stitute an additional character of the species. And
for those, we must enlarge our notion of specific
limits, and introduce elements which are generally
overlooked.

But I proceed to the illustration of the class of
Crustacea. These animals constitute, as they are
now circumscribed, a very natural group ^ though
it may be very difficult to assign general charac-
ters to it. And, indeed, on trying to find a practi-
cal traJt of character, a combination of structural
peculiarities, which should exclude any other an-
imal, and combine together all the Crustacea, I
have strongly felt that these animals were now
combined as they are, not from any anatomical
svidence, but from the very reason on which I
insist as the foundation of classification ; namely,
from various hints about the growth, the mode of
formation, and the transformations of their species.

They are so heterogeneous in their external as-
pect, as scarcely to indicate animals belonging to
one class. Who would suppose such a congrega
tion of large shells, (here the Professor exhibited
a large bunch of Barnacles,) to be Crustacean —
to have an animal allied, for instance, to the Horse-
Shoe, or to the Crabs, Lobsters, Shrimpa, and the
like. Nevertheless, it is certain, from what we
know of the metamorphoses of the Barnacles, that
they, too, as well as many Worm-like Parasites,
belong to the Crustacea.

The importance of Embryologies! studies, for a
correct understanding of the true character and
•lassification of animals, is so plain and so ob-
vious in the class of Crustacea, that I beg to be al-
lowed to illustrate more extensively this class, m
this respect, than I would otherwise.

I would begin this, by pointing out some pecul-
iarities in their form, which have reference to the
changes which these animals undergo during their
metamorphoses. Plate XVII represents various
animals, all of which belong to the class of Crus-
tacea,

In Plate XVII, ftg. A, is a Crab (Lupa dicantha)
seen from above ; and in Fig. B the same as seen
from below. You may notice the number of legs.
They are in pairs— the anterior pair of which con-
stitute powerful claws ; the others being termi-
nated by a simple joint at the erd. The body is
so contracted that the longitudinal diameter is
shorter than the transverse diameter. It is a pe-
culiarity of almost all Crabs, that their longitudi-
nal diameter, if not shorter, is scarcely longer than
the transverse.

Another peculiarity is, that the tail itself is
short and bent under the main part of the body.
^Plate XVII, fig. B).

The main body, which is neither a head nor a

,4TK XVfT— f!R4T?«J A ten

chest, but which is simultaneously both, and on
that account is called cephalo-thorax — the head-
chest— contains the main mass of organs ; the njrr.
yoas system, the alimentary canal, and the heart
as well as tbe respiratory organs, which are in
these animals attached to the legs. This peculiar,,
contracted form v/ill presently be found to have
reference to some changes,which are noticed in the
growth aad metamorphoses of Crustacea; aad- are-
therefore essential;

On the anterior portion of the body, there are
thread-like appendages, called antennss or palpi
Of thoser there are two pairs ; one an inner pai-r,.
and the other an external pair? and sideways from
those, are eyes. They are, iu these' Crab?f (Plate
XVII, figs. A, B) placed in a little depression on
the side of the shell, so that they cannot be seen in
tbe position in which this animal is drawn in this-
plate. To see the eyes, we should look into the
face of the animal, Between the eyes and palpi
are the jaws, consisting of a very powerful appa-
ratus of moveable appendages.

The position of the main organs is the more im-
portant, as it is reflected by the external covering j
so much so, that froro the outside,, ia various

LECTURES ON EMBRYOLOGY.

cics, toe position of the heart can be recognized by
definite outlines. These outlines in the shell
(Piate XVII, fig. A) cover the position of the heart.
This other outline indicates the position of the gills.
This becomes possible, from the fact that those or-
gans, though soft, are earlier developed than the
shell, which is modeled over the organs.

The position of the gills is important on one ac-
acount, being always connected with the legs ;
though they appear to differ widely in their posi-
tion in various Crustacea. Where they are cover-
ed, they are attached upon the thieb, the shield
extending over their point of insertion.

In other Crustacea, the gills are external— they
are attached to the external joints of the legs, and
seem to present an entirely different connection
from what we observe in the Crabs, But the mo-
ment we go to the bottom of the question, we see
that here also the respiratory organs are connect-
ed with the ieg, only that they are from the upper
portion, and covered by a shield, as it is devel-
oped.

In those Crabs, the nervous system presents a
very interesting arrangement Above the alimen-
tary canal there is a first mass, which gives threads
for the head proper, a kind of brain; a ring around
the alimentary canal connects this swelling with
ihf other swellings; but these posterior swellings
form only one uniform mass in the centre, from
which threads go to all the rings of the chest and
their appendages. And this position, this concen-
trated arrangement of the nervous mass, is observ-
ed in all Crabs. In other Crustacea, the nervous
centres under the alimentary canal are more or
less scattered, and correspond directly in their po-
sition to the rings which they furnish with nerves.
This structure of the nervous system plainly shows
that the Crabs must be considered as ranking
highest among Crustacea, if we remember whajt
has been observed in the Caterpillar, in which,
during its metamorphosis into a higher form, the
nervous swellings were observed to concentrate
gradually more and more into compacter and
fewer masse?. [PLATE VI page 41— LOBSTER.]

Let us now compare these Crabs with a Lobster,
(Plate yi) or with a Shrimp, (Plate XVII, fig. C) a
species of Shrimp which occurs in the Southern
States, called Peneus setifer. The general arrange-
ment of the parts is the same as in Crabs. Here
we see irst. the cephalo-thorax covering the main
organs, and the anterior pairs of legs, covering
also the mouth, and from which, on the anterior
part, arises the peduncle for the eye and those
appendages called the palpi. Next, we distinguish
the tail, which is continuous with the head-chest,
and forms a large part of the body ; a portion of
the body, which is as large as the cephalo-thorax,
or even larger, and which can be curved forwards,
but which is never permanently bent under the
cephalo thorax. Such an arrangement of parts is
also observed in the Lobster, (Plate VI) which
does uot differ materially in its structure from

what we have noticed in the Shrimp. The various
rings which constitute this cephalo-thorax and the
tail, are equally provided with moveable appen-
daeres, which are represented separate in Plate
XXL In the head we notice a short peduncle,
(Fig. A) terminating with a compound eye,consist-
ing of thousands of little lenses, each of which has
a crystaline lens, a nervous thread, and really is
a compound eye. Next, we have those two sorts
of palpi represented in figs. B, C. Next we have
six pairs of horizontal moveable jaws, (Figs. D, I,)
three of which are more powerful perhaps than
the others, and constitute what are called the jaws
(Figs. D, F); whilst the three others are called jaw-
feet, from their close resemblance to the legs in
many of these animals.

[PLATE XXI— APPENDAGES or CRUSTACEA.]

The three first pairs, which are near the palpi
are properly called jaws; and the three following
pairs are called jaw-feet, (G, H, I). They are call-
ed jaw-feet, for having internally, like the legs
proper, appendages which are modifications of the
apparatus which supports the gills proper. These
appendages, however, (Figs. G, H, L) instead of
being complicated gills, have only fringed mem-
branes, extending backwards, without performing
respiratory functions. So that, in these parts which
surround the mouth and act as jaws, we have the
same connection between the respiratory organs,
as that we observe in the legs under the chest.

PRtfF. AGASSIZ'S

So that, notwithstanding the functions of these
parts, which are used to crush food before it passes
into the alimentary canal, we see that they are a
modification of the same appendages which on
the side constituted simple legs. Here, jaws and
legs are really modifications of one and the same
type of appendages.

But the chest is not one continuous mass, (Plate
VI). It consists of several rings, united in the full
grown individuals, but distinct in the young, and
still distinct on the lower surface of the adult
These transverse rMges, (Plate XVII, fig. B) which
are noticed between the legs, indicate the foints,
•which, by their re-union, constitute the chest, or
eephalo-thorax. And so we cannot wonder that
there are as many pairs of legs as there are joints
united to form the eephalo-thorax. These five
pairs of legs of the chest are figured separately,
(Plate XXI, figs. I, K, L, M, N).

But, are we allowed to consider the cepbalo tho-
rax as consisting simply of five joints, and one for
the bead ? If it be true that every joint can have
but one pair of moveable appendages, then we
must admit that the head, however contracted, is
the result of the re-union of nine distinct joints.
The eyes, the palpi, the three pairs of jaws, and
the three pairs of jaw- feet. And indeed, so many
transverse divisions may be noticed in the interior
of the chest, in its anterior extremity, when ex-
amined closely ; it can scarcely be doubted, there-
fore that it is out of so many joints that the eepha-
lothorax has been formed.

At the posterior part of the body, under the tail,
we have other appendages, which assume the
shape of branched threads, as represented in Plate
XXI, figs. 0. P, Q, R, S. These are modified legs,
which are not used in locomotion, but to which
the eggs become attached when they are laid, and
as they remain suspended to the lower side of the
tail, they are carried about by the female Crabs till
the young are hatched. The fin-like appendages
at the extremity of the tail, (Plate VI), are still
other modifications of legs ; and so, throughout the
longitudinal axis of such an animal, whatever
shape ifcs body assumes, whether in Insects or
Crustacea, the appendages used as legs or as jaws,
are only modifications of one and the same sort of
organs.

It was important to come to this conclusion, in
order to be allowed to compare the various appen-
dages which were noticed on the side of many of
these other Crustacea, (Plate XVIII). For in-
stance, in Squilla, (Plate XVII, fig. D), we have a
kind of claw, of a very different nature. It is no
longer as we see it in the Crab, but it is the ter-
minal joint which is bent over the preceding one.
So that the claw here would resemble the motion
of my arm pressing against the shoulder, and
forming a forceps, not by the antagonistic action
of two articulations moving against each other, as
in the Lobster, but by the bending of the last joint
against the preceding one.

Many other modifications of these appendages
are noticed on the sides of the body of Articulata;
but the time will not allow me to give all these de-
tails ; I merely refer to them for the sake of further
comparisons. Let me only show that here in Sto-
mapoda or Amphipoda, there is a difference of ar-
rangement in plate XVII, fig. I>, and plate XVIII,
different from what we have in Crabs and Lobsters.
The gills are entirely internal in Lobsters and Crabs;
in the Squilla they are below the rings. Is there an
essential difference in such a position ? No, there
is not. If we look at the embryo Crawfish,as it has
been figured by Eathke, we shall know that the
shield, or the external covering, is gradually modi-
fied by the development of the shield, which grows
successively over the gills. The gills are external
where they are attached to the lower joints of the
legs, and are not different in their nature, but only
modifications of one and the same type.

All the Crustacea belonging to these two groups i
or rather to these three groups— the Crabs, the Lob"
sters, and the Squilla — are among the larger of the
class. The other types, represented (Plates XVIII,
XIX, and XX) are almost universally small— some
even microscopic. In the Amphipoda (Plate
[PLATE XVTTI— Low SPECIES OF CRUSTACEA.]

LECTURES ON EMBRYOLOGY.

XVIII, figs. A, B, and C), we have a structure re-
sembling the Shrimp in its general outlines ; but
in the eye, we have no longer a peduncle. The
eye is sessile — that is to say, it does not rise above
the surface of the body upon a peduncle.

In the others, Decapoda and Stomapoda, the
eyes proceed from a moving peduncle, and are
provided with the peculiar apparatus for seeing. —
Such eyes are, therefore, moveable upon the joints
of the peduncle; but in these Amphipoda the eyes
are flat upon the shield (Plate XVIII, fig. B). You
see that there is a diversity of legs among them,
and a peculiar kind of claws in the anterior part-
various appendages performing at the same time
the function of legs and gills, and the tail similiar
to that of Decapoda (Plate XVII, fig. B). One
modification, however, will strike you. There are
no longer many joints united to form a cephalo-
thorax, but all the joints are nearly equal. The
head constitutes only a joint similar to those of
the rest of the body. There is no concentra-
tion of legs in distinct regions. The number of
these animals which occur in this vicinity is very
great1; but they have, by far, not all been described-
A few only have been mentioned in Dr. Gould's Re-
port. Even genera which have not been described
at all, occur in the harbor of Boston. Here, for
instance (Plate XVIII, fig. C), is one of the new
species, a new generic type, which is very beauti-
ful. It is a curious fact that among these animals
there is such a variation of color. I have had a
good many of them drawn and painted, in order to
collect all the variatioas of colorations which exist.

It is scarcely possible to find two specimens which
acree in color ; and many differ in the distribution
of color so much, that if they were brought from
different countries, and if it was not known that
they lived together, Naturalists might arrange them
as different species. In various individuals of the
same species, (Plate XVIII fig. A) we find some
are red, and others (Fig. B) green, others bluish,
and- others still, with every variety of color. To j
this fact I shall call again your attention hereafter.

We have (Fig. E) others still different, in which
the different joints are so slender as to form an
elongated figure with outward appendages to it. —
The middle appendages are very simple; the anterior
ones have claws, while the posterior ones are mere
simple legs. But on the whole, they come near to
the Amphipoda, (Plate XVIII, fig. A.) As the
legs, however, show some modified combinations,
they have been considered as a peculiar family,
under the name of Loemodipoda.

In some Crustacea of another form, (Plate XVIII
fig. D) the rings are also not combined in distinct
regions, and the eyes arise equally from the level
surface of the shield ; but the legs are uniform, and
the uniformity goes on, increasing as we proceed
lower down, to the various forms of this type
which comprise the Isopoda.

All the Crustacea of which I have spoken, have
one common character— a thin calcareous shell :

[PLATE I— GERMS OF SCORPION.]

whence their common name of Malacostraca is de-
rived. Those of which I am now to speak are dif-
ferent in this respect, and have been called Ento-
mostraca. Some of them (Plate XVIII, figs. G and
H, and Plate XX, figs. F and L,) are Parasitic
Animals, in which we observe two long ap-
pendages, or ovaries, hanging down from the
posterior joints. The body in the Entomos-
traca is simply protected by a horny shield or
envelope, lining the back. There are some (Fig.
G) in which the body is elongated, in the shape of
a Worm, and in which the joints are almost en-
tirely gone; so much do they differ from the com-
mon character of Crustacea; and indeed,in such an
animal as the Lernea, (Plate XX, fig. L) there is
no joint at all to be distinguished; there are not
even gills to be observed ; there are no legs to be
found in any part of the body ; there is no heart ;
no one of the leading anatomical characters of this
class of animals is observed in the Lernea; and
nevertheless it is a Crustacean. It is one of those
Crustacea which have been long known in
their later condition of life, when they have be-
come attached Parasites, but which have not been
known in their earliest stages of life, when they
are free, moving, independent individuals, with all
the characteristics of other Entomostraca and
similar Crustacea. These young, however, have
the structure of Crustacea, inasmuch as they have
fringes, appendages to their rings; inasmuch as
there is a nervous system, presenting the arrange-
ment of the nervous system in the Cyclops. But,
when they have been freed fora certain time, they
become attached, and are then Parasites, and un-
dergo a most remarkable retrograde metamorpho-
sis, by which they lose all the peculiarities of their
structure, sink to a lower condition of life, and
producing a great number of eggs in this condi-
tion, finally die by a peculiar kind of bodily de-
cay, as it were, which we nevertheless cannot con-
sider as a decay, as it is ''in this curious stage of
these animals ^that their eggs are most rapidly
produced. It is really, as Rathke has considered it,
a true retrograde metamorphosis in after life. But
it is remarkable that there should be animals be-
longing to the class of Crustacea, which have so
entirely lost the aspect of Crustacea; which have
no one of their anatomical characters, and which,
nevertheless, belong to that class, as is shown by
their metamorphosis.

PROF. AGASSIZ S

[PLATE XIX— YOUNG CRABS, SHRIMPS, BARNA-
CLE AND CYPRIS 1

We may say the same of Barnacles, in which in
the final condition there is nothing of Crustacea in
their external appearance; but which when young
resemble common Shrimp-like Crustacea, to a very
great extent, as we see by comparing a Cypris.
(Plate XIX. fig. F, with a young Barnacle, fig. G).
There are several of these horn-shelled Crustacea
which have been described as peculiar animals ;
for instance, the species figured, which constitute
the genera Foda, Megalopa and Cuma, (Plate XIX,
tigs. A, B, C, D, E ) which are nothing but young
Crabs and Shrimps. Their resemblance to Cyclops,
or Calanus, (Plate XVI II, fig. F,) or to Cypris,
(Plate XIX, fig. F) is however striking. Here is a
species (Plate XIX, fig. F) of Cypris, for instance,
which resembles, not only the other young Crusta-
cea of figs. A, B, C. D, E, but even the young
Barnacles (Plate XIX, fig. G) most remarkably.
The young of a shelly animal, which in this early
condition of life is a little, free, moving Shrimp-like
Crustacean, with an elongated tail, with legs and
respiratory fringes, having eyes in the anterior
portion of the body,which is similar, in fact, to other
young Crustaceans, and which, after it has grown

to a certain size, becomes attached, and is trans-
formed into the remarkable Barnacle. Here are
some more, of the curious Entomostraca (Plate
XX) to which I shall call your attention. We have
(Figs. A to E) one species, (Figs. F to K) another
species. This latter one, resembles the Lernea in
many respects ; being attached by a sucker to the
gills of fishes, on which they live, but having still
a proboscis with jointed appendages, and having
also indications of rings in the posterior part of
the body, and having sacks of eggs hanging be-
hind. In the other, Apus, (Figs. A to L) the body

PLATE XX— ROTIFERA, AND

TACEA ]

PARASITIC CRUS-

LECTURES ON EMBRYOLOGY.

is free through life; but all undergo similar changes
in early life.

The Horse-Shoe Crab, though large, and in many
respects somewhat more complicated in its struc-
ture, belongs also to the Crustacea which have not
a calcareous, but a horny shell, and are called Ento
mostraca.

From these facts, you may observe that Natural-
ists divide the Crustacea into two great groups;
those furnished with a shield, like the Crab and
the Lobster, called Malacostraca, and such as are
not thus protected,called Entomostraca, which have
only a horny envelope, and in which all the parts
are less diversified.

I may mention more particularly one of these
Entomostraca (Plate XVIII, fig. F) a species of Ca-
lanus, which has a peculiarity of being phosphor-
escent, and of presenting a peculiar kind of phos-
phorescence which I am not aware has been ob-
served before. Here the nervous system, with the
eyes,is the shining part of the animal ; that nervous
system being not only phosphorescent, but the
substance of the nerves being of a highly red col-
or. The arrangement of the parts is precisely
the same as in the nervous system of the Crusta-
ceans in general, A close investigation of this
arrangement has shown me. that there can be no
mistake about it.

[PLATE XXII— EGGS OF PINNOTHERES.]

The embryonic growth of Crustacea has been
extensively studied. We have had numerous mo-
nographic investigations upon that subject, which
were made by the most eminent of the Embryolo-
gists of our day. Rathke, in particular, has in-
vestigated that subject to a greater extent than any
one else. However, the earliest changes which the
egg undergoes, have not been so completely exam-
ined. Therefore,allow me to call your attention for
a few moments to the transformations of the eggs
of the little Parasitic Crab, the Pinnotheres Os-
triun^which is found in Oysters, and lives as nPar-

asite between the gills of this animal. The whole
animal is so transparent that its growth and
changes can be very easily investigated. And
there we find eggs of various degrees of develop-
ment, some exceedingly minute, which consist of a
simplely vitelline membrane,with an absolute trans-
parent yolk, a small gerrninative vesicle and ager-
minative dot in the centre ; a few granules are no-
ticed in the yolk substance. Others will present
the same appearance in general structure, when
the germinative vesicle will be much larger, and
the germinative dot also much larger, being
swollen into a small vesicle. The same will be
universally observed in a series of changes, where
we notice that the germinative dot may groAY
much larger than it was before, and even form a
hollow vesicle within the germinative vesicle it-
self; the yolk granules having greatly increased
in quantity between the germinative vesicle and
the vitelline membrane. So that here it is perfect-
ly plain, that, the germinative dot can grow into a
hollow vesicle ; and from the condition of other
eggs, we may be satisfied that there is a period
when the germinative vesicle and the germinative
dot may disappear, to give rise to the formation of
another germinative vesicle containing more, nu-
merous granules; and that that vesicle may burst
again, and give rise to the formation of two germi-
native vesicles with their germinative dots, or we
may haye three germinative vesicles with their
germinative dots. And during this period of evo-
lution of cells within cells, there is an increase of
the mass of yolk taking place, an accumulation of
granules growing, by which that egg finally as-
sumes that degree of maturity! which precedes the
first formation of a germ.

[PLATE III— EGGS AND DEVELOPMENT OP
SHRIMPS ]

I have traced these eggs up to the moment when
the yolk had become a mass of somewhat opaque,
though not very compact yolk, and the first rudi-
ments of an embryo were formed, as a disc on
one side of the egg, growing around it, and pre-
senting all the changes which hare already been
described by Rathke and Erdl, as occurring con-
stantly in the growth of Crustacea, and to which I
will now allude, referring to the species which he
has figured.

