Paper - Contribution to the structure and development of the vertebrate head 2: Difference between revisions

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:Contents: [[Paper - Contribution to the structure and development of the vertebrate head#General Introduction|General Introduction]] | [[Paper - Contribution to the structure and development of the vertebrate head 1|Part I - Metamerism of the Head]] | [[Paper - Contribution to the structure and development of the vertebrate head 2|Part II - The Sense-Organs]]
:Contents: [[Paper - Contribution to the structure and development of the vertebrate head#General Introduction|General Introduction]] | [[Paper - Contribution to the structure and development of the vertebrate head 1|Part I - Metamerism of the Head]] | [[Paper - Contribution to the structure and development of the vertebrate head 2|Part II - The Sense-Organs]] | [[Paper - Contribution to the structure and development of the vertebrate head 3|Figures]]


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=Part II - The Sense-Organs (1895)=
=Part II - The Sense-Organs (1895)=


PART II.—-—THE SENSE—ORGANS.
The Sense-Organs of Vertebrates embrace all those structures that are endowed with Special sensibility They differ from one another mainly in degree of differentiation, and form a series, at the lower end of which are simple sensory papillæ, and at the upper end are complex organs like the eye and ear. We recognize two orders of Sense-Organs — simple generalized ones, the ganglionic Sense-Organs, and more specialized ones» belonging to the so—ca1led five senses.


The Sense-Organs of Vertebrates embrace all those structures
that are endowed with Special sensibility They differ from
one another mainly in degree of differentiation, and form a
series, at the lower end of which are simple sensory papillæ,
and at the upper end are complex organs like the eye and ear.
We recognize two orders of Sense-Organs — simple generalized
ones, the ganglionic Sense-Organs, and more specialized ones»
belonging to the so—ca1led five senses.


The question of the origin and relationship of these sense-—
The question of the origin and relationship of these sense-— Organs is full of interest. From the combined results of investigations on both Invertebrates and Vertebrates it seemsaltogether probable that the higher· Sense-Organs have been derived from those of a lower order, and, indeed, that they have all been differentiated from a common sensory basis, and are therefore related in a direct way.
Organs is full of interest. From the combined results of investigations on both Invertebrates and Vertebrates it seemsaltogether probable that the higher· Sense-Organs have been
derived from those of a lower order, and, indeed, that they
have all been differentiated from a common sensory basis, and
are therefore related in a direct way. i
546 Lock Ijvon XI.


. This view has been entertained by morpho1ogists for upwards
of ten years, and has a firm foothold in philosophical discussions. The evidence favoring such an interpretation has been
accumulating, and although still incomplete, the hypothesis
was never so well supported as at present.


Professor Whitmarks researches were among the first to
This view has been entertained by morpho1ogists for upwards of ten years, and has a firm foothold in philosophical discussions. The evidence favoring such an interpretation has been accumulating, and although still incomplete, the hypothesis was never so well supported as at present.
bring out this conception. Ten years ago he demonstrated in
1eeches the genetic relation between eyes and tactile papil1ae,
and since that time it has no longer been a matter of pure
assumption to say that certain end-organs are modifications of
a common sensory basis. He shows that the sensory papillæ
located on each bodycking in the leeches have a very interesting relation. In the hinder part of the body the papillæ are
purely tactile, but passing headward they become gradually
modified, and are at first mixed visual and tactile, and tinally
the anterior ten pairs are purely visual. We have thus had
for upwards of ten years a welbauthenticated case of the derivation of Sense-Organs of a higher grade from those of a lower
order.


In a more recent publication, «The Metamerism of Clep—
sine,« 1892, Professor Whitman has added to the def1niteness
of his earlier discoveries by observations on a new species, ask-r»
Clepsine hol1ensis. He shows by a comprehensive study of
the elements of the nervous System, the nerve distribution, and
the sensillæ the complete homodynamy of all the segments.


Regarding the relationship of the eyes to other metameric
Professor Whitmarks researches were among the first to bring out this conception. Ten years ago he demonstrated in 1eeches the genetic relation between eyes and tactile papil1ae, and since that time it has no longer been a matter of pure assumption to say that certain end-organs are modifications of a common sensory basis. He shows that the sensory papillæ located on each bodycking in the leeches have a very interesting relation. In the hinder part of the body the papillæ are purely tactile, but passing headward they become gradually modified, and are at first mixed visual and tactile, and tinally the anterior ten pairs are purely visual. We have thus had for upwards of ten years a welbauthenticated case of the derivation of Sense-Organs of a higher grade from those of a lower order.


Sense-Organs, he says: «0ne more peculiarity of this species


may be noticed The median rows of sensillæ are quite well
In a more recent publication, The Metamerism of Clepsine", 1892, Professor Whitman has added to the def1niteness of his earlier discoveries by observations on a new species, ask-r» Clepsine hol1ensis. He shows by a comprehensive study of the elements of the nervous System, the nerve distribution, and the sensillæ the complete homodynamy of all the segments.
developed as eyes, at least on segments IV, V, and VI. The
visual elements in Segment IV are quite numerous, and they
are placed in a pigment cup quite like that of the principal
eyes, only thinner and sma1ler, and directed backward instead


of forward. The sensi1la of Segment V is a little sma1ler, but
Regarding the relationship of the eyes to other metameric sense-organs, he says: "One more peculiarity of this species may be noticed The median rows of sensillæ are quite well developed as eyes, at least on segments IV, V, and VI. The visual elements in Segment IV are quite numerous, and they are placed in a pigment cup quite like that of the principal eyes, only thinner and sma1ler, and directed backward instead of forward. The sensi1la of Segment V is a little sma1ler, but still presents the same general features. In Segment VI the organ is still eye-lilce, butiits visual elements are fewer andthe pigment cup imperfectly represented by loose pigment. In Segment VII we find one or two visual eells and a little scat . teredpigment In the following segments we usually find the sensillae in about the. same condition. In no other species hitherto described do we find the sensillæ passing by such gradations into the principal eyes. The serial homology of these organs with the eyes is, then, a fact demonstrated not only by the embryological development, but also by the structural gradations exhibited in the adult animal" (pp. 39I, 392).


still presents the same general features. In Segment VI the
organ is still eye-lilce, butiits visual elements are fewer andthe
pigment cup imperfectly represented by loose pigment. In
Segment VII we find one or two visual eells and a little scat
. teredpigment In the following segments we usually End the
No. 3.] THE FEETEZJEH TE HEXE. 547


sensillæ in about the. same condition. In no other species
The existence of a series of rudimentary sense—organs in Vertebratos was brought to 1ight in 1885 by Froriep and Beard. While it is by no means clear that they are the homo1ogues of the sensory papillæ of anne1ids, it is nevertheless probab1e. It is now generally admitted that they constitute the basis from which the higher Sense-Organs of Vertebrates are derived. These organs are arranged in 1ongitudinal series. The series has become rudimentary or lost in the adult forms of higher existing Vertebrates, but they are present in the embryos Kupffer and others have shown that there are at least two series of these Organs, the upper one corresponding to the lateral line of comparative anatomy, and the 1ower one embrac— ing the so-ca11ed branchia1 Sense-Organs of Beard. From the upper series the ears and nose are probab1y derived, and possibly the eyes.
hitherto described do we find the sensillæ passing by such
gradations into the principal eyes. IX«- Fæiriaz Æomozogy cj
Zlzcxe wjgsmscx ZEJZZJZ Mk:- FJEF IF, HEXE, z: ja« okjrøoyzszkezxgci »« nie-s;åy ff« wyøåkjxologitczl xicexæ!oziwekx-skzz«, He« als» Zy «« Fxrzxcxzmczf gracixzziom egxlziåizezi i« Zlza rufe-Z! zwei-setz! « (pp. 39I, 392).


The existence of a series of rudimentary sense—organs in
Vertebratos was brought to 1ight in 1885 by Froriep and Beard.
While it is by no means clear that they are the homo1ogues of
the sensory papillæ of anne1ids, it is nevertheless probab1e.
It is now generally admitted that they constitute the basis
from which the higher Sense-Organs of Vertebrates are derived.
These organs are arranged in 1ongitudinal series. The series
has become rudimentary or lost in the adult forms of higher
existing Vertebrates, but they are present in the embryos
Kupffer and others have shown that there are at least two
series of these Organs, the upper one corresponding to the
lateral line of comparative anatomy, and the 1ower one embrac—
ing the so-ca11ed branchia1 Sense-Organs of Beard. From the
upper series the ears and nose are probab1y derived, and possibly the eyes.


Allis (’88), in his well-known memoir on the lateral line of
Allis (’88), in his well-known memoir on the lateral line of Amia, gives Hgures that show a connection between the epithelium of the nasal pit and that of the1ateral1ine. His Fig. I, P1. XXX, representing a larval form one day old, shows the nasal epithelium forming part of the lateral line. In subsequent growth it follows the same course that the surface organs of the lateral System do when they become canal Organs. Allis says, p. 537: The nasal pits are inclosed in the same way that the lateral cana1s are, and the short canal so formed is at first continuous with the canal inclosing organ 5 infra-orbita1.
Amia, gives Hgures that show a connection between the epithelium of the nasal pit and that of the1ateral1ine. His Fig. I,
P1. XXX, representing a larval form one day old, shows the
nasal epithelium forming part of the lateral line. In subsequent growth it follows the same course that the surface organs
of the lateral System do when they become canal Organs. Allis
says, p. 537: «The nasal pits are inclosed in the same way


that the lateral cana1s are, and the short canal so formed is at


first continuous with the canal inclosing organ 5 infra-orbita1.«
There is now genera1 agreement that the ears be1ong to the lateral line series, but there has been much dissent on the part of Some morphologists to admitting the eyes to the same category; but the grounds for the Opposition seem to me to be largely removed. It must be said, moreover, that the Organs of taste and touch have not as yet been shown to have any genetic relation to the ganglionic Sense-Organs.  


There is now genera1 agreement that the ears be1ong to the
lateral line series, but there has been much dissent on the part
of Some morphologists to admitting the eyes to the same category; but the grounds for the Opposition seem to me to be
largely removed. It must be said, moreover, that the Organs
of taste and touch have not as yet been shown to have any
genetic relation to the ganglionic Sense-Organs.
548 Lock: kvon XI.


I am g1ad to acknowledge my indebtedness to Minot’s intro—
I am g1ad to acknowledge my indebtedness to Minot’s introductory remarks to his chapter on the Sense-Organs. He says: "Very suggestive in this connection are the observations of H. V. Wilson («91), of a thickening of the nervous layer of the epiderrnis on either side of the head in the bass embryo (Serranus atrarius). This thickening forms a long, shallow furrow,
ductory remarks to his chapter on the Sense-Organs. He says:
«Very suggestive in this connection are the observations of
H. V. Wilson («91), of a thickening of the nervous layer of the
epiderrnis on either side of the head in the bass embryo (Serranus atrarius). This thickening forms a long, shallow furrow,


which subsequently divides into three parts, of which the first
which subsequently divides into three parts, of which the first


becomes a Sense-Organ over the gil1-cleft, the second, the auditory invagination, and the third, the Anlage of the sense-organs
becomes a Sense-Organ over the gil1-cleft, the second, the auditory invagination, and the third, the Anlage of the sense-organs of the lateral line. This peculiar development confirms the notion that all these organs belong in one series, but the appearance of a continuous thickening as the Anlage of them all has, as yet, been observed only in this fish, and may not indicate a corresponding ancestral condition Unfortunately, Wilson was unable to make out anything as to the connection of the sensory plate with the ganglia. The sense-organ above the gi1lcleft, although differentiated, is a larval structure only, and disappears in the adult."
of the lateral line. This peculiar development confirms the
notion that all these organs belong in one series, but the appearance of a continuous thickening as the Anlage of them all has,
as yet, been observed only in this fish, and may not indicate a
corresponding ancestral condition Unfortunately, Wilson was
unable to make out anything as to the connection of the sensory plate with the ganglia. The sense-organ above the gi1lcleft, although differentiated, is a larval structure only, and
disappears in the adult.«


A quite similar condition is now known to obtain in some elasmobranch forms. Mitrophanom in 1 89o, published a preliminary
report of his observations on the lateral line of Acanthias and
other Elasmobranchs In 1893 he published a full report of
the same, illustrated by many iigures He describes a continw
ous thickening of the epidermis along the sides of the head embracing the territory of the roots of the seventh to tenth nerves.
From this thiclcening there is separated the material for the
auditory saucer, the branchia1 sense-organs, and the beginning
of the 1ateral line. My observations on this region in Squalus
agree with those of Mitrophanow


In looking over the literature on the sense-organs one cannot
A quite similar condition is now known to obtain in some elasmobranch forms. Mitrophanom in 1890, published a preliminary report of his observations on the lateral line of Acanthias and other Elasmobranchs In 1893 he published a full report of the same, illustrated by many iigures He describes a continw ous thickening of the epidermis along the sides of the head embracing the territory of the roots of the seventh to tenth nerves. From this thiclcening there is separated the material for the auditory saucer, the branchia1 sense-organs, and the beginning of the 1ateral line. My observations on this region in Squalus agree with those of Mitrophanow.
fai1 to be struck by the remarkable uniformity of the sensescells
in the different kinds of sense-organs. , They are easily reducible to one type, and this, of course, favors the view that they


have been derived from a common form.
NO. 3.] THE FEÆTEBÆÄ TE III-TO. 549


I. THE. LATERAL Eis-Es.
In looking over the literature on the sense-organs one cannot fai1 to be struck by the remarkable uniformity of the sensescells in the different kinds of sense-organs. , They are easily reducible to one type, and this, of course, favors the view that they have been derived from a common form.  


Squalus acanthias is an especially favorab1e form for observing the beginnings of the optic vesic1es. They make their
==I. The Lateral Eyes==
appearance in this animal at a very early stage, while the neura1
p1ate is broadly expanded, and before the neural folds have
arched upwards in any part of the embryo. I should estimate
their appearance in this animal to be as early as in any other
anima1 in which they have been described. Previous to I889
it had been current opinion that the optic vesic1es appeared
very early only in the class of Mammala Bischoff, Kö11iker,
His, Van Beneden, Heape, and others had recorded their early
appearance in that group ; but it was regarded as a precocious
deve1opment for some reason confined to mamma1ian forms.


More careful observations on the earlier stages have
Squalus acanthias is an especially favorab1e form for observing the beginnings of the optic vesic1es. They make their appearance in this animal at a very early stage, while the neura1 p1ate is broadly expanded, and before the neural folds have arched upwards in any part of the embryo. I should estimate their appearance in this animal to be as early as in any other anima1 in which they have been described. Previous to I889 it had been current opinion that the optic vesic1es appeared very early only in the class of Mammala Bischoff, Kö11iker, His, Van Beneden, Heape, and others had recorded their early appearance in that group ; but it was regarded as a precocious deve1opment for some reason confined to mamma1ian forms.
shown that the optic vesic1es customarily arise in different
anima1s much earlier than was supposed. In I889 Whitman
called attention to the very early appearance of the optic vesicles in Necturus: « Its basis being discernib1e as a circular
area—-— after treatment with osmic acid, followed by MerkePs
fluid —- long before the closure of the neura1 folds of the brain."
Dura1noted the early appearance in birds. In 1893 Eyc1eshymer
gave notes upon their appearance and structure in Necturus
and other Amphibia His most noteworthy observations are
upon Rana pa1ustris, a form hitherto unstudied, in which he
ftnds a remarkably early deve1opment of the primary optic vesicIes. They are very evident from surface study in this form,
on account of the distribution of pigment granules in them, just
after the neural ridges have begun to form. cross-sections- in
these early stages show a considerable amount of histological
differentiation from the surrounding ce11s.


The early differentiation of the optic vesicles had not been
recorded in any e1asmobranch form unti1 the publication of my
preliminary papers in 1893 and I894, where the optic vesic1es
are described and their serial relation to other structures on
the cepha1ic plate is show-vix. Since then I have had opportunity to coniirm my observations on Squa1us, and to extend them
to some other forms ; for instance, in Diemycty1us and
550 Lock kvon x1.


Amblystoma the optic vesicles are well deve1oped in very
More careful observations on the earlier stages have shown that the optic vesic1es customarily arise in different anima1s much earlier than was supposed. In I889 Whitman called attention to the very early appearance of the optic vesicles in Necturus: « Its basis being discernib1e as a circular area—-— after treatment with osmic acid, followed by MerkePs fluid —- long before the closure of the neura1 folds of the brain." Dura1noted the early appearance in birds. In 1893 Eyc1eshymer gave notes upon their appearance and structure in Necturus and other Amphibia His most noteworthy observations are upon Rana pa1ustris, a form hitherto unstudied, in which he ftnds a remarkably early deve1opment of the primary optic vesicIes. They are very evident from surface study in this form, on account of the distribution of pigment granules in them, just after the neural ridges have begun to form. cross-sections- in these early stages show a considerable amount of histological differentiation from the surrounding ce11s.
early stages In Torpedo oce1lata they show fairly well, and in
the chick they may be readily made out while the groove is
wide1y open.


In Squalus acanthias the optic vesicles are the first ruchments of Sense-Organs to appear. counting out the gastricular
cavity (and the neural segments) they are the Hrst organs of
any kind to be established. Strangely enough, their early history in these animals has been entirely overlooked. Balfour’s
statement that « The eye does not present in its early development any very especial features of interest," seems to have
withdrawn attention from that organ in the elasmobranch
Hshes.


