Early stages of the brain of Acanthias

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EARLY STAGES OF THE BRAIN OF ACANTHIAS

BY

LESTER CARLTON VER NOOY

A.B. Amherst College, 1916

THESIS

Submitted in Partial Fulfillment of the Requirements for the

Degree of
MASTER OF ARTS

IN ZOOLOGY

IN
THE GRADUATE SCHOOL
OF THE
UNIVERSITY OF ILLINOIS

1918

Digitized by the Internet Archive
in 2013

http://archive.org/details/earlystagesofbra00vern

$10 /€s

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N59 UNIVERSITY OF ILLINOIS

THE GRADUATE SCHOOL

a iY 2 Oe

1 HEREBY RECOMMEND THAT THE THESIS PREPARED UNDER MY

SUPERVISION BY... eater Carltom Ver Nooy «§«. —«s_—

ENTITLED" -Seriy- Stages of ‘the Brain of Ac@nthiag ~

BE ACCEPTED AS FULFILLING THIS PART OF THE REQUIREMENTS FOR

THE. DEGREE OF -_ £

In Charge of Thesis

of Department

Recommendation concurred in*

Committee

on

Final Examination*

*Required for doctor’s degree but not for master’s

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TARL® OF CONT EYTS

Int roduct for —
Yaterials and Wethods
Historical
Nbservatiors
Wmbryo 2 mm.
Wmbryo 3,6 mm,
Wrbrvo 3.5 mm,
Mmborvo 4 mm.
mrbrvo 4.5 mm.
Wmbryo 5 mm,
Wrbrvo 6.2 mm. —
Wmbhryo 10.2 mm,
Developrant of Cranial “Nerves - ----------- = 25
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EARL Y STAGES .
OF T Hal BRAIN OF AWG ANT) TAB,

INTRO DUCTION,

Although considerable work has been done upon the sarly
brain of Acanthias in connection with the subject of metamoerism,
there has hesn no consecutive study of ths sarly stages from the
single standpoint of vertebrate caphalogenesis,

The investigation of which this paper is an account,
was carried on in the Zoological Laboratory o* the Mniversity of
Illinois under the direction.of Dr. J. S. Kingsley, to whom ths
Writer wishes to express his indebtedness for valuable advice

and criticism during the course of the work,.
MATERIALS AND METHODS,

The material employed was the series of prepared slides
of the University, both sections and toto mounts. Camera drawings
ware made and the most interesting stages in the davelopment were
reconstructed by Born's wax plate method. For several reasons,
study of the development of the brain was not carried beyond the

65 somite stage, or avout 11 mm, total length.
HISTORICAL,. ‘

Numerous investigators have usaqd Acanthias chiafly in

solving the quaston of the segementation of ths head. Ralfour (78)

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was the first to taka up the comparative study o% the head and

made the discovery of the continuous sxtension of the primary

— cavity into the head. In this he was followed by his pupils,
Marshall(''81) and Sedgwick ('928) but the great paper in this line
was by Van Wihje ('82) who confirmed the direct morphological
comparability between head and trunk and described a metameric
head of nine segments, four of these Punespragtie and five postoti¢
which number has been generally acceptsd by investigators to-day,
although Miss Platt ('91) showed the existance of another somits
(the "A" of later papers ) anterior to the first of Van Winje.

Locy ('95) discovered segments in the early blastoderm which he
traced into the formation of the neuromsres and then the later
divisions of the brain. These segments appear3d long bafore there
wore any segmental divisions of the midds germ layer. In Acanthias
he found the neural segments extending into the germ ring and, in
the chick, at times into the primitive streak, The segmants were
serially homologous, mora dafinitely shown in ths sctodsrm, with
the other layers only slightly affected. Locy's conclusion was

that the brain was distinctly metameric and consisted of 14
segments, nine in the hindbrain and fiva in the combined forebrain |
and midbrain. Neal ('98) through his studies of the primitive

nerves centers of the cranial nerves, the sarly segmentation of the
embryo and the formation of the visceral arches, concluded, ‘he
there was a primitive correspondance between neuromerism,

masomerism and oranchiomoerism,.