The earliest condition of these germs in Palse-
mon, (Plate III. fig. A.) after the egg itself has nn-

PROF. AGASSIZ'S

dergone all its changes, is the formation of a lay-
er of more animated substance, the beginning of
the young animal. We have here (Plate III, fig.
B) the germ as it flattens out at one end and is
contracted at the other part, divided as it were in-
to two connected discs, the larger assuming after-
ward another form (Fig. C), the smaller one grow-
ing laterally, when soon it is observed what has be-
come of these two extremities of the expanded
disc (Fig. D). One will be the head end of the
germ, and the other will be the caudal end of the
germ. Those serratures upon the posterior ex-
tremity of the animal, represent the divisions
in the animal layer, in the blastoderma, or germ,
which will give rise to the joints or rings of the
chest ; while the anterior disc will represent that
part of the body which properly forms the head,
growing larger and larger; these flat discs are drawn
backwards, forwards and on the side, so that it
gradually surrounds the yolk, having assumed a
more elongated shape (Fig. F) leaving the mass of
the yolk free at the dorsal side, so that when
seen from above (Fig. G), you have the margin of
the animal in sight, which is rolled over the yolk.
We have also here the eyes, which are forming at
the anterior portion of the germ ; and also the in-
dications of the formation of a heart

But from below (Plate III, fig. E,) we see how
the lower surface is changed ; the formation of
those parts which will represent the mouth, is
seen, and also the formation of those parts which
will represent ihe legs, and in addition, the parts
which will represent the tail. And those separ-
ations of different joints become gradually more
and more distinct, (Fig. G,) so that upon close
examination, you may find that the germ is now
a little animal, which soon escapes under the form
of fig. H. Here we have the young, which rises
from such a transformation ; and this young is the
young of a Palasmonof the character of Plate
XVII, fig. C. The young as it is hatched represents
the figure which is a general characteristic,
not only of the Macrouran Crustacean, but it has
more particularly the form of those Entomostraca
which have been described under the name of
Cuma (Plate XIX, figs. D and E) I have traced
many of those which occur in Boston harbor,
of Palasmon, of Hippolyte, even of Mysis,
and they all give rise to young which are species
of the genua Cuma, belonging to the Entomo-
straca of Carcinologists ; showing that there are
still extensive grounds to cultivate in the history
of Crustacea, and that they undergo metamor-
phoses. The subject of the metamorphoses of
Crustacea has been discussed very extensively,
Rathke denied positively that there are metamor-
phoses among Crustacea; while facts were col-
lected in Ireland which showed distinctly that such
metamorphoses take place.

Mr. J. V. Thompson, who has published many
interesting investigations upon the lower Marine
aaimals— the same to whom I have before referred

—and who discovered that the young Comatula
had a stem in its earlier condition, was also the
first to notice that the so-called Zoea (Plate XIX
fig. A and B), were not animals of a peculiar ge-
nus, but that they were the young of Crabs— of
Crabs of similar form to that figure, (Plate XVII,
fig. A.) Captain Tuckey of the British navy, ob-
served similar changes. He saw the transforma-
tion of the egg into those entomostracal germs,and
further changes, which left no doubt in his mind
that the Crabs underwent the above described met-
amorphoses.

The objections of Rathke arose from the fact.that
the Crawfish, a Crustacean, in which he studied
that embryology, does not undergo extensive chan-
ges of form during its embryonic growth. The
young Crawfish resembles very early the perfect
animal; so that by correct investigations this emi-
nent Embryologist was misled ; though he after-
ward acknowledged his error with reference to the
investigatioas of Thompson, in the most liber-
al and generous manner. These metamorphoses
have been traced extensively in other Crustacea.
Zaddach has published a monograph, in which
he has represented the changes which this animal,
Apus, (Plate XX, figs. A to E) undergoes, from its
primitive formation in the egg, up to its perfect
condition, (Fig. E.) In the beginning (Fig. B) it
has but few appendages ; and afterwards, others
successively, more numerous, are added under-
neath. Here (Fig. F) is a diagram of another ani-
mal, the Achtheres, in which similar embryonic
changes have been observed. First, there are
also but few appendages, but afterwards several
pairs have been added to form the various appen-
dages which exist in the adult (Fig. G.) How
similar Rotifera are to these various embryonic
conditions of Entomostraca, will not escape the
observer, who is simply reminded of the existence
of these microscopic animals, (Plate XX, fig.
0.) They resemble most remarkably those
Entomostraca in their earliest condition. But in
their embryonic condition, Crustacea — even
Crabs, as well as Lobsters have young which re-
semble perfect forms of those Entomostraca, be-
yond which certain Crustacea do not pass. We
have thus direct indication that they should be con-
sidered as the lowest ; and so would we place at
the lowest range, all the Rotifera and these vari-
ous kinds of Entomostraca and Parasites, (Plate
XX and Plate XVIII.) Next, we would have the
Malacostraca; and among them, those lowest, with
uniform rings, which are not combined into dis-
tinct regions; and next, those in which the rings are
also not combined, but the legs diversified, (Plate
XVIII, figs. A, B, C, E); and above all, those in
which the rings are combined in various ways,
which are still more diversified, (Plate XVII.);
placing the Lobster and Shrimp lower among
them ; but we should consider the Crabs (Fig. A)
the highest of all, because in these the concentra-
tion has gone to the extreme , the tail which was

LECTURES ON EMBRYOLOGY.

proportionably the greatest appendage, the longest
and most developed part of the body, in the earli-
est condition, being now reduced to the simplest
and lowest condition.

Such a classification agrees with the classifica-
tion which has been introduced into our natural
histories, from tfye general impression received
from these animals. Guided to some extent by
anatomical details, and also in some points by em-
bryonic data, the arrangement proposed has been
the same to which, from embryonic evidence, we
would arrive. Only, there is an objection to be
made to the division of Crustacea into two groups;
Entomostraca, passing by transformation into Ma-
lacostraca, as can be directly ascertained in the
case of Cuma,the young Pctlaemon. Therefore, that
division cannot stand as a natural division. We
must have a series of groups following each other,
according to their embryonic gradation, but not
two types of Crustacea; as the diflerences upon
which this distinction rests present only degrees of
one and the same thing.

But, there is another point in which the analogy
of gradation with embryonic growth is most re-
markably striking. It is the order of succession
of Crustacea in geological times. Crustacea
have existed from the earliest times. They are
found in the earliest formations, and found in all
subsequent beds.

[PLATE XXTIt— TRILOBTTE 1

The forms assumed are different. The oldest are
the so-called Trilobites of several types (Plate V).
There is a remarkable analogy between the forms
of various Trilobites, and the outlines of the germ
of Crustacea, as figured Plate III, the earlier stages
reminding us of Agnostus, and the like, whilst the
later agree more with the higher Trilobites; but the
most striking resemblance is noticed on comparing
these types with the embryo of the Entomostraca,
as they are represented (Plate XX, fig. A) within
the egg, before they are hatched ; the divisions of
the middle part of the body into three lobes, the

long, lateral appendages arising from the anterior
extremity. Every point of the structure agrees.
It is only, that in these ancient types there was a
permanent state of growth — a condition under
which this animal lived for ages, and reproduced
its species; whereas, in our lowest Crustacea we
find even such an arrangement in the ea;lier form
only, as the beginning of a metamorphosis.
Next, we have in the geological series, Horse-Shoe
Crabs. During the coal period, there existed seve-
ral genera of Crabs allied to the Horse Shoe,having
the same general features. There are also species
found in the Oolitic beds. If we trace the grada-
tion of types, we find that these (Plate XX fig. A)
the Apus, in their perfect state, are next in order.
Those which undergo a retrograde metamorphosis
or which agree with the embryonic stage of Apus,
as Trilobites, being altogether the 'owest. And so
we have the Horseshoe Crab, which is the second
type in the order of geological ages, ranking high-
est among Entomostraca; taat is,above those which
resemble the Trilobites.

During the deposition of the Oolitic and Creta-
ceous rocks, there existed a countless number
of Crustacea, but all of them were Lobster and
Shrimp-like animals. The earliest of all the Mala-
costraca is a long tailed animal, the Palinurus
Sueurii, resembling Lobsters and Shrimps. And
during all this time, we have only such animals —
and not one Crab is formed until afterwards. But,
during the later part of the deposition of chalk,we
begin to find Crustacea with short tails, belonging
to the type of Crabs. So that, in the order of suc-
cession of the more recent types, we have the same
evidence that the arrangement which is proposed,
from embryonic data, is also the order of progress
which has been introduced into the character of
these animals at different successive periods.

And I may add here, that the geographical dis-
tribution corresponds even to this gradation of
types, as far as it is understood. Crabs, for in-
stance, are not numerous on this shore. Few spe-
cies occur here. In the Middle States they are
more numerous. They occcur more frequently
and are very diversified in South Carolina ; and
still more numerous, in the tropics, where Crabs
prevail over Lobsters and Shrimps. And, though
these latter are extensively found in temperate
regions, it may be said, that the lower orders of
Crustacea (Plate XVIII, fig. A) are innumerable in
the northern regions, and much fewer in the trop-
ical regions. So that, in whatever point of view
we notice this subject, we see one plan, one com-
bination, one system, uniformly carried out.

PROF. AGASSIZ S

LECTURE IX

More than once I have alluded to the uniformity
of structure of the egg, in its primitive condition,
in all animals ; thus showing that there is a com-
mon starting point for their growth, throughout
the various classes of the animal kingdom, I
shall now illustrate more fully the physiolog-
ical process by which the egg, when matured,
gives rise to the formation of a germ. I do not
intend this evening to enter into more details than
I have already given, upon the formation of the
egg itself, but to illustrate the process by which
the egg gives rise to a germ. This process has
been traced in all classes of the animal kingdom ;
and it is found to consist of a very complicated se-
ries of changes taking place in the substance of the
yolk, when it has reached a certain degree of ma-
turity.

The condition, therefore, the first essential and
constant condition for the formation of a germ, is
the previous formation of an egg, and its being
matured to a certain degree. The size, the degree
of maturity, and changes which the egg itself un-
dergoes before the germs are formed, vary in dif
ferent classes. I will not allude to that point at
all, but only take now the germ as it is forming
within the egg, when the yolk has grown to a cer-
tain size.

[PLATE XXIV— EGGS OP

I cannot, however, omit mentioning a very curi-
ous mode of ovulation which is noticed in some
Worms. When, some months befo:e the laying o(
the eggs, we observe the ovary of the Nemertes,
we see in their interior, oblong, bottle-shaped
pouches forming, which fill with yolk substance,
that gives rise to the eggs. When these bottles
have attained their whole development, that is to
say, when they are completely filled with yolk
substance, a new process is introduced in them. —
The substance groups itself around several centres,
and forms a series of little spheres, whose number
varies. These are the eggs ; eggs which soon
have a germinative vesicle, and within it, a germi-
nal ive dot characteristic of the eggs in general.—
When this second progress is terminated, the bot-
tles are laid, under the shape of a chain, and the

eggs are thus contained in a transparent sub-
stance of shapeless appearance.

After the laying of the eggs, another series of
transformations is produced, as we shall see pres-
ently. Almost the same changes occur in the
Malacobdella, which is a Parasitic Worm found in
the Clam. There, also, we have observed yolk
bottles, as also the successive formation of the
eggs. Here there is no ovary proper ; we have
found the bottles distributed in the whole body
around the intestinal canal. Some contained only
one egg, and some not yet condensed yolk sub-
stance ; others contained two eggs ; others three,
four, and even a greater number were formed,
until the whole yolk was exhausted.

Plate XXV represents some of these phases. —
In the Planarise the mode of formation of the eggs
is the same, except the bottles.

[PLATE XXV— EGGS OF MALACOBDELLA ]

Let us return to the egg, when it is about enter-
ing another series of changes. In Piates XXIV
and XXV, we have eggs of different animals,
in which the process of the formation of the
germ is represented up to a certain degree of
its growth. The primitive egg consists, as you re-
member of a vitelline membrane containing yolk,
and within this yolk a germinative vesicle,and with-
in that a germinative dot, as shown in Plate XXIV,
A, B. The yolk becomes gradually more and
more condensed, thickened, and more and more
opaque; and at that epoch, the germinative vesi-
cle generally disappears ; the germinative dot dis-
appears also, and new changes begin to take place
within the yolk.

It has been questioned, whether the germinative
vesicle and the germinative dot precede, or follow
the formation of the yolk substance. There are
examples of ovarian eggs in which this vesicle and
this dot are very distinct, as also the yolk mem-
brane, at the time when the vitellus is yet very thin
and transparent in the sphere of the egg. We have
seen this vitellus increase and fill up the whole

LECTURES ON EMBRYOLOGY.

space and condense arounJ the germinative ves-
icle. So that there was no more possibility of
doubt that the vesicle and the germinative dot did
exist there before the vitellus* At ether times, the
srertninative vesicle alone has been observed in the
developing eggs. There are other instances where
the ovarian egg presents neither gerrainative ves-
icle nor germinative dot during the formation of
the yolk. This shows that even the question of
the fundamental structure of the egg, in order to
be sofved, calls yet for minate and serial research-
es.

In the interstices of the granules or little cellules
which compose the vitellus, is contained a transpa-
rent liquid more consistent than water, since it re^
sists a certain pressure. When the egg is formed
this liquid tends towards a centre and agglomerates
itself there under the form of a transparent sphere,
the appearance of which precedes the ordinary
phases of the dividing of the yolk.

Whether the progress is the result of the mix-
ture of the contents of the germinative vesicle and
the germinative dot; or the changes are intro-
duced simply owing to the fact that the egg has
arrived at its maturity ; whether it relies simply
apon the yolk to undergo those changes, is a point
which it is impossible to decide at present. Gen-
erally, when the yolk undergoes the first change
by which the germ is formed, the germinative
vesicle acd the germinative dot have already dis-
appeared ; bat in some instances^ the germinative
vesicle and the gerrainative dot have been ob-
served within the yolk, when another mass, (the
•clear sphere) which generally appears after those
have gone, had been formed in another portion of
the egg, as represented in PI. XXV, fig. Hi so that
changes which have been known to be connected
with the first formation,— changes giving rise
to the germ — such modifications are observed in
the yolk when the germinative vesicle is still
within.

Therefore, it cannot be absolutely said that the
bursting of the germinative vesicle, and the mix-
ture of the substance contained within it, is prop-
erly the cause of the changes now taking place. —
It may have an influence upon the yolk, by which
those changes are accelerated or facilitated; but
that it is properly tfee cause, cannot be main-
tained.

Well, to understand all these changes which take
place within the eggv they must be conceived as
successive modifications of substance. We know
that one sort of egg will only give rise to one sort
of animak Therefore we racist admit, that as an
«gg of ene kind gives rise only to one sort of ani-
mal, there must be an immaterial principle presid-
ing over these changes, which is invariable in its
nature, and is properly the cause of tke whole
process,

But now the changes which take place in the
yolk vary in different classes of animals. In some
fcbev consist <?f a division of the yolk, which is

successively repeated and repeated, till the whole
mass of the yolk has been so much subdivided as
then to consist of innumerable little masses, aris-
ing from the subdivision, from the repeated subdi-
vision of the primitive mass into successively more
and more numerous parts. In others, the division
is only partial. On one side of the yolk there is a
depression formed, which does not penetrate across
the whole mass, and then another, which will be
formed at right angles with the first, thus forming
four partial divisions; and that being repeated,
the surface of the yolk, on one side of this mass
may be divided into little fractions, though a great
portion of the yolk takes no part in this process of
repeated division and subdivision. In many ani-
mals the division of the yolk is most wonderfully
regular.

The dividing of the yolk is probably a general
phenomenon, appearing in all eggs, though obser«
vation has not revealed it to us in all classes with
the same certainty. Its generality, however, is
difficult to trace at present; as its various modifi-
cations have not been reduced to one common
type > however, the fact is already ascertained in
the class of Mammalia. la the Birds, the size of
the eggs has been an obstacle for this kind of ob-
servation. It has been noticed in the class of Rep-
tiles, and in that of Fishes. I have already men*
tioned the difficulty which observations encounter
in the class of Crustacea and Insects ; in regard to
which the data upon the dividing of the yolk are
deficient, although it has been observed in the in-
ferior Crustacea. It is easily traced in the Worms
and Mollusca; indeed it is nowhere easier to ob-
serve it, than in these two classes of animals. The
phenomenon of dividing of the yolk does not fol-
low the same course in every class at the same
stages of dev elopment. Perhaps it begins, in some
cases, even before the laying of the eggs. This
would explain, at least, why it has sometimes not
been observed. The process is sometimes slow,
sometimes very rapid ; and in this latter case it may
easily escape the attention of the observator. Nor
must we lose sight of the fact that embryogenie
science is a comparatively recent one, and in
this department there remains yet much to be
done— above all, with reference to the study of tis-
sues. This should especially be acknowledged, if
we consider that it is as late as the year 1834, when
Schwann made the discovery of the uniform cel-
lular structure of organic tissues, in the animal as
well as the vegetable kingdom.

There are animals, {and it has been more par-
ticularly observed among Worms, among Intestinal
Worms especially, by Dr. Bagge,) in which the
yolk first divides into two halves, which subdivide
and subdivide regularly till the whole mass of the
yolk is reduced into minute uniform yolklets. The
process of this division is also seen in Mollusca,
especially among naked Mollusca ; the whole mass
dividing into two halves, forming two distinct
masses. Next, each will be subdivided into two

PROF. AGASS1Z S

so that the primitive mass of the yolk will be
divided into four equal parts. And then those
segments will be subdivided and subdivided, till
the whole mass consists of small yolklets, each
surrounded by a membrane.

But the subdivision is accompanied by a pecu-
liar formation of other masses within those partial
spheres. Let me show you some diagrams repre-
senting this process. In Plate XXIV, fig. C, we
have the eggs of Planaria,ia which the yolk is divi-
ded into four masses ; and in Plate XXIV, fig.D, we
have it the same under slight pressure, when four
clear spheres are noticed within each of these seg-
ments.

In the next place we observe that besides the
four great masses there are four small ones, rising
in the centre.

Again we may observe in each of the small ones
such a clear sphere, and when the subdivision goes
on forming a greater number of these spheres, the
whole process is repeated, the large one being
greatly reduced, there being successively, 16, 32
or more. Such fragments are increased very reg-
ularly, and though many variations are observed,
they appear in multiples of two or four, and so on.
When it has gone on a certain time, instead of four
small ones and eight large ones, or vice versa, there
will be quite a number of minute ones, and all
alike in size, and the process will be repeated tiil
these divisions are so minute that it is no longer
possible to count them, they forming a mass of
little cells, filling the whole of the membrane of the
yolk.

What those clear spheres within the yolk are, it
is somewhat difficult to say, inasmuch as chemi-
cal analysis cannot reach them. The eggs are so
small that their composition has not been exam-
ined. It is only with the microscope that we can
reach these processes and determine the changes
of form and substance which take place, by the
various properties of these substances with refer-
ence to light.

The fact of their being more or less transparent
will make some appear different, under the mi-
croscope, from others. And that is the whole
ground upon which the changes can be ascer-
tained.

The manner in which the division takes place
when there are two forming, for instance, in the
intestinal Worms, has been described by Dr.
Bagge, as follows.

The primitive clear sphere in the centre is said
to assume an elongated form,, and then the centre
to be contracted, and finally the two ends become
independent by a separation of the middle part, so
as to form two spheres ; and then the yolk mass to
agglomerate around those transparent spheres; and
then a division to be formed in the vitelline mem-
brane; and that to go on and to divide the vitellus
into two spheres ; and in each the same process
having been repeated, to have transformed that
four. Assuming again an elongated form, and

then dividing completely, they go on and foraa
four masses. But that clear spheres within do nofc
always constitute or determine the separation of
the substance of the yol& into more and more nu-
merous masses, is shown by the example which I
have quoted, where a clear space exists in thecerr
tre of an egg, and the division takes place across
it. For instance, there will be such a mass as rep-
resented ia Plate XXV, Sg. H., and the division
will take place, a clear sphere accumulating on
one side of the mass, aad she yolk condensing on
the other side, and so on.

The fact is, that the subdivision of the yolk mass
and the formation of these clear spheres, is a pro-
cess which goes on .simultaneously, but which can-
not be considered as directly dependant on each
other. In proportion as this tendency of the yolk
to subdivide is manifested by a contraction of the
mass, and the division of the spheres into two-
spheres, in the same proportion the substance
within the yolk, which fills the space in the centre
of the yolk, accumulates in spheroid raasses, to*
give rise to partial spheres. And that beiag re-
peated, there are then numerous divisions of the
yolk successively introduced, and having been en-
tirely kneaded^ as it were, by this repeated divis-
ion, the substance of the yolk in process of time
becomes a germ.

For instance, in the Worm from which the dia-
grams in Plate XXIV are made, the germ (Fig,
A,} is a mass of very minute cells. Then from
the surface of those cells rises vibratory Cilia. We
know that cells can have vibrating Cilia on one of
their extremities. It is observed in the full-^rown
animals, and it is observed ia many germs, es-
pecially in Mollusks, tbat such vibrating Cilia,
are formed on the external surface of cells and
become an apparatus for locomotion, which Cilia
are voluntary, ceasing to move at intervals, re-
newing their motion at other times and transport-
ing the animal from place to place. But remark-
able a& it is, tbat the sphere is the fundamental
form of all animals, so rotation is the form, of the
action of all animals whea they begin- to move
within the vifelline raembrane

No sooner has the little Planaria (Plate XXIV)
been covered with vibrating Cilia, than it begins
to revolve upon itself ; it has then a spherical out-
line, and undergoes a rotatory, constant motion in
one direction to begin with. And whea it has
crown to assume a somewhat elongated form, by
which the prevailing longitudinal diameter will be
introduced, after that longitudinal diameter b&9
exceeded the transverse, then it will change the di-
rection. And as soon as it is hatched, then it wi!3
proceed in an onward and forward motion, which
will be the motion that will characterize the an-
imaH aad then comes tbe bilateral symmetry
which exists throughout the animal kingdom, even
where it is concealed under the radiated form of
so-called radiated animals.