In order to iix clearly the time when the optic vesicles first
The early differentiation of the optic vesicles had not been recorded in any e1asmobranch form unti1 the publication of my preliminary papers in 1893 and I894, where the optic vesic1es are described and their serial relation to other structures on the cepha1ic plate is show-vix. Since then I have had opportunity to coniirm my observations on Squa1us, and to extend them to some other forms ; for instance, in Diemycty1us and Amblystoma the optic vesicles are well deve1oped in very early stages In Torpedo oce1lata they show fairly well, and in the chick they may be readily made out while the groove is wide1y open.
appear in Squa1us, we must glance over the early steps in the
formation of the head-plate. Pl. XXVL Figs. I, 2, Z, represent three stages that immediately precede the formation of
the eyes. In Fig. I the embryo is well established; it is a
stage slightly older than Balfoufs stage B. The front part of
the embryo is not broader than the rest, and the curve of the
anterior margin is uniform and unbrokerr In Fig. 2 we note
two changes, vie» that the front end is broader than the part
just behind it, which is apparently constricted, and the anterior
margin is drawn out into a rounded median cusp. The headend continues to expand laterally (Figs. 3 andr4) until we may
(with some appropriateness) speak of it distinctively as a headplate. In the meantime the trunk region has grown longer,
and is relatively narrower, so that the entire axial embryo has
a Characteristik: appearance of a narrow body terminated by a
rounded plate-like head. When this stage has been reached the
first satisfactory traces of the eyes become visible ; when first
formed they present the appearance that might be produced by
pressing down lightly upon a plastic surface with two rounded
dies. They are circular areas slightly concave upwards, and
occupy a position far forwards on the cephalic p1ate. The
early stage of these vesicles may easily escape observation, as
they become evident only when the light strikes them properly,
and when they are in a favorable position for the observer.
No. 3.] III-I: PEJZTEFJEH TE Eis-ro. 551


Fig. 5 is engraved from a photograph in swhich the optic vesicles
show very well. The specimen from which the photograph was
made showed about six mesoblastic somites, and was ZHF mm.
in length The circu1ar areas may be made out satisfactorily
in slightly earlier stages, in which only three mesoblastic somites
are differentiated, and in which the neural fo1ds of both head
and trunk are not only broadly expanded, but are even ventrally
—curved. The circular depressions are at first separated from
one another by a raised welt. This has already been spoken
of in Part I as atonguedike process extending from the mediananterior tip bacliwards to two—thirds the length of the cephalic
plate. It continues to be a prominent feature of the cephalic
plate for some time. I can offer no suggestion as to its signifis
cance, outside of the obvious suspicion that it may represent a
proboscis of Some lcind, or that it may be related to the 1arge
notochord of this region. On this point 1 have no conjectures to
make. The depression to form the infundibulum starts a little
after the first appearance of the optic vesicles, and cuts the
median process in two, so that the anterior tip is separated from
the rest, and the depression for optic vesicles and infundibulum
combined reaches across the median plane of the cephalic
plate (Figs. 6, J, 8, 9, ers-J. The optic vesicles, however, do
not reach to the 1ateral margins of the neural folds ; the latter
are much expanded beyond them.


It is evident that we cannot, in a strict Sense, speak of
In Squalus acanthias the optic vesicles are the first ruchments of Sense-Organs to appear. counting out the gastricular cavity (and the neural segments) they are the Hrst organs of any kind to be established. Strangely enough, their early history in these animals has been entirely overlooked. Balfour’s statement that « The eye does not present in its early development any very especial features of interest," seems to have withdrawn attention from that organ in the elasmobranch Hshes.
such vesicles as «diverticula of the fore-brain.« The «diverticula" are found before the fore-brain. The depressions once
started grow deeper and press outward laterally ; when fully
formed they are cupped, almost rounded in out1ine, concave
from within, and they form rounded elevations where they come
in contact with the outer layer of epiblast. Figs. H, J, 8, 9, Io,
show very well the appearance presented by these primary
optic vesicles when viewed from above. Fig. 7 is magnified
higher than the others, and is, therefore, larger, but the others
are all uniformly en1arged. These figures all show the interior
of the optic pits. They represent a range of stages only slightly
older than that·shown in Fig. s. In all of them the depression
in which the optic vesicle lies extended across the median
552 Lock Ijvon x1.


p1ane, and in its median portion, the depression develops into
the infundibu1um.


The1atera1depressions, which form the optic vesicle, sink
In order to iix clearly the time when the optic vesicles first appear in Squa1us, we must glance over the early steps in the formation of the head-plate. Pl. XXVL Figs. I, 2, Z, represent three stages that immediately precede the formation of the eyes. In Fig. I the embryo is well established; it is a stage slightly older than Balfoufs stage B. The front part of the embryo is not broader than the rest, and the curve of the anterior margin is uniform and unbrokerr In Fig. 2 we note two changes, vie» that the front end is broader than the part just behind it, which is apparently constricted, and the anterior margin is drawn out into a rounded median cusp. The headend continues to expand laterally (Figs. 3 andr4) until we may (with some appropriateness) speak of it distinctively as a headplate. In the meantime the trunk region has grown longer, and is relatively narrower, so that the entire axial embryo has a Characteristik: appearance of a narrow body terminated by a rounded plate-like head. When this stage has been reached the first satisfactory traces of the eyes become visible ; when first formed they present the appearance that might be produced by pressing down lightly upon a plastic surface with two rounded dies. They are circular areas slightly concave upwards, and occupy a position far forwards on the cephalic p1ate. The early stage of these vesicles may easily escape observation, as they become evident only when the light strikes them properly, and when they are in a favorable position for the observer.


p downwards more rapidly than the median part of the depression,


and as a resu1t there is left a ridge or keel separating the optic
Fig. 5 is engraved from a photograph in swhich the optic vesicles show very well. The specimen from which the photograph was made showed about six mesoblastic somites, and was ZHF mm. in length The circu1ar areas may be made out satisfactorily in slightly earlier stages, in which only three mesoblastic somites are differentiated, and in which the neural fo1ds of both head and trunk are not only broadly expanded, but are even ventrally —curved. The circular depressions are at first separated from one another by a raised welt. This has already been spoken of in Part I as atonguedike process extending from the mediananterior tip bacliwards to two—thirds the length of the cephalic plate. It continues to be a prominent feature of the cephalic plate for some time. I can offer no suggestion as to its signifis cance, outside of the obvious suspicion that it may represent a proboscis of Some lcind, or that it may be related to the 1arge notochord of this region. On this point 1 have no conjectures to make. The depression to form the infundibulum starts a little after the first appearance of the optic vesicles, and cuts the median process in two, so that the anterior tip is separated from the rest, and the depression for optic vesicles and infundibulum combined reaches across the median plane of the cephalic plate (Figs. 6, J, 8, 9, ers-J. The optic vesicles, however, do not reach to the 1ateral margins of the neural folds ; the latter are much expanded beyond them.
vesicle and connecting the anteriopmedian tip of the headp1ate with the Central tonguelike process. Miss P1att, in her
studies on the head of this animaL says: « It may be here


cis-r 8. —-Eight transverse Sections of the embryo shown in Fig. s, Pl. XXVL d( about zo
diameters The nnmbers below the Sections refer to their Position in the set-les.


noticed that the downward growth in the Hoor of the brain,y
It is evident that we cannot, in a strict Sense, speak of such vesicles as "diverticula of the fore-brain." The "diverticula" are found before the fore-brain. The depressions once started grow deeper and press outward laterally ; when fully formed they are cupped, almost rounded in out1ine, concave from within, and they form rounded elevations where they come in contact with the outer layer of epiblast. Figs. H, J, 8, 9, Io, show very well the appearance presented by these primary optic vesicles when viewed from above. Fig. 7 is magnified higher than the others, and is, therefore, larger, but the others are all uniformly en1arged. These figures all show the interior of the optic pits. They represent a range of stages only slightly older than that shown in Fig. 5. In all of them the depression in which the optic vesicle lies extended across the median p1ane, and in its median portion, the depression develops into the infundibulum. The lateral depressions, which form the optic vesicle, sink downwards more rapidly than the median part of the depression, and as a resu1t there is left a ridge or keel separating the optic vesicle and connecting the anteriopmedian tip of the headp1ate with the Central tonguelike process. Miss P1att, in her studies on the head of this animaL says:  "It may be here noticed that the downward growth in the Hoor of the brain,y which gives rise to the infundibulum, is prirnitively bilateraL and not median," and I think, therefore, that she saw these circular optic pits in an early stage, but interpreted them as 1atera1 pockets of the infundibu1um. There might be reason to doubt whether they are the optic vesicles, if they could not be traced with perfect continuity into the eyes, and if there were not evidence of histological differentiation in their areas.
which gives rise to the infundibulum, is prirnitively bilateraL
and not median," and I think, therefore, that she saw these
circular optic pits in an early stage, but interpreted them as
1atera1 pockets of the infundibu1um. There might be reason to
doubt whether they are the optic vesicles, if they could not be
traced with perfect continuity into the eyes, and if there were
not evidence of histological differentiation in their areas.


By the time the neural folds of the head begin to rise the


optic vesicles are very prominent, and may be observed from
the outside and from within. Fig. I 3 shows an embryo viewed
No. 3.] THE »Er-E TEBJM TE JJEHJZ 5 5 3


obliquely from the 1eft side, and the optic vesicle of that side is
Cut 8. - Eight transverse Sections of the embryo shown in Fig. s, Pl. XXVL x about zo diameters The nnmbers below the Sections refer to their Position in the series.
seen from the external surface, where it presents the appearance
of a rounded eminence 0n the opposite side the optic vesicle
is seen from within. cut 9 shows transverse Sections of the
head of the same embryo. Figs. 16 and 17 show embryos in
which the optic vesicle Ho. A) shows from without, and also
from within («:)P.i-). 1n Figs. I9 and 22 the optic vesicle
shows clearly from the outside. Transverse Sections of the


CUT 9.-— Eight transverse Sections of the embryo shown in Pl. XXVL Fig. is. I( about so
characters. The accessory optic vesicles H. Ha) have been formed.


embryo photographed in Fig. 23, after partial closure of the
neural groove, are illustrated in cut I0.


These optic vesicles are so well developed at an early period
By the time the neural folds of the head begin to rise the optic vesicles are very prominent, and may be observed from the outside and from within. Fig. I 3 shows an embryo viewed obliquely from the 1eft side, and the optic vesicle of that side is seen from the external surface, where it presents the appearance of a rounded eminence 0n the opposite side the optic vesicle is seen from within. cut 9 shows transverse Sections of the head of the same embryo. Figs. 16 and 17 show embryos in which the optic vesicle Ho. A) shows from without, and also from within («:)P.i-). 1n Figs. I9 and 22 the optic vesicle shows clearly from the outside. Transverse Sections of the embryo photographed in Fig. 23, after partial closure of the neural groove, are illustrated in cut I0.
in Squa1us, that it seemed to me probab1e that similar early
formed vesicles might exist in Torpedo oce11ata, and have been
overlooked by the Zieglers, who have studied the early stages
of that form in 1892; accordingly I procured embryos of
Torpedo oce1lata from the Zoölogical Station at Naples, and
studied the head region with some care. The first thing noted
was that the embryos of Torpedo ocellata aire not nearly as
favorable objects for observations as those of SquaIus. They
are smaller and the beginnings of Sense-Organs, branchiæ, and
other structures about the head are by no means so clear as
5 5 4 LOC P. [Vo1«. XI.


they are in Squa1us. As far as I am ab1e to discern, there is
no very distinct differentiation of the optic vesicles, in the
nearly stages of this form; nevertheless, they are present at the
stage designated C, by the Zieglers, and perhaps a little earlien
In studying the actual specimens, I had occasion to note that


cui« 1o.—'I’we1ve transverse Sections of the embryo shown in Pl. XXVL Fig. 23. x about zo
diameters The number-s below the Sections reker to their positions in the series. In Section
14 the optic vesicles show below and the depressions for the mid-brain vesicles above.


the Zieglers’ models are admirable representations of the embryos of the form they are intended to represent The optic
Cut 9.-— Eight transverse Sections of the embryo shown in Pl. XXVL Fig. is. I( about so characters. The accessory optic vesicles H. Ha) have been formed.
vesicles are much fainter than they are in Squa1us, but net-ertheless are simi1ar to them. I have studied them in Torpedo
both in surface views and in Sections. One noteworthy fact is
that they have been sectioned and actua11y Hgured by Ziegler in
his Pl. IV, Fig. 19I. It will be noted in this iigure, that the
brain-wa11s bulge outwards on either side and these depressions
No. 3.] » THE IJEEIEEJZH IE Iris-IV. 555


mark the interior of the optic vesicles P1. XXIX, Fig. 65, is
a drawing of one of my Sections of Torpedo s1ight1y younger
than the one of Zieg1er’s just referred to. The optic vesicles
are indicated at pp.


The external appearance of the optic vesicles in later stages
These optic vesicles are so well developed at an early period in Squa1us, that it seemed to me probab1e that similar early formed vesicles might exist in Torpedo oce11ata, and have been overlooked by the Zieglers, who have studied the early stages of that form in 1892; accordingly I procured embryos of Torpedo oce1lata from the Zoölogical Station at Naples, and studied the head region with some care. The first thing noted was that the embryos of Torpedo ocellata aire not nearly as favorable objects for observations as those of SquaIus. They are smaller and the beginnings of Sense-Organs, branchiæ, and other structures about the head are by no means so clear as they are in Squa1us. As far as I am ab1e to discern, there is no very distinct differentiation of the optic vesicles, in the nearly stages of this form; nevertheless, they are present at the stage designated C, by the Zieglers, and perhaps a little earlien In studying the actual specimens, I had occasion to note that the Zieglers’ models are admirable representations of the embryos of the form they are intended to represent The optic vesicles are much fainter than they are in Squa1us, but net-ertheless are simi1ar to them. I have studied them in Torpedo both in surface views and in Sections. One noteworthy fact is that they have been sectioned and actua11y Hgured by Ziegler in his Pl. IV, Fig. 19I. It will be noted in this iigure, that the brain-wa11s bulge outwards on either side and these depressions mark the interior of the optic vesicles P1. XXIX, Fig. 65, is a drawing of one of my Sections of Torpedo s1ight1y younger than the one of Zieg1er’s just referred to. The optic vesicles are indicated at pp.
is shown in Pl. XXVII, Figs 34, 35, 36, 37. When the stage
representecl in Fig. 35 has been reached the lens shows externally ; it is forrned in the usual vertebrate way. The choroid
fissure shows in Figs. 36 and 37. My studies have not been
made to include the later stages of development of the optic
vesic1e, but are coniined to its earliest differentiation


Sections of the earliestckormed circular areas, show something in the direction of histo1ogical clifferentiation Mitoses
are more frequent and more of the cells are e1ongated and
pear-shaped than in the other parts of the cepha1ic plate. The
differentiation is by no means as marked as in the Rana
pa1ustris Hgured by Eycleshymer —- nevertheless indications
of change are not wholly lacking My Sections are too thick
(I0«- to I In) for satisfactory histological study, but one can see
in them that the middle of the wa11s of the optic cups are areas
of differentiation The differentiation does not progress far
until later, but the frequency of dividing cells, and their
change of form continue, in these early stages, to be marks for
distinguishing visual epithelium from that of the surrounding
brain—wal1s. The dividing cells are neurob1asts, and these are
known to be points from which thenervedibres spring. Their
presence in any considerable number would, therefore, indicate
a differentiation in the direction of increase of sensibi1ity.


The eyes are the highest developed of the Sense-Organs, and
in that particular stand at the head of the series. But, do the
eyes belong to the same series with the other Sense-Organs, or
do they occupy a position by themse1ves, can they be homo1ogized with the restk This is a puzzling question, over which
there has been much controversy, and upon which there is
still much difference of opinion among anatomists The stock
objection to their being classed with the other Sense-Organs, is
this:—— they have apparently been derived from a different
basis, and their nerves are developed in a different way. The
556 Lock. Not. x1.


eyes are formed out of neural-plate material as diverticula of
the fore-brain, while the other Sense-Organs arise outside the
neural plate ; they have an independent epiblast origin.


It is questionable, whether any particular stress should be
Cut 10. — Twe1ve transverse Sections of the embryo shown in Pl. XXVL Fig. 23. x about zo diameters The number-s below the Sections reker to their positions in the series. In Section 14 the optic vesicles show below and the depressions for the mid-brain vesicles above.
put upon that argument, as the neural plate is at best only a
part of the modified epiblast ; moreover, the suggestion that the
neural plate is, undoubtedly, very much widened and expanded
from its ancestral condition, and that certain sensory area,
originally lying outside, may have been included in it, does
much to 0ffset that objection. The neural plate has been a
prodigiously long time in forming, and the eyes have been
brought into closer relationship with it. The plate is broadest
in front, and the eyes are the most anterior sense-organs, and
have become included in this expansion While there is not
sufficient data to give a wholly satisfactory answer to the question propoundech the assumption that the eyes are closely
related to all the others is not without foundation, and we are
now in the attitude of awaiting further facts.


II. AccEssoRv Oprxc VEs1cLEs.


The neura.1 plate, while still in very young stages, and while
the neural grooves are widely open, becomes the seat of some
accessory differentiations that resemble the optic vesicles. So
far as I know no observations upon these structures have been
recorded except those in my preliminary account in the
JOURNAL OF« MORPHOLOGL I quote from that paper: But,
more interesting than the fact of their (the optic vesicles’) very
early appearance in Elasmobranchs, is their apparent relation—
ship to other depressions that are formed upon the cephalic
plate, behind the already established optic vesicles. The new
involutions referred to, make their appearance upon the
cephalic plate just baclc of the optic vesic1e. Two of them
(Fig. 13, as. v) and Fig. U, as. Eil, ask. IN) take precedence of all
others in deve1opment, and are so distinctly formed as to afford
a good basis for cornparison with the optic vesicles. They are
circular depressions formed in front of the latter, and they
produce upon the exterior corresponding rounded e1evations.
No. 3.J THE »Es« Paar« JIE HEXE. 5 5 7


The optic vesicles are formed first, and when, at a very litt1e
The external appearance of the optic vesicles in later stages is shown in Pl. XXVII, Figs 34, 35, 36, 37. When the stage represented in Fig. 35 has been reached the lens shows externally ; it is forrned in the usual vertebrate way. The choroid fissure shows in Figs. 36 and 37. My studies have not been made to include the later stages of development of the optic vesic1e, but are coniined to its earliest differentiation.
later stage, the others arise behind them, it appears as if the
process of eye-formation were repeating itself serial1y.