OBSERVATIONS,
imoryo 8 mm,

In the present study an embryo with five mesodermic

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somites, about 2 mm. in total length, begins ths series. At this

stage, (fig. 1) the medullary plate is widely exranded anteriorly
to form the anterior part of the cephalic plate and, with ths rest
of the embryonic part of the germ, is raised somewhat above the

blastodermic area. The nsural plate is nearly flat at this stage,

but its edges are bent slightly ventrally, while along the sagittal}

line of the dorsal surface is a shallow groove, most marked behind |
which indicates the line of concrs3scencsa of the two halves of the
embryo. From this the sdg9s slops gradually upwards and outwards,
the whole of this part of the plate forming the neural groove,

In his study of Amblystoma, Griggs ('10) finds a similar median
groove which persists until the neural tube is formed. But he
also describes two germinal depressions, ths "blastogroove"® and
“peripheral groove" for which 1 find no parallel in Acanthias
embryos, and hencs they need not be discussed here, It may be

that study of fresh or living embryos would bring out similar
markings. The notochord, lying in the axis just beneath the neural
plate, causes a slight elevation in the floor of this medial
groove. In this stage the notomord passes in front into a mass

of tissue which later is differentiated into entodsrm and mesoderm
whils the entodermal part of the alimentary canal gxtends far
forward beyond the end of the notochord. Near the anterior end

of the neural plate are a pair of slight depressions either side
of the median line. These are thse first indications of the
developing optic vesicles and are fairly well indicated in the
reconstruction. The edges of the cephaiic plate are irregularly
lobulated, the lobules being the nsuromares of Locy, who believed

, that they wera the remnants of a primitive msatamarism o* the

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vertebrats body; that ones established in these very sarly stages

they could be traced onward until they became thse neuromeres

other investigators and showsd definite relations to ths spinal
and cranial nerves. Hill ('00) found an early appsaranes of neural
segments in his studies on chick embryos and claimed that they
were identical wtth the neuromsres of latar stages.

Neal ('98) is a very severe critic as regards the
neuromeres of Locy and Hill. He found no constancy in tha number o*f
segments in different individuals; nor agreement in number or
position upon the two sides of the cephalic plate of a single
individual. In no case were the segments symmetrical nor was he
able to detsrmine definite relations between theses neural segments
ard the somites. Neal balieved that ths irregularity am
inconstancy of the segments precluied a phylogenetic interpretation

In the restoration which was made, (fig. 1) the lobes

1 the two sides of the cephalic plate do not correspond either
s to number or positiom, In some casss it is almost impossible
determine their boundaries. Such an irregularity of so called
neural segments is totally at wariance with the neuromeres of
later stages.

There now arises the question o* the limit of the cephalic
Plate. Locy ('95) has stated, that be*ore the closure of tha
neural tube, head and trunk could be distinguished, Neithsr in the
preserved embryos nor in ths restoration which was made was any
evidencs of a line separating the expandsd portion o”° the neural
plate from that of the unexpanded portion, to be found, However
near the hinder end of ths expanded portion of the cephalic plate

‘is the region of greatest ventral flexure. Neal has traced this

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point into later stages wheres the neural tube is transformed into
a closed tube and found that it corresponded exactly with the

hinder boundary of encephalomerse VI, the hindbrain noeuromersa,

Embryo 3.2 mm,

In Stage 2 (3.2 mm., fig. 2)) the edges of the neural plate
have begun to turn dorsally to form the medullary folds. This
procass begins at the anterior em, so that while the anterior pert |
of the neural plate has become U-shaped, the postsrior part is stil]
somewhat flattoned. At the anterior end the primitives groove is

| very shallow, growing deeper as it runs caulally, until, in the
posterior brain region, it is dewply V-shaped. The optic vesicles
have become deeper cup-like depressions, growing outwards as well a¢

| downwards and forming evaginations of the lateral wajls. Just behi:
theses primary optic vesicles is anothsr pair of deprassions which |
have pushed out the lateral walls... According to Locy ('93) who
found such accessory vesicles in his stwy of Acanthias, and Hill

('00) who describes similar pits in his study of the chick, these

depressions may be a pair of accessory optic vesiclsas which later
develope to form the doraal part of the walls of the diencephalon
and its principal outgrowth tho pinealis,.