A remarkable comparison might b« lmsfcitRt«fl

LECTURES ON EMBRYOLOGY.

between the embryogenic phenomena, as we have
just described them, and what is known of the ce-
lestial bodies, in their combinations, upon an im-
mense scale. First, we have primitive cells, com-
bining and condensing to form the mass of the
egg, like clusters of nebular stars. After the yolk
has undergone the various phases which precede
the formation of the embryo or germ, this new be-
ing with a spherical form, which is also the form
of the primitive egg, begins to assume a rotatory
movement, under the influence of life, as the ce-
lestial bodies rotate under the influence of univer-
sal gravitation. At last the progressive, onward
movement is introduced, which characterizes ani-
mal life properly, and is the first step in the series
of progress, which, in man, ends with intellectual
freedom and moral responsibility.

But this form of the division of the yolk is not
the only one which is observed among animals.
In Fishes, for instance, we have a division of the
yolk, which differs considerably from that just de-
scribed. In these there will be first a transverse
depression upon the yolk, so that, seen from above,
the yolk will seem divided in two halves. And
then it will be divided again at right angles, so
that there will be two furrows at right angles,
forming a division which remains superficial. So
that in a profile view these furrows do affect the
yolk but very little, and the whole mass below re-
mains unaffected.

But only the superficial layer undergoes this
change ; the lower portion and the central par^s of
the yolk remaining unchanged, but being gradu-
ally introduced into the process— being gradually
absorbed by that part of the germ which is already
formed, and finally totally absorbed by the germ ;
or if not introduced into the substance of the germ
as a part of its body, it is finally introduced as a
sac from the lower part of the body into the di-
gestive cavity, and is digested. So that we have
all possible steps, from total division of the yolk,
which is entirely changed into a germ, to a super-
ficial furrowing giving rise to a germ which rests
upon a modified yolk. In the first instance, by
repeated subdivision, the whole substance of the
yolk is prepared to become a germ ; or, in the sec-
ond, only a part of it is modified to form a layer
upon the yolk, which grows and gradually absorbs
the remainder of the yolk.

In those animals in which the division of the
yolk is only partial, as in fishes, the divisions
where they have been multiplied have nevertheless
finally given rise to cells. In the beginning, those
divisions are only separations of the superficial
mass. But those masses not being entirely sur-
rounded, do not form distinct spheres or parts of
spheres ; but at last, when they have repeatedly
multiplied, then each particle is surrounded by a
membrane, and thus transformed into a distinct
cell. So that the germ, in whatever manner it is
produced— whether by total or partial division of
the yolk— is finally, when formed, constituted of

numerous small cells, The changes \vhich those
cells undergo — the manner in which additional
cells are derived from the yolk, either by division
or by evolution from those already formed,— con-
stitute the phases of the embryonic growth of each
animal. But it is by a uniform process of division
that the germ itself is first formed, The degree of
maturity which the germ has reached when it is
hatched, varies extraordinarily. There are ani-
mals in which the germ is hatched in a degree of
development which is so distant from what the
animal will be finally, that it cannot be recognized,
and that the type of the parent is not at all indi-
cated even in the outline, in the form, or in the
structure of the germ when born. There are other
animals in which, on the contrary, the germ is not
hatched before it has grown within the egg to
assume the external forms of the mature animal,
and has even attained to a very considerable size,
in many of them.

It is perhaps from not having considered suffici-
ently those differences that so many mistakes have
been made in the study of the changes which those
animals undergo. Had it been supposed that ani-
mals were born in a condition in which they differ
so widely from the parent, they might have been
watched longer before they were described as dis-
tinct animals, on the sole ground that they were
free moving. And we should not find that animals
of the same species would be described under so
many different names if this had been more gene-
rally known.

A great many larvae of Worms are undoubtedly
simply those small animals described as Infusoria ;
and I have myself seen eggs of Planaria give rise
to some of these Infusoria called Pararusecium,
Annellides, Here, for instance, is one (Flare XXVI,
figure E), remarkable for its sucker-like discs
[PLATE XXVI— PARASITIC WORMS.]

and the Cilia by which it moves. Tlie \oung
Planaria resembles closely *this species. And it
is more than probable— it is altfiost certain— that a
great number of those so-called Infusoria, are no-
thing more than the moving germs of Worms.
Here is, for instance, a young Planaria, in which
we have such a sucker, and in which the general
form reminds us of the Infusoria very striking-
ly. (Plate XXVII, fig. B). The change which

FROF. AGASSIZ S

those germs undergo in various families of Worms
seem to differ \\idely ; and indeed, among Worms
every where, there are types which are so widely
different in their outlines as scarcely to afford char-
acters by which to combine them.

[PLATE XXVII- YOUNG WORMS J

Is will be a great difficulty to find Anatomical
as well as Zoo'ogical terms to constitute in?o one
class all these various forms, (Plates XXVIII,
XXIX and XXX) an<5 those which are represent-
ed there, (Plates XXXI and XXXII ) Neverthe-
less, in tracing the> intermediate forms, we are
compelled to bring them into one and the same
group.

[PLATE XXVIIT -WORMS WITH COLORED BLOOD]

The class of worms, as I circumscribe it here,
contains numerous and very diversified types, as
well hy their internal .structure, as by their exter-
nal form: so that it is difficult to assign to all of
them common characters. The Intestinal Worms,
formerly considered as a class hy themselves, can-
not be separated from the true Annulata. There
;ire intermediate forms between the two groups —
For instance the Trematoda, which are closely al-
lied to Planaria, the AscariX which resembles Lum-
bricus, imd so on. The Intestinal Worms, gener-
ally speaking, have their body naked 5 the Acan-
thocephala only have hooks of fringe-like appen-
dages. Among Annulata there are, however,
types which cannot be compared with any of the
Intestinal Worms; as the Tubulibranchiata and
Dorsibranchiata. Among these there are some in
which the lateral appendages of the body are uni*

[PLATE XXIX— VARIOUS WORMS.]

[I'L\Tf XXX E » RTH \V'l)-RM AND IJT- A \' • ); I A .

form tor its wuule length; mothers, ttie appen-
dages of the anterior, middle and posterior region
of the body differ among themselves, and assume
even an entirely different character. In some, the
rings are generally provided only with a few stiff
hairs, whilst the head is surrounded with tufts of
respiratory fringes, and other appendages, in va*
rious degrees of development. Nevertheless,
through all that diversity, there is a common type
which can be easier understood than properly de-
scribed or defined.

The development of the class of Worms varies
according to its types. In some, the yolk sub-
stance, after having been indefinitely subdivided
into homogenous little spheres or cells, assumes a
rotatory movement, sustained by vibrating Cilia,
which have been formed upon its whole sur-
face. Such are the Planarioe, &c., &c., whose

LECTURES ON EMBRYOLOGY.

[PLATE XXXI— INTESTINAL WORMS.]

PLATE XXXLl— INTESTINAL WORMS J

young are InfUiOrum^. in others, the develop-
ment resembles more that of Crustaceans and In
sects, there being an animal layer formed upon the
lower side of the yolk sphere, which surrounds
gradually the yelk and encloses it, so that the
narle is dorsal. Such a growth has been observed
in a worm of the Leech family, which occurs in
Fresh Pond, (Plate XXXIII) as well as in a marine
V\'orm of the bay of Boston, belonging to the ge-
nus Pasithae.

I wish only to make some remarks upon the va-
rious metamorphoses which the Worms undergo.
Among the Intestinal Worms we have forms which
are cylindrical, and which present no extreme di-
visions in the body (Plate XXXII, fig. C).

We have others which are also cylindrical. (Pen-
tastoma, Plate XXXII, figs. A, B) but in which we
have transverse ridges. There are very numerous
forms of the kind, which are flattened as the
Tapeworm. We have others in which the differ-
ent parts of the body (Plate XXXI, fig. C,) differ
widely— the Cysticercus. Tiiere are others in
which the articulations are srill more distinct, and
there are again others (Plate XXVI, fig. E) in which
the articulations are scarcely distinct at all, but
which constitute really compound animals, as
there are always two united together — Diplozoon.
There are again others, which are flat, (Distorna,
Plate XXVI, figs. A, B, C, D) and entirely unartic-
ulated, unless we should consider as articulations
those folds on the margin, which can scarcely be
considered so; but owing to the arrangement of
their parts, particularly that of their nervous sys-
tem, we find that they must be referred to the clnss
of Worms. Indeed although these animals have
been placed in a special class, owing to the fact
that they are Parasites, they cannot be grouped to-
gether with all other Intestinal Worms, nor fnrm a
class by themselves. They have little in common
with other Parasites, but this mode of existence.

la fact, Intestinal Worms constitute various types,
of which the main common trait of character is to
live upon other animals, rather than to resemble
each other in their structure. But between Planaria
(Plate XXX, fig. B) there is the most remarkable
affinity. This is a Distoma. (Plate XXVI, figs. C
D) an internal Parasite, and we find that every
thing agrees in the structure with Planaria (Plate
XXX, fig. B). There is an alimentary canal, first
a simple tube, which divides afterwards into two,
and from which arise innumerable branches rami-
fying in the substance of the animal.

The same structure exists in Planaria, an animal
which has been referred to another class, but the
resemblance is so great that it is now no longer
possible to separate them ; and very recently, Mr.
Blanchard has proposed to combine them, under
the name of Aneurosi ; and previously Professor
Owen had intimated the propriety of uniting them
with those broad Intestinal Worms. Their ner-
vous system agrees m6st remarkably, and agrees
not only with that of other Intestinal Worms, but
when properly understood, shows that the nervous
system of the Intestinal Worms, though seemingly
so peculiar, is reallv constructed upon the same
plan as that of other Articulata in general. In Ar-
ticulata in general, the nervous system consists of
a series of swellings, as I have shown before (Plate

[PLATE XXXni— NERVOUS SYSTEM OF WORMS.]

XXXIII, fig. A). In Malacobdella (Plate XXXIII
fig. B), and in all intestinal worms, the nervous
system consists of a main mass about the alimen-
tary canal, and two longitudinal threads extending
along the two sides of the body, from which arise
other threads. We have now only to conceive that
the two parallel threads are brought nearer to-
gether, and combined in one continuous thread by
transverse commissures, to have the same uniform
system, which characterizes the higher Articulata in
which those swellings are combined. We have
again in Planaria the nearest possible approach to
the nervous system of the Intestinal Worms, which
really brings them much closer than they could be
brought before, and combines them all into one
class.

The manner in which these animals are found is
very remarkable. The Distoma, as we have it
here, (Plate XXXIV, figs. 2.3.4) lives as a parasite
in the cavity of other animals— upon their liver — is
very frequently met with in the cavities of higher

PROF. AGASSI2 S

[TLA.TE XXXIV— ALTERNATE GENERATIONS OF

\) I STUM A. |

animals, but is also often found upon fresh water
mollusks in the intestinal cavity,as well as upon their
abdominal organs around their liver and upon the
anterior portion of the mantle. And it has been
recently ascertained by Mr. Steenstrupp that these
are free animals at certain seasons of the year, and
that they undergo metamorphoses, of which we
had no conception before his observations were
published.

Let me give the history of these various changes
to some extent. Wherever fresh water shells
occur, of the genus Lymneus and Planorbis, we
find around them in June a great many little worms
of which we have here a figure (Plate XXXIV, fig
Z) which has been described as Cercaria. They
move with great ease in curved motions describ-
ing constantly the figure 8 when moving. Within
are various organs whose functions are not fully
understood. Whether these branches lead to the
alimentary canal, or to one of the glandular ap-
pendages belonging to the alimentary system, is
not fully ascertained. There is another appara-
tus on the side, whose real physiological functions
are also not precisely known. But whatever may be
the anatomical structure of the&e animals, so much

is known; that at a certain period of the summer
they move around the freshwater shells, and final-
ly fix themselves in great numbers upon the mu-
cus and burrowing into the mucosity of the ani-
mal until they are entirely surrounded in it, they
seem to move freely, but cast their tails under vio-
lent contortions. They are now surrounded by a
cyst of mucus in which they fall as it were into a
state of sleep, or into a state similar to that of the
pupa of Butterflies remaining motionless in the
cyst of mucus. (Plate XXXIV, fig. 1.) During
their rest in their little cavities they undergo chan-
ges. The part which represents a kind of head in
the Cercari^, is uow surrounded by a circle of folds.
This part becomes more and more prominent, and
when they leave their sacs they come out with a
sucker around the mouth, provided with little
hooks by which they can attach themselves. The
alimentary canal is very distinct, and in this form
we recognise a single Distoma. So that such a
sucking animal as that of (Plate XXXIV, fig. 2) is
finally transformed into a perfect Distoma, (fig. 4)
and this Distoma is finally found in the cavities of
the animal. After they have left the sac they
gradually penetrate into the abdominal cavity.

The process of the metamorphosis of the Cerca-
ria lasts rather long. During the winter it is
scarcely perfectly accomplished. But now the
question is: How did such a Cercaria arise?—
Where did it come from ? We have here an Intes-
tinal Worm (Plate XXXIV, figs. N, 0,) as it ap-
pears in the same fresh-water shell, before the Cer-
caria are observed, in one of which (Fig. P.) we
however notice small Cercaria. How are these
Cercarise formed? In June we find in the Worms
before mentioned, (Plate XXXIV, figs. N, 0, P,)
a great many little bodies distending them so as
nearly to cause their envelope to burst. If we trace
many of them, we may find in some which are
younger, that there are some with such bodies,
(Plate XXXIV, figs. Q, R, S, T, U,) and on close
examination these bodies are found to be eggs
which develope like those of other animals, and
finally give rise to little Worms, which grow to
the full size of Cercarias. These Worms (Figs. N.
and 0,) are therefore the mothers, or, as they have
been called, the nurses of the Cercaria, producing
a generation which is freely moveable, while they
themselves are constant parasites, and this free
generation is changed into Distoma.

But this is not yet the whole of the process,
How were the Worms of figs. N, 0, formed ? Still
earlier in the season, another kind of Worm is ob-
served in the same animal in which those nurses
are noticed, and having some anatomical differ-
ences ; for instance, their stomachs being larger,
(Plate XXXIV., figs. C, and D.,) and having some
other slighter differences ; and in their body we
observe in early spring or latter part of the winter
a series of transformation of eggs or germs which
grow gradually to all the changes of germs (Plate
XXXIV, figs. E, F, G, II, I, K, L, M ;) and finally be-

LECTURES 'ON EMBRYOLOGY.

tYieiTirsrses,so that the nurses are born from
another kind of Worms, living equally as parasites
in those shei's, and which are on that account
••called grand nurses, so that we have now three
generations °, Grand nurses observed in the early
part of the year giving rise, by a series of devel-
opment of their eggs to so called nurses, in which
chere are asrain eggs produced which undergo all
the changes ef a regular developement, and are now
•v>orn as Cercaria. And when these Cerearia have
2ived as free animals for a certain time, they un
•dergo the changes which produce Distorfia

It is a remarkable fact, that the nurses of Cer-
-carias bring forth a great many Cercarise, which re-
main as parasites; a great many of them being
-developed within the body of the shell fish, into a
Distoma. WQ have, therefore, three successive
generations which differ. The grand nurses give
Tise to a generation which resemble them in a cer
lain degree, but not in every respect. And the
nurses which give rise to*Cerearise ; and by meta-
morphosis the Cercariss are transformed into Dis-
toma. How the grand nurses are formed, has
not been observed directly. But it is known from
•other species, and it has been observed by Siebold,
that the Distoma will mature eggs which will give
rise to other ttnimals similar to our grand nurses
These which will either grow within the maternal
Distoma body, as in this form. (PI. XXXIV, figr. B)
where we have here Disroma, {PI. XXXIV, fig, A )
and here, {Fig B) we have its progeny. Butastbis
progeny is so different from the parent, there can-
not be a doubt but that at a certain period the
Distoma lays eggs, and that there is a certain gen-
eration which resembles the first starting point of
the animal. But whatever may be these changes,
there will be always a ^period when the animal
will lay eggs. And whatever may be the number
of these intervening generations, there will be al-
ways a period when the animal will come back to
the fundamental type of its species.

In the Tape-worm, a curious observation has been
made by Prof. Eschricht, who has ascertained that
the head, when it is furrowed by innumerable
joint?, will from time to time cast these joints, and
at regular periods reproduce them. The joints
present a remarkable uniformity of structure, in
each joint there being the various apparatus —
ovaries and other organs, which are developed in
these animals.

So that each joint is, in certain respects, an indi-
vidual by its structure, but remains united with its
other joints, forming a series of articulations In
such a condition of things, we have certainly an
approach to or at least some analogy with what
we have observed in the Medusae, which form
those piles of individuals called Strobila^ which
become free and give tise to as many individuals.
En Intestinal Worms such transverse divisions take
place ; the animal being free and each ring be-
coming as nearly as possible a peculiar individual
terming a kind of compound animal, but in a dif-
10

ferent sense from what we have observed among
•polyp-i, ti'K at a certain period of the year, they
castthese rings and scatter about the innumerable
eggs which they produce. The quantity of eggs
which are produced in each of these animals, and
the quantity of eggs which are produced by each
individual Warm, is amazing.

Prof. Owen has computed, that in one single full
grown female Ascaris, there were sixty-four mil-
lions of eggs developed. Now as it has been ascer-
tained by several Entomologists, that Intes-
tinal Worms a-nd their eggs have a more persis-
tent life than other animals, we should not won-
der tbat they have a chance to re-enter the bodies
of animals in which they live. It is a remarkable
fact, that Intestinal Worms are found generally
in particular animals, and that the same spe-
cies is not developed in every kind of animal,
even if they live under the same circumstances.
And now the chance which these various kinds of
Tape-worms have of being introduced into ani-
mals of the same species as those from which they
have been removed, is very great.

In the Fishes, for instance, the Parasites become
a part of the food of the Fishes, and in this way
they are transfered into the animals in which they
live. Some ef these Intestinal. Worms have ua-
dergone the action of boiling water without being
killed. Their eggs have been pat under the influ-
ence of strong acids without being destroyed. So
that we should not wonder, after such experiments
have been made, that these animals, having been
introduced into the alimentary canals of animals
should live to grow and reproduce their species,
instead of being digested.

The eTternal Worms — such as live in the water
or the earth — when they are hatched, present al-
ready transverse divisions. They early "assume
(Plate XXVII, fig- A | the shape of common artic-
ulata. Professors Milne-Edwards, Loven, and KoK
liker have traced the changes of several of those
Worms. But I see that I have scarcely time to state
the leading facts of their history, and I must go on
to another subject,

I shall now endeavor to show that there is a uni-
formity of type among the Worms, notwithstand-
ing the external differences we observe among
them. In these various external Worms (Plates
XXVIII and XXIX) we may notice some in which
there are no external appendages at all (Plate
XXIX, figs. A and B)— for instance, theNemertes
— which is very common on these shores where I
have first noticed several species. In Planaria
there are also no external appendages (Plate XXX,
fig. B).

In the earth-worm (Plate XXX, fig. A) we hav
appendages upon their rings, and although very
simple, we have here the first step toward those
complicated appendages which we notice in oth-
ers. The complications grow out of modifications
of those appendages themselves. Instead of stiff
hairs scattered about, we may have a brush of

PROF.. A€ASSiarS

those hairs arising from definite parts, or the
brashes may not arise immediately from the rings
of the animal, but there maybe vesictes into which
the blood-vessels may run, and from which arise
various hairs. And the manner in whieh these
hairs are combined with the vesicles, aad the ves-
sels and the little hooks which may be appended
to them, will constitute the most complicated ap-
pendages which can be imagined.

And, indeed, there are no animals in whieh the
appendages are so complicated as they are observ-
ed to be in some of the Anaulata. The anterior
part may have one kind, the middle part m&y
have another kind, the posterior past may
have a third kind; or those of the head may
be very prominent, and those of the pos-
terior extremity of the body may be scareely dis-
tinct. And these are the more remarkable, as we
may find in the earlier condition of those aaJraals
that they are uniform. For instance, in this worm,
(Plate XXVIII, A) which is a new genus, whieh I
have called Pleigopththalmus, we have little brush-
es of stiff hair, and what is still more curioas, a
pair of eyes to each rJng, And when the animal
grows larger and larger these eyes vanish succes-
sively and there is only one pair left in the anterior
portion of the body, and one on the posterior part
of the body, and the intermediate ones are ^ one.

And here (Cirrhatulas, Plate XXVIII, fig. B)
are not merely eyes, but several colored dots to
each ring, and along the whole body uniform vas-
cular threads. Eyes which have a crystaline
lens may gradually be found to pass to simple co-
lored dots. This is the case, foy instance, in the
Planaria (Plate XXIX, fig. E), where we have no
longer an eye, but we have a great accumulation
of black dots »pon the skin, some of which are
larger than others, which can no longer be consid-
ered as eyes — which can no longer be considered
as organs of sight— but which are doubtless an ap-
paratus simply to receive an impression of the
light.