These circular pockets not only arise in a simi1ar way, but
structura1ly resemb1e the optic vesic1es. In cross and 1ongitudina1 Sections the ce11s of the sunken patches are similar to
those in the eye pockets. They may be designated access-»Jozjszz«c TIZFZ«CZFF.


The assumption that they are primitively visual in character
Sections of the earliestckormed circular areas, show something in the direction of histological differentiation Mitoses are more frequent and more of the cells are e1ongated and pear-shaped than in the other parts of the cepha1ic plate. The differentiation is by no means as marked as in the Rana pa1ustris Hgured by Eycleshymer —- nevertheless indications of change are not wholly lacking My Sections are too thick (I0«- to I In) for satisfactory histological study, but one can see in them that the middle of the wa11s of the optic cups are areas of differentiation The differentiation does not progress far until later, but the frequency of dividing cells, and their change of form continue, in these early stages, to be marks for distinguishing visual epithelium from that of the surrounding brain—wal1s. The dividing cells are neurob1asts, and these are known to be points from which thenervedibres spring. Their presence in any considerable number would, therefore, indicate a differentiation in the direction of increase of sensibi1ity.
may be going too far, but the basis upon which it rests will be
shown directly. The 1east that can be said of them (from their
structure, the place and manner of their appearance), I think,
is that they are segmental sensory patches.


These structures begin to appear soon after the eye vesic1es.
There are several pairs of them stretching in rows baclc of the
eyes, as stated above, the two anterior pair are the most prom—
inent. They gradually grow fainterz the anterior one is the
best differentiated; the second one is not so clear, and the
succeeding ones grow fainter as we pass backwards I have
counted as many as four pairs in surface studyz but Sections
show that the series is more extensive and contains not less
than eight pairs of these circular pits. The two anterior ones
are well shown in Figs. 9 and ro. These are the earliest surface
indications. In Fig. V, which is slightly more advanced, the
accessory structures are clearly developedz the ftgure shows
four pairs behind the eyes. In longitudinal Sections of the
same specimen, Pl. XXIX, Figs. y6——87, eight pairs may be
counted. The form of the depression, as seen at first from
the surface view, is ovate; they soon become circular patches,
and form themselves into concave, shallow cups. I have seen
them in many specimens.


It will be noted that they make their Erst appearance while
The eyes are the highest developed of the Sense-Organs, and in that particular stand at the head of the series. But, do the eyes belong to the same series with the other Sense-Organs, or do they occupy a position by themse1ves, can they be homo1ogized with the restk This is a puzzling question, over which there has been much controversy, and upon which there is still much difference of opinion among anatomists The stock objection to their being classed with the other Sense-Organs, is this:—— they have apparently been derived from a different basis, and their nerves are developed in a different way. The eyes are formed out of neural-plate material as diverticula of the fore-brain, while the other Sense-Organs arise outside the neural plate ; they have an independent epiblast origin.
the neural plate is broadly expanded, and spring into prominence while the neural folds are growing upwards It is during
the rise of the neural folds that similar, but fainteky vesicles
can be made out, in surface study, behind the two anterior pairs.


Pl. XXX, Figs. 88 to 112 show twentyckive transverse sections of an embryo very slightly older than that represented in
5 58 LOCPI IJVo1.. XI,


Pl. XXVL Fig. 16. The Sections 88 to 100 lie in the region
It is questionable, whether any particular stress should be put upon that argument, as the neural plate is at best only a part of the modified epiblast ; moreover, the suggestion that the neural plate is, undoubtedly, very much widened and expanded from its ancestral condition, and that certain sensory area, originally lying outside, may have been included in it, does much to 0ffset that objection. The neural plate has been a prodigiously long time in forming, and the eyes have been brought into closer relationship with it. The plate is broadest in front, and the eyes are the most anterior sense-organs, and have become included in this expansion While there is not sufficient data to give a wholly satisfactory answer to the question propoundech the assumption that the eyes are closely related to all the others is not without foundation, and we are now in the attitude of awaiting further facts.
of the cepha1ic plate. The following Sections, 105 to 1I2, lie
in the neck and trunk region. These Sections are remarkable
in bringing to light serial depressions a1ong the walls of the
neural folds. They show that the serial cup-like differentia—
tions extend back of the cephalic plate. I have not been able
to determine the number of serial differentiations of this character in the embryo, but it is clear that there are several pairs
behind the cephalic plate upon which I have noted, in surface
study, four pairs in addition to the primary optic vesicles.


My Sections are not favorable for a critical study of the
==II. Accessory Optic Vesicles==
histological conditions, but it is clear, from them, that« a
differentiation starts in the anterior patches simi1ar to that
mentioned above, for the true eye vesicles. There is a greater
frequency of dividing cells and many of the cells become
elongated and pear—shaped.


comparisons of the two anterior pairs with the eye vesicles
give the following points of resemblance They are formed
in precisely the same way, and present the same general
appearancez viewed from within they are all similar concave
cups; they produce corresponding elevations on the outer
surface, and, if observed from the« outside, they form a series
of rounded knobs serially arranged, the eye being the anterior
terminations of the row. The marked differentiation in the
eye takes place in later stages. There are, however, the
histological features spoken of above that serve to distinguish
it in early stages, and the accessory vesicle shows the same
characteristics


These resemblances, coupled with the existence of serial
The neura.1 plate, while still in very young stages, and while the neural grooves are widely open, becomes the seat of some accessory differentiations that resemble the optic vesicles. So far as I know no observations upon these structures have been recorded except those in my preliminary account in the JOURNAL OF« MORPHOLOGL I quote from that paper: But, more interesting than the fact of their (the optic vesicles’) very early appearance in Elasmobranchs, is their apparent relation— ship to other depressions that are formed upon the cephalic plate, behind the already established optic vesicles. The new involutions referred to, make their appearance upon the cephalic plate just baclc of the optic vesic1e. Two of them (Fig. 13, as. v) and Fig. U, as. Eil, ask. IN) take precedence of all others in deve1opment, and are so distinctly formed as to afford a good basis for cornparison with the optic vesicles. They are circular depressions formed in front of the latter, and they produce upon the exterior corresponding rounded e1evations.
eyes in Some of the Invertebrates, has led to the assumption
that in Vertebrates these rudirnents represent accessory optic
vesicles. The fate of the anterior pair would also favor this


view ; they pass into the pineal sense—organ, a structure whose
visual character is acknowledged


If the view expressed above is true, we have a multiple-eyed
The optic vesicles are formed first, and when, at a very litt1e later stage, the others arise behind them, it appears as if the process of eye-formation were repeating itself serial1y.
condition in the embryos of these animals. This is common


enough in Invertebrates, but has not been previously noticed
in Vertebrates.
No. 3.] THE« IJEXTEEKH TE III-Hm. 5 59


These accessory structures are present for a brief time only.
These circular pockets not only arise in a simi1ar way, but structura1ly resemb1e the optic vesic1es. In cross and 1ongitudina1 Sections the ce11s of the sunken patches are similar to those in the eye pockets. They may be designated accessory optic vesicles.
That region of the head, behind the optic vesicles and in front
of the ears, becomes the seat of great modifications, and, as
the neural groove closes, the structures described give way to
later formed ones. They are, therefore, not only embryonic
structures, but are also transitory, It is a truism in develop—
mental history that the more primitive characteristics appear
first, and the secondary modiiications come in later. The
structures described are among the most primitive that have
been preserved in this very ancient group of Vertebrates, and
should be of signiiicance in indicating the ancestral relations
of the eye. I have spoken of the disappearance of the Organs,
but I have been able to trace the anterior pair further along,
an account of which will be given in the next Section on The
Pineal Sense-Organs.


I have also traced out simi1ar rudimentary organs in the
embryo chick, at about the conventional 24-hour stage ; there
is a series of seven vesicles behind the optic vesicles These
structures are rudimentary, and very quickly fade away.


All this acquires new interest when taken in connectiion
The assumption that they are primitively visual in character may be going too far, but the basis upon which it rests will be shown directly. The 1east that can be said of them (from their structure, the place and manner of their appearance), I think, is that they are segmental sensory patches.
with the researches of Whitman on the eyes of leeches. As
indicated above, he showed that the eyes of the leeches are
derived from segmental sensory papillæ. It now appears that
we have traces of a somewhat simi1ar line of segmental sensory
organs in Vertebrates It is essential to say Drecke-F, for these
structures are transitory, and do, not rise to a high grade of
differentiation, except in the case of the pineal Sense-Organs.
They may never, in the vertebrate group, have been raised to
the rank of eyes, but it is certain that in Annelids simi1ar
segmental sensory patches have been so raised. It is, however,
reasonable to suppose that members of this series have been
functionaL even in Vertebrates, when we recollect the highly
deve1oped condition of the pineal eye in Some forms.


There is really very little direct evidence to support the
proposition that the Vertebrates are derived from multiple—eyed
ancestors, although it is aconception that has been in the
minds of morphologists for Some time. Besides the indications
of such a condition afforded by the facts of this paper there
560 LOCPT [Vo1.. XI.


has recently come more evidence bearing in the same direction. This consists in the discovery of multip1e pineal eyes, in
These structures begin to appear soon after the eye vesic1es. There are several pairs of them stretching in rows baclc of the eyes, as stated above, the two anterior pair are the most prom— inent. They gradually grow fainterz the anterior one is the best differentiated; the second one is not so clear, and the succeeding ones grow fainter as we pass backwards I have counted as many as four pairs in surface studyz but Sections show that the series is more extensive and contains not less than eight pairs of these circular pits. The two anterior ones are well shown in Figs. 9 and ro. These are the earliest surface indications. In Fig. V, which is slightly more advanced, the accessory structures are clearly developedz the ftgure shows four pairs behind the eyes. In longitudinal Sections of the same specimen, Pl. XXIX, Figs. y6——87, eight pairs may be counted. The form of the depression, as seen at first from the surface view, is ovate; they soon become circular patches, and form themselves into concave, shallow cups. I have seen them in many specimens.
several distinct forms, and, in one case, the existence of three
distinct nerves to the pineal organs (see Section on The Pineal
Sense-Organs)


Whitman, in his paper just cited, says: ssAlthough the
evidence appears to me conclusive that the eyes and the
segmenta1 papillæ were, originally, morpho1ogical as well as
physiological equivalents, it does not, of course, follow necessarily that both Organs now have the same fnnctional signifk
cance. The original papillæ may have represented Sense-Organs
of a more or less indifferent order, among which, inthe course
of the historical development of the leech, a division of Iabor
was introduced, a few at the anterior end becoming specialized
as lighbperceiving Organs, the rest either remaining in their
early indifferent condition or becoming specialized in Some
other direction.


«The discovery that these papillæ are Sense-Organs might
It will be noted that they make their Erst appearance while the neural plate is broadly expanded, and spring into prominence while the neural folds are growing upwards It is during the rise of the neural folds that similar, but fainteky vesicles can be made out, in surface study, behind the two anterior pairs.
lead us to speculate on aHinities of a distant and somewhat
uncertain nature, such as are supposed by the writerz in
common with many others, to exist between annelid worms
and Vertebrates At all events, the existence of such organs
in the leech furnishes a broader basis for the discussion of the
question whether the Vertebrates and Annelids have been
derived from a common form possessing metameric Senseorgans, as was first argued by Dr. Eisig of the Naples station.
Assuming that the Sense-Organs of the Vertebrate and the
segmental papillæ of the leech may be traced to a common
origin in some remote ancestral form, it does not follow that
they should now present close structural resernblances It is
far more important to show that they possess certain general
features in common. The most important of their common
features is undoubtedly their metameric origin. The nervesupply forms another feature of fundamental importance, in
which, according to the interesting observations of Mr. Beard,
on the segmental Sense-Organs of the lateral line (Z·:)x)Z.
des» VIL Nos. 161 and I62) of the Vertebrate there is
essential agreement. The developmenta1 history of these
No. 3.] THE »Der-Tagen TE Erst-ro. 561


latera1 organs in the Hsh, where they make their first appearance as FFFJJHFJZZZZZPZZPJJZZE in the strictest sense of these words,
cannot be explained on a more satisfactory hypothesis."


I1I. THE: PINEAL SENSE-ORGANS.
Pl. XXX, Figs. 88 to 112 show twentyckive transverse sections of an embryo very slightly older than that represented in 5 58 LOCPI IJVo1.. XI,


I. Gram-J- oj lfisssozeyZecige ragmwssbøg IX«- Piøeaaz Same-Organs.


The pineal outgrowth has attracted much attention since the
Pl. XXVL Fig. 16. The Sections 88 to 100 lie in the region of the cepha1ic plate. The following Sections, 105 to 1I2, lie in the neck and trunk region. These Sections are remarkable in bringing to light serial depressions a1ong the walls of the neural folds. They show that the serial cup-like differentia— tions extend back of the cephalic plate. I have not been able to determine the number of serial differentiations of this character in the embryo, but it is clear that there are several pairs behind the cephalic plate upon which I have noted, in surface study, four pairs in addition to the primary optic vesicles.
discovery, in I886, of its eye-like structure. Since then it has
been accepted by morphologists as a rudimentary Sense-organ.
The earlier 1iterature bearing on the subject has been theroughly reported on by Spencer («86) and Francotte («89).
several recent1y published papers taken together put the
subject in a new light, and, as Ritter («94) has said, «At no
time since the epiphysis and pineal eye have been the topic of


investigation has it been a more interesting or a more inviting


one than it is just now."
My sections are not favorable for a critical study of the histological conditions, but it is clear, from them, that« a differentiation starts in the anterior patches simi1ar to that mentioned above, for the true eye vesicles. There is a greater frequency of dividing cells and many of the cells become elongated and pear—shaped.
The growth of our knowledge regarding this remarkable


Sense-organ, scattered through the various publications of
comparisons of the two anterior pairs with the eye vesicles give the following points of resemblance They are formed in precisely the same way, and present the same general appearancez viewed from within they are all similar concave cups; they produce corresponding elevations on the outer surface, and, if observed from the« outside, they form a series of rounded knobs serially arranged, the eye being the anterior terminations of the row. The marked differentiation in the eye takes place in later stages. There are, however, the histological features spoken of above that serve to distinguish it in early stages, and the accessory vesicle shows the same characteristics


Spencer, Beraneclg Francotte, Strahl and Martin, Leydig, and
These resemblances, coupled with the existence of serial eyes in Some of the Invertebrates, has led to the assumption that in Vertebrates these rudirnents represent accessory optic vesicles. The fate of the anterior pair would also favor this view ; they pass into the pineal sense—organ, a structure whose visual character is acknowledged.
others, may be reduced to the following summary : i
First came the demonstration of its eye-lil(e structure
in Amphibia, by De Graaf, and in Lacertilia by Spencer.
De Graaf’s publication is much briefer, and was published
only a few months before Spencer’s extensive study. Both
authors investigated the structure in adult forms. Spencer
studied its condition in twenty-eight different species of
Lacertilia, and demonstrated very clearly that even in the


adult forms of these animals, this organ has an eye-like Struc
If the view expressed above is true, we have a multiple-eyed condition in the embryos of these animals. This is common enough in Invertebrates, but has not been previously noticed in Vertebrates.  
ture, with a 1ens, a pigmented retinal layer, and a nerve. It
seems doubtful, in the light of recent work, whether the
kibrous Strand connecting the eye-capsu1e with the epiphysial
Stall-i, and designated the nerve by Spencer, is really nerve or
not.


Following the study of the adult structure came investigations of the embryonic conditions The first studies dealingv
with the embryonic development of this organ, and with its.
5 62 LOCK [Voi:.. XI.


minute structure in the early stages, were those of Strahl and
These accessory structures are present for a brief time only. That region of the head, behind the optic vesicles and in front of the ears, becomes the seat of great modifications, and, as the neural groove closes, the structures described give way to later formed ones. They are, therefore, not only embryonic structures, but are also transitory, It is a truism in develop— mental history that the more primitive characteristics appear first, and the secondary modiiications come in later. The structures described are among the most primitive that have been preserved in this very ancient group of Vertebrates, and should be of signiiicance in indicating the ancestral relations of the eye. I have spoken of the disappearance of the Organs, but I have been able to trace the anterior pair further along, an account of which will be given in the next Section on The Pineal Sense-Organs.
Martin. Along this same line were the studies of Båraneclg
Francotte, Leydig, and others. By these investigators the
organ was traced backwards to a certain point in its history,
that is, to the time when it springs from the roof of. the thalamencephalon as a tubular outgrowth, like the tip of a finger
of a glove. To this period also belongs the discovery of a
nerve of transitory existence The investigations were coniined almost entire1y to two groups of animals, Amphibia and
Reptilia. In the embryos of these animals many points of
minute structure were worked out, such as the distribution
of the nerve within the eye—like capsule, the division of the
retinal part into distinct layers, the fact that the pineal eye is
higher deve1oped in the embryonic periods than later, etc.


The next step was the discovery that there is more than one
I have also traced out simi1ar rudimentary organs in the embryo chick, at about the conventional 24-hour stage ; there is a series of seven vesicles behind the optic vesicles These structures are rudimentary, and very quickly fade away.
epiphysial outgrowth. Se1enka, Leydig, Eycleshymen Hill,
successive1y ca11ed attention to the existence of two distinct
outgrowths, or vesicles, from the roof of the thalamencephalon
in Anguis, Amblystoma, and Teleosts.


Hi11’s studies on these two vesicles are the most complete
All this acquires new interest when taken in connectiion with the researches of Whitman on the eyes of leeches. As indicated above, he showed that the eyes of the leeches are derived from segmental sensory papillæ. It now appears that we have traces of a somewhat simi1ar line of segmental sensory organs in Vertebrates It is essential to say Drecke-F, for these structures are transitory, and do, not rise to a high grade of differentiation, except in the case of the pineal Sense-Organs. They may never, in the vertebrate group, have been raised to the rank of eyes, but it is certain that in Annelids simi1ar segmental sensory patches have been so raised. It is, however, reasonable to suppose that members of this series have been functionaL even in Vertebrates, when we recollect the highly deve1oped condition of the pineal eye in Some forms.
He has shown the existence in Teleosts and Amia of two
vesicles, anterior and posterior, and has traced them through
the stages of deve1opment. The posterior develops into the
epiphysis and the anterior degenerates. The dista1 portion of
the former is histologically very complex, and receives a nerve
from the posterior commissure. Hill made out the distribution
of nervedibres in this part of the epiphysis. He traced quite
clearly the history of the vesic1es.