The notochow at this stage shows distinctly its origin
from the entoderm of the archenteron. At its very antsrior em it
is a broad oval plate of cells which havs been given off from the
dorsal side of the archentsron. As it runs postsriorly it grows

smaller amd more cylindrical, still unattached to the vantral side

of the brain floor until about the beginning of the hindbrain regio

1

‘ wherg its cells ara in close contact with those of the main trunk

of the neural tube, being separated mersly by a basilar membrane,

Embryo) 3.5. mm..

Gradually the folding of the msdullary plate increases,
the lateral parts assuming a more vertical position and the margins
near the anterior end begin to fuse. Next, a fusion occurs in the
| hind brain region which gradually extends forward towards the firstj|

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until thse whole is converted into a cate oneept at the anterior

| end where the canal ramains open as the anterior neuropore, (fig.3)}

This opening persists until the embryo is 4 mm. long ami has about
19 mssodermic somites. The primary or mesencaphaiic flexure has
begun at this stags and has continued so far that the anterior
part of the brain, including the optic vesicles, now lies ina
plain at right angles to the axis of the hindbrain, The brain as
yet is very simple; very slight constrictions divide it into the
three typical regions, forebrain, midbrain and hindbrain. The
walls are fairly uniform in thickness throughout. The forebrain

is somewhat expanded and the anlagen of the optic vesicles project
from it, outward and cawlally. The midbrain is a simple enlargement |
of the neural tube, and the hindbrain, as long as the fore and
midbrain together, is only a simple tube, with a slight lateral
expansion anteriorly. At the anterior end of thse bindbrain the
roof has begun to widen a little, the result being a soreawhat
triangular outline in cross-section. Tha notochord becomes free
from the basilar membrane which connects it with the neural tube
at about the middle of the hindbrain; from thence it runs forward
to just below the midbrain and ends a little dorsal and in front

of the tip of the forebrain as ths latter is bent downward,

Embryo 4 mm,

next great change is the closing of the anterior

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neuropore and with this, as seen in figure 4, is associated an
increased primary flexure, The forebrain has ircreased markedly
in length. The optic vesicles have begun ito grow dorsally as well
as laterally amd are now separated from the forebrain in front by

a shallow groove. The walls have become thinner and the cavity

has become larger, There is little change in ths midbrain, although

its sides are dilated somewhat laterally. In ths hindbrain the
floor and sides are of about the same thickness while ths roof
is composed of only two or three layers of cells. The notochom,
growing smaller as it runs forward becomes free from the basilar
membrane which connects it to the neural tube, about the middle
of the hindbrain and gradually bends downward as it approaches

the anterior end of the forebrain.

Embryo 4.5 mm,

In an embryo of 4.5 mm. (fig. 5) the cranial flexure is
more pronounced, causing thea posterior portion of the midbrain
to form the extreme anterior part of the long axis of the embryo.
As a result of the increased flexure the forebrain now extends
further posteriorly, decreasing the angle between its ventral
side and the ventral side of the hindbrain. The optic vesicles
are larger and extend more towaid the anterior end of the forebrain
The midbrain has grown laterally, the roof becoming thinner as
the sides expand, while the enclosed ventricle, a simple cavity,
is scarcely distinghishable from that of the hirdbrain, At this
stage the first indications of the division of the hindbrain
into cerebellum and medulla are recognizable. Tha roof of the