These animals, without eyes properly, but simply
with colored dots, must have merely impressions
of light. The eyes are merely to concentrate the
light. In Cirrhatulus, we have simple vascular
threads (Plate XXVIII, fig. b> to each ring ; but in
Terebella, which is the perfect state of the same
animal (Plate XXVIII, fig. C< they are reduced to
complicated gills behind the head. The vessels of
the anterior gills, which occur in the anterior part
of the body are indeed only modifications of these
vascular threads. In the young animal (Plate
XXVIII, fig. B), which has been described as a pe-
culiar animal, under the name of Cirrhatulus, we
have the threads all along the body, and the pos-
terior threads, gradually disappear first, and the
anterior ones are branched and transformed into
gills ; and in the beginning there are vascular
threads, one to each ring.

Let me now add another fact referring to this
animal, that this Cirrhatulus, when young, as it is

represented here (Plate XXVIII, fig. B / is
resceat. The adult, which has been described as a
Tersebella, is also phosphorescent. But in the last,
phosphorescence is only noticed in the long
threaelSjbutin Cirrhatmlus it is noticed all along the
body. On close examination I have satisfied my-
self that the blood vessels are the pborphorescent
apparatus. Some such threads separated from
the bedy when acted upoa by alcohol, or some
other strong reagent, would throw oat faint light
when no other part of the aaimal would emit
it. So that we have here an example of phos-
phorescence in a position of the body different
from another whieh we have mentioned before —
This phosphorescence proceeds from the blood
vessels. We have had aa exam-pie from the ner-
vous system* I may Quote others ; for instance,
some Insects in whieh the respiratory organs, those-
Traeheal organs, those aerial sacs, will emit
light; and tbe focts are such that we perceive a
connection between coloration and phosphoresenee
and sight, as tfoere is between electricity, heat and
light. The physical phenomena are parallel to the
phenomena in the animal kingdom, only it is more
difficult to show their connection ; but I hope to
show that there are at least among the Molluscav
some types in which it may be demonstrated that
euch a connection exists.

[PLATE XXXV— CATERPILLAR.!

L.C. im- aud one more )etnark, thai the Ca,ierpil-
lar, with all its appendages, (Plate XXXV) should
be eomjmyea! with the Worms. "What are the di-
versified hairs which are observed upon so many
Caterpillars 1 They have been usually considered
as hairs; but they are connected with the organs
of locomotion and respiration, as in the Annulata.
We should, therefore, institute upon the Caterpil-
lar a regular comparison, to ascertain whether they
are not in some respects analogous to the various
appendages of the Worms. This comparison I
have not instituted. It remains to be done ; but I
cannot he)p thinking, on noticing the close resem-
blance there is between the diversified aspect of
Caterpillars and Worms, that in their analogies
there will be also a type discovered, as it. has been
noticed in the appendages of Worms ; and thas
Caterpillars will only be another modification more,
of one and the same type.

[PLATE XXXVI —SYMBOLICAL FORMULA OF AR-
TICULATA ]

LECTURES ON EMBRYOLOGY.

vntrocKiced in one of cay preceding lec-
tures symbolical formulas for the three classes of
Radiate animals, 1 deem it usefEl to do the same
:for the Articslata. Thus the symbol of the whole
•department will be an Omega (Plate XXXVI. fig.
A) representing th-e curious mode of formation of
the embryo at the inferior part of the vitellus, of
fcfee twe sides arise ia^r^er to -envelope the

vitellus. For the class of Worms we will have the
same figure slightly opened at the summit, (Fig. B.)
IFiff.'C, an Omega with a transverse bar, will repre-
sent the class of Crustacea, where two regions are
already distract. Finally, Fig. D, with two trans-
verse bars, for the class of Insects, in which the
body is divided in three regions,

LECTURE X

tracing the nrst formation and the growth
'of animals, there is one point, which never should
"oe lost sight of. It is, that at various periods of
this growth, the substance of which the animal
consists gradually changes.

We have seen that in the beginning the germ
•consists of simple cells, derived from a modification
'of the yolk, Such is the first condition of all
•germs. Now, from this starting-point we may ar=
Vive at animals so complicated as Man.

In other animals, throughout the series of the
animal kingdom, in which the most complicated
structures are observed— la which structures very
distinct are successively foreied, — <fiesh, blood,
'nerves, skin, hairs, scales, and aR possible struc-
tures so different as scarcely to be compared—
how are these formed *? Are they new things fa-
'troduced during the growth of the germ— or are
'they only modifications, simple changes o'f one
and the same fundamental element, modifications
•of the cellular tissue which characterized the germ
\vben forming 1

This is a question which can be answered by
'facts which have been entirely investigated by one
gentleman, a young physiologist of 'Germany, Pro-
fessor Schwann. Ten years ago he began tc exam-
ine the subject of animal tissues, and up to that
time it was believed that animals and plants drSer-
•ed widely,— that their substance had nothing simi-
'lar, — that cells existed only in plants. Such was
the condition of things in i&SS, when Sc&wann,
taking tip the beautiful investigations which
Schleitien !!iad just published upon the structure
and growth of vegetable cell-s, came to the conclu-
sion that animal tissues consisted equally of cells,
and that whatever may be the cdtn plication of this
substance in the animal — whatever may be the ex-
ternal form of the various parts in the animal tis-
sues— they all originate from cells, and are, after
-all, only modified cells.

?u tbis absolute form, .perhaps the -results lctf

Schwann will have to 'be sotnewnat modified, 'otft
in foe main all subsequent investigations have
only gone to confirm his unexpected result, and a't
present tnere is no student in Anatomy, who has
not seen these cells o'f animal tissues, who is not
able to £ nd them out, even with microscopes of a
very inferior quality. But it required the sagacity
of the able and persevering investigator whose
name I have mentioned, to start such an investi-
gation—to go through with it— to give Unfinished, tc
the world, and then to remain silent for ten years
through all the attacks he has "had to undergo.

Since Schwann published the volume containing
the results of his investigations, he has net been
heard in the debates which are still going 'onnpon
this subject, It is a remarkable instance of con-
fidence in his theory, and of a desire not to inter-
fere with that which contradictory investiga-
tions might bring about. Still it is known by ;his
friends that he is pressing on, and preparing new
investigations, which may iea& to as important re-
sults as his preceding labors.

His efforts now go to ascertain 'how these ceils
are combined to form individuals of different kinds.
Indeed, he has undertaken nothing less than to in-
vestigate, if possible, the principle which combines
those cells into individual cells, — to ascertain the
nature of that power which we call vital power,— to
find oat what kind of influence it is which consti"
tutes individual, independent 'and progressive be-
ings.

i have delayed introducing this subject up to the
present evening, became there is no class in which
the cellular structure of animal tissues can be so
fully and easily illustrated, as among Mollusca.—
In their tissue when full grown, in their egg when
forming, the celmlar structure is perfectly plain
and easily ascertained.

To what important results for Physiology the
final investigations on this subject will lead, cas
scarcely be foretold now. 'For siace it has bees

PRO*1.- AGASSI2TS

ascertained that the animal tissues are. in their
fundamental structure, identicalwith the vegetable
tissues,. we may expect that l>eianical investiga-
tion may tlnow as much light upon the animal
kingdom, as the study of animals may threw upon
the vegetable kingdom.

Easy as it has- been- to study- the s-tractt»8 of
vegetable tissues, so diScu-lt has it been to ascer-
tain their functions — to ascertain the working of
the various organs 5fl plants The most different
and contradictory opinions are entertained upon
vegetable functions, upon the circulation of their
sap, upon their respiration, and the acti©n of res
piratibn upo-n their fluids.

On the contrary, in animal structures, tfae func-
tions are easily traced1. The combined action of
various functions upon each other, can be easily
asct-rtained It was the stractare— the intimate
stiuctwe— vvhii-h it wasd'ificult to investigate. And
now, by re1 erring the result fiora one kingdom to
the other, it is to be hoped that much more rapid
progress will be obtained than before.

One unexpected resnlt has airead'y been ascer-
tained—namely, that cells are properly the organs
of living beings; that all functions are influ-
enced by life, by the independent life of isolated
cells. It is not the stomach, as a whole, which di
gests; digestion is influenced' by the cells which
line the internal surface of the stomach.

The life of individual cells may be compared to

•

the action of several Farce organs combined into
one system, as a whole. Ifow much independence
there is really in the life of individual' cells, can no
where be better shown thaa in some of the germs
&f Moll asks.

Let me for a moment illustrate the various figures-
which are represented in Plate XL.

They show the changes which a Mollnsk may
undergo; a species of Eolis, a naked Mollusk,
found in Boston harbor, of which tkere is a figure
in Plate XLIT. fig. C Several species of these
Mollirsks occur in Boston harbor, and can at any
rime be obtained far investigation. Several eggs
which contain a single yolk, are first noticed
((Plate XL), and in tiie same plate are represented
all the changes whrch the yolk undergoes in the
process of dividing, up ro the period when the
whole mass of yolk is transformed into ranumer-
able cells, as represented here

The divisions of these masses are not always so
reeulai? as thay havg been described. In this
Eolis, it does not constantly tak'e placeby a reg-
ular division iatotwo halves. "?ou see that the Swo
halves are more or less different in their size;
sometira-es the division takes place into three
spheres, two of which are sra«iler than the other,
and not even equal among themselves-. In others,
there are th-ree equal spheres 5 in others, fou-? cq,ual
spheres ; in others are four less equal; in oShers
are five almost equal ; aad still in others, five, all
®f which are small. Many irregularities ocour.
Ihers is tia invar iable

|Pr ATE XL— CHANGES OF THE YOCTNG

[.PLATE XLII J

You iiiav

ures ot ibe &aine,

('Plate XL) that the process of dividing the yolk
is very regular, there being first two, then four
equal divisions, out of which may arise on one
side £QUE ottn.es large spheres, and on the

LECTURES ON EMBRYOLOGY.

side four smaller ones. We have still in another,
less regularity. Four less spheres are formed, and
between them two large ones, and two very small
ones; and soon, hy multiply ing the divisions, we
arrive finally at the state of the yolk, when it is
composed of a mass consisting of many yolk cells,
in each of which there is a clear sphere, As there
is one forming in each division when the process
of dividing the yolk has only divided the mass into
fewer spheres.

About the time when the whole mass is reduced
into small cells, there are vibrating Cilia coming
out from the surface of some of these eggs (Plate
XL, figs. A, B, C, D.)

But the most curious phenomenon which takes
place is this : that the whole yolk does not con-
stantly go on to form one single individual, But
there may be instances when the mass of yolk
which has been subdivided into cells, is itself divi
ded into two, or three or more masses, which grow
independently, several individual animals arising
from one yolk;— several individual animals aris-
ing from one mass of yolk, which thus divides.—
And in this process of the division of a whole mass
into several individuals, there are isolated cells,
which are separated from the main mass, and
continue to live and to rotate by the agency
of their vibratile Cilia with the main mass. And
in such a case we have the wonderful sight of two
or more germs, having been derived from the di-
vision of one unique mass of yolk, constituting
two or three, or more individuals, each moving for
itself and rotating with the others in one yolk
membrane, and isolated cells which also rotate be-
tween. So that individual loose cells maintain for
a time a separate life, and continue to live during
the whole period of growth of the larger animals
within the egg membrane; and those isolated, scat-
tered cells die only when the larger germs, which
will grow into perfect animals, have been hatched,
or pressed out from the vitelline membrane.

Nothing could show more distinctly that there
is independence of life in the cell than the fact of
this isolation. But what the combining power is
between those cells which grow and form individ-
ual animals, can scarcely be understood under
such condi ions. Whence the action of the vi-

tal principle which keeps the cells together, oiigi-
nates, escapes our intelligence. Indeed, nothing is
more astonishing than to see that under slight
pressure, such a germ may be resolved into loose
cells, whose Cilia will continue for a short time to
vibrare,in the same manner as a nebular mass seen
through a powerful telescope may be resolved into
individual stars, which nevertheless form a pecu-
liar cluster of isolated bodies ; similar to the cells
with individual life, which constitute, as it were,
similar clusters. And when they have gone beyond
this period of life, then they have undergone a
more intimate connection, which prevents their dis-
solving again ; and then they go on constituting a
new being. Then during the further changes, by

which they now assume the form of the parent
animal, there are constantly isolated cells cast from
the main body, which revolve for a short time, and
then die, This process, which is exemplified here
In the early condition of life, and under a simple
condition of structure, is well known to take place
in many animals, which east their skin repeatedly
during life, as the caterpillar ; or Mollusks, which
cast their external coating under the form of mu-
cus; or other animals, which cast their hairs; or in
our own body when the epidermis is cast and oth-
er cells are formed to take the place of those which
fall off in the form of small scales. So that you
see the remarkable phenomenon of the isolated
cells of Eolis, is only what we have on a still great-
er scale in higher animals, where millions and mil-
lions of cells are constantly east from the surface
of fuil grown individuals.

These cells consist permanently and uniformly of
an external envelope, a thin membrane containing
a fluid, within which there is another vesicle called
the nucleus, and in the centre of which,there is still
another called the nucleolus,so that a perfect cell in
its perfect condition is a sphere enclosing two other
spheres, the innermost one being the smallest, ap-
pearing like a granule. In such cells as are represen°
ted in Plate XXXVII, we have figures with which
we have been familiar from other illustrations. A
cell in its perfect condition has the same structure
as an egg in its primitive formation. Here we ar-
rive at a most unexpected, but universal, uniform
structure, not only of cells, but of the primitive
substance of wh'ch new individuals are to be form-
ed. What we call eggs in their simple condition,
are cells of a peculiar structure, formed in a pecu-
liar part of the body, destined to undergo peculiar
modifications, by which the body is not enlarged,
by which no particular function is performed, but
by which a new individual is formed. So that in
every point of view we find unity in the structure
of animals, even in the structure, compared with
the mode of re-production ; the cells of which the
tissues consist being identical in structure with the
eggs by which new individuals are produced.

There is a question which may be asked, and to
which I hope to give at least a partial answer.
How are these cells formed? and how are these
eggs formed? We have examined the mode of
formation of the germs. Let us now examine the
mode of formation of the eggs.

I have been fortunate enough to trace them
through all their phases of formation in Mollusks,
and I think there has not been a link in their trans-
formation which has escaped my attention. So
that the whole process of their multiplication has
been directly observed. Tracing the formation of
eggs will be tracing the formation of cells, the mo-
ment it is understood that cells and eggs have the
same structure. When examining very young ova-
ries—for we must not take the egg when laid— we
must not take them when formed within the ovary
—we must not take even a full grown ovary—

PROF. AGASS1Z,;S

take the ovary when forming and examine what is
produced. There we observe that the ovary consists
of pouches (Plate XXXVIII, figure A)— folds of
[PLATE XXXVIIi.— OVISAOS AND EGGS OF As-

CIDIA.]

membranes, in each of which bottle-shaped pouch-
es (fig. B ) there are masses of eggs and other sub*
tances— granulated substances— and complete eggs
in the larger ones. You may perhaps distinguish
from the distance that in such a pouch (fig. A.)
which is circumscribed by a membrane, there is a
mass of little granules and a number of eggs, each
having a vitelline membrane with its germinative
vesicle and its germinative dot. The smaller of
ihe.se pouches contain the same elements. These
smaller ones will contain fewer eggs. The still
smaller one will contain also eggs, but they are
not so v/ell defined. And we may find some pouch*
es in which there are no distinct eggs, but a bag
full of uniform, clear liquid.

Here is the starting point. And if we examine
under a very high power what is going on in these
pouches, we may observe all the changes which are
represented (Plate XXXVII) in these various fig-
ures. First a little ba<? is observed, but perfectly
transparent and homogeneous. Others may grow
larger, but still contain transparent homogeneous
fluid. All these figures are represented under the
same magnifying power. Then we may find one
in which the membrane surrounding the liquid di-
vides. This process of dividing is observed in the
yolks when fully grown, giving rise to the embry-
onic cells; here it takes place to form numerous

[PLATE XXXVII —FORMATION OF GERMS.]

eggs, giving first rise to two continuous vesicles,
one larger than the other, which may grow to an
equal or to an unequal size— the one dilating, the
other growing less, may give rise to two'half vesi-
cles* Next, they may grow larger. Next, we ob-
serve that granules are formed. Here we have the
first element of heterogeneous substance. Granules
are formed within. How such changes are
brought about is not understood. It is a mystery
in the subject of our investigation. But that is
takes place can be easily seen.

Now, these bags being full, no longer of a
uniform liquid, but of a granulated liquid, will un*
dergo the same change. They will divide into
two sacs, which will grow equally or will remain
unequal, and we shall have the process of separas
tion as observed here. But as soon as granules
have become numerous, there is a condensation
taking place in some point. These granules ag£
glomerate in some point ,without having a mem=
brane about them. There is simply a dense con-
densation of granules in one point. And this con-
densation will grow larger, so that the condensed
sphere within the granulated liquid will successive1'
ly be larger and larger ; or by the side of the large
one there will be several small spheres developed,
growing at some distance from them, and remain^
ing isolated. And perhaps some two such spheres
will begin to separate, or a separation of the part
which contains only clear granules from the part
in which a condensation has taken place, will occur
in this way, and then those spheres with two cen-<

LECTURES ON EMBRYOLOGY.

tres of concentrated mass will begin to separate,
as we have here (Plate XXXVII, fig. 2) where we
have two distinct spheres, with a concentrated mass
in each. At this period, each of these concentrated
masses is without an envelope. And now there
will be an envelope formed around it. And here
it will grow into a hollow vesicle ; and as soon as
this last process has taken place, we have a free
egg. Around the spheres of condensed gran-
ules a membrane is formed, and some one or sev-
eral of the granules within growing larger, give
rise to a perfect egg. And so we see in the larger
and still larger, those concentrated collections take
place and go on developing as we have them here
(Plate XXXVIII, fig. B.) with a mass of con-
densed yolk,swimming in a granulated liquid. And
then the eggs escape from these pouches, and are
laid, under their normal form. Then begins the
series of modifications and repeated divisions
and subdivisions which give rise to the formation of
a germ to form a new individual. Now, the
changes of these eggs illustrate the same time
the formation of cells. They are multiplied by the
division of vesicles containing a simple liquid.
Condensation takes place within and around this
collection of granules, and a membrane is produced.
Then will appear again some granules growing
within, which will be the nucleoli.

It can now no longer be doubted, that the pro-
cess of formation of eggs and the process of
formation of cells, are identical, as it was under-
stood that eggs and cells, in their perfect formation,
were similar organizations.

I would now proceed to illustrate the further
changes of the germ of Mollusks— to show how the
young of the Mollusks are developed — how they
successively assume the form of the perfect ani-
mal, and how their various organs are developed.
Here is a diagram (Piate XXXIX), which gives a
general view of the rapid successive changes which
the eggs of Cuttle Fishes undergo, in which the
germ is formed around the yolk (Fig. B). After
some changes, the outline of the young animal is
formed (Fig. E), and after some other changes
(Fig. F), it begins to resemble the ful! grown ani-
mal (Fig. G) ; and before the animal is hatched, we
see it really does resemble the Cuttle Fish. (Plate
XXXVI, fig. A).

You see ( Plate XXXIX, fig. G.) the body,the eyes
the tentacles, &c. But in order to show that all
Mollusks have the same mode of formation, not-
withstanding their apparent diversity, I must be-
gin by showing you that the perfect animals them-
selves are constructed upon the same plan. And
this is no easy task. There is no group of the ani-
mal kingdom which has been more studied, and no
one which is less understood than that of the Mol-
lusca in their morphology. I do not say that
there is no group in which species are less known.
On the contrary, few departments of the animal
kingdom have been more extensively studied in
the details— in the distinction of genera and species.

[PLATE XXXIX— EMBRYOS OF THE CUTTLE

FI-H I

ot them have heeu vvt-H ti_;urcu C4ii-i de-
scribed. But the correspondence of their parts,
from one class to another— the analogv of the dif-
ferent organs in their various positions— this is
what is not understood in this class of animals.

That all Mollusks agree in the softness of their
bodies, is well known. And this character has
.been constantly insisted upon as the distinguish-
ing character of Mollusca — a soft, contractile body
without articulation. This is the general charac-
ter assigned to the type of Mollusca. And in ad-
dition to their character, derived from the external
appearance, is usually added the fact that they
have a nervous system, consisting of a circular
ring around the entrance of the alimentary canal,
with a swelling above and below, forming a single
ring without a chain of repeated swellings, as is
observed among the Articulata. But that Mollusks
agree beyond this, in their structure, is so little
understood, that in our descriptions,we find groups
contrasted in which it is said that the gills are

PROF. AGASSIZ S

[PLATK XXXVI— CUTTLE Frsn.J

upon the UUCK ; OUICMS in which ic is said ttiat the
gills are upon the sides, or on the lower side of the
animal ; and others in which the eyes are said to
be in an entirely different position from what is
observed in others. Indeed, no analogy has been,
nor can properly be traced between these animals.
I have, however, taken pains to trace analogy,
and if I am not mistaken, have succeeded in
making it out. But if I shall equally succeed in
satisfying you, is anotherquestion, which you may
decide after my illustrations have been made. Let
us begin with an animal well known in its form and
structure. Let us take the Oyster or the Scallop.
If we lift one shell, we see that it is lined inside
with a membrane called the mantle. The two
valves of the Scallop (Plate XLIV, fig. A) as you
see them drawn here on a large scale, are both
lined with the mantle. On opening these two
valves, you see the mantle on both sides. The
membrane, as it lines the valve of the right side, is
seen in Fig. B. The membrane which lines the
opposite valve, which is removed, and which cov-
ers the internal organs, is removed with the shell.
These two membranes lining the shells hang on
the two sides of the animal. So that the mass of
organs, the gills, the muscles, the liver, and alimen-
tary canal— the whole structure is contained, as it
were, between those two folds — those two mem-
branes— as the contents of a sac within its walls. !
Or I may compare the shell to the coat, the lining
membrane to the waistcoat, and the organs to the
body within.