Two vesic1es, an upper and a lower, have long been known
There is really very little direct evidence to support the proposition that the Vertebrates are derived from multiple—eyed ancestors, although it is aconception that has been in the minds of morphologists for Some time. Besides the indications of such a condition afforded by the facts of this paper there has recently come more evidence bearing in the same direction. This consists in the discovery of multip1e pineal eyes, in several distinct forms, and, in one case, the existence of three distinct nerves to the pineal organs (see Section on The Pineal Sense-Organs)
to exist in Petromyzon, in both embryos and adu1ts. It is
much to be regretted that we do not know their embryonic
history, for much depends on this. The most recent paper on
the epiphysial Organs in Petromyzon—that of studniöka——
does not clear up the question of the origin of the two vesic1es.
Histological material was 1acking in just the stage required.
The two outgrowths in embryos outside the Cyclostomes have
been designated epiphysis, for the hinder, and pineal eye for
the anterior. Studniöka designates them pineal and parapineaL
No. 3.] THE NEJZTEEJEA If: fix-w. 363


respectively, in the Cyclostomes As I shall show latet, the
so-ca11ed paraphysis is a different Structur-e.


Beraneck showed that the pineal eye and epiphysis have
Whitman, in his paper just cited, says: ssAlthough the evidence appears to me conclusive that the eyes and the segmenta1 papillæ were, originally, morpho1ogical as well as physiological equivalents, it does not, of course, follow necessarily that both Organs now have the same fnnctional signifk cance. The original papillæ may have represented Sense-Organs of a more or less indifferent order, among which, inthe course of the historical development of the leech, a division of Iabor was introduced, a few at the anterior end becoming specialized as lighbperceiving Organs, the rest either remaining in their early indifferent condition or becoming specialized in Some other direction.
been confused, and, on account of the existence of the nerve
from outside sources, he argued for the complete independence
of the pineal eye.


Klinckowstrom has p1aced himse1f in Opposition to this
opinion, showing the eye vesicle is formed as an outgrowth
from the epiphysis.


Next to the discovery of two or more epiphysial outgrowths
"The discovery that these papillæ are Sense-Organs might lead us to speculate on aHinities of a distant and somewhat uncertain nature, such as are supposed by the writerz in common with many others, to exist between annelid worms and Vertebrates At all events, the existence of such organs in the leech furnishes a broader basis for the discussion of the question whether the Vertebrates and Annelids have been derived from a common form possessing metameric Senseorgans, as was first argued by Dr. Eisig of the Naples station. Assuming that the Sense-Organs of the Vertebrate and the segmental papillæ of the leech may be traced to a common origin in some remote ancestral form, it does not follow that they should now present close structural resernblances It is far more important to show that they possess certain general features in common. The most important of their common features is undoubtedly their metameric origin. The nervesupply forms another feature of fundamental importance, in which, according to the interesting observations of Mr. Beard, on the segmental Sense-Organs of the lateral line (Z·:)x)Z. des» VIL Nos. 161 and I62) of the Vertebrate there is essential agreement. The developmenta1 history of these latera1 organs in the Hsh, where they make their first appearance as segmental papillae in the strictest sense of these words, cannot be explained on a more satisfactory hypothesis."
may be mentioned the existence of accessory pineal eyes The
ear1iest publication containing an account of such Structur-es is
that of Duva1 and Kalt, in I889. In a brief note (Des Yeux
Pinåaux Multiples chez 1’Orvet) these authors record the
fjnding of two or three accessory pineal vesic1es in Anguis


They do not say whether adu1t or embryonic forms were


studied
==III. The Pineal Sense-Organs==


carriere («89), in the same year, noted the occurrence in embryos of Anguis of a very rudimentary accessory pineal vesic1e.
===1. Growth of Knowledge regarding the Pineal Sense-Organs===


Leydig («90) observed these structures independently in
The pineal outgrowth has attracted much attention since the discovery, in I886, of its eye-like structure. Since then it has been accepted by morphologists as a rudimentary Sense-organ. The earlier 1iterature bearing on the subject has been theroughly reported on by Spencer («86) and Francotte («89). several recent1y published papers taken together put the subject in a new light, and, as Ritter («94) has said, «At no time since the epiphysis and pineal eye have been the topic of investigation has it been a more interesting or a more inviting one than it is just now." The growth of our knowledge regarding this remarkable sense-organ, scattered through the various publications of Spencer, Beraneclg Francotte, Strahl and Martin, Leydig, and others, may be reduced to the following summary : i First came the demonstration of its eye-lil(e structure in Amphibia, by De Graaf, and in Lacertilia by Spencer. De Graaf’s publication is much briefer, and was published only a few months before Spencer’s extensive study. Both authors investigated the structure in adult forms. Spencer studied its condition in twenty-eight different species of Lacertilia, and demonstrated very clearly that even in the adult forms of these animals, this organ has an eye-like Struc ture, with a 1ens, a pigmented retinal layer, and a nerve. It seems doubtful, in the light of recent work, whether the kibrous Strand connecting the eye-capsu1e with the epiphysial Stall-i, and designated the nerve by Spencer, is really nerve or not.
embryos of the same anirnals He showed that there is much
variabi1ity, both as to the presence and the histological Structure of these accessory capsu1es. In Some individuals they
are lacking, and in others of the same age present and well
deve1oped. He records the occurrence, in Some cases, of two
accessory Organs, a. larger and a smaller one. The larger one
resembles the pineal eye in structure and arrangement of pigment. The smaller one is very rudimentary Leydig com—
pared them to the ocelli of insects. It is c1ear from his account
that variabi1ity and inconstancy are characteristics of these
accessory Organs.


Prenanh in the latter part of I893, again records the occurrence of one accessory pineal eye in the embry0s of Anguis
He directs attention to the fact that up to that time they had
been discovered only in a single species, VIII» Anguis fragilis,
and further, that they were all in ernbryos, and not in adults.
Ritter (-94) has recently recorded the, occurrence of such an»
accessory org-an in the aclult Phrynosoma
564 Lock: kvon XI.


The above observations, taken together, give suflicient positive data to establish the fact that there appears, from time
Following the study of the adult structure came investigations of the embryonic conditions The first studies dealingv with the embryonic development of this organ, and with its minute structure in the early stages, were those of Strahl and Martin. Along this same line were the studies of Båraneclg Francotte, Leydig, and others. By these investigators the organ was traced backwards to a certain point in its history, that is, to the time when it springs from the roof of. the thalamencephalon as a tubular outgrowth, like the tip of a finger of a glove. To this period also belongs the discovery of a nerve of transitory existence The investigations were coniined almost entire1y to two groups of animals, Amphibia and Reptilia. In the embryos of these animals many points of minute structure were worked out, such as the distribution of the nerve within the eye—like capsule, the division of the retinal part into distinct layers, the fact that the pineal eye is higher deve1oped in the embryonic periods than later, etc.
to time, in some animals the vestiges of accessory pineal
Organs, and that they are variab1e and inconstant in their
occurrence.


The most recent advances consist in carrying the history of
these organs baclcwards, and showing them to be connected
with patches of sensory epithelium that arise on the cephalic
plate in very young stages, and in the discovery, in Iguana, of
three distinct nerves connected with the epiphysial outgrowths.
The former work was done by the present writer, and the latter
by Klinckowstrom.


As indicated in the preceding Section, there are several pairs
The next step was the discovery that there is more than one epiphysial outgrowth. Se1enka, Leydig, Eycleshymen Hill, successive1y ca11ed attention to the existence of two distinct outgrowths, or vesicles, from the roof of the thalamencephalon in Anguis, Amblystoma, and Teleosts.
of cups on the cephalic plate back of the optic vesicle, formed
while the neural groove is widely open. I have designated
them accessory optic vesicles In the process of closure of
the neural groove the anterior pair are brought together, and
form part of the thalamencephalon, from the roof of which the
pinea1 outgrowth is derived. This process will be described
more in detai1.


Klinckowström (-98) has recent1y shown in Iguana the presence of two nerves connected with the anterior outgrowth or
pineal eye, and sometimes a third connected with the posterior
soutgrowth or epiphysis.


These observations suggest new interpretationsz still it is
Hi11’s studies on these two vesicles are the most complete He has shown the existence in Teleosts and Amia of two vesicles, anterior and posterior, and has traced them through the stages of deve1opment. The posterior develops into the epiphysis and the anterior degenerates. The dista1 portion of the former is histologically very complex, and receives a nerve from the posterior commissure. Hill made out the distribution of nervedibres in this part of the epiphysis. He traced quite clearly the history of the vesic1es.
not an auspicious moment to indulge in speculation. It is
clearly evident that we need more data regarding these organs
and their relationships But the trend of these discoveries is
to strengthen the suggestion already made in this paper, that,
primitively, the Vertebrates were multiple-eyed. The presence
of accessory pineal eyes, the discovery of serial sensory areas
on the cephalic plate from which epiphysial outgrowths arise,
and the presence of two or three pinea1 nerves, are all consistent with this interpretation so far as the evidence goes,
there is more than one epiphysial outgrowth, and therefore I
have headed this Section, The Pineal Sense-Organs. In what


way it will be necessary to qualify this proposition will depend
on subsequent discoveries.
NO. 3.] THE« IJEJBTEBJEÄTE HEXE. 565


The results of all the embryo1ogica1 studies on the epiphysia1
Two vesic1es, an upper and a lower, have long been known to exist in Petromyzon, in both embryos and adu1ts. It is much to be regretted that we do not know their embryonic history, for much depends on this. The most recent paper on the epiphysial Organs in Petromyzon—that of studniöka—— does not clear up the question of the origin of the two vesic1es. Histological material was 1acking in just the stage required. The two outgrowths in embryos outside the Cyclostomes have been designated epiphysis, for the hinder, and pineal eye for the anterior. Studniöka designates them pineal and parapineal, respectively, in the Cyclostomes As I shall show latet, the so-ca11ed paraphysis is a different structure.
outgrowth between 1887 and 1893 were to settle on one point
of common agreement regarding its origin. Each investigator
successiveiy found it to arise as a tubular outgrowth from the
roof of the tha1amencepha1on comparative1y late in ontogeny,
after the parts of the brain are estab1ished. No earlier trace
of it had been found; but there was a previous undiscovered
history, and it is now time1y to make the inquiry, What is
the very beginning of the pinea1 Sense-Organs?


Z. Las-Hofe Fragt-«» czf Mk· Pisa-se! O«z«g-o2e-x-Tz.


As already indicated, there are upon the cephalic plate accessory eye vesicles which have made their appearance while
Beraneck showed that the pineal eye and epiphysis have been confused, and, on account of the existence of the nerve from outside sources, he argued for the complete independence of the pineal eye.
the neural groove is widely open; and in tracing their fate I
shall attempt to f111 up that gap in the history of the pineal
outgrowth from the time these cup—1ike structures appear upon
the cepha1ic p1ate to the time the tubu1ar outgrowth begins
from the tha1amencephalon.


As the wa11s of the neural groove grow upwards these Cuplike structures are carried up vvith them; and there comes a
time (Figs. I3, 3I) when they may be seen from the outside


as rounded eminences, and from the inside as concave cups.
Klinckowstrom has p1aced himse1f in Opposition to this opinion, showing the eye vesicle is formed as an outgrowth from the epiphysis.


When the edges of the neural groove meet in the middle line
the cups are approximatech and come to form part of the tha1amencepha1on.


While these changes have been going on, further differentis
Next to the discovery of two or more epiphysial outgrowths may be mentioned the existence of accessory pineal eyes The ear1iest publication containing an account of such Structur-es is that of Duva1 and Kalt, in I889. In a brief note (Des Yeux Pinåaux Multiples chez 1’Orvet) these authors record the fjnding of two or three accessory pineal vesic1es in Anguis. They do not say whether adu1t or embryonic forms were studied.


.szations have been taking place in the wa11s of the brain. The


bulging of the wa11s to form the mid-brain vesicle has come on
Carriere ('89), in the same year, noted the occurrence in embryos of Anguis of a very rudimentary accessory pineal vesic1e.
insidious1y, and has taken a position behind the vesicles of
the paired eyes, in apparent1y the same position previously


ssoccupied by the accessory vesicles. These transformations are


confusing, as the wa11s of the mid-brain resemble the accessory
Leydig ('90) observed these structures independently in embryos of the same anirnals He showed that there is much variabi1ity, both as to the presence and the histological Structure of these accessory capsu1es. In Some individuals they are lacking, and in others of the same age present and well deve1oped. He records the occurrence, in Some cases, of two accessory Organs, a. larger and a smaller one. The larger one resembles the pineal eye in structure and arrangement of pigment. The smaller one is very rudimentary Leydig com— pared them to the ocelli of insects. It is c1ear from his account that variabi1ity and inconstancy are characteristics of these accessory Organs.
optic vesicles grown largerx 1n an ear1ier published paper I
made the mistake of identifying the mid-brain with the accessory optic vesicles; but it was merely an error of identification,
and did not affect the main contentionof that artie1e.


In working out the details of the formation of pineal outgrowth in Squa1us, it soon becomes apparent that the surface
Prenanh in the latter part of I893, again records the occurrence of one accessory pineal eye in the embry0s of Anguis He directs attention to the fact that up to that time they had been discovered only in a single species, VIII» Anguis fragilis, and further, that they were all in ernbryos, and not in adults. Ritter (-94) has recently recorded the, occurrence of such an» accessory org-an in the adult Phrynosoma.
566 Lock. kvon XI.


contours of the head are considerably altered by the distribution of mesoblastic Hcells lying between the external layer of
epiblast and the brain-waIIs. The mesoblast forms a pad of
varying thickness in close contact with the brain—wa1ls; the
depressions are tilled, and in some places thick patches of
mesoblast give rise to external rnarkings that render the surface appearances untrustworthy. There is, for instance, a
deep pad of mesoblast tilling up a depression at the sides of
the cerebellum, and also extending forwards on to the midbrain; at the external surface the pad assumes a circular form,
and it is that pad of mesoblast which is seen in Fig. 32. The
true mid-brain vesicle in that Hgure is the eminence marked
wo. The accessory optic vesicles are present, but are not
well marked externally. They are in front of the protuberance
marked »O.


In order to get a view of the brain-walls I have found it
The above observations, taken together, give suflicient positive data to establish the fact that there appears, from time to time, in some animals the vestiges of accessory pineal Organs, and that they are variab1e and inconstant in their occurrence.
necessary to remove the mesoblast and its coverings of epib1ast, and thus to complete1y expose the brain-wal1s. The
brain thus laid bare, enables one to see with complete satisfac—
tion its different parts and their relation to one another. A
very interesting relation comes to light through the dissections.
Somewhere between the stage with an open neural groove
(Fig. 3I) and the completely closed groove (Fig. 33) the midbrain vesicle insidious1y takes the former position of the accessory optic vesicle while the latter is carried forward by the
pr0cess of cranial flexure During the formation of the midbrain vesicle the two structures become incorporated into one
faintly bi1obed protuberance (PI. XXVIIL Figs. 38 and 39).
The anterior part is the accessory optic vesicle, and the posterior one the mid—brain. The separation soon becomes com—
p1ete, and by the stage represented in Fig. 40 the two are
completely separatedz but owing to the arrangernent of the
mesoderm the accessory optic vesicle is rendered indistinguishable from the outside. When the mesoderm is entirely removed
it may be seen. All this takes place in very brief intervals of
time, and a complete series of embryos representing the different phases of the closure of the neural groove is required to
see it all. I must say also that the changes are so perplexing
No. z] THE YEETEEJZH THE: III-m. 567


that there exists in my mind Some doubt as to the adequacy
The most recent advances consist in carrying the history of these organs baclcwards, and showing them to be connected with patches of sensory epithelium that arise on the cephalic plate in very young stages, and in the discovery, in Iguana, of three distinct nerves connected with the epiphysial outgrowths. The former work was done by the present writer, and the latter by Klinckowström.
of the above account. 1t may have to be modified, but I have
given the facts as they now appear to me.


F igs. 38-54 show a series of embryos that have been partly
As indicated in the preceding Section, there are several pairs of cups on the cephalic plate back of the optic vesicle, formed while the neural groove is widely open. I have designated them accessory optic vesicles In the process of closure of the neural groove the anterior pair are brought together, and form part of the thalamencephalon, from the roof of which the pinea1 outgrowth is derived. This process will be described more in detai1.
dissected in the way just indicated They show characteristic
changes in the brain-wa1ls as seen from the exteriotr


In Fig. 38 the vvalls of the neural groove have not yet met
in any part of their course, and the thalamencephalon cannot
be distinguished from the rest of the brain. The first pair of
accessory vesic1es are visible; they soon become inc0rporated
in the walls of the tha1amencephalon. I have not been able
to determine whether it is the epithe1ium of the first pair of
accessory vesicles only, or whether epithe1ium from the second
pair is also included. Either the epithe1ium of the two pairs
is incorporated into the walls of the tha1amencepha1on by
being carried together, or the epithe1ium of the second pair
fades into the surrounding substance of the brain-wa11, which
almost immediately grows into the vesicle of the mid-brain.
Although I am doubtful as to whether the second pair of accessory vesicles pass into the inter——brain, I am sure that the
epithe1ium of the anterior pair is so included.