hindbrain becomes thinner toward the posterior smd. Just in front

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of the thickened epithelium, which marks ths vory osarly formation
of the otocysts, the roof is composed of several layers of cells,
from which has grown out the anlage of ths acustico-facialis
nerve. Posterior to this thickened epithalium the roof is only a
gingle layer of cells. The lataral walis of the hindbrain are so
+hickened that the ventricle is hour-glass shaped in transverse
section, thus dividing the central canal into two enlargements,
dorsal and ventral, almost equal in size, connectsd by a narrower
portion. Gradually, passing back from the hindbrain into the
medulla, the ventricle becomes a simple cavity ovoid in transverse
section, with the broader portion near the floor, Ths notochormd
remains in contact with the floor of the brain until just in
front of the beginning of the acustico-facialis anlage. Its origin |

from the entoderm of the mesenteron is clearly seen, and, as it
extends dorsally from ths ventral side of the forsbrain, it makes
almost 2a right angle in relation with the primary flexure. At its
anterior end the notochor is laterally compressed, gradually
becoming cylindrie¢al and at the same tims increasing in diameter

as it proceeds cawially.

Emoryo 5 mm,

The conditions in an embryo of 5 mm. (fig. 6) area not
essentially different from those in the stage just described,
Forebrain, .midbrain and hindbrain are separated by very faint
dorsal constrictions. The forebrain has been pushed a littls more
caudally, thus dscreasing the angle at the ppa wees The optic

vesicles havs pushed out laterally and becomsa extended mors

dorsally. The midbrain is only slightly dilated, but is somewhat

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broader on the ventral surface than on the dorsal, Theres is only

a single layer of columnar cells in the roof of the hinibrain
while the lateral walls have becoms greatly thickenad, The
ventricle in the anterior portion of the hindbrain is kite-shaped
in transverse section, while in ths posterior portion it has the
shape of an hour-glass. The notochord only extends as far forward
as the anterior edge of the acusticus-facialis anlage and is
attached to the neural tube throughout its length except fora

short space at the anterior em,

Embryo 6.2 mn.

In a 6.8 mm. embryo (fig. 7) some very decided changes
cecur. The forebrain is greatly enlargad, with the vertical and
horizantal axes approximately the same. A well defined dorsal
groove separates it from the midbrain. The optic vasiclas stand
out prominently from the brain wall on a well defined optic stalk.
The outer wall of the vesicle has invaginated to form ths optic
cup in ene lies a spherical lens. At this stags the Ae es A
appears as a slight evagination from the floor of the forebrain
and lies directly beneath the urper end of thse hypophysis. The
hypophysis has arisen from the sctodsrm of the mouth cavity and
in a 5.2 mm. embryo, Scammon ('11) describes it as a thickened
hypophyseal plate of epithelium in contact with the ventral
surface of the forebrain. In a 5.8mm. embryo he dascribss it as
an evagination of the oral cavity which has extended inward so
far as to be in contact with the brain at the median line, from
the recessus ~raeopticus nearly to the tuberculum postesrius, In

the restoration which was made (fig. 7) the posterior (upper)

end of the hypophysis is separated from the surface of the

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forebrain and is slightly notched on the upper side to form a |

shallow pocket into which the tip of the notochom has extended,
This may be the beginning of the inferior sacs as described by
Raungartner ("15) although he finds no such constriction urtil
the 22 mm. emoryo.

The axis of the midbrain is almost at right angles to
that of the hindbrain. In the anterior part the walls ard floor
of the midbrain are composed of five or six layers of calls aw
as it extands posteriorly ths roof becomes gradually thinner until
it is only made up of a single layer of cells. In cross-section
the ventricle is rectangular. The thickness of the walls has
increased with hhe lengthening, A shallow dorso-ventral groove,
which is in line with the anterior end of the hypophysis, marks
the beginning of this hindbrain region. Just behind it the roof
ofthe brain is composed of several layers of cslls which is the
beginning of the cerebellum. This thickened portion extends just
posterior to ths end of the trigeminus anlage, at which point the
roof becomss thinner, until 1t is composed of only a single layer |
of cells. This thin portion is the beginning of the tela chorioides
of the fourth ventricle which extends throughout the rest of the

brain region,

Embryo 10.2 mm,

Between the 6.8 mm. embryo and the last stage to be
described, an embryo of 10.2 mm. (fig. 8) the anterior brain regiom
has undergone considerable changes. In ths dorsal portion of the
forebrain two expansions haves appeared, causing a division into