[PLATE XLTV— PECTEN— SCALLOP-SHELL J

The position of the eyes is very remarkable in
this animal. There is a series of eyes (Plate
XLIV, fig. B) all around the margin of the mantle
— about forty or fifty, or more, in number. And
you see that they occur upon both sides, so that
it is like a row of buttons along the coat, forming
here two rows of eyes, [laughter] ; and this posi-
tion 5s so extraordinary that we may not expect to
find any analogy with the Cuttle Fishes, (Plate
XXXVI. fig. A), where we have two large eyes up-
on the sides of the head, or with Strombus, as we
have in Plate XXXIII, where we have two large
eyes, upon peduncles, on the two sides of thepro-
[PLATE XXXIH— STROMBUS.]

LECTURES ON EMBRYOLOGY.

, which comes out from the mouth. We
enight not expect to find these eyes abottt the head
in any way analogous to the large number ef eyes
^vhieh surround the margin 07" Che mantle.

Nevertheless, if I have understood £he structure
of MollKsca,! shall show that these eyes are £.11 the
same as those of the Oyster, the same as those of
the Cuttle Fish, the same as those of the Strotnbus,
the same as those of all ether Mollusea. And I
will try to reduce all these different forms to a few
simple types, and then compare these few simple
types together, in order to find, if possible, &e
•common uniform type. The Scallop, which £ have
already mentioned, belongs to the so called bivalv-
-ad shells — to the Acepiiala. And there are many
kinds with regular or irregular shells, the two
valves being equal in some, as is the Clam (Plate
XXXV, fig. B) for instance, and ia other hard-
shelled animals; one being deeper than the other,
one exceeding the other, and forming a beak over

it, or being unequal, as the Oysters are unequal,

•
[PLATE XXXV.— ACE?HALA— CLAMS.]

All th^se differences will not modify the general
arrangement of parts as seen in tbe Scallop.
There is a mantle around the whole body; next two
pairs of gills : the fleshy mass is in the centre ;
and above are grouped all organs, as the liver, ali-
mentary canal, &CM &c. (Plate XLIV, fig. A )
11

[PLATE XXXIV— GASTEROPODA.]

In the Snail-like Mollusea, or Gasteropoda, we
have, on the contrary, a large fleshy mass below,
on which the animal walks. At the anterior part
of the body, there is a pair of eyes upon tentacles
a-nd above the foot, the main mass of organs — the
stomach, t-iie liver, the gills— generally protected
by the shell. If the shell be removed and the man-
tle split, we have the gills on one side, the liver on
the other, the intestines winding within, the heart
being near by, and the whole mass within the shelU

But anaong these Gasteropoda or Snail-like ani-
mals, there are many in which the body is not so
complicated, or at least net twisted, but straight as
in Eolis, Boris, Patella, Chiton, Emarginula, Fis-
surella, &c. &c. And in some of them we see that
the body, for instance in Eolis, (Plate XLII fig. C)
has respiratory appendages symmetrically on both

[PLATE XLV.— TERBRATULA AND SPIRIFERA.]

PROF. AGA&SIZ'S

sides, all along the upper surface of the body. So
also Glaucus on both of the sides, (Plate XLII, fig.
A.) So it is also in Doris, where, however, the
mass of gills is placed only at the posterior extrem-
ity of the body, and has long tentacles at the an-
terior extremity.

But, without entering into more details, youlmay
have already remarked that whatever differences
exist between these animals in the inequalities of
the two sides, we can reduce their symmetry to the
regular arrangement of parts on the two sides of
the body, more or less developed on one aide than
the other. And passing from these Snail-like Mot-
3usca to the Cephalopoda— to the Cuttle Fishes— we
shall have again (Plate XXXVI fig. A) all the parts
analagous to the symmetrical Gasteropoda, the
eyes and the gills are here again in pairs on the
two sides of the animaL

But how will Cephalopoda and Gasteropoda
compare with the Acephala, is the great question.
The fleshy mass which is in the centre in Acepha-
la, is below the mass of organs in Gasteropoda. —
We have the liver, we have the alimentary canal,
and we have the heart all shown. Those main
organs are above the fleshy mass, and hanging over
the fleshy mass, we have only the gills and the
mouth.

Let us for a moment suppose that the mantle
was not so long, and would not hang in such large
folds on the two sides of the body, but be shorter.
And let us at the same time suppose that this
fleshy central part was not so contracted, (Plate
XLIV, fig. B) but stretched down, and you see
at once what analogy we have. You may change
at once such a bivalve shall into a univalve (Plate
XLIII, fig. A) with a single shell. Suppose the two

[PLATE XLIIf.— MARGARITA.]

valves were united, and you will have what we
observe in Patella, where there is a shell spreading
on the back of the Mollusk, without any spiral on
the summit; and among bivalves there are several
in which the two valves are immovable ; the 'di-
vision is well marked in youth, but they unite to-

gether in old age. This is observed in the
of Naiades, among those which constitute the
genug Alasmodonta. And that the cover be shield-
like or divided into two valves, does not indicate a
great difference.

[PLATE XLYL— NAUTILUS.]

We have already noticed the little value of such
differences when speaking df the Crustacea, in
which we had among the Entomostraca, some
whose bodies were covered with Sat shields, and
others in which the bodies were enclosed between
two moving valves, as in Cypris. Suppose this
Patella was articulated in the middle, and the man-
tle was drawn down, there would be the first ap-
proach to the Scallop or the Oyster. Suppose that
the foot was reduced to one central fieshy mass,
and the analogy would then be almost complete ;
only the difference between the eyes and tentacles
would remain.

That this is no vague supposition Jo admit of
such a division, is showa by some shells, in which
there is a notch on one side, in the longitudinal di-
ameter of the shell, for instance in Parmophorus,
and in Emarginula, there is realty a deep fissure.
So that we pass almost gradually into the type of
two connected valves, and into those which have
moveable parts. Now for the eyes and for the
other parts which are modified in this structure.
The eyes are here (Plates XLII, XLIII,) placed
around the mouth. The mantle in many of the
Mollusk univalves, extends all along the shell, as
you will observe in Phasianella, in Buccinum. &c,
But there are no eyes except in the head. Last
winter, however, it was my good fortune to meet
with a little Margarita in Boston Harbor, in which
we have (Plate XLIII, fig. A) tentacles all along
the body ; and at the base of each tentacle, are
dark spots similar to the eye which is observed in
the anterior part of the animal. On examination,
I noticed that the mantle is constructed as it is in
the Scallop. (Plate XXXVI, fig. B). We have,

LECTURES ON EMBRYOLOGY.

therefore, here, series of eyes all around the mantle.
We have even the series nearly as completely de-
veloped, as in the Pecten, and we have a fully de-
veloped eye on the anterior part of the head, on
ihe side of whicb, there is one larger tentacle ob-
served ; making the analogy perfect.

But let the lateral radijTientary eyes disappear
•and the anterior pair remain, and we have the
•ordinary condition of Gasteropoda^ so that the
question whether there is any similarity between the
Acephala and other Mollusks, must be answered
toy the assertion that the analogy is as complete as
ean ever be expected between animals of the same
great department, but belonging to different classes.
Endeed, in tracing the differences between the man-
tle of Margarita, (Plate XLIII,) and that of the
Acephala, we notice the anterior part of the mantle
has larger fringes corresponding to the region
where those larger eyes occur. So that we have an
uninterrupted series from these in which there are
•©yes all around, gradually to those which have
•eyes only a part of the way round,, and to those
which have only two eyes. Tracing, however, this
structure further down, we come from Pecten to
shells, as in Mya, where there are no eyes at all. But
•even in these, there are colored specks at the open-
ings of the mantle. So that we have a natural ap-
paratus with compound eyes, with perfect tensesjn
one order of Mollusca, as they exist in vertebrata,
down to those which have eyes with a rudimentary
crystalline lens, and still further down to those
specks which can enable the animal hardly ,if at all,
to distinguish between light and darkness.

Here we have a new species of a so-called soft
shelled Clam, (Ascidia) (PlateXLI,) in which the
animal is included within a sac, and leaving only
two openings at one end. Now on the ends of
these openings we have ia tkis— a new species,

[PLATE XLI— ASCIDIA OR SOFT-SHELLED CLAM.]

Ascidia scutella — which I have observed recently
in New Bedford— colored dots, What are they 1
The last indication of the lowest condition of eyes
on the margin of those tubes, through which water
is introduced into the body, And through these,
and through the open tubes of Clams, we pass
gradually to those more complicated organs, as
they are seen in the higher species,with a pair of
eyes. From those in which we have eyes, to those
in which we have only colored dots, we have grad-
ual steps.

And in this way from the most regular Cephalo-
poda (Plate XXXVI, fig. A.) down to the Ace-
phala, (Plates XXXV, XLIV and XLI) we have
the multiplication of these organs, tending to
transform well-defined organs into single colored
specks.

In my next lecture I shall say a few words more
upon the structure of Moilusca, and then proceed
to illustrate their embryonic growth.

LECTURE XI

En every type of the animal kingdom, there have
been some forms observed which have perplexed
Naturalists, and whose natural positions have not
been ascertained until after extensive investiga-
tions. You remember with what difliculties we
struggled when examining the natural circumscrip-
tion of the type of Radiata^ how, many animals,
which had been considered as Polypi, had to be
excluded from that class, as it must be circum-
scribed by the observations of modern investiga-
tor?. Among Articulata, we felt the same difficul-

ties, owing to the peculiar structure of many par*
asitic Worms, of many parasitic Crustacea, which,
when full grown, differ so widely from their em-
bryonic condition,that they cannot be arranged with
them, unless the whole history of their metamor-
phoses be ascertained by embryonic investigations.
The same difficulty occurs with Mollusks.

If we had only to deal with animals with bivalve
shells, with the Snail-like Gasteropoda, or with the
Cuttle-fishes, as I showed in my last lecture, the
general structure could be traced in their outlines1.

1'ROF. ASAS'SIZ'S

and there woui'd be no d'oiibt left as to the final
circumscription of that group.

But there are animals which mast be referred to
the type of Mollusks, according to our present
knowledge of their structure, which differ so wide-
ly in their appearance from Mollusks, that, at first,
when mentioned, this combination seems utterly
unnatural and unfounded • and indeed, leaving the
impression as if there could be no foundation for
a natural system, if such combinations were to be
considered as natural Nevertheless, I think that
the association of some animals which I am aboat
to illustrate, will be found to rest on real aMnity ;
and that the external differences in form will have
no influence upon the impression which such a
combination will leave.

[;See Plate XXXIX, page S3.}
We have here in Plate XXXIX, and in several oth-
er diagrams [which the Professor exhibited to the
audience,] Polype-like animals, resembling Folypi
very much by their stems, with cells ia which there
are living animals extending and contracting in a
manner similar to Polypi, with tentacles around
their mouths, which a«t in a manner resem-
bling Polypi still more than the stem in -v?hich they
are included. And these animals do not belong to
the type of Polypi ;: they are true Mollasks. The
discovery of their internal structure was made
almost simultaneously by Ehrenbrg, by Milne-Ed-
. wards, and by Mr. Thompson, of Corkrso that their
relation to Mollasks is now known to be very
close. They have a relation to the radiated type
of Polypi by the fringes around the mouth. —
But the arrangement of their whole system is truly
bilateral.

This ngure (Plate XXXIX, fig. C)' represents the
alimentary canal, which differs very much from
the Radiata, ia being curved upon itself, in having
distinct openings, a large sac which represents the
stomach, and a structure which comes very near
that of some animate which have never been sep-
arated from Mollusks. If we were only to consid-
er those, perhaps the resemblance to Mollusks
might have escaped observation.

But let me now trace further than I did before
the analogies which exist between Mollasks. I
compared the Gasteropoda with the Acephala and
the Cephalopoda; I showed that there was one
type in the bivalves, ia the univalves and in
Cephalopoda. But between the Ascidias (Plate
XLVII), and the Clams, (Plate XXXV), there
are only slight differences. Suppose the shell of
the Clam (Plate XXXV) to disappear, the mantle
to be almost entirely removed, the respiratory
tube to be shortened, and the two openings to be
somewhat remote, and we shall have such an an-
imal as is represented in Flate XLVII, fig. H, en-
closed ia a sac with two openings, which'are not
the openings of the alimentary canal, but are the
openings which lead into a cavity ia which all the
©rgans are coatained.

Plate X%XV, page 78.}

And BOW going further, we may have all possi-
ble modifications of this type whea it is contracted
and when the peduncle is attached. Plate XLVII;
f g. A, represents a fixed Ascidia, the peduncle be-
ing only a prolongation of the sac-like envelope.
Here we have two openings of the sac, correspond-
ing to ths two openings of the clamshell Eeyond
this type, we may have oae ia which several indi-
viduals ere u-aited by their base. And then, from-
single animals, we pass to compound animals
combined by their attachment on one spot, (Plat©
XLVII, fig. F) or by a gelatinous envelope which
keeps the eggs together, (Tig. B), and constitutes'
compound animals.

The interaal structure of these Ascidia, (Plate
XLVII, fi'g. C.}:, is so like that of Clams, that there
is ao difficulty about their analogy. Now, one
step further, and suppose that the gelatinous en-
velope which unites these individuals seeretes-
calcareous substance. Suppose further,, that each
individual is much smaller, and ia addition,,
that one extremity, instead of presenting fringes
at its opening, is- surrounded by threads ; theis
you have the structure of the Bryosoa, (in Plate
XLTIIIK with' a calcareous stem, with a sym-
metrical alimentary canal, but with serrated tenta-
cles round its anterior aperture, coastituting a pe-
type — th$ Bryosoa. And tliaS t&ey

LECTURES ON EMBRYOLOGY.

Polypi is shown when examining their embryolo-
gy. Here (Plate XLVIII) are changes in one of

[PLATE XLVIII— CHANGES OF THE BRYOZOA J

these Bryozoa, which have been investigated by
Professor Van Beneden, (Fig. A). The bud-like
egg which arises from the main cavity does not
produce a terminal germ, from the lower centre
of which the main cavity proceeds, but produces
(Figs. D, G) a division of this yolk-like mass, un-
dergoing all the processes of division which we
have elsewhere observed, and finally assuming an
elongated form. From the beginning it exhibits
the peculiar character of Mollusks, which distin-
guishes them from Radiata. Their bilateral form,
on the longitudinal axis, is observed in these
germs. And thus, going on further, the margin
becomes serrated, (see Fig. E), the internal cavity
growing deeper and deeper, introducing the whole
mass of yolk within, (Figs. H, K) with appendages
above. These appendages will soon open, and
you will have (Plate XLVIII, fig. C) a large alimen-
tary canal, with a central cavity placed in a dis-
tinct cavity of the body, with tentacles round the
opening ; so that this structure is distinct from that
of the Radiata.

But I must dissent from the conclusions which
Professor Van Beneden has deduced from his ob-
servations. From the manner in which the yolk
is placed in the interior of the alimentary canal, he
concluded that there is no difference between the
Radiata and the Mollusca in their embryonic
growth, as the yolk is formed around the cavity,

and aa the yolk is introduced from the lower side
in both. But he overlooks that in Radiata the
centre of development is really the centre of the
mass, and that the further growth takes place in
all directions simultaneously, by a uniform, all-
sided development; whilst in Mollusks there is
from the earliest period this bilateral and longitu-
dinalaxis. We might just as well say that the
Vertebrated Animals do not differ from the Radi«
ata, because in the former the yol k also is introduced
from the lower side into the animal. But we have
here another difference among Vertebrates. Be-
sides the lower cavity, there is an upper one form-

ed ; and so we must admit that the type of Mollash
is a distinct type from Radiata, and not to be uni-
ted with them

Professor Van Beneden being one who has
traced these investigations extensively, and who
has tried to characterize the leading groups of
animals upon the first changes of the embryo, I
thought it proper to make these remarks now.

[PLATE LV— SYMBOLICAL FORMULA or MOL-
LUSCA I

In order to show more fully the distinguishing
characters of Mollusca, compared with the other
departments of the animal kingdom, I think it
useful to mention here the symbols which I shall
use to designate in future that type. la accord-
ance with the mode of development of the germ
in Cephalopoda, a narrow crescent, placed verti-
cally, (PI. LV, fig. A) would give the best image of
these animals. and contrast their growth with that of
Radiata, which are represented by a horizontal
circle. Let this crescent be closed, it may repre-
sent the Acephala, (Fig B,) with outward turned
margins the Gasteropoda, (Fig. C,) in allusion to
the wide gape of the lobes of the mantle, and with
a transverse division, (Fig. D,) the Cephalopoda,
alluding to the complete separation of the head.

The propriety of admitting this sign, a closed
crescent, to represent the type of Mollusca, will ap-
pear much stronger, when I mention that among
Cephalopoda the yolk is not entirely transformed
into a germ, as it is among the Bryozoa, Acephala
and Gasteropoda, only a part of the 3Tolk being
modified, so as to form a germ around the yolk. —
The yolk remains for a great part unchanged, and
enters the lower side of the embryo into the ab-
dominal cavity, (PI, XXXIX, figs. D, E ) So that
we have really in outlines the form of the embry-
onic sign, which I would preserve for the type of
the Mollusca.

Among the Bryozoa there is a genus called Pe*
dicellina, which is minute, and has a still more
regular form than the Eschara and Retepora, rest-
ing isolated upon small stem?, -with fringes all

PROF. AGASSIZ'S

around, and a structure of the alimentary canal
similar to that of the other Bryozoa. This Pedi-
cellina shows that among the so called Infusorial
animals, the Vorticelloe, placed among those which
have been up to this time considered as a natural
group, should be separated from them. In my
opinion, Vorticellseare closely allied to the type of
Bryozoa, living separately, and constitute a fresh
water genus of the type Bryozoa.

Among these Bryozoa, (Plate XLVIII,) there are
some which are very remarkable for the curious
appendages which surround the main parts of their
body. Generally, there are some large cells, and
around them smaller ones, (Plate XLVIII, fig. J,)
independent buds, as itwere,with threads, (Fig.F,)
or with articulated joints, which shut and open
like the beak of a bird, (Fig. B). What these are
is scarcely understood, and I shall hardly venture
to express my opinion about them.

Buds which rise from a common stem, and which
differ from other buds, we have observed among
MedusjB , even buds which perform different
functions from others. And I can scarcely help
thinking, that in these Bryozoa there are buds
formed upon the same stem which will not grow
in the same manner as the main individuals, but
assume an entirely different shape, and will be
'* catching individuals,'' living to supply the stom-
ach with food by seizing upon little animals, and in-
troducing them into the cavities of the main body.
To consider those appendages as parts of the main
animal is out of the question, (Plate XLVIII, fig.
C), as they have no true connection with them.—
To consider them simply as peculiar appendages
to those animals, would not be more rational. But
when we see that these Bryozoa can form stems so
complicated, or rather containing so many indi-
viduals, I do not see why we should not recognize
imperfect individuals, like buds, assuming a
peculiar form of their appendages, and then ad-
mit that they are analogous to the compound in-
dividuals of Medu?ie. in which the isolated indi-
viduals do not perform the same functions, and
do not resemble strictly each other. If this view
be correct, I would also venture to hint at the
probability of Pedicellaria in Echinoderms being a
kind of budding of very imperfect individuals, re-
sembling the lowest forms of the class, the Crl-
noids, and living as a sort of low parasites upon
the parent, from which they differ more than any
other kind of buds. Thus exemplifying those
higher beings to which all sorts of parasites attach
themselves constantly.

Ascidise have a mode of development which re
sembles in its modifications, somewhat, the Bry-
ozoa, but in other respects differs. The eggs of
Ascidise, which have been observed, are free when
laid, and from them arise free germs. You may
trace in Plate XLIX, all the phases of the devel-
opment: first, the development of the yolk into
spheres, which grow to form a uniform germ, (Fig.
D) which divides by a deep depression (Fig. C) into

[PLATE XLIX. — DEVELOPMENT OP

an anterior and a posterior mass; the anterior be-
ing transformed into a sort of head (Plate XLIX,
fig. A ). When hatched they resemble very much
Tadpoles, and they move like Infusoria, or rather
like Cercarias. Next appendages are developed or
the two sides of the animal (Fig. E ). We see that
here, even in these compound Ascidiaus, the bi-
lateral symmetry of the parts is still characteristic.
There are no great modifications or changes in any
of them after they have grown to a certain size.
This external coating, which is not an egg-shell,
gradually enlarges and separates more extensively
from the germ itself, and finally is transformed
into a shell like, or rather a membranous envelope,
like this (Plate XLVII, fig. H),and the germ is trans-
formed into an Ascidiaproper,with all the structure
which characterizes the perfect state of those ani-
mals— there being those two openings to the exter-
nal covering (Plate XLIX, fig. J),and with those the
external masses of the animal proper; so that
these Ascidise closely resemble the Bryozoa, and
we pass at once from them to the bivalves. How
the compound Ascidise are developed, is not fully
known, though their embryology has been traced
in some instances, (Plate XLVII, fig. B.) Facts of
great importance have yet to be ascertained,
and I do not suppose that the facts which have
been studied on the growth of compound Ascid-
ians are completely understood, except in one
class — in the Salpa — in which a wonderful alter-
nation of generations has been observed. We have
here (Plate L), those curious animals known under
the name of Salpa, a kind of Acephala, which are

LKCTURES ON EMBRYOLOGY.