Fig. 39 shows the first accessory optic vesicle and the midbrain vesicle forming a common protuberance, but they are
Klinckowström ('98) has recent1y shown in Iguana the presence of two nerves connected with the anterior outgrowth or pineal eye, and sometimes a third connected with the posterior soutgrowth or epiphysis.
very quiclcly separated, and the anterior vesicle goes into the
tha1amencepha1on. e


In Fig. 4o, which is somewhat o1der, this has occurred The
lateral wa11s of the thalamencephalon now consist of two shallow cups, approximated so that the structure looks lenticular
when viewed from above. As a usual thing, the thalamencephalon is not evident at this stage before the removal of the


overlying tissues. Fig. 4I shows, however, a specimen in
These observations suggest new interpretationsz still it is not an auspicious moment to indulge in speculation. It is clearly evident that we need more data regarding these organs and their relationships But the trend of these discoveries is to strengthen the suggestion already made in this paper, that, primitively, the Vertebrates were multiple-eyed. The presence of accessory pineal eyes, the discovery of serial sensory areas on the cephalic plate from which epiphysial outgrowths arise, and the presence of two or three pinea1 nerves, are all consistent with this interpretation so far as the evidence goes, there is more than one epiphysial outgrowth, and therefore I have headed this Section, The Pineal Sense-Organs. In what way it will be necessary to qualify this proposition will depend on subsequent discoveries.


which it could be seen before any dissection. Compare this
with Fig. 32, which is the more usual appearance of an embryo


of this age.
The results of all the embryo1ogica1 studies on the epiphysia1 outgrowth between 1887 and 1893 were to settle on one point of common agreement regarding its origin. Each investigator successiveiy found it to arise as a tubular outgrowth from the roof of the tha1amencepha1on comparative1y late in ontogeny, after the parts of the brain are estab1ished. No earlier trace of it had been found; but there was a previous undiscovered history, and it is now time1y to make the inquiry, What is the very beginning of the pinea1 Sense-Organs?


Fig. 42 is a dissection of the embryo shown in Fig. 41.
===2. Remote Origins of the Pineal Outgrowth===
The mid-brain, which, from external view, appears like a
single rounded eminence, is shown after exposure to be


"bilobed.
As already indicated, there are upon the cephalic plate accessory eye vesicles which have made their appearance while the neural groove is widely open; and in tracing their fate I shall attempt to f111 up that gap in the history of the pineal outgrowth from the time these cup—1ike structures appear upon the cepha1ic p1ate to the time the tubu1ar outgrowth begins from the tha1amencephalon.
568 Lock. Not. x1.


Figs. 43, 44, and 45, which are successively o1der, show an


increase in the size of the thalamencephalon, and marked
As the wa11s of the neural groove grow upwards these Cuplike structures are carried up vvith them; and there comes a time (Figs. I3, 3I) when they may be seen from the outside as rounded eminences, and from the inside as concave cups.


changes in the mid-brain.
When the edges of the neural groove meet in the middle line the cups are approximatech and come to form part of the thalamencepha1on.


In Figs. 46 and 47 a faint furrow has appeared in front that
serves to mark the boundary between the thalamencephalon
proper and the cerebral lobes. This furrow is clearly deiined
in Figs. 48 and 49, so that we have now the thalamencepha1on
distinctly marked off from the rest of the fore-brain.


In Fig. 48 the optic vesicle and all the mesob1ast have been
While these changes have been going on, further differentiations have been taking place in the wa11s of the brain. The bulging of the wa11s to form the mid-brain vesicle has come on insidious1y, and has taken a position behind the vesicles of the paired eyes, in apparent1y the same position previously ssoccupied by the accessory vesicles. These transformations are confusing, as the wa11s of the mid-brain resemble the accessory optic vesicles grown largerx 1n an ear1ier published paper I made the mistake of identifying the mid-brain with the accessory optic vesicles; but it was merely an error of identification, and did not affect the main contentionof that artie1e.
removed, and the view is talcen from the right side. The brain—
walls show very clearly. The tha1amencephalon is now bounded
anterior1y and posterior1y by furrows. The two furrows run
nearly parallel to one another, and they serve to mark oft« the
inter-brain very distinct1y.


The roof of the thalamencephalon is at this stage raised into
two rounded eminences of nearly equal dimensions, an anterior
and a posterior one. The posterior is, from the beginning,
somewhat in advance of the other. It becomes the pineal out—
growth, while the posterior eminence goes on deve1oping. The
anterior one becomes greatly reduced by compression from the
rapidly growing adjacent brain-walls. It very soon loses its
rounded Character, and becomes pressed into a semicircular
fo1d lying in front of the epiphysis. It is, I thinl(, equivalent
to the Zirbelpolster of German author-s. If longitudinal sections be made at the stage represented in Fig. 48, the two
e1evations show as in Minot’s Fig. 3I9, p. 5y2, of his Hasen«
EmZ--;1-·c)!og·j-. In that Hgure they look like two equal vesicles
springing from a single elevation of the brain roof, and«opening by a common wide passage into the brain vesicle No
especial signif1cance, I think, is to be attached to the fact that


there are at first two equal embossrnents from the roof of the
In working out the details of the formation of pineal outgrowth in Squa1us, it soon becomes apparent that the surface contours of the head are considerably altered by the distribution of mesoblastic Hcells lying between the external layer of epiblast and the brain-waIIs. The mesoblast forms a pad of varying thickness in close contact with the brain—wa1ls; the depressions are tilled, and in some places thick patches of mesoblast give rise to external rnarkings that render the surface appearances untrustworthy. There is, for instance, a deep pad of mesoblast tilling up a depression at the sides of the cerebellum, and also extending forwards on to the midbrain; at the external surface the pad assumes a circular form, and it is that pad of mesoblast which is seen in Fig. 32. The true mid-brain vesicle in that Hgure is the eminence marked wo. The accessory optic vesicles are present, but are not well marked externally. They are in front of the protuberance marked »O.


tha1amencephalon. The anterior one is converted into a fold
of the roof of the ’tween—brain that has long been recognized in
different anima1s. It is designated Zirbelpolster by Burckhardt
It does not in any sense correspond to the anterior epiphysial
vesicle discovered by Hill in Teleosts, but rather to the fold
of ’tween-brain that lies in front of both epiphyses. Studniölca


has, however, marked the fold in diagramrnatic figures of Tele—No. 3.] THE PEETEBJZA TE XII-IV. 569
In order to get a view of the brain-walls I have found it necessary to remove the mesoblast and its coverings of epib1ast, and thus to complete1y expose the brain-wal1s. The brain thus laid bare, enables one to see with complete satisfac— tion its different parts and their relation to one another. A very interesting relation comes to light through the dissections. Somewhere between the stage with an open neural groove (Fig. 3I) and the completely closed groove (Fig. 33) the midbrain vesicle insidious1y takes the former position of the accessory optic vesicle while the latter is carried forward by the pr0cess of cranial flexure During the formation of the midbrain vesicle the two structures become incorporated into one faintly bi1obed protuberance (PI. XXVIIL Figs. 38 and 39). The anterior part is the accessory optic vesicle, and the posterior one the mid—brain. The separation soon becomes com— p1ete, and by the stage represented in Fig. 40 the two are completely separatedz but owing to the arrangernent of the mesoderm the accessory optic vesicle is rendered indistinguishable from the outside. When the mesoderm is entirely removed it may be seen. All this takes place in very brief intervals of time, and a complete series of embryos representing the different phases of the closure of the neural groove is required to see it all. I must say also that the changes are so perplexing that there exists in my mind Some doubt as to the adequacy of the above account. 1t may have to be modified, but I have given the facts as they now appear to me.


osts, the anterior vesicle of Hi1l, but this identification is not
right.


We have in this specimen the first appearance of the tubular
outgrowth, which, accorciing to previous authors, marks the
beginning of the epiphysia It is somewhat important to fix
the stage at which this outgrowth begins. Its very first appearance as an elevation of the roof of the thalamencephalon is in
the stage represented in Fig. 36 after the saucersstage of the
ear is past.


In Fig. 49 the posterior protuberance of the roof of the
Figs. 38-54 show a series of embryos that have been partly dissected in the way just indicated They show characteristic changes in the brain-wa1ls as seen from the exterior.
thalamencephalon is assuming a tubular form. It becomes
the epiphysis The surface of the mid-b1«ain is now considerab1y changed. Two furrows have made their appearance, which
divide the 1ateralwalls into three nearly equal 1obes. These
do not show at all from the exterior before the removal of the
epidermis and the mesoblast.


The gradual reduction of the thalamencephalon by compression between the Central hemispheres and the mid-brain is
shown consecutively in Figs. 53, 54, II, If, and 6o. In
Fig. 54, and 1ater, the epiphysis appears pear—shaped from
the side, but from in front the enlargement on the distal end is
seen to be broader than thick. It is borne upon a hol1ow
sta11(.


Fig. 57 represents the brain of an embryo considerably older
In Fig. 38 the vvalls of the neural groove have not yet met in any part of their course, and the thalamencephalon cannot be distinguished from the rest of the brain. The first pair of accessory vesic1es are visible; they soon become inc0rporated in the walls of the tha1amencephalon. I have not been able to determine whether it is the epithe1ium of the first pair of accessory vesicles only, or whether epithe1ium from the second pair is also included. Either the epithe1ium of the two pairs is incorporated into the walls of the tha1amencepha1on by being carried together, or the epithe1ium of the second pair fades into the surrounding substance of the brain-wa11, which almost immediately grows into the vesicle of the mid-brain. Although I am doubtful as to whether the second pair of accessory vesicles pass into the inter——brain, I am sure that the epithe1ium of the anterior pair is so included.
than in Fig. 56. The thalamencephalon is now very much
compressed. The anterior half of it, which originally had a
rounded protuberance growing from its roof, is now reduced to
a small semicircular fold in front of the epiphysis.


Fig. 59 is a view upon the epiphysis of the same embryo
from directly in front. It was obtained by removing the cere—
bral 1obes, which lie in front of, and partly hide the epiphysia
The epiphysis is seen to be composed of a Stall( and an en1arged
dista1 extremity ; leading from the Stall( on either side are the
epiphysialpedunc1es. In this hgure we are looking through
the ventricle of the thalamencephalon into that of the midbrain. On the inside of the lateral walls of the opening are
two enlargements, the beginnings of the thala1ni optici. These
contain the ganglia hebenulæ
570 Lock. s kvon XI.


In this specigmen the paraphysis had made its appearance as
Fig. 39 shows the first accessory optic vesicle and the midbrain vesicle forming a common protuberance, but they are very quiclcly separated, and the anterior vesicle goes into the tha1amencepha1on.
an outgrowth from the cerebrum. It is relatively late in arising, and is entirely distinct from any outgrowth from the
thalamencephalon The lateral wal1s of the mid-brain are no
1onger10bed as in earlier stages. Fig. 58 is the same brain
viewed from above ; we are looking directly into the cavity of
the fourth ventricle.


Fig. 6o shows the brain of a still older embryo. It is the
largest embryo of squalus in my collection, measuring 35 mm.
in length. The cerebral lobes are divided into right and left
halves by a sha11ow furrow. From the back of the cerebrum
rises the paraphysis (P); just behind this is seen the semicircu1ar fo1d, belonging to the ’tween-brain, and behind that is
the epiphysis, rising above and resting upon the anterior wal1
of the mid-brain. Fig. 6I shows a front view of the cerebral
lobes after being removed from the rest of the brain ; the
paraphysis is also brought into view. Fig. 62 is the same


I brain with the cerebral lobes removed, and p1aced so as to give
In Fig. 40, which is somewhat o1der, this has occurred The lateral wa11s of the thalamencephalon now consist of two shallow cups, approximated so that the structure looks lenticular when viewed from above. As a usual thing, the thalamencephalon is not evident at this stage before the removal of the overlying tissues. Fig. 4I shows, however, a specimen in which it could be seen before any dissection. Compare this with Fig. 32, which is the more usual appearance of an embryo of this age.


a ful1-face view on the epiphysisz the stalk is considerably
longer than in Fig. so.


I have not had materia1 to work out the subsequent history
Fig. 42 is a dissection of the embryo shown in Fig. 41. The mid-brain, which, from external view, appears like a single rounded eminence, is shown after exposure to be bilobed.
of the epiphysis In specimens about six inches long it is situated in the wall of the cranium, directly over the cerebral lobes,
and is connected to the roof of the ’tween-brain by a long
threadJilce stallc The enlarged distal end is about the same
size as in Fig. 62. cattie shows it in the adult with several
capsules on the end of the stalk


Sections of the stages just studied serve to coniirm the
observations made from the outside. They do« not add very
much to its features, speaking strictly morphologically The
surface views have been prepared in such a way that they may
stand for reconstruction, but they are more accurate than any
actual reconstruction can be, as they show the relations entirely
undisturbed.


Figs. I I9—I24 show a series of sagittal Sections covering the i
Figs. 43, 44, and 45, which are successively o1der, show an increase in the size of the thalamencephalon, and marked changes in the mid-brain.


period of the first appearance of the epiphysis, the reduction of
the anterior part of the thalamencepha1on, and the beginning of
the paraphysis and the choroid plexus
N0.3.] THE FEÆIIEBÆÄTE HEXE-D. 571


Fig. I19 is a specimen of the same age as that shown in
In Figs. 46 and 47 a faint furrow has appeared in front that serves to mark the boundary between the thalamencephalon proper and the cerebral lobes. This furrow is clearly deiined in Figs. 48 and 49, so that we have now the thalamencepha1on distinctly marked off from the rest of the fore-brain.
Fig. 48. It shows that the roof of the thalarnencephalon is
raised into two nearly equa1 protuberances.


Fig. I20 shows the beginning of the reduction of the anterior of these elevations, and the increased growth of the
posterior one. In Fig. 121 the posterior e1evation, or epiphysis,
has become a tubular outgrowth, while the anterior one is
reduced and depressed.


In Fig. 122 the tubular Stall( of the epiphysis is considerably
In Fig. 48 the optic vesicle and all the mesob1ast have been removed, and the view is talcen from the right side. The brain— walls show very clearly. The tha1amencephalon is now bounded anterior1y and posterior1y by furrows. The two furrows run nearly parallel to one another, and they serve to mark oft« the inter-brain very distinct1y.
elongatech while that part of the roof of the thalamencephalon
in front of it has become reduced to a narrowfo1d that has
been carried ventracL The paraphysis has now arisen from
the roof of the prosencepha1on. The beginning of the choroid
plexus extends from the structure down into the ventric1e of
the fort-brain, and its posterior part is connected with the fold
in front of the epiphysis The anterior and posterior Commissures are now distinguishab1e.


Fig-s. 123 and 124 exhibit snbstantially the same relations ;
the choroid plexus has considerably increased in extent, and is


rapidly becoming much convolutedz the brain vesicles have
The roof of the thalamencephalon is at this stage raised into two rounded eminences of nearly equal dimensions, an anterior and a posterior one. The posterior is, from the beginning, somewhat in advance of the other. It becomes the pineal out— growth, while the posterior eminence goes on deve1oping. The anterior one becomes greatly reduced by compression from the rapidly growing adjacent brain-walls. It very soon loses its rounded Character, and becomes pressed into a semicircular fo1d lying in front of the epiphysis. It is, I thinl(, equivalent to the Zirbelpolster of German author-s. If longitudinal sections be made at the stage represented in Fig. 48, the two e1evations show as in Minot’s Fig. 3I9, p. 5y2, of his Hasen« EmZ--;1-·c)!og·j-. In that Hgure they look like two equal vesicles springing from a single elevation of the brain roof, and«opening by a common wide passage into the brain vesicle No especial signif1cance, I think, is to be attached to the fact that there are at first two equal embossrnents from the roof of the tha1amencephalon. The anterior one is converted into a fold of the roof of the ’tween—brain that has long been recognized in different anima1s. It is designated Zirbelpolster by Burckhardt It does not in any sense correspond to the anterior epiphysial vesicle discovered by Hill in Teleosts, but rather to the fold of ’tween-brain that lies in front of both epiphyses. Studniölca has, however, marked the fold in diagramrnatic figures of Teleosts, the anterior vesicle of Hi1l, but this identification is not right.
become reduced. In Fig. 124 the dista1 end of the epiphysis
is inserted into a cavity in the roof of the cranial Wall, that is,


the mesenchyme forms a cup over its rounded end.


Z. Cw-e,7)-z«·.ck)-F Hexen-we EPHOÆIFZZIZ O2««g-c)2e-z«JzF.
We have in this specimen the first appearance of the tubular outgrowth, which, accorciing to previous authors, marks the beginning of the epiphysia It is somewhat important to fix the stage at which this outgrowth begins. Its very first appearance as an elevation of the roof of the thalamencephalon is in the stage represented in Fig. 36 after the saucersstage of the ear is past.


The recent studies have done much towards establishing a
basis for comparison between the epiphysial outgrowths in
different groups of anima1s. Beraneclg Francotte, and others
claim that the anterior epiphysial vesic1e—common1y designated the pineal eye — is formed independently, having no
generic connection with the posterior one, or epiphysis
Hill’s observations on the Teleosts coincide with this view.
Klinclcowström, on the other hand, claims that the anterior
one is formed at the expense of the posterior one, and Substantiates his conclusions with figures of Sections of 1guana,
showing the two vesiclesin union with one anothetc It is not
slifficult to conceive how the condition represented by Klinc572 Lock: [vo1..xI.


kowström might be brought about, and still not be inharmonious with the other observations. The two vesicles might, in
In Fig. 49 the posterior protuberance of the roof of the thalamencephalon is assuming a tubular form. It becomes the epiphysis The surface of the mid-b1«ain is now considerab1y changed. Two furrows have made their appearance, which divide the 1ateralwalls into three nearly equal 1obes. These do not show at all from the exterior before the removal of the epidermis and the mesoblast.
ear1ier stages than he represents, have been formed indepencL
ently, side by side, and thrown into communication by an
upward growth of the brain roof involving them both. Or,
indeed, there may be variations in the details of the formas
tions of these two vesic1es.