*@ large anterior amd a small posterior portion. Ths anterior

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expansion is the telencephalon which is to develop into the

cerebral hemispheres and olfactory lobes. The roof of this anterior|

portion is composed of ten to twelve layers of cells am grams

thinner as it approaches the epiphysial structures roofing in the

third ventricle. At the point where the velar fold begins, the roo

is only three or four layers of cells in thickness, increasing as

it proceeds caudally. The velum transversum is in a line almost

parallel] with the lower border of the optic cup amd extends a

little posteriorly into the brain cavity for a distance of about

150 p- This fold separates the telencsphalon from the postertor

portion, which is to form the thalamencephalon. The walls and

roof of the posterior expansion are about of the stme thickness

and enclose a simple cavity except at ths anterior em, whsre

there is a slight trace of anothor invagination, in front of

which is to form the parancephalon ( the "Zirbelpolster" , of the

Germans) behind which the pinealis develops. The midbrain ig

separated from the thalamencephalon by anothsr fold which extends

into the brain cavity about 100 po
The optic cup is now connected with the twixtbrain by a

Slender optic stalk, on the ventral side of which is the first

the later chorioid fissure. The two walls

indication of a groove,

of the lens of the eye have become differentiated into a thin

exterior and a thick interior am thse lens now lies about in the

aperature of the optic cup. From being but an inconspicuous

invagination in the 6.2 mm. stage, the infundibulum has greatly

increased and now forms a sac-like projection, extending somewhat

backward fromthe floor of the forebrain in a curve; higher in fronf

and sloping downward till it blends with the floor of the forebrai

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12.
having its cavity in a broad open connection with the ventricle.

The hypophysis has increased in size. The grooves which was so
marked in the previous stage has completely disappeared, leaving a
single tube with its closed dorsal snd in contact with ths
infundibulum. The notochord is in contact with the central nervous
system throughout its length; the anterior snd curving slightly
downwaris towards the dorsal end of the hypophysis. It increases
in diameter backward as far as the brain extends.

In the midbrain the roof 1g becoming progressively
thinner and the sides thicker. Dorsally the midbrain is only one
simple expansion, while ventrally and laterally three slight
|} expansions are found. These are probably the three segments of the
primary midbrain, described by Zimmermann ('91) and Neal ('98),
The anterior expansion is bounded in front by the primary
constriction between the forebrain and the midbrain and behind by
a lateral constriction extending ventrally from the point of origin
of thse oculomotor nerve. The secomd expansion has for its posterior
boundary a ventral constriction beginning half way up the side of
the midbrain and extending a little postero=vertrally into the
primary flexure. Rehind this a third expansion which extends back
to the beginning of the himbrain, The pounde ry between the
midbrain and the hindbrain is a marked dorso=-vsentral constriction
which lies just in front of the trigeminus anlage. The hindbrain
is still a fairly uniform structure with a simple cavity. At the
anterior end the roof is composed of several layers of ecslls which
indicates the rudiment of the cerebellum, which has extended

posteriorly so that, just in front of the origin of the acustico-

facialis anlage the roof becomes thinner, marking the beginning of

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the medulla,

DEVELOPMENT OF THE CRANIAL NERVES,

In direct relation to the formation of the brain comes
the formation of the cranial nerves and ie réconst rhetiore which
were made have incluijed these structures. The remaimer of this
paper deals with the early stages of the nerve development from
the time of their first appearance up to about the 65 somite stage,

Neal ('98) states that in an early stage, when the
cephalic plate is still widely oxpanded, there is a disassociation
of the neural crest cells, the fundament of the trigeminus, which
is clearly differentiated from that portiot of the neural plate
which is destined to form the neural tube. However not until the
stage when the cephalic plate is closed except in tha region of
the forsbrain did I find any differsantiation in the neural creast..
As shown in figure 3,,the neural crest is very distinct, extending
close along the upper side of thse brain, from a point a little
behind the closed portion of the neuropore to the middle part of
the hindbrain. The cells which compose it have already extended
half way down the side of the neural tube.