[PLATE L— SALP.E OR SOFT-SHELLED MOLLUSCA
— ACEPHALA.]

not all compound animals. Salpaa are soft shelled
Mollusca, in which the transverse muscular fibres
•(Fig. C) are very distinct, and of which two kinds
are observed. And this is the peculiarity of Sal-
PSB, that some of them are constantly found to
form long chains of distinct individuals, united by
peculiar appendages,' and in two rows— united side
by side, and back by back, so that in a chain there
are always two rows— one (Plate L, fig D) in which
individuals are placed side by side ; another where
two such rows are united by their backs. These
compound individuals swim freely about in connec-
tion together, and are never known to separate or
live isolated, except, perhaps, after accidental sepa-
ration. But there are other Ascidians observed
which move free, and which are never found to
unite, but which, nevertheless, in other respects,
resemble so closely the former, that in tracing the
internal anatomy, no difference whatever is ob-
served. The arrangement of muscles, for instance,
in such a compound Ascidia (Plate L, fig. A), or
the arrangement of muscles in such isolated indi-
viduals, (Plate L, fig. C,) is identical. The size of
the individuals is even so similar, that this resem-
blance has struck observers ever since the Salpse
have been studied.

Chamisso, the poet naturalist, who accompanied
Admiral Kotzebue around the world, ascertained
that there was among these animals a most extra-
ordinary mode of reproduction ; that this resem-
blance of individuals, attached and free, could be
fully accounted for. He found that within those
compound Ascidian Salpag, there were only isola-

ted eggs developed, as you can see, (Plate L. fig.
A) ; that in the internal cavity there is one single
egg developed from the main cavity in each of the
compound individuals. And Chamisso has seen
those eggs born, developed and transformed into
isolated Salpae, which would grow to the size of
their parents, and when fully developed would not
produce isolated eggs and isolated individuals, but
a chain of individuals (Plate L, fig. C) arranged in
a similar manner to the compound animals, and
growing till they are born as a chain, and finally
developing to the size of their grandparent, with-
out separating, and living (as long as the observa-
tions were traced) in this compound arrangement,
to reproduce in themselves isolated eggs, without
ever one generation resembling the preceding. So
that the compound Ascidians would always pro-
duce isolated eggs, from which free individuals are
born 5 and those free individuals would always
produce chains containing numerous individuals,
which individuals would never separate in life, but
each of which would reproduce free ones.

Over forty years these facts have been known
and fully described. Chamisso has traced these in
more than one instance without one link in the
investigation escaping his attention. Nevertheless
these facts were so astonishing, so different from
every thing that was known in the other classes of
the animal kingdom, that, up to this present mo-
ment, they are not generally believed or under-
stood. There are recent publications dated from
last year, in which these statements are not admit-
ted ; though the accuracy of Chamisso is unques-
tioned among Naturalists— he having published
other investigations which show how accurate an
observer he is; and even after the investigations
of Chamisso have been confirmed by other obser-
vers, there is still doubt entertained upon the cor-
rectness of the views derived from those facts.

Dr. Krohn, a German Naturalist, has traced the
same phenomena in some species, which he obser-
ved on the shores of Italy. He has traced, as
Chamisso did, their whole series of alternate gene-
rations without one single interruption. Steenstrup
has traced similar changes. These facts have even
been the starting point of his views upon alternate
generations, of which I have spoken more at length
before. Still more recently Mr. Sars, of whom I
have so often spoken, bas published the complete
history of the alternate generations of several spe-
cies of Salpa, in which the whole development
through alternate generations is studied and con-
firmed, so that we have no longer any ground to
doubt these observations. We must come up to
the conclusion that there are alternate distinct
generations in various classes of animals. We must
admit that there are animals in the Mollusca as
well as in the other departments, in which the
young never resemble the parent, but resemble
constantly and throughout life their grandparent,
as alternately these generations of compound and
free Salpa are observed.

PROF. AGASSIZ S

But remarkable as it is, there are no metamor-
phoses observed in these animals. Very early in
the young,the form of the adult is fully developed.
Here is a young Salpa (Plate L, fig. B) developed
within a compound chain, in which you observe
all the principal organs as they are in the perfect
animal— the muscular fibres below, as they are
observed here, (Fig. C) the gill as it is also seen,
the heart as it is observed in (Fig. B). The appa-
ratus of the liver and alimentary canal, (Fig. B)
separated here, but combined here (Fig. C) in one
mass.

Now the phenomena of alternate generation, of
which I have spoken, in the class of Worms, is
more complicated than in the Mollusca, as in the
Worms we have not only alternations in the gen-
eration, but also metamorphoses in each genera-
tion ; this opens a field of investigation, which
will present endless difficulties and endless details
to ascertain, but which will certainly go to enlarge
our views of animal structures and of individual
life.

The embryology of the Bivalve Shell has not
yet been traced to that extent to which other classes
have been traced. It is remarkable that, though
they are so common — though we have fresh wa-
ter bivalves — though we have so many marine
bivalves, and some of them so exceedingly abun-
dant, their development has not been traced with
any degree of precision. The growth of the Oyster,
which might be traced every where, has never been
watched by any one. Even the Muscles are very
imperfectly known. Prof. Carus has observed the
fact, that from a very early period, the germ (Plate
LI, fig. A) of Anodonta has a tendency to divide
on one side, and the other side to flatten ; so that
the animal assumes an oblong shape with a disc
covering it from above.

[PLATE LT.— GERMS OF ANODONTA ]

Professor Beneden says he has ascertained that
those germs have been mistaken for Infusoria, and
that the Leucophrys Anoclonta of Ehrenberg is
only a germ of a fresh water Clam. So that we
would thus have another evidence of the hetero-
geneous nature of that class of Infusoria. Perhaps
we should not insist so strongly upon these mis-
takes, when we remember how much Ehrenberg
has done to illustrate the lower animals. That
mistakes must have occurred constantly when the
metamorphoses of the more perfect animals were
less understood, is very natural. A peculiarity of
the Bivalves, in their growth, consists in the fact
that even those which have a foot developed as
a large fleshy mass between their two valves, have,
when young, only a small transverse bundle of

fibres uniting the two valves, (Plate LI, fig. D)
and throw out a kind of byssus, which we observe
between the muscle, in Plate LI, fig. D. This
fact is important, as it shows that the shells which
have a byssus above the foot, should be considered
as lower than those in which the foot is more
largely developed, and can be expanded and con-
tracted between the two shells.

Lastly I would mention the changes which Gas-
teropoda or snail-like animals undergo. During
their growth they have been traced in several
types. The changes of snails were early observed,
more recently the metamorphoses of naked Mol-
lusca. As they have very recently been more ful-
ly investigated, (Plate XLVIII) I would rather
mention them than refer to the ancient investiga-
tion upon Pulmonatse Prof. Vogt has traced these
investigations in a species of Action more exten-
sively than anybody else. He has noticed that the

[PLATE LIL— ACTJEON.J

division (Plate LII,fig. E,) of the yolk goes to form
a germ consisting of homogeneous cells and that
after many more than twenty-four cerls had been
formed the external or peripheric cells assume a
somewhat different aspect from the internal
which would centre in the interior. And at that
time the peripheric cells (Fig. F) would forma
sort of envelope to the inner cells and then a di-
vision take place in the inner mass so that here al-
so the body assumes very soon a bilateral syme-
trical disposition. But what is curious is that on
the sides of the anterior portion of the body, (Plate
LII, fig. I) there are remarkable rotary appenda-
ges formed and between them a rudimentary foot.
The upper portion (Fig. G) of the body is soon
separated from the lower portion so that before
the animal leaves his shell, we have (Fig. H) an
upper part and a lower part and lateral wheels, by
which the animal moves like the Rotifera, and a
sort of foot and a sac (Fig. J) containing the va-
rious organs. Then the shell begins to be devel-

LECTURES ON EMBRYOLOGY.

&s a very thin membrane within the external
Boating (Fig. K) of the animal.

Next the yolk mass within the animal gives rise
to an alimentary canal, and at that time the aci-
aial is hatched (fig. M). Before it is hatched, it re-
sembles by no means the perfect animal. It has a
shell. This is a most remarkable fact, It has a
shell, though when fully grown it will resemble the
Hollusca, without any shell, This shell is entirely
!ost before the form of the perfect animal is
assumed, (Plate XLII, fig. M). These prgans
around the mouth are not yet distinct. Early in
life, however, a hearing apparatus exists — a kind
•of sac, resembling the lowest form of ears, which
disappears almost entirely in the perfect animal. —
And the eyes are not yet seen. Kow these changes
are brought about, has not yet been established, as
the intermediate steps from this condition to the
perfect animal have not yet been traced. Indeed,
there is a great difficulty in all embryonic investi-
gations, in tracing the further growth of the germ.
After they have been hatched, they die generally
in confinement. It is much easier to trace it at
first, than te trace it when it is undergoing its me-
tamorphosis to assume the final form of the ma-
ture animal. And in this there is more left to in-
vestigate than in any other department of Zoolo-
gy. It is even to be expected that many animals
described as perfect, will be found to be only the
young state of other well known animals in their
fall grown condition. I cannot, for instance, help
thinking that the new genus established by Prof.
Muller, under the name of Astinotrocha (Plate L,
fig, F), is only a yoang Gasteropod of the family
of Doris.

However, we can learn one great result from this
fact, here— that the shell in Gasteropoda is not a
character of superiority ; that those animals which
have a shell, so far from being of a superior type,
ought to be considered as the lower ones, as there
are many which have shells in their embryonic
condition, and afterwards cast them. It has been
ascertained by Prof. Loven that all the naked Mol-
iusks have a shell when young, and that they all
cast this shell as soon as they leave their embry-
onic envelope. But though I would now consider
the Gasteropoda which have shells as uniformly
•inferior to those which are naked, this conclusion
will probably not be admitted without controversy
roy Zoologists,

But when we consider the peculiar forms which
existed among the shells of former geological
ages, especially in the oldest periods, we may sat-
isfy ourselves that this conclusion is correct.

In the first place, let me observe that these em-
bryonic shells have a simple margin (Plate Lll.fig.
K). Their opening is entire as are the shells of
many other Gasteropoda, the Helix, the Trochus,
the Turbo, the Natica, &c. But there are many in
which the margin of the shell is prolonged into a
fetbe, a respiratory canal, as the Fustis; Murex,

&c., through which the respiratory tube can be
protruded and retracted.

Now if we are allowed to consider the order of
succession of fossil animals as of any value, we
would have a hint to appreciate the value of these
shells. On a former occasion I proceeded precise-
ly in a reverse way from the investigation of the
types as they are well known. From their anato-
my in the present epoch, I proceeded to show that
the order of their appearance in geological time
agrees with the gradations as they are formed in
oar days ; and concluded that the more ancient
were the lowest, because they resembled most the
lowest of our days. But having once ascertained
such facts very extensively, we are prepared to
compare the most ancient shells with our shells, to
ascertain which in the present creation should be
considered as lowest, and we find that the more
ancient univalves or Gasteropoda have a simple
shell. And that those with a notch are of more
recent date. And this is so constant that Paleon-
tologists have not yet found one shell with a respi-
ratory tube among those of the oldest deposits.—
Then we should conclude that those which have
an entire opening are lower ^ and those which
have a notch are higher; and as those ancient
shells resemble the type of the embryonic shells of
the present age, we should further conclude that
those which have a shell at all are lower than
those which have none} and that our naked Gas-
teropoda should be considered as the highest in
the group. But there is one point in the structure
of Mollusca which is worth our attention, and
wliich throws light upon embryonic phenomena ia
general, to which I will allude before concluding.

The circulation of the blood in Mollusks does
not take place as it is observed in other animals.
We have here {Plate LHI, fig. B) no continuous
blood vessels passing from the heart into arteries,
and then dividing gradually into some branches to
unite again into complicated tubes — to open into
the heart again— to form veins — indeed we have
not a closed circulation. It is but a few years
since it was known that the blood can be circulated
through the body without being moved by a closed
system of vessels. The impression has universally
been that the circulation is regulated from a cen-
tral organ propelling the blood which is circulated
through vessels which go on branching into small-
er and smaller tubes, and then the blood is col-
lected again and brought back again into a central
cavity ; so that the circulation would imply a reg-
ular circle of this movement of the blood.

Now in Mollusca, in Haliotis for instance, (to de-
scribe only one case) we have blood which is mov-
ed by the heart into a tube (Plate LHI, fig. B)
which is not gradually branching, and which sends
out only a few vessels and then enlarges into a
wide cavity. Indeed the vessel is lost, and the
blood is emptied into a large cavity in the anterior
part of the body. And from this cavity, arise va-
rious little vessels, which circulate through all th«

PROF.

L1II— CIRCULATION ov HA&IOTIS, A GAS-

TBKOPOD.]

parts, which will then unite together in the veins,
and those veins, before they empty into the heart,
will again open into another cavity of the body,
and fill it with blood ; and from that cavity, the
blood is introduced by tabes into the heart. Only
in certain parts of the body— for instance, along
the gills, and upon the glandular organs — there
are regular arteries and veins (Plate LIU, fig. A)'
But the main portion of the blood is emptied into
the abdominal cavity, or emptied into another cav-
ity around the mouth. So that in this Haliotis, the
main stem arising from the heart, ends in a sack
in which there is the centre of the nervous system,
(Fig. B ) the brain of these animals, in which there
are the muscles of the tongue and the beginning
of the alimentary tube in one and the same cavity
in which the main mass of the blood is emptied.
So that the brain sv/iras in blood — the muscular
apparatus which moves the tongue sv?ims in blood
— and the main track of the alimentary canal, the
alimentary tube and the other intestines swim in
venous blood in the posterior cavity of the body —
the most unexpected structure and apparatus of
circulation, which lias ever been observed among
animals. And this peculiar unconnected disposi-
tion of the blood system, discovered by Prof. Milne
Edwards, has been successively observed by him
and by Prof. Yalenciennea and Mr. Quatrefages,
in all Mollusks. In the Cuttle-Fish there is a great
sac in which the intestines are placed, in which
they move freely, which contains venous blood,and
giils and gland ttlarorgams, with their proper

circumscribed with membranous tubes. What
be inferred from such a state of things, for the'
understanding of the embryonic changes which-
animals in general undergo ?

\7e see every where in the beglnnias these an-
imals consisting of uniform cells— of uniform ma-
terials,, And out of these uniform materials may
grow the most complicated structures, Fluid
should be circulated in the parts in order that new
elements should be introduced into the bodyu
And this must bs considered as brought about icj
the following manner.

Some of these cells will become loose, and when
loose, the fiuid, accumulated in the intercellular
spaces, will unite in fiakes, and those free cell&
swim within a liquid. This is blood. This blood
is nothiag but an accumulation of cells, which be-
come blood corpuscles, Soating in fluid within the
body. Let us have the cells of an embryo, and:
let there be a fiuid of a certain kind, and let the
cells and Suid all move about, and there will be ss
real movement of blood. First, it is only moved
forwards and backwards ; but channels are gradu-
ally formed vrithin the substance ; and thos« chan-
nels may be lined with membrane by the coagula-
tion of a part of the fluid. But this may take place
in such a manner as to form a central cavity, which-
will be a heart; and to form radiating tubes, which?
will be arteries and veins ;• or to form largs cavities*
around the main organs. Those large cavities
cannot be considered as formed in another way
than by the dissolution, as it were, of the embry-
onic substance of which they consisted primitive-
ly, and by the changes cf this substance into-
moveable blood. The moment that the embryo-
has come to this point of development, it is so far
advanced in its other changes that it takes food ;•
it is hatched, and at that time new substance is in-
troduced as food into the alimentary canal. Be-
ing digested, the result of digestion is mixed with*
that blood, and so the new substances are brought
into tile system, to undergo the changes by which?
it is so complicated as finally to form a most
heterogeneous mass.

That the heart must be formed from the disso'-
Istion, as it were, of parts of the substance of the
germ, is plainly shown by its peculiar position ir*
so many animals. In some of the Mollirsca it sur-
rounds the alimentary canal, forming various sacs
hi rruvny parts of the cavity,, And this shows-
plainly that there we have no regular development
but a sort of decomposition of the animal sub-
stance, which is gradually restored, by the forma-
tion of more and more blood, by the process of di1-
gestion.

The classification of Mollasca which should be'
admitted if we base our classification upon embry-
onic d?ata, would differ to some e:itent from whaS
has been generally acknowledged.

Generally they have been divided into six clas&-
es. The Cephalopoda or Cuttle-fishes, the G-astero-
snail-like Mollasca, the Pteropsda-

LECTURES ON EMBRYOLOGY,

are very few which differ from the Gasteropo
•da by having En-like appendages on both sides of
She body. Then the bivalve shells, called Lamelli-
branchia, and the Terebratula, under the name of
Brachiopoda, and also the Barnacles orCirripe-
dia. That the Barnacles belong to the great type
•of Articulata I have already shown. The Brachi-
opoda ought to be combined with LameUibranchia,
having the same structure and differing only by
secondary modifications, and the Pteropoda united
with Gasteropoda, being merely an embryonic type
•of that class. In that manner three classes only
remain : the Acephala or clam-like shells, the Gas-
teropoda or snail-iike, and the Cephalopoda or cut-
tle fish-like,

The order of gradation of these three classes,
according to their organisation, is very •easily es-
tablished. Among Acephala the Bryosoa are the
lowest, next the Ascidia, and next the bivalves:;
the Brachiopoda being irregular, lower than the
regular ones, as the Clams.

[PLATE LIY— COMPOUND AND SIIKJLE BRTO-

ZOA |

Among Gasteropoda, the Pteropoda would be the
lowest, being, as we have stated before, a mere
embryonic type, reminding us of the germs of the
snail-like Mollusca. which, when full grown, have
a flat foot upon which they walk, among which
we would place lower those which have a shell du-
ring all their life ; nest, and above all, the naked
Mollusca, which cast their shells at an early period
of their life. It is a very curious fact that the na-
ked Mollusca are born with a shell which they often
cast afterwards, showing that the shell is a char-
acter of real inferiority. And even among the

with, a shell,those are inferi-

or which have the aperture entire. The order of
succession in time shows that those shelly Gastero-
poda with an entire aperture appeared before those
which have either a long tube, or a mere notch for
introducing the water to the respiratory organs^

The class of Cephalopoda will stand highest, as
has always been admitted by naturalists. But we
shall consider as the lowest type, those which are
provided with a shell. That the chambered shells
were innumerable in the former geological periods,
especially in the older and middle ages, is very
well known, whilst the naked ones occur most
abundantly in our days; and only two genera oc-
cur in the present creation, with, a shell divided
into two chambers, united by a siphon.

The order of succession in time is a guide quite
as safe to appreciate the gradation of types as
organization itself 4 so that where the information
upon the one point is deficient, we can refer to the
other. In more than one instance we have seen it
coincide in so striking a manner with the series
which the intimate structure had revealed to us,
that we can no longer resist such a conclusion. —
We could say with confidence, that the order of
succession corroborated the inferences derived
from organic gradation, and, vice versa, that or-
ganic gradation illustrates the order of succession
in time.

For the class of Acephala we have been able to
establish a natural series, upon evidence derived
from internal organisation. Succession in time
gives us the same series ; that is to say, the Bryo-
zoa are highly numerous in the oldest fossil-
iferous strata; then among the Bivalves, the Bra-
chiopoda, which have a shell with two unequal
sides, and the anterior and posterior extremities
symmetrical, follow next in innumerable variety;
the groap of Oysters comes after the Brachiopoda,
and finally the regular Bivalves, with equal sides
and unequal extremities, tending towards a well
marked bilateral symmetry, with diversified ends
of the body, come last.

We see here the order of succession in time
agrees fully with the order given by organic gra-
dation.

It remains now only for me in my next lecture,
to present a rapid and condensed sketch of the
embryology of vertebrated animals, in order to
show that there we have a uniform type even
among the highest living beings— to conclude this
course of lectures.

PROP. AGASSIZ'S

LECTURE XII.

We have now to examine the highest group of
animals, which Naturalists have called Vertebrata
Upon this type more embryonic investigations
have been made than upon any other. It was, in-
deed, with this type that embryological studies
began— when Dollinger, the Physiologist, traced
the growth of the young Chicken within the egg,
and for the first time showed how important for
physiological investigations it would be to under-
stand the manner in which organs were formed,
in order to arrive at more precise conclusions upon
their functions. And though Dollinger has writ-
ten nothing upon this subject himself, those who
are conversant with the history of Embryology,
acknowledge him as the first and most eminent
among those who have devoted themselves to
these investigations. Indeed, his pupils have, un-
der his directions, curried out his views, and de-
veloped the new science up to the point at which
it has arrived at the present day.

The most eminent among Embryologists, in this
special department, (C. E. Von Baer) has been a
pupil of Dollinger ; and Pander and d' Alton, who
first published extensive researches upon the
growth of the germ within the egg of the Hen,
traced their investigations under Dollinger's im-
mediate superintendence.