However this may be, the resu1ts of the different observers
all go to show conclusively that there are two (not counting
the paraphysis) distinct outgrossvths from the roof of the thalas
mencephalon of Petromyzon, Teleosts, and Lacerti1ia. The
fate of the anterior vesicle varies in the different groups, while
the posterior one persists in all as the epiphysis The anterior
one may be formed as a rudimentary organ of transitory existence, as in Teleostsz or it may be developed into the pineal
eye, in front of the epiphysis, as in Lacertilia


The sources of nervesupply to the two vesic1es have been
The gradual reduction of the thalamencephalon by compression between the Central hemispheres and the mid-brain is shown consecutively in Figs. 53, 54, II, If, and 6o. In Fig. 54, and 1ater, the epiphysis appears pear—shaped from the side, but from in front the enlargement on the distal end is seen to be broader than thick. It is borne upon a hol1ow sta1k.
made known in Cyc1ostomes, Teleosts, and Lacerti1ia, and
these facts are important in establishing homologies. It has
been shown (studniåka, Gaskelh KlinckowströrrO that the


upper capsule of Petromyzon receives its nerves from the


posterior commissure, and that the 1ower vesicle is innervated
Fig. 57 represents the brain of an embryo considerably older than in Fig. 56. The thalamencephalon is now very much compressed. The anterior half of it, which originally had a rounded protuberance growing from its roof, is now reduced to a small semicircular fold in front of the epiphysis.
from the left ganglion hebenu1a. Hi1l, in embryos of Te1eosts,
has traced a nerve from the posterior commissure into the
epipipzysis and has studied its distribution in detail in that
structure The anterior vesicle of Te1eosts disappears before
it has formed any nerve connection with the brain.
Klinckowström («9s) has observed very interesting conditions
in embryos of Iguana. In this anima1 he found in one case
three nerves distributed to the parietwpineal Organs: one
coming from behind and entering the epiphysis, and two
coming from right and left ganglion hebenula, respective1y,
and entering the pinea1 eye. He found the right and left
nerves in three different embryos of Iguana eighteen days old.
In one case the nerve from the left ganglion hebenula was
nearly the same size as that from the right, but in the other


two individuals the left nerves were much reduced l It should


be added that Klinckowstrom has found the nerve to the pineal
Fig. 59 is a view upon the epiphysis of the same embryo from directly in front. It was obtained by removing the cerebral 1obes, which lie in front of, and partly hide the epiphysia The epiphysis is seen to be composed of a Stall( and an en1arged dista1 extremity ; leading from the Stall( on either side are the epiphysialpedunc1es. In this hgure we are looking through the ventricle of the thalamencephalon into that of the midbrain. On the inside of the lateral walls of the opening are two enlargements, the beginnings of the thala1ni optici. These contain the ganglia hebenulae.
N0.3.] THE TXEÆTEBJPÄTE ABBE.  


eye in this animal to come normally from the right ganglion
In this specigmen the paraphysis had made its appearance as an outgrowth from the cerebrum. It is relatively late in arising, and is entirely distinct from any outgrowth from the thalamencephalon The lateral wal1s of the mid-brain are no 1onger10bed as in earlier stages. Fig. 58 is the same brain viewed from above ; we are looking directly into the cavity of the fourth ventricle.
hebenu1a, while in Petromyzon it comes from the ganglion of
the left Ziele. But, his discovery of the occasiona1 presence of
two nerves in Iguana, the left one being smallen affords an
interesting transition between the two. It also enables us to
account for the marked asymmetry in the ganglia hebenulæ
in Petromyzon, on the hypothesis that the right ganglion, in
Cyclostomes has, for some reason, ceased to have any nerve
connection with the pineal Organ, while the left ganglion has
retained its connection.


As Beraneck has shown, a nerve leading to the pineal eye
arises in front of the epiphysis, and has but a transitory existence


Putting the facts together and basing relationships on thenervesupply, and comparative study of the vesic1es, we may express
Fig. 6o shows the brain of a still older embryo. It is the largest embryo of squalus in my collection, measuring 35 mm. in length. The cerebral lobes are divided into right and left halves by a sha11ow furrow. From the back of the cerebrum rises the paraphysis (P); just behind this is seen the semicircu1ar fo1d, belonging to the ’tween-brain, and behind that is the epiphysis, rising above and resting upon the anterior wal1 of the mid-brain. Fig. 6I shows a front view of the cerebral lobes after being removed from the rest of the brain ; the paraphysis is also brought into view. Fig. 62 is the same brain with the cerebral lobes removed, and p1aced so as to give a ful1-face view on the epiphysisz the stalk is considerably longer than in Fig. 59.
the probable homologies in the following diagrams and tab1e :




PETRoMYzoN. TELEosTs. LACERT1LIA.
I have not had materia1 to work out the subsequent history of the epiphysis In specimens about six inches long it is situated in the wall of the cranium, directly over the cerebral lobes, and is connected to the roof of the ’tween-brain by a long threadJilce stallc The enlarged distal end is about the same size as in Fig. 62. cattie shows it in the adult with several capsules on the end of the stalk.
CUT 1 I.


I. Uppser vcsicle in Petkomyzon, epiphysis in Teleost and Lacerti1ia.
 
II. Lower vesicle in Petrotny2on, HiIPs anterior vesicle in Teleosts, pineal eye in laccrtilia
Sections of the stages just studied serve to coniirm the observations made from the outside. They do« not add very much to its features, speaking strictly morphologically The surface views have been prepared in such a way that they may stand for reconstruction, but they are more accurate than any actual reconstruction can be, as they show the relations entirely undisturbed.
IV. Nerve to posted-im« seminis-sure. XI. Nerve from lower vesicle in Petromyzon to Gan—
 
glion hebenuliy embryonic nerve of transitory existence in Lacertiliix
 
Figs. 119—124 show a series of sagittal Sections covering the period of the first appearance of the epiphysis, the reduction of the anterior part of the thalamencepha1on, and the beginning of the paraphysis and the choroid plexus.
 
 
Fig. I19 is a specimen of the same age as that shown in Fig. 48. It shows that the roof of the thalarnencephalon is raised into two nearly equa1 protuberances.
 
Fig. I20 shows the beginning of the reduction of the anterior of these elevations, and the increased growth of the posterior one. In Fig. 121 the posterior e1evation, or epiphysis, has become a tubular outgrowth, while the anterior one is reduced and depressed.
 
In Fig. 122 the tubular Stall( of the epiphysis is considerably elongatech while that part of the roof of the thalamencephalon in front of it has become reduced to a narrowfo1d that has been carried ventracL The paraphysis has now arisen from the roof of the prosencepha1on. The beginning of the choroid plexus extends from the structure down into the ventric1e of the fort-brain, and its posterior part is connected with the fold in front of the epiphysis The anterior and posterior Commissures are now distinguishab1e.
 
 
Figs. 123 and 124 exhibit snbstantially the same relations ; the choroid plexus has considerably increased in extent, and is rapidly becoming much convolutedz the brain vesicles have become reduced. In Fig. 124 the dista1 end of the epiphysis is inserted into a cavity in the roof of the cranial Wall, that is, the mesenchyme forms a cup over its rounded end.
 
===3. Comparisons between Epiphysial Outgrowths===
 
The recent studies have done much towards establishing a basis for comparison between the epiphysial outgrowths in different groups of anima1s. Beraneclg Francotte, and others claim that the anterior epiphysial vesic1e—common1y designated the pineal eye — is formed independently, having no generic connection with the posterior one, or epiphysis Hill’s observations on the Teleosts coincide with this view. Klinclcowström, on the other hand, claims that the anterior one is formed at the expense of the posterior one, and Substantiates his conclusions with figures of Sections of 1guana, showing the two vesiclesin union with one anothetc It is not slifficult to conceive how the condition represented by Klinckowström might be brought about, and still not be inharmonious with the other observations. The two vesicles might, in ear1ier stages than he represents, have been formed indepencL ently, side by side, and thrown into communication by an upward growth of the brain roof involving them both. Or, indeed, there may be variations in the details of the formas tions of these two vesic1es.
 
 
However this may be, the resu1ts of the different observers all go to show conclusively that there are two (not counting the paraphysis) distinct outgrossvths from the roof of the thalas mencephalon of Petromyzon, Teleosts, and Lacerti1ia. The fate of the anterior vesicle varies in the different groups, while the posterior one persists in all as the epiphysis The anterior one may be formed as a rudimentary organ of transitory existence, as in Teleostsz or it may be developed into the pineal eye, in front of the epiphysis, as in Lacertilia.
 
 
The sources of nervesupply to the two vesic1es have been made known in Cyc1ostomes, Teleosts, and Lacerti1ia, and these facts are important in establishing homologies. It has been shown (studniåka, Gaskelh KlinckowströrrO that the upper capsule of Petromyzon receives its nerves from the posterior commissure, and that the 1ower vesicle is innervated from the left ganglion hebenu1a. Hi1l, in embryos of Te1eosts, has traced a nerve from the posterior commissure into the epipipzysis and has studied its distribution in detail in that structure The anterior vesicle of Te1eosts disappears before it has formed any nerve connection with the brain. Klinckowström («9s) has observed very interesting conditions in embryos of Iguana. In this anima1 he found in one case three nerves distributed to the parietwpineal Organs: one coming from behind and entering the epiphysis, and two coming from right and left ganglion hebenula, respective1y, and entering the pinea1 eye. He found the right and left nerves in three different embryos of Iguana eighteen days old. In one case the nerve from the left ganglion hebenula was nearly the same size as that from the right, but in the other two individuals the left nerves were much reduced l It should be added that Klinckowstrom has found the nerve to the pineal eye in this animal to come normally from the right ganglion hebenu1a, while in Petromyzon it comes from the ganglion of the left Ziele. But, his discovery of the occasiona1 presence of two nerves in Iguana, the left one being smallen affords an interesting transition between the two. It also enables us to account for the marked asymmetry in the ganglia hebenulæ in Petromyzon, on the hypothesis that the right ganglion, in Cyclostomes has, for some reason, ceased to have any nerve connection with the pineal Organ, while the left ganglion has retained its connection.
 
 
As Beraneck has shown, a nerve leading to the pineal eye arises in front of the epiphysis, and has but a transitory existence.
 
Putting the facts together and basing relationships on thenervesupply, and comparative study of the vesic1es, we may express the probable homologies in the following diagrams and tab1e :
 
 
I. Uppser vcsicle in Petkomyzon, epiphysis in Teleost and Lacerti1ia. II. Lower vesicle in Petrotny2on, HiIPs anterior vesicle in Teleosts, pineal eye in laccrtilia IV. Nerve to posted-im« seminis-sure. XI. Nerve from lower vesicle in Petromyzon to Ganglion hebenuliy embryonic nerve of transitory existence in Lacertiliix


Z.P. «Zirbelpolster."
Z.P. «Zirbelpolster."


s I
+++Reformat table below+++
PETRoMYzoN. THE-USE. . LACERTILIA.
I H . . . «
Posterior epiphysial Is the superior Is the epiphysis Is the epiphysis
vesicle of I-Ii1l. vesicle. Nerve- with nerve con- net-ve- s u p pl y


niislca. « posterior com— posterior com- commissure.
s I PETRoMYzoN. THE-USE. . LACERTILIA. I H . . . « Posterior epiphysial Is the superior Is the epiphysis Is the epiphysis vesicle of I-Ii1l. vesicle. Nerve- with nerve con- net-ve- s u p pl y
Epiphysis of others. [ missure. missure.
 
niislca. « posterior com— posterior com- commissure. Epiphysis of others. [ missure. missure.


Pinealorgan of stucl- I. 1 supply from necting it to from posterior
Pinealorgan of stucl- I. 1 supply from necting it to from posterior
Line 1,145: Line 339:
Parapineal Organ of Ell. groups, but with a nerve
Parapineal Organ of Ell. groups, but with a nerve


studniislca. j l aborts. which early cle[ generates
studniislca. j l aborts. which early cle[ generates  
5 74 LOCR [VOL. XI.
 
 
If these comparisons are well taken, we have in the Fette— myzon, the epiphysis in its highest state of development Its distal end is enlarged into an eye—like capsu1e that is more differentiated than their inferior vesicle which represents the pinea1 eye. This is the reverse of what is true in most Lacertilia, and, on this account the upper capsule of Petrornyzon has usually been eompared with the pineal eye of Reptilia and Amphibia But the fact that the superior capsule in Petrornyzon receives its nervesupply from the posterior commissure, corresponding in this particular with the epiphysis of other animals, has much more weight in determining its homo1ogies than any purely structural resemblances We are on a better foundation, therefore, in comparing the upper capsule of Cyclostomes to the epiphysis of other animals, than in taking the other Position. It may be noted, in passing, that the superior capsu1e is in the same relative position as the epiphysis, and the inferior one corresponds in position with the pineal eye.
 
 
This view would make the epiphysis eye—like in Character, and there are many facts that weigh in that direction. Hil1’s studies speak strongly for the visual nature of the epiphysis He shows a high degree of differentiation in the nerve (:ells to which the nerve from the posterior commissure is distributed. There is also formed in Iguana, Plica, and Phrynosorna an eyelike enlargement at the distal extremity of the epiphysis.


If these comparisons are well taken, we have in the Fette—
Between the condition in Petrornyzon and that in the Lacertilia many gradations have already been brought to light.
myzon, the epiphysis in its highest state of development Its
distal end is enlarged into an eye—like capsu1e that is more
differentiated than their inferior vesicle which represents the
pinea1 eye. This is the reverse of what is true in most
Lacertilia, and, on this account the upper capsule of Petrornyzon has usually been eompared with the pineal eye of
Reptilia and Amphibia But the fact that the superior capsule
in Petrornyzon receives its nervesupply from the posterior
commissure, corresponding in this particular with the epiphysis
of other animals, has much more weight in determining its
homo1ogies than any purely structural resemblances We are
on a better foundation, therefore, in comparing the upper
capsule of Cyclostomes to the epiphysis of other animals, than
in taking the other Position. It may be noted, in passing, that
the superior capsu1e is in the same relative position as the
epiphysis, and the inferior one corresponds in position with the
pineal eye.


This view would make the epiphysis eye—like in Character,
In Teleosts, the anterior vesicle arises, as in the other groups, but it is transitory, ancl soon degenerates It would appear from a recent paper by Ritter, that the anterior vesicle in Phrynosoma is more persistent than in Teleosts, but does not reach the grade of differentiation exhibited in the Lacertilia. He gives a Hgure showing the rare occurrence of the anterior vesicle in an adult Phrynosoma, and although large this capsule is not so highly differentiated as the capsule behind it, but to compensate for this the distal end of the epiphysis is highly developed.t· ln other korms of Lacertilia,  the anterior vesicle reaches a high grade of perfection, while the epiphysis is not well developed. In most cases, however, there is a deposit of pigment in the distal Position of the epiphysis. The nerve distribution in that organ is known only in the Teleosts.
and there are many facts that weigh in that direction. Hil1’s
studies speak strongly for the visual nature of the epiphysis
He shows a high degree of differentiation in the nerve (:ells to
which the nerve from the posterior commissure is distributed.
There is also formed in Iguana, Plica, and Phrynosorna an eyelike enlargement at the distal extremity of the epiphysis.


Between the condition in Petrornyzon and that in the
Lacertilia many gradations have already been brought to
light


In Teleosts, the anterior vesicle arises, as in the other
We might carry our comparisons further and inquire whether the structure present in selachians is to be homo1ogized with the epiphysis or the pineal eye of other forms. The nerve relations are not known in the Se1achians and, theref0re, we have not, as yet, a satisfactory basis for comparison. In its structure and persistent attachment to the brain roof by a sta1k, the outgrowth in Se1achians resembles the epiphysis and one would be inc1ined to say that only the epiphysis is represented in the Elasmobranchs, and that the pineal eye is lacking. Klinckowström has offered the ingenious suggestion that in Se1achians and birds the second or anterior epiphysis is never ful1y separated from the posterior, and this is worth thinking about. There is no positive evidence to support it.
groups, but it is transitory, ancl soon degenerates It would
appear from a recent paper by Ritter, that the anterior vesicle
in Phrynosoma is more persistent than in Teleosts, but does
not reach the grade of differentiation exhibited in the Lacertilia. He gives a Hgure showing the rare occurrence of the
anterior vesicle in an adult Phrynosoma, and although large
this capsule is not so highly differentiated as the capsule
behind it, but to compensate for this the distal end of the
epiphysis is highly developed.t· ln other korms of Lacertilia,
No. 3.] THE PEXTEBJZH H; Erst-IF. 5 7 5


the anterior vesicle reaches a high grade of perfection, while
the epiphysis is not well developed. In most cases, however,
there is a deposit of pigment in the distal Position of the
epiphysis. The nerve distribution in that organ is known only
in the Teleosts.


We might carry our comparisons further and inquire
However the above question may be settled, it seems to me that the fundamental features of the morphology of the epiphysis are tolerably clear, and that future work on this subject will be main1y in the direction of ampliiication and discovery of details.
whether the structure present in selachians is to be homo1ogized with the epiphysis or the pineal eye of other forms. The
nerve relations are not known in the Se1achians and, theref0re,
we have not, as yet, a satisfactory basis for comparison. In its
structure and persistent attachment to the brain roof by a
sta1k, the outgrowth in Se1achians resembles the epiphysis and
one would be inc1ined to say that only the epiphysis is represented in the Elasmobranchs, and that the pineal eye is
lacking. Klinckowström has offered the ingenious suggestion
that in Se1achians and birds the second or anterior epiphysis is
never ful1y separated from the posterior, and this is worth
thinking about. There is no positive evidence to support it.


However the above question may be settled, it seems to
me that the fundamental features of the morphology of the
epiphysis are tolerably clear, and that future work on this
subject will be main1y in the direction of ampliiication and
discovery of details.


The invertebrate homologies of the pineal eye are not
The invertebrate homologies of the pineal eye are not known. Spencer supposed it to be homologous with the median eye of Tunicates, but that relation is a strained one.
known. Spencer supposed it to be homologous with the
median eye of Tunicates, but that relation is a strained one.