In an embryo of about 19 somites, figure 4., there are
two of these regions of cell proliferation, the beginnings of the
fifth and seventh=eighth nerves. The fifth arises as an outgrowth
from the extreme dorsal summit of the posterior part of thse midbra
It extends ventrally, covering the entire side of the neural tube

am is in close contact with its walls. At this time only one

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14,

continuous plate of cells has dsveloped and thse two later branches,

namely the offithalmie and the mandibular ars not yet differentiated,

The seventh and eighth nerve is outlined shortly after the fifth

as is shown by the fact that in this stage the cells have only just

begum their descent. These facial-acustic ganglion cells are

located. a little distances behind and indererdent of the fifth and

like the trigeminal, they arise from the extreme dorsal summit of

the neural axis. It is only later that the seventh am eighth

nerves are differentiated from this common anlage.

In the next stage, (fig. 5) the trigeminal anlage is a

continuous neural ridge, lying dorsal to the optic vesicle, but

subdivided ventrally into two somewhat distinct portiors, Of these

the anterior is the smaller; it passes in front of the midbrain

vesicle toward the optic evagination. The larger postsrior portion

is quite elongate and extends ventrally into the region of the

mandibular arch. The acustico-facialis anlage extenis ventrally

over half of the neural tube in the direction of the hyoid arch,

somewhat triangular in shape

It is now only a thin shest of cells,

as viewed from the side.

From the dorsal and posterior .part of the hindbrain now

the first indication of the

comes a third proliferation of cells,

ninth and tenth nerves. They arise, like the others, from the very

summit of the neural axis. They have just begun their latearo-

ventral extension am now appear only as a small ridge, no

differentiation into separate nerves having taken place,

Some important changes occur in an embryo of 30 somites,

(fig. 6). Owingto a further *lexure of the head the trigeminal

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15.
crest does not extend so far forward. Theres is a slight thickening
of the anlage ami the first indication of a division into two
branches. The anterior portion or ophthalmic branch is in close
contact with the walls of the midbrain and only extends a short
| distance ventrally along its side. The posterior portion of the
nerve, lying almost in the constriction between mid and himbrain,
now extends into the mandibular arch. A slight differentiation
has begun in the ecustico-facialis anlage together with a decided
thickening of the cells, which have grown out to mest the
thickening of the auditory spithelium, In the vagus anlagea a
differentiation has likewiss taken place. Two distinct portions
are visible, the anterior, the early stage of the glossorharyngeus,
the posterior, the vagus proper. In this figure they extend
ventrally only half way down over the side of the neural tube.
Only the beginning of the tenth nerve has been shown and at this
stage there is no evidence of the four branches which later arige
fromwits

In an embryo of 50 somites, (fig. 7) the trigeminus
anlage has become very much shortened ard is now connected to the
neural tube by a root of about 75 p in length. The cells have
increased until the nerve is about thres times as thick as in the
previous stage. The single rwiimont of the seven and eight is also
somewhat enlarged and it has begun to divide into two distinct
parts. Leaving the side of the neural tubs the main trunk runs
ventrally and a little posteriorly, dividing into the branch which |
is to form the seventh nerve and a posterior acusticus. The facialis

continues to extend ventrally. The awitory nerve begins to widen

and at the same time expands, like a cup, around the ventral sides

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16...

lof the awlitory vesicle with the epithelium of which it is fused,

Rehimd the ear capsule the glossopharyngeus anlage forms @ long

| thin strand which arises from the midlateral portion of the neural

| tube. Immediately behind this come ths cells of the vagus, now

| completely separated from the former. Anteriorly the anlage is only | '

ia faint ridge projecting from the side of the neural tube. Ranbortad
it becomes a broad sheet of cells extending ventrally betwsen the

| ectoderm and the mesoderm in the region of the pha tyexowhere the

| latter begins to be divided by the formation of the visceral clefts.

