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brain of the future animal come from these foul' small
THE DEVELOPMENT OF A MOLLUSK.
Scattered all along the Atlantic coast, from Labra­
dor to Florida, is a genus of mollusk known to zoology
as Crepidula which, despite its abundance, has been so
little 'studied
that
only
cor;:tparatively
recently
has
anything definite about its embryology been known.
G. Conklin, of the University of Pennsyl­
To Prof. E.
vania, we owe the first account of the life history of
crepidula.
With the results of Prof. Conklin's inquiry
before him, Dr. Dahlgren, head of the department of
preparation of the American Museum of Natural His­
tory, has
cells.
In FIg.
9, the beginning of the fourth cleavage i,,;
indicated by the separation from
the larger cells of
another quartette of small ectoderm cells.
These de­
velop until they assume the size and shape shown in
FIg.
.
10.
process
As the animal continues
of
division,
the
first
to
grow by this
quartette
which
was
formed in the stage shown in Fig. 8 has been split up
into eight cells, so that we now have twelve ectoderm
cells in all
(Fig.
11).
The karyokinetic
figures
in
photographs.
illustrations
The velum
as­
mushroom-like
ap­
Finally we have the fully
Here we see the velum with
to
a
sessile
mollusk,
be­
In Fig. 21, we have a median
The velum,
which is a highly characteristic cili­
of
ated formation of the molluscan em­
bryo serving as an organ of locomo­
dark zone represents the
tion in that stage when the embryo
is called'a veliger.
the lower or more
It is usually soon
lost, but in some cases it is retained
lightly tinted 'part is the yolk, con­
permanently in a modified form.
stituting the vegetal pole of the egg.
invariably
New Fire-Borup or Ga" Indi('ator.
thrown off during the maturation of
precedes
its
the spermatozoon.
A new automatic apparatus for in­
union
Every
nu­
cleus contains a substance known as
forms various
colored
dicating
the presence of
illuminat­
ing
or
the
gas
sphere
chromatin, which, in the process of
d.ivis;;:m,
a
as its name indicates, is a veil or sail
animal pole of the egg, or the proto­
with
19, we find these
The terms velum and veliger re­
what is known as the division spin­
which
gland which secretes
quire some explanation.
or sperm and egg, nuclei which do
egg,
the
erly labeled.
spots represent the male and female,
the
with
mollusk, each of the parts being prop­
The two dark
are
center,
In Fig.
almost
20.
cies.
1 we see the undeveloped
bodies
the
section of the completely developed
rise to a particular organ, or part, of
polar
in
them for the distribution of the spe­
the developed animal.
Two
are
cause the animal is dependent upon
cells has its own peculiar shape, size
plasmic portion;
These outgrowths
nothing but swimming organs highly
Each of these first-formed
The
foot
essential
and position, and it invariably gives
dle.
18.
its several rows of cilia, which are
cells in the early division stages of
appearance
of Fig.
Fig.
be traced back to certain individual
the
striking features
the head at the top and the velum which lies below it;
pearance.
more they show that many of the or­
before
17 develop
Certain lines of cells shown in Fig.
into outgrowths or projections, constituting the more
developed laFa or veliger; shown in
Further­
gans of the fully-formed animal may
fuse
The embryo now assumes a more decidedly ovoid
sumes
changes which are designated by. the
not
shape.
ultimately assume.
living cell in the process of division,
In Fig.
17).
developed into the forms they will
show those series of active changes
egg of the crepidula.
Soon they completely surround
outgrowths have more pronouncedly
which take place in the nuclei of a
the egg.
rounding the embryo.
it, leaving only a very small opening-the mouth (Fig.
the shell.
are
The models excellently
general name karyokinesis.
Finally, in the stage repre­
16, these cells are shown aU but sur­
the bottom a
history of crepidula, of which models
accompanying
sented in Fig.
mouth a mere cavity above it; and at
series of models to illustrate the life
the
the top of the entire egg.
the
directed the making of a
13 and 14 the ectoderm
further we find that in Figs.
cells have incr,eased by simple division until they cover
has
Messrs.
fig­
After the chromatin has been dis­
The
of -a
The First Stages of a Mollusk's Growth.
tributed equally to the two poles of
very
carries
tight
the division spindle the cell body be­
been
Hauger
Paris.
ures, such as disks and threads.
fire-damp
at
in
brought
and
sensitive
recipient
end
by
Pescheux,
apparatus
one
atmo­
out
is
balance
of
of
composed
which
the beam
containing
a
ordinary
2.
air, while it is balanced at the olher
The egg elongates and the· entire cell
end by a plate having the same sur­
gins to divide, as shown in Fig.
face.
is constricted about the central axis.
equal
portions,
which
are
at
normal
first
nearly spherical and touch each other
only
at a
comparatively small
face, as shown in Fig.