Various authors have compared the pineal outgrowths to the
ocelli of Arthropods Leydig compares them to the stemmata
of Insects, and shows that there are many resemblances to
support the comparison


Patten has recent1y studied the deve1opment of the eyes of
Various authors have compared the pineal outgrowths to the ocelli of Arthropods Leydig compares them to the stemmata of Insects, and shows that there are many resemblances to support the comparison.
Limulus, and argues for a homo1ogy between the median eyes
of these Arthropods and the pineal eye of Vertebrates He
shows the median eye of Limulus to. arise by the fusion of
paired eye-stallcs, giving us a case of undoubted union of originally paired Sense-Organs. I have c1aimed a similar occurrence
in the epiphysis of Squa1us. But it seems to me-that the
576 Lock: kvon XI.


evidence is too circumstantial at present to bear the interpretation that we already know the invertebrate homologue of the
vertebrate pinea1 Organ.


4. Tätig» Don-He Max-»r- oj II«- EPZPÆJFZF
Patten has recent1y studied the deve1opment of the eyes of Limulus, and argues for a homo1ogy between the median eyes of these Arthropods and the pineal eye of Vertebrates He shows the median eye of Limulus to. arise by the fusion of paired eye-stallcs, giving us a case of undoubted union of originally paired Sense-Organs. I have c1aimed a similar occurrence in the epiphysis of Squa1us. But it seems to me that the evidence is too circumstantial at present to bear the interpretation that we already know the invertebrate homologue of the vertebrate pinea1 Organ.


receives support from two sources. It is diflicult to interpret
===4. The Double Nature of the Epiphysis===
Klinckowströmk diScovery of a double nerve in Iguana on any
other hypothesis That interpretation is also strengthened by
the claim I have made of tracing the epiphysis in Se1achians
back to a pair of epithelial cups on the cepha1ic plate.


Hill (-94) regards the two vesicles he has discovered in
Receives support from two sources. It is diflicult to interpret Klinckowströmk diScovery of a double nerve in Iguana on any other hypothesis That interpretation is also strengthened by the claim I have made of tracing the epiphysis in Se1achians back to a pair of epithelial cups on the cepha1ic plate.
Teleosts as having been primitively side by side; and Ritter
(«94) has recently suggested that the epiphysis and the parapineal organ of Studniälca are right and left mates rather than
independent eyes of double origin. It will be remembered, in
this connectiorn that Klinckowström found three nerves in one
individual of Iguana, two from the ganglia hebenulae, and one
from the posterior commissure; that Leydig discovered two
accessory pineal organs in Anguis ; Duval and Kalt found as
many as three of these Organs, and I have found several pairs
of epithelial patches back of the eyes on the cephalic plate, and
have traced one pair into the epiphysis It seems to me that
all this evidence is more favorable to the idea that the pineal
Sense-Organs are multip1e, and individually of paired origin, than
to the idea expressed by Ritter.


s. Posapkysisx


considerab1e confusion has arisen in identifying the paraphysis in different forrns. As soon as it was made known that
Hill ('94) regards the two vesicles he has discovered in Teleosts as having been primitively side by side; and Ritter («94) has recently suggested that the epiphysis and the parapineal organ of Studniälca are right and left mates rather than independent eyes of double origin. It will be remembered, in this connectiorn that Klinckowström found three nerves in one individual of Iguana, two from the ganglia hebenulae, and one from the posterior commissure; that Leydig discovered two accessory pineal organs in Anguis ; Duval and Kalt found as many as three of these Organs, and I have found several pairs of epithelial patches back of the eyes on the cephalic plate, and have traced one pair into the epiphysis It seems to me that all this evidence is more favorable to the idea that the pineal Sense-Organs are multip1e, and individually of paired origin, than to the idea expressed by Ritter.
there are two outgrowths from the roof of the fore-brain, it
was at once assumed that the most anterior one is the paraphysis, and the posterior one the epiphysis But there is now
evidence that there are, at times, more than two outgrowths
Hill has shown in Amia the presence at one and the same time
of three tubular outgrowths from the roof of the fore-brain.
Two of these come from the thalamencephalom andthe anterior one arises from the prosencephalon He points out that
N0.3.] THE« IZEÆTEBJEÄTE JJEATZZ


the 1atter is the paraphysis proper, and that in Amia we have
===5. Paraphysis===
to deal with two other outgrowths from the roof of the thalamencepha1on. These he makes homo1ogous with the two
vesicles he has described in Teleosts. Hill concludes that in
the 1atter group the paraphysis is lacking on account of the
nomdevelopment of the choroid plexus This identification is
in harmony with the original description of the paraphysis
Francotte, who described it in 188 J, and Selenka (-90), to whom
the credit of its discovery is usually accorded, both state that
it arises from the prosencephal0n. Selenka says: « Wie das
Zwischenhirn seine Epiphyse, so hat das Vorderhirn seine
Paraphyse.« Francotte suggested that it represents the beginning of the choroid plexus p


Minot (-92), p. 69o, designates the anterior vesicle discovered
Considerab1e confusion has arisen in identifying the paraphysis in different forrns. As soon as it was made known that there are two outgrowths from the roof of the fore-brain, it was at once assumed that the most anterior one is the paraphysis, and the posterior one the epiphysis But there is now evidence that there are, at times, more than two outgrowths Hill has shown in Amia the presence at one and the same time of three tubular outgrowths from the roof of the fore-brain. Two of these come from the thalamencephalom andthe anterior one arises from the prosencephalon He points out that the 1atter is the paraphysis proper, and that in Amia we have to deal with two other outgrowths from the roof of the thalamencepha1on. These he makes homo1ogous with the two vesicles he has described in Teleosts. Hill concludes that in the 1atter group the paraphysis is lacking on account of the nomdevelopment of the choroid plexus This identification is in harmony with the original description of the paraphysis Francotte, who described it in 188 J, and Selenka (-90), to whom the credit of its discovery is usually accorded, both state that it arises from the prosencephal0n. Selenka says: « Wie das Zwischenhirn seine Epiphyse, so hat das Vorderhirn seine Paraphyse.« Francotte suggested that it represents the beginning of the choroid plexus.
by Hill (-91) in Coregonus the «paraphysis," but the Structure in question arises from the roof of the thalamencepha1on,
and between it and the prosencephalon is a marked downward
fold in the brain-wal1, which in many other forms is present
behind the paraphysis. It is more probable, as Hill suggests
in his1ater paper («94), that the paraphysis is lacking in the
Te1eosts.


It is to be understood that in some forms there are two
epiphysia1 outgrowths from the thalamencepha1on that are
entirely independent of the paraphysis When the latter Structure is present in conjunction with the former two, as in Amia,
there are then three separate outgrowths from the roof of the
fore-brain.


As already indicated in this paper, the paraphysis is present
Minot ('92), p. 69o, designates the anterior vesicle discovered by Hill (-91) in Coregonus the «paraphysis," but the Structure in question arises from the roof of the thalamencepha1on, and between it and the prosencephalon is a marked downward fold in the brain-wal1, which in many other forms is present behind the paraphysis. It is more probable, as Hill suggests in his1ater paper («94), that the paraphysis is lacking in the Te1eosts.
in Squalus as an outgrowth from the prosencephalon, and is
connected with the choroid plexus


IV. THE BEcnNNmG oF THE AUDrroRY Gras-IN.


The formation of the auditory saucer is preceded by a thickening of the epidermis along the sides of the head in the
It is to be understood that in some forms there are two epiphysia1 outgrowths from the thalamencepha1on that are entirely independent of the paraphysis When the latter Structure is present in conjunction with the former two, as in Amia, there are then three separate outgrowths from the roof of the fore-brain.


auditory region. The thickening portion occupies a larger area »


than that used by the ear vesicle when it is Erst formed, and it
As already indicated in this paper, the paraphysis is present in Squalus as an outgrowth from the prosencephalon, and is connected with the choroid plexus.
is coniiuent with the epidermal thickening just above the
578 Lock. » [Vor«. XL


branchiae As Mitrophanow has pointed out, this thickenecl
==IV. The Beginning of the Auditory Organ==
patch of epithelium is composite in character; it embraces not
only the epithelium for the formation of the auditory plate, but
that to form the so—called branchial Sense-organs, and also the
beginning of the lateral line. I had noted this relationship in
Squa1us before seeing Mitrophanoufs paper, and 1 lind no com—
parison that my resu1ts correspond —— so far as the earlier Stages
are concerned—very closely with his. He has, however, carried his studies considerably farther, and has shown how the
different branches of the lateral-line System arise.


The general relationship noted between the auditory plate,
The formation of the auditory saucer is preceded by a thickening of the epidermis along the sides of the head in the auditory region. The thickening portion occupies a larger area than that used by the ear vesicle when it is Erst formed, and it is coniiuent with the epidermal thickening just above the branchiae As Mitrophanow has pointed out, this thickened patch of epithelium is composite in character; it embraces not only the epithelium for the formation of the auditory plate, but that to form the so—called branchial Sense-organs, and also the beginning of the lateral line. I had noted this relationship in Squa1us before seeing Mitrophanoufs paper, and 1 lind no com— parison that my resu1ts correspond —— so far as the earlier Stages are concerned—very closely with his. He has, however, carried his studies considerably farther, and has shown how the different branches of the lateral-line System arise.
the branchia1 Sense-organs, and the lateral line is very similar
to what Wilson found in sea bass. In that form the Connection between ear, branchial Organs, and lateral line is evidently
more c1ear on account of the presence of a distinct sensory
furrow and the way in which the separation into three parts
takes place.


The general thickening in the sharks is not so well circumscribed. The fact that these different organs proceed from a
common epithelial thickening indicates relationship of a fundamental kind. After separation the ear gives further evidence
of its relationship with the lateral organs by preserving its
canal (endolyrnphatic duct) connection with the exterior and
developing in a manner that is characteristic of the canal organs
of the lateral line.


Mitraphanow departs from the usual point of view that the
The general relationship noted between the auditory plate, the branchia1 Sense-organs, and the lateral line is very similar to what Wilson found in sea bass. In that form the Connection between ear, branchial Organs, and lateral line is evidently more c1ear on account of the presence of a distinct sensory furrow and the way in which the separation into three parts takes place.
organs of the lateral line are metarnericz and in that particular,
I think, I should be inclined to follow him.


The auditory plate is at first differentiated from the general
thickening, and its epithelium then becomes gradually rounded
up into a circu1ar area, that is depressed in the centre—the
auditory saucer. This structure sometimes covers the space
of three neuromeres, and I have one surface preparation in
which the outer epithelium shows three bars running vertically
across it; these bars correspond with the underlying neuromeres.


We may interpret these superficial bars either as being
The general thickening in the sharks is not so well circumscribed. The fact that these different organs proceed from a common epithelial thickening indicates relationship of a fundamental kind. After separation the ear gives further evidence of its relationship with the lateral organs by preserving its canal (endolyrnphatic duct) connection with the exterior and developing in a manner that is characteristic of the canal organs of the lateral line.
moulded upon the underlying neural segments, or as being
No. 3.] THE IXEJBIEBJZAIIE READ. 579


due to the same general cause as the neura1 segments I am
of the opinion that the latter alternative is the correct one.


When iirst formed, the auditory saucer is opposite the ninth
Mitraphanow departs from the usual point of view that the organs of the lateral line are metarnericz and in that particular, I think, I should be inclined to follow him.
neuromerez but subsequent1y, while it is still in the saucer
condition, it shifts backwards, and cornes to lie at the side of
the tenth neural segrnenh and later still, as a capsule, it lies
opposite the eleventh neuromere


The history of the auditory vesicle in sharks has been worked
out beyond this period by Ayers (-92), and it is very clear from
its mode of growth that it is directly related to the canal organs
of the lateral 1ine.


A consideration of the so—ca11ed branchia1 Sense-Organs and
The auditory plate is at first differentiated from the general thickening, and its epithelium then becomes gradually rounded up into a circu1ar area, that is depressed in the centre—the auditory saucer. This structure sometimes covers the space of three neuromeres, and I have one surface preparation in which the outer epithelium shows three bars running vertically across it; these bars correspond with the underlying neuromeres.
their ganglia is reserved to be published later with the part on
the Nerves


NOT-E. ——I have been indebted to the persons mentioned below for material that has helped jill up the gaps in rny collection of embryos, and I
desire here to express Iny appreciation of their lcindness


To Miss Julia B. platt, for the loan of Sections of Acanthias ; to Dr. E.
We may interpret these superficial bars either as being moulded upon the underlying neural segments, or as being due to the same general cause as the neura1 segments I am of the opinion that the latter alternative is the correct one.
L. Mark, for young stages of Squalus ; to Mr. F rank smith, for a considerable number of embryos ; to Professor J. E. Reighard for the loan and
free use of his entire collection of elasmobranch embryos; to Professor
A. D. Morrill, for early stages of squalus


LAKE FoREsT, ILL1No1s,
January, I895.
580


’8s
When iirst formed, the auditory saucer is opposite the ninth neuromerez but subsequent1y, while it is still in the saucer condition, it shifts backwards, and cornes to lie at the side of the tenth neural segrnenh and later still, as a capsule, it lies opposite the eleventh neuromere.
 
 
The history of the auditory vesicle in sharks has been worked out beyond this period by Ayers (-92), and it is very clear from its mode of growth that it is directly related to the canal organs of the lateral 1ine.
 
 
A consideration of the so—ca11ed branchia1 Sense-Organs and their ganglia is reserved to be published later with the part on the Nerves.
 
:Note. — I have been indebted to the persons mentioned below for material that has helped jill up the gaps in rny collection of embryos, and I desire here to express Iny appreciation of their Kindness.
 
:To Miss Julia B. platt, for the loan of Sections of Acanthias ; to Dr. E. L. Mark, for young stages of Squalus ; to Mr. F rank smith, for a considerable number of embryos ; to Professor J. E. Reighard for the loan and free use of his entire collection of elasmobranch embryos; to Professor A. D. Morrill, for early stages of Squalus.
 
Lake Forest, Illino1s,
 
January, I895.
 
==Literature==


«84
«84 ’88
’88


’92
’92


’28
’28 ’85
’85


’88
’88


’92
’92 ’84
’84


«87a
«87a ’87b
’87b


’92
’92
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LITERATURE
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Eint-Bonn, F . Ueber die Segmentation des Wirbelthier—I(örpe1-s.
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ALLE, E. P., Jr. The Anatomy and Development of the Lateral Line
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AYERS Hei-Man. Vertebrate Cephalogenesis Il. A contribution
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to the Morphology of the Vertebrate Ear, with a Reconsideration
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BEARIV JOHN. The System of Branchia1 Sense-Organs and their
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KUPFFER, C. vors. Primäre Metamerie des N euralohrs der Vertebraten. Eis-Z. ei. Aktien! a» XII-wägen, PJx·)-.r.-Mk2z·-E. Cz, vom
KUPFFER, C. vors. Primäre Metamerie des N euralohrs der Vertebraten. Eis-Z. ei. Aktien! a» XII-wägen, PJx·)-.r.-Mk2z·-E. Cz, vom F. December.
F. December.


Passe-»F.
Passe-»F. 5 8 2 «91 see -94 seo 298
5 8 2
«91
see
-94
seo
298


VIII!
VIII!


’94b
’94b ’94o ’90 78 ’81 ’82 «« "92 PS
’94o
’90
78
’81
’82
««
"92
PS


VI!
VI!


Ja?
Ja? see
see


’88
’88
Line 1,611: Line 592:
Lock( krank. XI.
Lock( krank. XI.


Kur-Eisen, C. vors. Die Entwickelung des Kopfnerven der Vertebraten.
Kur-Eisen, C. vors. Die Entwickelung des Kopfnerven der Vertebraten. Kerkers-XX. X. Heer. Geer-MAX. München.
Kerkers-XX. X. Heer. Geer-MAX. München.


KUPFFEIIH c. vors. Studien der vergleichenden Entwickelungsgeschichte des Kopfes der Kranioten Heft t.
KUPFFEIIH c. vors. Studien der vergleichenden Entwickelungsgeschichte des Kopfes der Kranioten Heft t.
Line 1,618: Line 598:
Kopf-THE, c. von. Ibid. Heft 2. München und Leipzig.
Kopf-THE, c. von. Ibid. Heft 2. München und Leipzig.


Las-»Die, F Im. D·as Parietalorgan der Amphibien und Reptilien
Las-»Die, F Im. D·as Parietalorgan der Amphibien und Reptilien Anatomisclkhistologische Untersuchung. Erkennt-II. as. Fee-take«åeyg. Mai-»F( GFXFZZFCL XVI, Z. S. 44I-55I.
Anatomisclkhistologische Untersuchung. Erkennt-II. as. Fee-take«åeyg. Mai-»F( GFXFZZFCL XVI, Z. S. 44I-55I.


L0cY, WM. A. The Derivation of the Pineal Eye. -4-.c2r.«4«ae-Jgey.
L0cY, WM. A. The Derivation of the Pineal Eye. -4-.c2r.«4«ae-Jgey. ix, S. I69—-I8o, F iigs. Nachtrag zu diesem Aufsatze Anat Ase-Zeiger. ist, S. 23I—232.
ix, S. I69—-I8o, F iigs. Nachtrag zu diesem Aufsatze Anat
Ase-Zeiger. ist, S. 23I—232.


L0cY, Wirt. A. The Optik: Vesieles of Elasmobranchs and their Serial
L0cY, Wirt. A. The Optik: Vesieles of Elasmobranchs and their Serial Relation to 0ther Structur-es on the cephalic Plate. · Ins-m. ej May-»Ja. Vol. ix, pp. II5—-I22, 6 tigs.
Relation to 0ther Structur-es on the cephalic Plate. · Ins-m. ej
May-»Ja. Vol. ix, pp. II5—-I22, 6 tigs.


L0cY, WM. A. Metameric Segmentation in the Medullary Folds
L0cY, WM. A. Metameric Segmentation in the Medullary Folds and Embryonic Rim. Erz-«. Lege-Yes. IX, S. 393—-41F, II läge. LocY, Wm. A. The Mid-Brain and the Accessory Optic Vesicles.
and Embryonic Rim. Erz-«. Lege-Yes. IX, S. 393—-41F, II läge.
LocY, Wm. A. The Mid-Brain and the Accessory Optic Vesicles.