In the next stage, an embryo of 10.2 mm. (fig. 8) the

first appearance of the oculomotor nervs is seen. At first it is

| a thin process extending from the base of the midbrain ventrally

| to join the ophthalmic profundus branch of the fifth just before
the latter enters the optic cup. The trigeminus crest h@s constrictad

| and four branches are recognizable, Anteriorly there is a slight
projection which is to form the ophthalmicus superficialis. Just
below this is a short ganglionic commissure which connects the

| mesocephalic and the malmganglion. From the mesocephalic ganglion |
the ophthalmicus profundus branch passes anteriorly ard ventrally
into the optic cup. Just back of the mesocefhalic ganglion is the
larger branch of the fifth, which breaks up to form the maxillaris |
in front and ths mandibularis posteriorly, as it runs ventrally
and a little cauially.

In the acustico-facialis anlage four branches are now

developed. Anteriorly in cloSs connection with the skin, the

ophthalmicus superficialils has begun to send out its sensory fibers|

| which lie almost parallel with the notochord, Directly bsalow this

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tls buccalis branch. has just begun to develop. Posterior to the

| buce@lis branch, extending further ventrally and curving a little

| posteriorly, is the mixed hyoid branch of the facialis which goes
to the developing muscl3s and skin of the sacond visceral arch.
Somewhat dorsal and behind the hyoid branch is the acusticus branch

which is connected with the median and vantral side of the otic

capsuls, The glossopharyngsus is practically unchanged and Is still |

|a slender strand arising from the mid-latsral portion of the neural |
tube. The anlage of the vagus is connectsd to the ninth nerve by a
thin ridge of.cells. Ventrally the vagus divides into four branches
| (only the anterior tWo are shown in the figuras) each of which goes
to the skin and muscles of the visceral arches. Posteriorly two

| other branches are given off, @n upper one which continues as the

; lateralis along the lateral line ami a lower one, the viscoralis,

| which extends a little ventrally as it proceeds cauially. Its

| posterior extent was not traced,

- 7
roO.180 5G

18
BIBLIOGRAPHY

Balfour, ¥.M..
'78,, A Monograph on the Development of Blasmobranch
Fishes .. London, XI 295pp..,. 20 Pls,.
Baumgartner, E,A.
'15,. The Development of the Hypophysis in Squalus Acanthtas
Jour. Morph., Vol. 26, pp. 391-446, 43 Figs...
Griggs, Le.
110, arly Stages in the Development of the Central
Nervous System of Amblystoma punctatum., Jour. Morrh,,
Vol. 21, pp. 425-485, 12 Figs., 1 Pl.
Eill,, Ci.
*00,. Developmental History of Primary Segments of the
Vertebrate Head, Zool. Jahr., Bi. 13, pp.393-447,
4 Figs., Pls. 28-30.
Locy, W.A,
'93, The Derivation of the Pineal Eye. Anat. Anz, Bd. 9,
No. 5 u. 6, pp 169-180, 5 Figs. |
Locy, W.A.
'95, Contribution to the Structure and Development of the
Vertebrate Head. Jour. Morph., Vol..11, No. 3, pp..
497-594, Pls. 26-30,
Marshall, A.M,.

"81. On the Head Cavities and Associated Narves in

Elasmobranchs. Quart. Jour.. Micr. Sci., Vol. 21, pp,
72-97, Pls. 5,6..

storbrds!