3.
sur­
the air,
the
COlltact becomes larger, so that each
ings, shown in Fig.
pearing
near
the
animal
pole
meres
pole,
of
forming
four
approximately
dow,
the
which
The Last Stages of a Mollusk's Growth.
6, the second cleav­
This second cleavage lies in
future animal
and divides
There
'
have been two cleavage planes at right angles to each
other; but now another phenomenon takes place.
small
cells at the animal pole of the egg.
the larger cells indicate that the separation of a third
quartette of ectoderm
cells
is
about
to
take
place.
The second and third quartettes give rise to all the
The
protoplasmic
These are the
head.
A large cell is now formed at the posterior
pole of the egg (Fig.
13) which contains all the sub­
stance of the middle layer or mesoderm of the future
animal.
The four lower large cells, which now, after
separation from those of the ectoderm and mesoderm,
first beginnings of the ectoderm, which is subsequentiy
consist
to
form the internal -lining of the future mollusk's diges­
envelope the
whole
egg
ment of the ;:;.nimal's body
and
(Fig.
become the
9).
as
to
Thus
alarm
bell at the
mixture.
In
order
to
are mounted upon the
integu­
The head and
tive
entirely
organs.
of
entoderm
Tracing
the
cells,
are
process of
scale
One of these is used to corchamber which
acts on a multiplying lever so as to
ectoderm of the future animal except tlla.t over the
Up to this time, cleavage has been equal.
gives rise to four
so
As the needle is regu­
sists of an aneroid
THE DEVELOPMENT OF A MOLLUSK.
its body into right and left portions.
next cleavage
arranged
rect the pressure variations and con­
the production of which also marked
the median plane of the
is
gaseous
beam.
Each
In Fig.
the
of temperature and pressure, the in­
nucleus during division breaks into
2).
and
where
ventors use two compensating devices
y!hat are known as division spindles,
the first cleavage (Fig.
down,
neutralize the atmospheric influences
size.
the process of cleavage, as illustrated
age is shown complete.
case
atus work for any desired degree of
The same elongation of the two cells
2, again takes place.
the
lated at will, we can make the appar­
and
which characterized the beginning of
in Fig.
is
ringing an
same time.
ap­
blasto­
equal
is
If
automatic device for opening a win­
running around until they reach the
vegetal
the vessel
destroyed.
current can be made to operate an
Two fairly inde­
produced,
that
can be of great utility for fllines. In
the case of private apartments the
is about to occur in a plane at right
are
In this case as the
wo.rked even at a distance, and this
4 by the four
reached this stage, another cleavage
furrows
ac­
a bell can be rung or other apparatus
egg, indicate that when the cell has
pendent
air
and close an electric circuit.
mark­
black spots at the upper pole of the
angles to the first.
o.f
plunge a needle into a mercury cup
the posterior half.
karyokinetic
the
modified
the vessel goes
reverse
The balance
One of these cells forms the anterior
peculiar
is
mixture becomes denser than the air.
4).
half of the future animal, the other
The
respiration.
the foreign elements are lighter than
Later the two cells are more closely
(Fig.
density
tight, the balance is
pressed together and the surface of
hemisphere
its
variable, seeing
having each given up half of its ma­
a
chll:nge,
for
composition
air which the vessel contains is in­
1
terial to form one of the new nuclei.
forms
the
is mixed with it.
halves thus formed has its own nu­
cell
condition
Should
cording to the amount of gas which
Each of the
cleus, the two nuclei shown in Fig.
Thus the apparatus is at rest
when the surrounding air is in the
. The cell body then divides into two
destined
evolution
to
still
displace a rider along the scale beam, thus keeping the
balance constant under varying air
pressur�s.
Heat
influences are compensated by a composite metal spiral
which acts upon a lever and shifts a rider upon the
beam in the same way.
When once it is accurately cali­
brated, the instrument is invariable at different tem­
peratures and pressures.
• ·e· •
The power generated in a modern steamship in a
single voyage across the Atlantic is enough to raise
from the Nile and set in place every stone of one of
the; great pyramids.