That the discovery of the unity of structure
among these highest animals, in their earliest con-
dition, has not excited more interest — that the dis-
cove'ry of the egg among Mammalia has not been
more spoken of, and has not been considered as
one of the most brilliant points in the history of
Physiology is, perhaps, owing to the want of a
general understanding of these matters, and the
difficulty there was before of comparing, properly,
the egg in the various classes of Vertebrata, or of
introducing this subject before the public at large,
as it was done among professional men. But
really, it was in Physiology a great discovery
when it was ascertained that all Vertebrata, that
Fishes as well as Reptiles, as well as Birds, as well
as Mammalia, arose from eggs, which have one
and the same uniform structure in the beginning,
and proceed to produce animals, as widely differ-
ent as they are in their full-grown state, simply
by successive gradual metamorphoses ; and these
metamorphoses upon one and the same plan, ac-
cording to one and the same general progress.

The unity of structure in Vertebrated animals
has been ascertained, has been understood, and
well understood, long before Embryology had ad-
ded anything to show how deep this unity of plan
was impressed upon that type. By the investiga=

tions of Comparative Anatomy, it had been
tained that the external differences which charac-
terize the class of Fishes, that of Reptiles, that of
Birds, and that of Mammalia, were only modifica-
tions of one and the same structure— that the head
of Fishes, for example, though apparently so dif-
ferent from that of Man, was made up of the same
bones, arranged in the same manner, only subdi-
vided into more distinct points of ossification,
with modified proportions, most of them remain-
ing moveable for life, but after all, arranged upon
the same uniform plan.

It was especially in Osteology, that is, in the in-
vestigation of the bones, that this unity has been
traced at full length. It is, therefore, to that sub-
ject I would particularly call your attention with
reference to these general realizations, although
I cannot enter here into details of an illustration
of the facts. Anatomy has shown us the grada-
tion which exists among these animals to such a
degree of perfection, that in the leading and fun-
damental divisions there will be very little to im-
prove, though the details may be considerably im-
proved under the influence of embryolosical re-
searches.

The order which is now assigned to the different
classes of Vertebrata is, to place Fishes as the low-
est class ; next Reptiles ; then Birds ; and Mam-
malia at the head. And this order of classifica-
tion, established from anatomical evidence, is
also confirmed by the differences which exist in
the mode of growth. Though starting from a
common structure in the primitive egg, the differ-
ent classes undergo metamorphoses,which proceed
in such a manner as to end in the establishment of
those final differences which characterize the struc-
ture of the different classes of Vertebrata. It is,
indeed, by the difference in the process of growth
that those differences which characterize the full
grown animals are brought about.

It may therefore be said with perfect propriety,
that the higher Vertebrates undergo changes
through which, in different periods of their life,
they resemble the lower ones; that there is a pe-
riod when the young bird has the structure, not
only the form, but the structure, and even the fins,
which characterize the Fish. And of the young
Mammals the same may be said. There is a pe-
riod in the structure of the young Rabbit, (in
which the investigations have been traced more
extensively than in any other species,) when the
young Rabbit resembles so closely the Fish, that
it even has gills, living in a sac full of water
breathing as Fishes do. So that the resemblance

LECTURES ON EMBRYOLOGY.

is as complete as it can be, though each of these
types grows to a complication of structure, by
which the young Mammal, for instance, leaving
behind this low organization of the lower types,
rises to a complicated structure, to higher and
higher degrees, and to that eminence even which
characterizes mankind.

As it is out of the question for me to introduce
an illustration of all the phases of these changes, I
will only introduce such points of the subject,
as bear upon classification, and upon the succes-
sion of types in former geological ages, in or-
der to show that the principle which I intend to
introduce, as the fundamental principle of classifi
cation, is really of value, in all departments of
Zoology.

In these diagrams you have representations of
the changes which animals of the four classes of
Vertebrata undergo. Here (Plate I, page 7) is the
history of a Fish, (a White Fish from Lake Xeuf-
chatel), as represented by Dr. Vogt, from the egg
(Fig. A) up to the period when the young Fish
(Fig. F) is hatched. The close resemblance be-
tween this form (seen in fie. H) and other classes,
is more striking. Here (Plate II, figs. F to 0) we
have the history of a Frog, (also from a paper of
Dr. Vogt,) from the first moment of its formation
(Fig. F.) up to the period when the young Tadpole
(Fig. N) is hatched. In Plate II, figs A to E, are

[PLATE II— EGGS OF SNAIL AND FROG.]

the changes which a Snail undergoes, according
to the illustrations of lUthke; and in this fig-
ure (Fig. B,) it is represented, as it appears,
taken out of the egg, and deprived of its external
envelope, in order to compare it with the form of
the young Tadpole, (Plate II, fig. L,) or the form
of the young Fish (Plate I, fig. H.) You see the
Snail, in its early condition, resembles the young

Tadpole, closely, as you may ascertain by compar-
ison of the figures. The resemblance with the
Fish is not less striking, as you see on comparing
also the figures of the young Fish.

[PLATE VII.— EGGS or BIRDS AND HENS.]

And if we go on, we shall find the same agree-
ment in Birds and Mammalia. We have here (in
Plate VII) the Hen's egg. Here, (Figs.E to K) we
have the different changes within the egg, as fig-
ured by Pander and Baer. We have here the dif-
ferent modifications of the young Chicken within
the egg; and we have here (Fig. K) the young
Chicken, already formed, at one of its earlier peri-
ods of growth, when it has yet undergone slight
changes of form in its progressive development ;
and here, (Plate IX,) as we proceed further, we
have the history of the changes of the embryo «f
a Rabbit, from the remarkable work of Prof. Bis-
choff. I have not figured the outlines of all peri-

[PLATE IX — EGGS or RABBITS.]

PROF. AGASS1Z S

ods ; but compare these earlier forms (Plate IX.fi gs.
L to Q) with those of the Chicken, (Plate VII, figs.
E to I, ) and you see again that the outlines are iden-
tical. I might have had a figure of a young Rabbit,
when the head has come to be more distinctly de-
veloped, and when the posterior part is more con-
tracted, and the resemblance would be still greater
than you notice at present, on comparing those
lower figures, (Plate IX, fig. P, and Plate VII, fig.
H.)

It is not only in these forms of the germ that we
have an identical structure, an identical form, an
identical process of the formation of the eggs, and
the same identical functions; we have the same
modifications in the egg to bring about the forma-
tion of a germ ; and this process by which the
germ is formed is identical in. Vertebrata with
what we have observed in Radiata, in Mollusca, in
Articulata. The resemblance does not go further,
but here the identity is complete. The egg of a
Fish (Plate I, fig. A) consists of a yolk mass, en-
closed in its membrane, and containing a germina-
tive vesicle and a germinative dot. If we pass on
to the classes which we would suppose to differ
most— the Mammalia, for instance— we find that
the egg of a Rabbit (Plate IX, fig. A) consists, as
you see, of a vitelline membrane, a germinative
vesicle, and within it a germinative dot. This is
the egg in its primitive formation, as it was dis-
covered by C. E. Von Baer, in the year 1827, when,
for the first time, the unity which exists in
the starting point of growth of all animals, was
clearly ascertained. You remember that we ob-
served an identical structure among other classes ;
and if I were to enter into the details of the
changes which the egg undergoes, I should seem-
ingly repeat only what I have said about other
types. If you have not forgotten what I said about
the divisions of the yolk, you will remember that
the egg showed (as represented in Plate I, fig. B,)
a successive division into cells, which is even more
regular in Mammalia than in any other class. It is
here ( Plate IX,figs.C to F) symmetrical in the Rab-
bit, with the division into halves, and the subdivi-
sion into quarters, into eighths, into sixteenths,
and into thirty-two parts, and so on, as represent-
ed in the various figures in this plate. And in-
deed, Professor Bischoff, who has traced all the de-
tails, represents the yolk as dividing first into two
masses, then into four, then into eight, sixteen,
thirty-two, and even sixty-four. Then the whole
vitellus is transformed into a uniform mass of
minute cells. Next a mass of somewhat different
substance is circumscribed. (Fig. G,) to be the
germ ; so that in describing the changes of sub-
stance occurring in Mammalia, we have the same
thing that we have noticed in the Worms, the Crus-
tacea, in the Shells, and which we know to take
place among Radiata. We know further, that these
changes (Fig. B) will go on to form organs, and
finally to produce the perfect animal.

Such a germ is formed here in Birds, (Plate VII,

fig. C). The mass in the middle of the yolk is a
part of its fatty contents. There is no funda-
mental difference, however, from what we observe
in the Mammalia; but as the substances are not
so completely mixed up as in other classes, the
figure, of course, represents the details as they ap-
pear. The young germ within the egg of the Fish,
(Plate I, fig. C), and also in the Frog (Plate III, fig.
A,) and the Snail, (Plate IF, fig. A,) represents also
the same condition. So that after the egg has
undergone its particular changes, the germ is
formed upon it, and has an identical aspect in the
four classes of Vertebrates. (PI. III. page S.)

Now this germ will grow by a double process
of extension. It is thickened by the assimilation
or by the transformation of successively larger
portions of the yolk, which is introduced into the
mass by transformation of substance, and next it
expands over successively larger parts of the sur-
face of the yolk. Then the thickened substance
of the germ soon divides into several layers, as is
figured in the diagrams in Plate VII, fig. D, where
the germ having grown larger, we may distinguish
a circular outline, and another of a more elonga-
ted form ; and in a transverse section of such a
germ, (Plate VIII, fig. A), we may observe that
the upper portion of it has a somewhat different
size from that of the lower and middle layer. So
that in its thickness the germ begins to show it-
self by the changes which it successively under-
goes. As soon as this separation of the germ into
several layers has taken place, then each layer for
itself will undergo changes. The upper layer, es-
pecially, is thickened within the centre ; and in the
middle layer there is an accumulation taking place
about the centre also, and the lower layer is
growing very soon beyond the upper, and beyond
the middle layer. So that from the figure seen
from above, (Plate VII, fig. D),you see the out-
lines of the three layers at once. Successively the
layers will enlarge, so as to cover the greater por-
tion of the yolk.

How the germ extends, gradually, more and
more, so as to cover a greater and greater portion
of the yolfc, is seen here (Plate IX, fig. J,) where
the layers are seen hanging over the yolk, and
finally, (Fig. K,) enclosing it in the lower cavity of
the germ ; or the upper parts cover only one por-
tion of the whole surface. To shorten this illus-
tration, let me at once say, that the upper layer is
the foundation for all those organs by which an-
imal life, properly, is maintained ; by which ani-
mal life, properly, is expressed. It is from this
layer that the whole frame which is acting in life,
will be developed. It is from this part that
the head, the legs, the walls of the body, the
flesh, the bones, the brain, the organs of sense, are
successively developed. The upper layer (Plate
VIII, fig- A,) gives, indeed, rise to the organs of
animal formation of all life ; and the middle layer,
on the contrary, gives rise to the formation of the
heart, and the blood vessels ; to the organs of the

LECTURES ON EMBRYOLOGY.

circulation in general. And the lower layer gives
rise to the organs by which life is maintained ; in
the first place to the various sacs of the alimenta-
ry canal, to the various parts of the digestive ap-
paratus, the stomach, with its glandular system ;
the lung, which is only a sac derived from the
alimentary tube, and all the other various glandu-
lar organs which are connected with the alimenta-
rv canal, and also to the liver, in connection with
the blood system, and to the system of reproduc-
tion.

I need not enter into these anatomical details ;
but in order to show how this is brought about, let
us trace, for a moment, these longitudinal sections,
in which the same layer (Plate VIII, fig. E,) is rep-
resented in its proportional development. The
blue mass represents the upper layer of the germ,
as it has grown thicker in the anterior part of the
germ. These red masses represent the middle
layer (Plate VIII, fig. C,) as they have grown
thicker in various parts by an accumulation of
blood. And this greenish mass represents that
which begins to give rise to the alimentary canal,
This is a longitudinal section of the germ, (Plate
VIH, fig. E ) seen from above, where we may
plainly observe the upper layer. We observe that
it is only elongated, and there is a depression fig-
ured here (Plate VIII, fig. A.) and below, there is a
collection of cells, forming what will give rise to
the development of the back bone.

[PLATE VIII— LAYERS AND LONGITUDINAL SEC-
TIONS OF THE EGGS OF BIRDS 1

of the tube into three large cells which ccmrnuni •
cate widely with each other (Fig. H), and will rep-
resent the three principal parts of the brain.

The sides of this tube are seen first to give rise
backwards to some consolidation in the mass °, be-
ing not yet organized, but being cells of a peculiar
form (Fig G). The number of tbese is gradually
increased, and in a profile view (Fig. H), you may
see how the cavity (Fig I) represents the cavity of
the spinal marrow, and how the head is bent down-
wards, and the tail is also gradually bent down-
wards, so that the germ is raised above the yolk,
and no longer rests flat upon the yolk, as it did in
the beginning. In the earlier periods, the germ
rests flat upon the yolk, and at first (Plate VIIL figs.
E) the embryo lies transversely across the egg— the
head and the tail bend towards each other down-
wards, the right hand side toward the pointed end
of the egg, and the left hand side toward the other,
the blunt end.

The successive modifications of the system thus
sketched out, will give rise to the formation of these
various systems of organs which characterize the
mass of the body. The parts of animal life will
be developed in the upper primitive uniform sub-
stance ; solid parts will be deposited around in
other places, and so the difference between hard
bones and contractile substance will be introduced,,

It is, perhap?, proper for me to show how this is
brought about, by pointing to some figures of the
modifications of cells, as they occur in the animal
body,

[PLATE V— VARIOUS FORMS ov CELLS.]

But the two sides of this depression growing
successively thicker and higher, will form, the
walls of a longitudinal furrow, and this furrow will
finally be shut up by the growth of the sides, and
then we have the cerebral tube gradually develop-
ed (Plate VII, fig. H). And you see that the head
is scarcely indicated (Plate VII, fig. G) by a some-
what greater dilatation of that upper cavity. And
after a while it is subdivided by the contractions

In the beginning, the embryo consists of
uniform cells ; but they may be elongated ; they
may undergo other forms by branching; they
may be so elongated as to give rise to tubes ; there
may be various substances deposited within them,
and in that way they may constitute the various
modified different parts which compose the animal
body.. There are even some of these cells, which,

100

PROF. AGASSIZ S

becoming loose, form the blood corpuscles, and
are circulated through the system. Here are cells
which line (Plate V, fig. A) the inner surface of
the cavities of the body, forming the so-called
epithelial membranes upon the different organs,
(Fig. B } The irregularity of the outlines of some
of the forms, with their nuclei and nucleoli,
still preserved, may be seen in these different dia-
grams, (Fig. C). In some, the cell membrane is
contracted, and assumes therefore a simple thread-
like shape. There are others (Fig. D) which form
a sort of pavement, and preserve their regular nu-
cleoli and nuclei. There are some of these cells
which have thus been elongated, upon whose
broader flat end there are vibrating Cilia formed,
which preserve, nevertheless, the nucleus and nu-
cleolus within. That these may present different
sizes in different layers, you see here (Plate VI,
fig. A) in the skin of the Frog, where the external
one constitutes the epidermis, and are successively
cast from the surface of the skin ; and the lower
cells grow successively, and form a new layer,
which will be again cast, and so on. How these
cells may combine to form a new tissue, is here
represented, where the walls of the cells are trans-
formed into regular threads, with swellings from
distance to distance. Here is another portion
(Plate VI, fig. C) which we may consider as an in-
tercellular space, with blood corpuscles circulating
in it, or becoming a fleshy mass by the fixation of
the nuclei, in which the walls (Plate V, fig F)
of the cells having united, will form the fibrous
portion of the flesh, and in which nuclei remain
for a time distinct ; and here they are still more de-
veloped, (Plate VI, figs. H, I, J.) The flesh of the
young animals is not yet completely fibrous; the
elements of the cells constituting those masses
are still to be distinguished.

Now if the cells themselves become loose and
move between the spaces and other cells, then we
have blood currents without walls, at first. But if
there be fluid between the spaces of cells, (Plate
VI, fig. C) they will form tubes, and in these tubes
blood corpuscles will be circulated. The nervous
substance consists still of similar elements, as we
perceive here, (PlateVI,fig. D) where the nuclei are
separated from the fibrous part of the nerve. Here
are other cells, which, from the regular form they
have in the beginning, (Plate V, fig. F) have grown
into branching ramifications. And these are filled
with colored matters. All those soots upon Fishes,
particularly the bright spots seen upon Trouts, for
instance, are only cells in which there are various
colored pigments, usually different sorts of oil of
various colors, and the forms of the cells differ
widely as you see here. Side by side, you may
have cells of different size and of different form.
Even in the bone, (Plate VI, fig. F) you may have
the same kind of cells, and also in the cartilages,
which finally make up the hard parts of the body.
There is, however, still a mystery in the manner in
which these parts are introduced and carried to

[PLATE VI— MODIFICATIONS or CELLS.]

the special parts of the body in which they have to
remain. That it is the food which supplies the
body with every additional particle of substance
all must see, from the fact that, by eating, animals
as well as men grow and increase the bulk of their
various organs. But from ?o uniform food, there
are such diversified organs produced, and with
such special properties, that we cannot but wonder
at the process by which it is made possible ; for in=
stance, that at some point of the body we have
bones produced from the metamorphoses of food—
of just those precise substances which are fit to
become the peculiar substance of that particular
part. How it is, for instance, that the brain is
nourished, and that always those parts of the blood
which can be transformed into brain substance,
are carried in greater proportion into the bead and
into the cavity of the skull, than those parts of the
blood which form and restore the fleshy massed.
It is a common experience, that with the use of
the arm, the fleshy mass is shortly increased. One
who has not been in the habit of practising the
muscles of his arm, if he begin to do so, will in a
very few hours feel pain. But after a few weeks,
he will notice a very considerable increase in the
substance of the flesh which forms the muscular
part of the arm. And this is brought about by the
accumulation of those particles of the food in dif-
ferent parts of the body, which are fit to nourish,
them. That every organ has such an assimilating

LECTURES ON EMBRYOLOGY.

101

power, is one of the most mysterious facts of phy-
siology, for which we have not yet any clue.

The middle layer of the germ is that which pro-
duces the organ of circulation. First, the blood ap-
pears by a simple process of liquifaction of the
the cells. It can be seen under the microscope
how the particles, or the cells of that layer (Plate
VIII, fig. H) begin to be loose at the outer margin,
and to move between themselves, and to run in
particular directions, and to combine into currents,
and those currents to assume particular directions,
and then introduce a regular circulation before
there is a heart, and before there are blood-vessels.
It can be seen in every chicken under so low a
magnifying power that no one should lose the op-
portunity of seeing this wonderful sight. When
blood corpuscles move from the centre towards
the margin of the germ, the other cells which be-
come loose in the periphery of the germ (Plate
VIII, fig. I) begin to move towards the centre. In
the beginning, (Fig. H) there being no cur-
rent circulating, the two collections of fluid meet,
and finally (Plate VIII, fig. K) become regu-
lar currents by means of channels, through
which the blood runs for a regular circulation. —
But there are constant changes in this current, and
even the ramifications of the blood-vessels are
constantly changing. One channel is left, and ano
ther is formed. It is like a river on the flat lands,
where the channel is constantly changing. And
here the channels of the blood are constantly dis-
appearing and are constantly reproduced.

The lower layer forms, (Plate VIII, fig E) in the
more circumscribed parts of the body, a tube
which is to be the alimentary canal; and that con-
tracted portion of the tube opens into the yolk, or
a passage is formed, through which the alimentary
canal communicates with ths remaining yolK.

It would lead me too far to notice the successive
changes which these organs undergo. I will only
show that we have some remarkable differences in
the successive transformations of the different
classes of vertebrated animals. In the Fish, (Plate
I, fig. H,) the same changes occur which we
have already noticed, but the final development
does not come up to what we have in the Bird.

The germ, during the whole growth, is not sur-
rounded by special envelopes derived from its own
body (Plate I, fig. G) ; iris only the vitelline mem-
brane which surrounds it as it rises from the yolk
when the head and the tail is separated from the
yolk; the germ being never enclosed in any other
membrane. But now in Snakes, in Turtles, in Liz-
ards, in Birds, and in all Mammalia,there is an ad-
ditional envelope all around the embryo. And
this envelope is derived from an extension of the
margin of the enlarged upper layer, (Plate VIII,
figs. E,F, G) which folds itself up around the germ,
forming the sac of the so-called Amnios. You
may trace this outline here as it extends from the
lower part of the head around the navel, and fold-
ing backwards and upwards, and from behind the

same, and from the side the same, and when these
folds unite upon the back, as they do in fig. F, the
germ is enclosed in another sac, though it is at the
same time surrouaded by the viielline membrane,
and the other substances contained in the shell.

There is, therefore, a double protection to the
germ, and it is by the special developemt of this
sac that these birds are placed in a cavity full of
liquid, and during their development in this cavi-
ty, gills are formed similar to those which exist in
fishes.

But there is soon another sac formed to pro-
tect once more the germ, rising in the form of a
vesicle from the lower and posterior part of the
body ; first, a small sac growing larger, and then
stretching (Fig. G) between the former sack and
the envelope of the yolk membrane, so as to sep-
arate both and to extend all around the germ,
forming another sac around it, the so-called Allan-
tois. And at this period the germ is enclosed within
two sacs ; first, one formed from the extension of
its own margin, and then another which rises as
a vesicle from its lower cavity ; and within these
envelopes, the young chicken is developed within
the hard egg- shell. Now in the Mammals, we have
a somewhat different adaptation. The same pro-
cess is at first going on, the same sacs enclose the
germ. Suppose only the enclosing membrane of
the egg not to grow hard, but to remain (Plate IX
fig. K) membranous ; and then we have the differ-
ences which characterize Mammalia, in which the
egg remains attached to the maternal body by a
peculiar development of the blood-vessels of the
Allantois ; when in this connection the germ un-
dergoes all its metamorphoses before the young is
born.