$x sae}
r~
ve *
,
:
2
‘ e 7
*

OO I a i SA
*

9 .
ani? ic treage levee edit me Ps ie 2 re
: lit ,.@7eee It yaaa Botte 23
ALS geoniag
p@ ev! etsydtegyl ed¢ to txemgelevel eae Sl? iba

°

“iG «Wg . oS Lov va lift oe eC

ec ‘wurcinved ens. 7) Gelert vite’, on
.Uletaug Beoteyldmas tO cede aucyre? 3
a5 peeplt SI y~ORPSeRe Joe he . ey |
2D tM
viesic. ie yrptes ledtreage leeds, 68%
Ol . 38 ».cae#G Lone «feel e¢eidemay
FE-82 :e29% cian i~
she
-6¢® eect! en: Yo nelvevised ant :8ee
ef a
Ae wed
ed tre « oir. 79 ef? of not die eed oe
; ? . eet U0 » SBOE etaicdeteo? ’

VO-ce ,o oc ~T op

we fiveD ope ed ae
5 ; owl See) setinendéie a
» 8-89

624 Py

1g
Neal, H.V,

*98,. The Segmentation of the Nervous System in Squalus
Acanthias. Bull. Mus. Comp. Zool., Vol.31, pp. 148=—
295. Ji: Fige., @ Ple..

Piatt, J.B.
'91,. A Contribution to the Worphol ogy of the Vertebrate
Head, based on a Stwy of Acanthias vulgaris. Jour..
Morph., Voll. 5, DR. 78-112. Ple. 46
Scammon, RE.

'11.. Normal Plate of the Develomment of Squalus Acanthias,.

Normt. z. Entwicklungsgeschichte d.. Wirbelthiers, H. I
Sedgwick, A,

'92. Notes on Elasmobranch Development. Quart. Jour. Mic

Sei., You. 33, Bt. 4 pp. 559-586. F135.
Wijje, J.W.van..

"82. Ueber die Mesodermsegments und die Entwicklung doer
Nerven der Selachierkopfes, Natuurk. Verh, d. K, Akad,
Wissensch.. Amsterdam, Deal 22, 50pp., 5 Taf., 1883.
Also separate, Amsterdam, 1882, 50) pp. 5 Taf.

Zimmermann, W.

*91. Ueber die Msetamoria des Wirbslthierkopfes. Verh. —

Anat. Gesellsch., V (Mfinchen), pp. 107-113..

ABBREVIATIONS,

ac. acusticus nerve
AG. f2e. acustico—facialis anlags
ae. Gin. Vs accessory optic vesicle
be. buccalis nerve
cerbell um
neural crest
facialis nerve
forebrain
primary flexure
mesocephalic ganglion
glossopharyngeus nerve

hindbrain

hyoid nerve

hypophysis
infunbdibul um

lens...

midbrain
mandibularis nerve
medullary plate
medullary folds
medulla oblongata
maxillaris nerve
neural groove
notochoxd

anterior neuropore

oculomotor nerve

Re ey

“ph Ite eblersat<as ers

6 folaev' bitte vtownegon

evier ellaocud
on 1 LetineD
Seete. [river ott
evrer etLaipg?
o heide4 cit "

CTIKGIT VASES

é.
~

Cp. C.
Op.Vv.
OPhef.
oph. Ss.
oph. s*..
oO. S.
tel,.
thal.
trig.
VE.

Wie. ti.

optic cup

optic vesicle

ophthalmicus profundus (Vth)

ophthalmicus superficialis (Vth)

ophthalmicus superficialis (VIIth))

optic stalk.
telencephaion
thalmencephalion
trigeminus anlage
vagus anlage

velum transversum

7? Pere?
' - 7

; - rN :
ae Hie j i FH
_ = bia | a" ?

Bas 7a

1A!
<s

. Wax

. Wax

Wax
Wax

Wax

PLATE I..

Explanation of Figures

reconstruction of @ 2mm. embryo, Dorsal view. (x

reconstruction of a 3.5 mm. embryo.

reconstruction of a 4mm, embryo,

reconstruction of a 4.5 mm. embryo.

reconstruction of a 5 mm. embryo.

PLATE 2,

Explanation of Figures

Side view. (x

Side
Side
Sids

reconstruction of a 3.2 mm. embryo. Dorsal

reconstruction of a 6.2 mm. embryo.

reconstruction of a 10.2 mm. embryo.

Side
Side

view,
view.

view.

view. (x
view. (x

view. (x

halle
>

on
é
. , a
= ve a
a

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mo.

cb. es ee =
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