THE PERIBLAST

YI. SEGMENTATION

VII. THE PERIBLAST

The

origin of this layer, together with man}' of its peculiarities of structure, has been noted in the descriptions of the plates. Itis not

my

intention to

go now

intoany extended discussion of its formation and fate.

However,

it will be well to describe briefly the various

NO.1431.

BREEDING HABITS AND EGG OF PIPEFISH— GUDGER. 491 modes

^of its formation in other Teleosts, and to

show

under which of these cksses the pipefish

egg

falls, and finally togiv^e referencesto a

few

of the

more

valuable papers on this subject.

In Teleosts the periblast layer seems to be

formed

afterthreetypes:

(1). In eggs, ⁱⁿ which the tirstfurrowcuts through tothe yolk, the periblast is

formed

by a thin protoplasmic sheet extending inward

from

the ^'"''Band.^'

Henneguy

(18S8, lig. 63)shows this very plainly

for the trout.

(2). In eggs,in which there is no layei"ofoil drops underthe germ- disk, or those in whicli the protoplasmic mass separates sharply

from

the yolk, the periblast is

formed when

the inner ends of the cellsin thefourand eightcelled stagesare cut outand lifted

from

the under- lyingthin protoplasmic sheet. This is the

mode

of formation in Ser- ranus (Wilson, 1801), Ctenolabrus (Agassiz and

Whitman,

1885), and Belone (Kopsch, 19U1).

(3). In eggsin which thereis an imperfect separation of

germ

disk and yolk, or in which there is a layerof oil drops under the blasto- disk, the central periblast hasa verypeculiar

mode

of origin. Cells are cutout of the protoplasmic disk in successive layers

from

above

downward

and the central periblast isthe renmant of blastodisk left

when

this process has ended.

The

explanation for thisis that the protoplasm continues to flow outof theyolk into the

germ

diskuntil segmentation has progressed

some

distance. Kupfl'er (1868)noticed that the

germ

disk

was

not fully

formed

in a

European

Syngnathus

until after the four- celled stage. Thisformation forthe central peri- blast is described

by most

workers on the Salmonoids, notably

by

Zeigler (1882), and

Hoflmann

(1888), for the salmon, and latest of all

by

His (1898) for the salmon and trout.

Kowalewski

(1886) found essentially the

same

formation in Cdrassius and I*(//(/acanthit.s.

The

central periblast nuclei, in types 1 and 2, originate

by

divi- sion of the ''Rand'''' nuclei and migrate centralwards in this layer.

In

Type

3 they are the direct descendants of the segmentation

cells.

In Sijdio.sfo/ita forldpe there are found the

two methods

of central periblast formation described in

Types

2 and 3 above. In figs. 10, 45, 46, 47, 48, and 52 for the eightand sixteen-celled stages, there is

shown

^a

mode

of formation for the periblast which negatives the idea that

from

^it there could ever

come any

""after-segmentation."

On

the other hand, infigs. 53,54,55,68, 59, 60,61, and 62, the central periblast is the protoplasmic

remnant

of the primary

germ

disk, left after all the blastoderm cells have been cutout of it. It is well to note here thata migration of nuclei into the marginal regionand the formation of a

"wreath"

by the disappearance of cell walls has, because of the opacity of the Qg^^ not been seen in the pipefish.

Whether

ittakes place or notI can not say.

492 PROCEEDINGS OF THE NATION

jiX

MUSEUM.

^vol.xxix.

The

difficult (juestion, whether, in the

egg

of the pipehsh, cells are

budded

^oil'

from

the central periblast and added to the blastomeres, can not herebe taken up.

However,

this

would seem

to bealegiti-

mate consequence of such a

mode

of cell formation as that

shown

in

Type

3 above, and apparently finds conlirmation in hgs, 75, 77, 79,

and 82, in which a perfectly definite periblastlaj^er hasbeen formed.

If these figures are

compared

with His's (1898) ^tigs. 10 and 12, this matter will be

made

clearer.

For

a fuller discussionof the origin of the periblast and its nuclei, and of the fateof the latter, the reader is referred to

Brook

(1887),

Kowalewski

(1886),

Hoffmann

(1888), Fusari (1890), Berent (1896), Zeigler (1887 and 1896), His (1898),

and Hertwig

(1903).

At

this point, the

work

on the developmentof the pipefish will have to rest. Ithas been the intentionof the writer to carry itfurther, at leasttothe closure of the blastopore,and forthispurposethe sections have been cut, but the difficulties

met

with have caused so

many

delaysthat it has been impossible tocomplete it.

The

Q,ggof the pipefish isvery different

from most

other teleostean eggs in the

form

ofits segmentation and the dualorigin of its peri- blast, together with the "after-segmentation" of^cellstherefrom.

So marked

are these differences that it seems proper to say that the figuresin this paperare representative of the sections of athousand or

more

eggs, obtained

from

thirt^^-threefishesduringthreesummers.

The

slides containing the sections

from

which these figures were

drawn

have beenpresented to the U. ^S. National

Museum.