All the difference which exists between these ani-
mals and the lower ones of the same type which
lay eggs, is only this— that in the lower classes the
egg is laid with a hard envelope which is cast af-
terwards; while in the Mammalia the envelope of
the egg goes on growing within the germ, and is
not cast until the young is born. Other differences
do not exist. But these changes go on to produce
the great differences which we have noticed among
the different classes. Unfortunately, up to the
present day, Embryologists have been satisfied to
have traced the first outlines of the germ, and have
never considered it of any importance to trace the
changes which the very young undergo to assume
their final form. It struck me that it might be of
some interest, and during the last Spring, I opened
several eggs, at rather a late period, to see what
changes they would undergo. And I was surprised
to find that the first forms which are developed
are not those which will be permanent. Indeed, to
say it in a few words, that for instance, when the
legs and wings of birds are formed, they are not
formed under the shape of legs or wings. When
the paws of Rabbits are developed they are not de-
veloped with their fingers as they finally grow, but
appear first under the form of fins.

1 O

102

PROF. AGASS1Z S

[PLATE X— LEGS OF MAMMALS, BIRDS AND REP-

'TILES]

Here is the bind leg of an adult Squirrel, (Plate
X, fig. A), and here it is in an earlier stage of de-
velopment, (Fig. D) Here is the foot of a Robin
(Plate X, fig. B) with its covering, scales and claws ;
and here is the foot of a Robin at an earlier period,
(Fig. E). In the Frog here, it is with distinct fin-
gers, (Fig. C) and in the earlier periods still more
different, (Fig. F).

What do we observe here ? Animals which
have so different organs of locomotion as the foot
of a Squirrel, or the foot of a Bird, or the foot of a
Frog, all have the same form originally. We may
therefore say that the organs of locomotion, how-
ever diversified in their perfect state, begin under
the same form. Could I have illustrated all those
which I have already traced, I should have shown
that the wing of a Robin is also in the beginning a
fin. And indeed, in all animals, which I have
traced with reference to these transformations of
the young, I have found that at the first period of
appearance,their organs of locomotion are through-
out identical— in the Swallow as well as in the Spar-
row, the Wading Birds, the Birds of Prey, and in the
Swimming Birds, in all the legs and wings in their
earlier condition. The legs and wings are similar to
the lowest appendages or fins which we observe in
Fishes, showing also that even in the first outlines
of their organs there is the greatest uniformity. I
may say the same with regard to the structure of
the heart, of the first formation of the lungs, which

are so complicated in Mammalia. It is a simple
sac in young Birds, and a simple sac in Fishes,
which have also that organ in a rudimentary con-
dition, as an air bladder, just to preserve the uni-
form harmony which exists in all these classes. It
is obvious what important consequences such facts
must have for the study of Zoology. They show
at once that all animals of any given group which
have webbed organs of locomotion, must be con-
sidered lower in their group than those in which,
this apparatus has become free— has grown inde-
pendent.

To make direct application of these results to
the classification of birds ; let me allude to the
fact, that at the present day, in our classification of
birds, all the birds in which the toes are united by
a web, are combined into one group— however dif-
ferent they may be in the structure of their wings
or feathers, in the development of their inner or-
gans, or in their mode of living. We have, in fact,
in that group, the lowest birds of various types
united into one very unnatural family — as we have
there brought together the Penguin, having imper-
fect wings used as fins, with the fleetest birds.
If we compare the bills, the predatory habits of
some, and the low mode of living of others, — if we
consider the difference between the bills of Ducks
and of the Gulls, — we may conclude, after the facts
above mentioned have been once ascertained, that
it is likely we have combined in one group the low-
est types of various families, which should be se-
parated ; and it is probable that we shall soon see
a re-arrangement of the class of birds, which will
be classified on other grounds, and leave in each
group some swimming types and some types with
divided fingers, which may be combined by some
higher characters.

Already among the Vultures we have some re-
semblance to the Gulls, and it was only from their
curved claws and hooked bills that they could not
be brought together. But when I mention that
within the egg, the young Robin also has a hooked
bill, we see that the difference between the birds of
prey and the web-footed Gulls cannot be so very
great— particularly when we notice the appear-
ance of the joints and fingers in their embryologi-
cal formation, and the fact that so many birds of
prey have the outer finger united by a rudimen-
tary web. Without entering into minute details, I
may state that the knowledge of the changes
which each germ undergoes to assume the form of
the full grown individuals, will obtain for us a
complete key to the natural arrangement of the
perfect animals.

Contrary to what happens in the Radiata, Mol-
lusca and Articulata, where all classes appear sim-
ultaneously from the first times of development of
animal life, the four classes of Vertebrata appear
successively in different epochs of the development
of our globe,in the order of gradation which Anat-
omy and Embryology assign to them. The class of
fishes appears first-, it is followed in a later epoch by

LECTURES ON EMBRYOLOGY.

103

the class of Reptiles; then come the Birds and Mam-
malia. The class of Fishes, which I have studied
more particularly, has shown me that the first
types appeared under forms and with an organi-
zation peculiar to embryos of that very class in the
present epoch, proving thereby with perfect evi-
dence the inferiority of the first created types, as
well in their peculiar class as in their department.
But though of a lower order, these types of an-
cient ages bore in themselves, from the beginning,
the impression of the plan that was to be succes-
sively developed in the different epochs which have
preceded the order of things existing at present,
and by whose realization have been brought about
those numerous families of Fishes, Reptiles, Birds
and Mammalia which live now on the surface of
the earth. According to this plan, a certain num-
ber of families were to be extinguished before our
epoch ; these families are known to us only through
their fossil remains, which researches in the crust
of our globe have brought to light. Other fami-
lies, less numerous, have lived through all the rev-
olutions of the globe, and have preserved some
representatives, a sort of reminiscence of a past
order of things, confined upon a few spots of the
present surface of the earth.

It is worth while to notice that Northern Amer-
ica is the present home of several of those ancient
types. Such are, in the class of Fishes, the Lepi-
dostei, which perpetuate the order of Ganoids, in
our days, an order so numerous in the fossilif erous
strata of a former world, and the genus Percopsis
of Lake Superior, which represents a family which
prevailed in ancient times in Central Europe, dur-
ing the epoch of the deposition of the chalk. We
observe the same relation among the trees of
Northern America, which resemble much more the
vegetation of the tertiary period than the trees now
growing in Europe.

The time has past which was allowed for this
course. I must come to some conclusions without
giving any further details upon the subj ect.

My object has been to bring the present knowl-
edge which is possessed upon Embryology, into
one point of view. If I have succeeded in show-
ing that there is a common development to all
animals, however diversified, I have succeeded in
illustrating, perhaps, in a more philosophical view,
the different data which have been acquired upon
this point. All animals arise from uniform eggs,
however different their final development may be.
But however like they are at first, we soon notice
the difference. The growth of the germ in Radi-
ata does not take place in the seme manner as
that of Mollusca ; nor does it take place as in
Articulata ; and we have again seen that the growth
of the germ in Vertebrata takes place in a different
manner. And to make this more prominent by
figures,we can represent the Vertebrata,as we have
done with the other great types, as follows: —
by a double crescent in two opposite directions,
showing that there is a special cavity containing

the brain and the main organs of sensation, and a
lower cavity containing the intestines and respira-
tory organs. And this symbol will be only a copy
of the outlines of the embryonic growth of any of
these vertebrated animals.

But we have found these metamorphoses to
agree in many instances with the gradation which
structure had illustrated. We may therefore in-
fer from the successive development of structure,
the order in which animals should follow in a
natural arrangement, as ascertained by the
knowledge of metamorphosis. So that, vice versa,
the study of Embryology will improve our classi-
fication, as derived from anatomical data, as well
as anatomical investigation will go to complete
the inferences derived from Embryology.

I think I have particularly been able to show
that classification in its details may be improved
by Embryological evidence. And it is upon this
point I would insist : that a more extensive knowl-
edge of young animals will be extensively useful
to the further progress of Zoology, as affording, by
the comparison of successive changes, the means
to assign to full grown animals their respective
places in any given group.

The facts are already numerous enough to allow
us to consider this principle as the fundamental
principle of classification, which should overrule
the information derived from Anatomy in the de-
tails, as here Embryology assigns a value to the
external forms for which comparative Anatomy has
no understanding. Comparative Anatomy has not
been able to value the external forms, to assign to
them any importance. But Embryology, by the
metamorphoses which take place in animals, as-
signs now a value to external forms, and not only
assigns to them a value, but a chronological value,
by which it is possible to consider as lower those
animals which agree with the earlier forms of the
germs.

These remarks would lead me to make some ob-
servations upon what is next to be done in these
investigations. That a greater number of animals
must be investigated than has been done before, is
obvious. There are several animals, upon which
we have no information.

But these results should not be traced simply
with reference to Physiology, as it has been hither-
to. All Physiologists have traced them with refer-
ence to the structure of the organs, to the structure
of the tissues, to the structure of their various
systems, and not with a view to understand their
forms. Simple sketches of the outlines of various
forms of germs from various families,with their de-
scription, would be a highly valuable contribution
to the stock of our knowledge at present, and would
afford, as rough as thev may be, the means to place
in a natural position many genera which are now
placed in a most arbitrary order in our method.

I do not undervalue our past labors in classifica-
tion, but I make a distinction between what has
been done in an arbitrary manner, and what has

104

PROF. AGASSIZ'S LECTURES.

been systematically done upon real data. And
when I see the possibility of leaving aside this ar-
bitrary method of classification, I insist upon the
value even of superfical comparison of all embry-
onic forms. And when this has been done more
extensively than it has been done up to the pre-
sent time, then it will be time to reconsider the
whole department of Paleontology, in order to
compare the forms of former periods with the early
stages of growth of the animals of the present cre-
ation.

All the information about the fossils— all the in-
formation of former ages, will have to be compar-
ed with those embryonic forms, in order to under-
stand more fully the analogy which exists between
these earlier types, and the successive changes

which those of our day undergo to assume their
final form. If I am not mistaken, we shall obtain
from sketches of those embryonic forms more cor-
rect figures of fossil animals than have been ac
quired by actual restoration.

The few hints which I have been able to give ai
only indications towards what is to be done. The
comparison, step by step, between the various fos-
sils of all Geological periods— between the great
changes which all families undergo, step by step-
also remains to be done, and then the plan of the
creation will be better understood. Then we shall
more fully acquire an insight into these harmo-
nies, by which the whole is combined and carried
through ages to the perfection which has : llowed
man to stand at the head of Creation.

Erratum, — A line is omitted from the bottom of the 26th page. After the word wkick, last line,
read as follows : " indeed does not come within the plan of the present course."

We subjoin a specimen of Phonography from Dr. STONE'S notes. It is the first four lines of the
twelfth Lecture.

A-/C

THE PRINCIPLES OF ZOOLOGY:

TOUCHING THE

STRUCTURE, DEVELOPMENT, DISTRIBUTION, £ NATURAL ARRANGEMENT

OF THE

RACES OF ANIMALS, LIVING AND EXTINCT:

WITH NUMEROUS ILLUSTRATIONS, FOR THE USE OF SCHOOLS AND COLLEGES.

PART I. -COMPARATIVE PHYSIOLOGY:

— BY —

LOUIS AGASSIZ AND AUGUSTUS A. GOULD.

PRICE, ONE DOLLAR.

The design of this work is to furnish an epitome of the leading principles of the science
of Z >OLOGY, as deduced from the present state of knowledge, so illustrated as to be
intelligible to the beginning student. No similar treatise now exists in this country, and,
indeed, some of the topics have not been touched upon in the language, unless in a
strictlv technical form, and in scattered articles. It has been highly commended, by the
most eminent men of science, and by the public press. A few of which are here given,
together with a sample of the cuts illustrating the work.

tJSrO

" This work has been expected with great interest. It is not simply a system by which
we are taught the classification of Animals, but it is really what it professes to be — the
1 Principles of Zoology,' carrying us on, step by step, from the simplest truths to the
comprehension of that infinite plan which 'the Author of Nature has established. . .
This book place-" us in possession of information half a century in advance of all our
elementary works on this subject. ... No work of the same dimensions has ever
appeared in the English language, containing so much new and valuable information on
the subject of which it treats."— -Professor James H Albany.

PRINCIPLES OF ZOOLOGY.

" A work emanating from so high a source as the
' Principles of Zoology,' hardly requires commendation
to give it currency. The public have become acquainted
with the eminent abilities of Prof. Agassiz through his
lectures, and are aware of his vast learning, wide reach of
mind, and popular mode of illustrating scientific subjects.
In the preparation of this work, he has had an able coad-
jutor in Dr. A. A. Gould, a frequent contributor to the Transactions of
the Boston Society of Natural History, and at present engaged upon the
department of Conchology, for the publication of the late exploring expe-
dition. The volume is prepared for the studtnt in zoological science ; it is
simple and elementary in its style, full in its illustrations, comprehensive
in its range, yet well condensed, and brought into the narrow compass
requisite for the purpose intended." — SilUmaris Journal.

" The reading of the work has afforded us double the satisfaction it would otherwise
have done, on account of the implicit confidence we felt in the statements and illustra-
trarion* of the talented authors.

Besides what we have already written, we cannot help urging readers generally, and
especially those who are collecting libraries and are fond of good books, to add this, to
their catalogue." — Christian World, Boston.

" Such books as this fasten upon our

minds the disagreeable impression that we

have come into the world half a century

too early. The schoolboys of the next gen-
ration can scarcely escape, even with great
•itre, the catastrophe of becoming learned.

The volume before us must introduce a new

epoch in the study of this branch of natural

science. It combines all the essential ele-

ments of a good text-book ; being at once

comprehensive, even to exhaustion of the subject, yet concise
and popular. The beauty of the paper, and typography, and illustrations, will aid the
fascination which the contents exert upon the mind. A single glance at a chapter on
Embryology, bound us with a spell which we could not shake ojf, till we had looled through
the volume. The names of the authors are vouchers for the merits of the work. —
Professor Agassiz is without a rival in his department of science. His associate is widely
known by his valuable contributions to the Conchology of Massachusetts, which have
won favorable notice from the savans of Europe. We hope the approbation of the public
substantially expressed, may encourage the authors to complete the series so auspiciously
commenced. ' ' — Philadelphia, Chronicle.

" This work is designed as a text book for Schools and Colleses, and as an exposition
of the interesting science of which it treats, it has many obvious advantages over any
other treatise extant. It is the joint production of two gentlemen, whose researches in
Natural History have enlarged the domain of human knowledge, and one of whom stands
confessedly at the head ot the science of the age. It hence contains the latest and most
approved classifications, with explanations and illustrations, borrowed from the forms of
animated nature, both living and extinct, and made accurate and perfect by the fullest
acquaintance with the present condition of Zoological science. As a text book it is ad-
mirably conceived.

" The presence of Prof. Agassiz in the United 'States, has given a new impulse to
every branch of Natural History, and we are happy to find him thus associated with Dr.
Gould — one of our leading American naturalists — in explaining his favorite science to
the youth of our Schools and Colleges." — Providence Journal.

" No such work had previously appeared in our country. The production is worthy
of the great names under whose care it has been prepared. Schools and Academies
will find it opens up a new and attractive study for the young ; and in no country is
there a finer field opened np to the naturalist than in our own." — Christian Alliance, Bos-

ton.

•4-

PRINCIPLES OF ZOOLOGY.

^\s*^+*s^/^s^s*^\s**s^/\s\s^**r*s**s**f**s**s^s*^^^

"A new and highly valuable publication, intended for a school book, but which will

be found equally interesting and important for all to study Such a work as this

has long been a great desideratum, and we rejoice that a want so strongly felt, has now,
at length, been so well and so completely supplied." — Boston Atlas.

" The work is admirably adapted to the use of schools and colleges, and ought to be
made a study in all our higher seminaries, both male and female."-— New- Tor k Observer.

" To the testimony which is furnished by their distinguished
scholarship, we may add, however, that the classifications of the
work are so admirably arranged, and its des-
criptions given with so much simplicity and
clearness of language, that the book cannot
fail of its practical aim — to facilitate the pro-
gress of the beginning student. It is a work
for schools." — New-York Recorder.

" The announcement of this work some time
ago, as being in a course of preparation, ex-
cited a high degree of interest among teachers,
students, and the friends of science. The names
of its authors gave ample assurance that it was no compilation drawn from other works
no mere reconstruction of existing materials. The work will undoubtedly meet the
expectations that have been formed of it, and already it has been adopted as a text-book
in several colleges. ' It breaks new ground; as is said in the preface, 'some of its topics
have not been touched upon in the language, unless in a strictly technical form, and in
scattered articles.' The volume exhibits throughout great labor and care in preparing it
for the public eve, and for the use of students. As it has no rival, we suppose its adop-
tion will be almost universal in literary institutions, and it will do much to awaken in the
minds of multitudes an enthusiastic love of natural history." — Christian Reflector $
Watchman.

" This is entirely a new field in Arnei-i-

can elementary literature, no similar

treatise existing in this country- At first

sight, the work appeared to us too ab-
struse for beginners, and for the use of

fhose whom the authors aim to benefit —

the scholars in our common schools. A

more careful examination convinces us

that any teacher or scholar, who is in

eai-nest to understand the subject, will
find the application necessary at the com-
mencement comparatively trifling, while the subsequent benefit will be immense. This
is tiie first volume of the work, and is devoted to Comparative Physiology, on which branch
it is exceedingly complete. It is freely illustrated with the necessary wood cuts. The
names of the authors will be a higher guarantee for scientific accuracy than any judgment
we might pronounce." — New- York Commercial Advertiser.

" It is designed chiefly for the use of schools and colleges, and as an epitome of the
subject on which it treats, contains more in a small space, than any book of the kind that
has yet fallen under our notice." — Saturday Gleaner, Philadelphia.

PRINCIPLES OF ZOOLOGY.

*v^/n^^^r^lr^S^r>*S^f**^^ii*~i~*f*'^rv'^^

" On almost every subject we have scores of new books without new principles, but
not so with the work before us; indeed several of the highly interesting topics presented
and illustrated have no treatise in the English language. It contains a large amount of
valuable information, and will be stud'ed with profit and interest by those who have made
respectable attainments in Natural History, as well as by those just commencing this
science. This volume is finely executed, and should find a place in every library. As a
text-book for schools and colleges it is far superior to any work before the public." —
New- York District School Journal.

"Professor Agassiz stands confes-
sedly at the head of Zoological science,
and his coming among us is everywhere
hailed with delight and enthusiasm,
and the influence of his mission is
everywhere felt already, and it will
continue for a century to come. Dr.
Gould is one of the most indefatigable
and accurate investigators in natural
science of our country, and we greet
with real pleasure the association of
his name with that of Prof. Agassiz in
the preparation of this work.

Our space will not allow anything

like a review of this admirable and to us novel work. The plan is
quite unlike those elementary works which teach us the mode of clas-
sifying animals by a few important characteristics. It commences by
explaining the sphere and fundamental prinicples of Zoology, and
follows by showing what are the general propertie- of organized bod-
ies ; the functions of organs in animal life ; the nervous system, the

senses, motion, nutrition, circulation,

chapter on Embry-

ology alone is of more actual interest in philosophical Zooloyy, than all*
that has ever appeared on the subject of Zoology, in our country. And as we before re-
marked, this knowledge is nowhere else to be had in the English language. The geograph-
ical distribution of animals forms another important feature of very deep and general
interest." — Albany Argus.

" I have read with the greatest satisfaction the volume on the principles of Zoi'logy.
It is such a book as might be expected from the eminent ability of the authors, Professor
Agassiz and Dr. Gould. So far as I know it is the most comprehensive and philosophical
elementary treatise on the subjects of which it treats, which has yet appeared.

It is well adapted to the purpose of being used as a text-book in schools, and I shall
employ it in preference to any other in my own school, whenever I have a class in the
elements of Natural History, and 1 can strongly recommend it to other teachers." — George
B. Emerson, Esq., Chairman of the Boston School Committee on

G. K. & L. have in the press PROFESSOR AGASSIZ'S "TOUR TO THE LAKES."
It will contain an interesting narrative of the excursion, by Elliot Cabot, Esq., and the
Scientific Researches of Prof. Agassiz, with elegant illustrations, in one volume, octavo.

CONTRIBUTIONS TO THEOLOGICAL SCIENCE :

BY JOHN HARRIS, D. D.

I. THE PRE-ADAMITE EARTH: 1 volume, 12rno. cloth. Trice, 85c.

II. MAN: His Constitution and Primitive condition. \Vithaportraitoftheauthor.

"His copious and beautiful illustrations of the successive laws of the Divine Manifes-
tation, have yielded us inexpressible delight.'' — London Eclectic Review*

Internet Archive Audio

Featured

Top

Images

Featured

Top

Software

Featured

Top

Books

Featured

Top

Video

Featured

Top

Mobile Apps

Browser Extensions

Archive-It Subscription

Save Page Now

Full text of "Twelve lectures on comparative embryology, delivered before the Lowell Institute, in Boston, December and January, 1848-9"

See other formats