Hans-«. Exzesse-«. ix, S. 486—-488.
Hans-«. Exzesse-«. ix, S. 486—-488. MCCLURFY CHARLES The Segmentation of the primitive Vertebrate Brain. Jene-». ej« Alex-Hefe. its, pp. 35—56, I pl.
MCCLURFY CHARLES The Segmentation of the primitive Vertebrate
Brain. Jene-». ej« Alex-Hefe. its, pp. 35—56, I pl.


Maximal-L, A. M. Development of the cranial N erves in the chielin
Maximal-L, A. M. Development of the cranial N erves in the chielin Leser. Jena«-». Wiss. sc. xviii.
Leser. Jena«-». Wiss. sc. xviii.


Magst-ihn, A. M. Observations (with W. B. Spencer) on the
Magst-ihn, A. M. Observations (with W. B. Spencer) on the cranial Nerves of sag-Eines. »Es-as. »Ja-»He. ej· Wiss. sc. xxin
cranial Nerves of sag-Eines. »Es-as. »Ja-»He. ej· Wiss. sc. xxin


Marien-ihr» A. M. The Segmental Value of the cranial Nerven
Marien-ihr» A. M. The Segmental Value of the cranial Nerven Jene-». DIE-mit. ais-d EVEN. Vol. XVI.
Jene-». DIE-mit. ais-d EVEN. Vol. XVI.


MIHALxov1cs, Entwielcelungsgeschiehte des Gehirns. Nach Unter—
MIHALxov1cs, Entwielcelungsgeschiehte des Gehirns. Nach Unter— suchungen an höheren Wirbelthieren und den Menschen. Leipzig.
suchungen an höheren Wirbelthieren und den Menschen.
Leipzig.


Mucor, CHARLES S. Human Ernbryo1ogy. New York.
Mucor, CHARLES S. Human Ernbryo1ogy. New York.


MITnoPIIAN0w, Paul» Etude embryogenique sur les Selaciena
MITnoPIIAN0w, Paul» Etude embryogenique sur les Selaciena Zins-E. «? Zeig! Enge. T. i, pp, 16t—22I. ·
Zins-E. «? Zeig! Enge. T. i, pp, 16t—22I. ·


PATTEJHH WILLIAIW On the Morphology and Physiology of the
PATTEJHH WILLIAIW On the Morphology and Physiology of the Brain and Sense-Organs of Zier-Max. gern-«. Jan-». Mit-«. sc. muss, pp. I—96.
Brain and Sense-Organs of Zier-Max. gern-«. Jan-». Mit-«. sc.
muss, pp. I—96.


0RR, H. A contribution to the Embryology of the Li2ard. Jene-».
0RR, H. A contribution to the Embryology of the Li2ard. Jene-». cjMeyjblk Vol. I.
cjMeyjblk Vol. I.


Ost-man, HENRY F. A contribution to the Internal Structure of the
Ost-man, HENRY F. A contribution to the Internal Structure of the Arnphibian Brain. jin-www. ej« May-pl« ii, pp. 5I—96.
Arnphibian Brain. jin-www. ej« May-pl« ii, pp. 5I—96.


0WsJANNIKOw, P. H. Ueber das dritte Auge bei Paxsnwjawzjkixøsiizs
0WsJANNIKOw, P. H. Ueber das dritte Auge bei Paxsnwjawzjkixøsiizs Ziizsssx nebst einigen Bemerkungen über dasselbe Organ bei anderen Thieren. Mär-z. cis.- Zhslcad list-Käf. n?- Fz«.-Pe’xey.5e3o»yg. xxxvh p. at, I Taf. ·
Ziizsssx nebst einigen Bemerkungen über dasselbe Organ bei anderen
Thieren. Mär-z. cis.- Zhslcad list-Käf. n?- Fz«.-Pe’xey.5e3o»yg. xxxvh p.
at, I Taf. ·


Faktor-kann, S. A. La glande pineale et le troisidme oeil des Ver—
Faktor-kann, S. A. La glande pineale et le troisidme oeil des Ver— tebres These. Paris, I887. No. 3.] THE FEÆTEBÆÄ IJE III-TO. 583
tebres These. Paris, I887.
No. 3.] THE FEÆTEBÆÄ IJE III-TO. 583


’9s EIN-ZEIT, A. sur Pæil parietal access-one. XIV-er. Away-Zeiss. ix.
’9s EIN-ZEIT, A. sur Pæil parietal access-one. XIV-er. Away-Zeiss. ix. S. los-I I2.
S. los-I I2.


«94 PRIZNANT, A. Les yeux parietaux accessoires dDslwgzxix jsczgz·!2«.s.
«94 PRIZNANT, A. Les yeux parietaux accessoires dDslwgzxix jsczgz·!2«.s. BZZZZQJFXEPJL XII-Cz. No. H, N ov.-Dec., pp. I—7.
BZZZZQJFXEPJL XII-Cz. No. H, N ov.-Dec., pp. I—7.


’89 PLATL JULIA B. studies on the Primitive Axial segmentation of
’89 PLATL JULIA B. studies on the Primitive Axial segmentation of the Chick. FULL. XII-«. case-P. Zoäz Vol. xvii, pp. I7I—I9o, 2 p1s.
the Chick. FULL. XII-«. case-P. Zoäz Vol. xvii, pp. I7I—I9o, 2 p1s.


’8s RAE-L, C. Bemerkungen iiber die segmentierung des Hirns. Zcswk
’8s RAE-L, C. Bemerkungen iiber die segmentierung des Hirns. Zcswk -4-.-«.28-Jg.e-«.
-4-.-«.28-Jg.e-«.


’50 REMMZ Untersuchungen iiber die Entwickelung der Wirbelthiere.
’50 REMMZ Untersuchungen iiber die Entwickelung der Wirbelthiere. Berlin, 18Fo—-5F.
Berlin, 18Fo—-5F.


VI. RITTER, W. E. The Parietal Eye in Some Lizards from the Western
VI. RITTER, W. E. The Parietal Eye in Some Lizards from the Western United States. BUT-IX. Markt. Ost-»F. XVI. Vol. XX, No. Z, 4 pIs.
United States. BUT-IX. Markt. Ost-»F. XVI. Vol. XX, No. Z, 4 pIs.


«94- RITTER, W. E. On the Presence of a Parapineal Organ in Pfzyymck
«94- RITTER, W. E. On the Presence of a Parapineal Organ in Pfzyymck Eos-m. XIV-XX. Eises-Xexes. irr, S. 766—772, I Hg.
Eos-m. XIV-XX. Eises-Xexes. irr, S. 766—772, I Hg.


’85 RYDER, JOHN. On the F ormation of the Embryonic Axis of the
’85 RYDER, JOHN. On the F ormation of the Embryonic Axis of the Teleostean Embryo by the concrescence of the Rim of the Blastos derm. ANY. Max. sei-c, pp. 614—6I s, I jig.
Teleostean Embryo by the concrescence of the Rim of the Blastos
derm. ANY. Max. sei-c, pp. 614—6I s, I jig.


«87 Sc0TT, W. B. The Embryology of Pciskiwkygow Jomw ojxlsosplk
«87 Sc0TT, W. B. The Embryology of Pciskiwkygow Jomw ojxlsosplk Vol. i, No. 2.
Vol. i, No. 2.


’90 SELENKA, E. Das Stirnorgan der Wirbelthieren Bis-Z. Texts-EIN.
’90 SELENKA, E. Das Stirnorgan der Wirbelthieren Bis-Z. Texts-EIN. Bd. X.
Bd. X.


’86 SHIPLEIL 0n Some Points in the Development of Pexyomjzosx
’86 SHIPLEIL 0n Some Points in the Development of Pexyomjzosx JE"««z-z·czxz«ZeTr. gis-m: »Am-». Mehr. sc. xxv.
JE"««z-z·czxz«ZeTr. gis-m: »Am-». Mehr. sc. xxv.


"’94 SORENSEIV A. D. comparative Study of the Epiphysis and Roof of
"’94 SORENSEIV A. D. comparative Study of the Epiphysis and Roof of the Diencephaloth Jan-»O Cis-»F. New-CI. Vol. its, pp. I2-72; I53—I7o.
the Diencephaloth Jan-»O Cis-»F. New-CI. Vol. its, pp. I2-72;
I53—I7o.


’86 SPENCER, W. B. On the Presence and structure of the Pineal Eye
’86 SPENCER, W. B. On the Presence and structure of the Pineal Eye in Lcxc--z«-·Zx"cx. Lichts. Joixrw Miit-c sc. Vol. Jst-ev, p. 239.
in Lcxc--z«-·Zx"cx. Lichts. Joixrw Miit-c sc. Vol. Jst-ev, p. 239.


’88 STRAHL und MARTIKL Die Entwickelung des Parietalauges bei
’88 STRAHL und MARTIKL Die Entwickelung des Parietalauges bei ÄAFYFJJTAFZFF und Lerci-»Sk- TJFQØEJW Ast-E. J. xljmt «. Ewkza S. I46—I6I, I Taf.
ÄAFYFJJTAFZFF und Lerci-»Sk- TJFQØEJW Ast-E. J. xljmt «. Ewkza
S. I46—I6I, I Taf.


’82 VAN WIJHE. Ueber die Mesodermsegmente und die Entwickelung
’82 VAN WIJHE. Ueber die Mesodermsegmente und die Entwickelung der· Nerven des Selachier-Kopfes. Mai. Feffzcxxrizl X. Eis-Es. Läg-Z. -Hi--zsz«a«icwx. xxii.
der· Nerven des Selachier-Kopfes. Mai. Feffzcxxrizl X. Eis-Es. Läg-Z.
-Hi--zsz«a«icwx. xxii.


’92 WATERS B. H. Primitive segmentation of the Vertebrate Brain.
’92 WATERS B. H. Primitive segmentation of the Vertebrate Brain.


gzzxxjc Jus-IX. Miit-c sc. xxxiiih pp. 457—475, I pl.
gzzxxjc Jus-IX. Miit-c sc. xxxiiih pp. 457—475, I pl.
’84 Wann-ais, c. O. The External Morphology of the Leech. Preise. Hin-g. Ärmel. Er« am! sc. Vol. XX, No. F, pp. 76—87, I pl.


’84 Wann-ais, c. O. The External Morphology of the Leech. Preise.
’92 WHITMADH c. O. The Metamerism of cis-Preises. Fuss-HEXEN gar« sz·cöc»zzsgsrc» Geåicyxxzwge Ækcciozj lL«--»c-kæ-«!.s. S. 385-39F, 2 Taf.
Hin-g. Ärmel. Er« am! sc. Vol. XX, No. F, pp. 76—87, I pl.
 
’92 WHITMADH c. O. The Metamerism of cis-Preises. Fuss-HEXEN gar«
sz·cöc»zzsgsrc» Geåicyxxzwge Ækcciozj lL«--»c-kæ-«!.s. S. 385-39F, 2 Taf.


VI. ZIMMERMADL Ueber die Metarnerie des Wirbe1thierlcopfes. Prslxxrmiä
VI. ZIMMERMADL Ueber die Metarnerie des Wirbe1thierlcopfes. Prslxxrmiä


rä XIV-ex. Gereizt-Ä. F. VersamtnL München. S. 1o7-—It3.
rä XIV-ex. Gereizt-Ä. F. VersamtnL München. S. 1o7-—It3. 584 » Lock: kvop XI.
584 » Lock: kvop XI.
 
EXPLÄNATTON OF FIGURES
 
END-»m- May-Es.
 
Ue, F, H, etc. Metameric Segment. »Ja-r. wisse-blast.
 
XII« line drawn in front of the ante- Max. metameric Segment.
rior neuromere of the vagus d km. nasal epitheliutth
net-vie. OF. ». Optik: net-se.
 
tm. auditory vesicla ex. .k. optic stalk
acn ». accessory Optik: Ists-Ziele. pp. z« « optic vesicle.
 
«. s. anterior portion of alimentary F. paraphysis
can-Eil. 4 w. serial Sense-Organ.
 
cåä cerebe11um. III. tha1amencepha1on.
Mk. prosencephaloxx « Mel. ex. thalamus opticus.
Hei. epiphysis · New. 4 fourth ventric1e.
II. c. manclibulax head cavitzk Zp. Zirbe1po1ster.
ynä mid-brain.
N().3.] THE ZEÆTEBÆÄTE HEXE.
 
EXPLANATION OF PLATE XXVL
 
The ligures are from untouched negatives and are noteworthy in showing the
very early condition of the optic vesicle, the accessory vesicles and in some cases
the primitive rnetameric segrnenta They are all Photographs of spart-«« irre-träfe.and, with the exception of Fig. J, are X about 2o diameters
 
Fig. 1. PhotograPh of embryo between Balfour’s stages i? and c.
 
FIg. 2. somewhat older embryo. The embryonic rim on the left side shows
faint1y Some of the metameric segments (not reproduced by the artist).
 
F1g. Z. Slightly older embryo to show the forrnation of the heachplate and the
Central wedgeshaped Process thereon. This is the stage in which the neural
folds are started along the margins of the body. They are ventrally curved.
 
Fig. 4. stage with a rounded head-Plate when the optic vesicles first become
evident.
 
Fig. s. Another embryo, about the same age as the Preceding, in which the
optic vesic1es and one of the accessory vesicles are shown.
 
F1g. H. Somewhat older embryo showing the infolding for the optic vesicles
extending across the median Plane.
 
Fig. 7. Older embryo somewhat higher magnifred The headsplate is very
broad, the trunlc narrow. The neural folds of the head lie nearly in the horizontal
Plane. The rnetarneric segments show well on the left margin of the head-P1ate.
They exist in the earlier stages, but are very diiiicult to catch with the camera.
 
FIg. 8. speeirnen in which the dePressions for the optic vesicles show very
distinctly
 
FIgs. 9 and ro. Two ernbryos older (although smaller) than the preceding. In
both two Pairs of accessory optic vesicles are to be seen on the cephalic Plate
baclc of the Primary optic vesicles.
 
Fuss. II and Ia. Two specimens slightly older than the preceding two, seen
from different Point of observatiow
 
FIg. r Z. Embryo after the neural folds have begun to grow uPwards, seen
obliquely from the left sicle. Gives an external view of— the vesicles on one side
and an internal view of them on the oPPosite side of the neural folda
 
FIg. I4. Embryo from which Fig. 29, Plate XXV1I, is drawjx Shows metaIneric segments on the exPosed ventral surface of the neural folds.
 
FIg. I s. Embryo of same age as Fig. 13 and Fig. Hi, Plate XXVII. shows
metarneric segments on the neural folds in front of the eye vesiclea
 
FIg. ro. Embryo of the same age as Fig. 9 just above it. Seen in a Position
more favorable to bring out the accessory vesicles on the cephalic plate. The
optic vesicle on the right side shows as an external Protuberance.
 
Fig. r7. somewhat older embryo viewed obliquely from above. shows the
optic vesicle of the right side as an external rounded Protuberance, and that of
the left side from within as a cup. Behind the optic vesiele on the left side of
the cephalic place are four accessory vesiclea They show their serial reiation
with the Primary optic vesicle.
 
FIg. 18. specimen showing a large development of the central tongueislilte
Process. Embryo slightly older than that in Fig. 7.
586 Lock.
 
Fuss. 19 and 2o. Side view of two heads of embryos vvith open neural groove
to show the external appearance of the optic vesicle
 
FIG. 21. Embryo of same age as Fig. 13 viewed obliquely from above.
 
F1G. ge. Side view of embryo of the same age (with broadly open neural
gross-re) to show the external appearance of the optic vesicie and the other vesicles
behind it.
 
Mo. 23. Embryo after partie-l closure of the neural groove Showing two
openings, an anterior and a posterior one, into the neural canaL
 
FIG. 24. Some-what older specimen with head broken off. showing three
openings (sma1ler than those in the preceding embryo) into the neural canaL
.Jc«i».««Z M· xwxyfzofozgzy !«F2!..-l7.
 
THE-Fast. s« Dir» e.- s SICH-rege: Fmxcskfnnfål
THE-is. Lock. know.
DIE IJEJE III-Fa re« III-Hi D. 587
 
EXPLANATlON OF PLATE XXVII.
 
All the figures are of Fee-Z« cxcxxmbäiax ,- they are drawn with the aid of thecamera, and are all d( 45 diameters.
 
FIG. 2 s. Young embryo intermediate between Balfoufs stages B and C. The
embryo so far as formed is divided into eight pairs of metameres and these arecontinued without brealc, or any change in Character, into the halves of the
embryonic rim. The eleventh Inetamere which, in later stages, lies in kront of
the vagus nerve is now on either side the third one from the axial embryo.
 
Fig. 26. Somewhat older embryo, showing the change in form of the head
region. The axial embryo now includes about fifteen pairs of rnetamerea
 
Fig. 27. Slightly older than the precedingz there are about eighteen pairs of
rnetameres in the axial part of the embryo and,·as in the forrner instances, are
continued into the embryonic rim. Two longitudinal marginal furrows have
appeared, separating two marginal bands from the rest of the embryo. Along
the line of these furrows are seen four depressions that mark the very beginning
of segmental Sense-Organs. ·
 
FIG. as. View from the upper side of the same embryo illustrated in Fig. 29.
The cephalic plate is now clearly marked off from the more slender trunk region.
The depressions for the optic vesicles (c-P.) have made their appearancqz
 
F1G. 29. View of the same embryo from the ventral aspect. The yolk has
been cornpletely removed, and we get a view directly into the gastrular cavity.
There are eleven pairs of metameres in the broad part of the cephalic plate. The
neural folds are ventrally curved The outlines of the figure and the neural segments are too syrnrnetricaL
 
FIG. Ho. Older embryo with neural folds lying in the horizontal plane. The
broad cephalic plate is in marked contrast with the slender trank.
 
Fig. Hi. Embryo in which the neural folds have nearly attained the vertical
Plane. The neural groove is still open. The optic vesicle (qk).) and the combined
vesicle of mid—brain and accessory optic Eis-z. -l- A. tax-L) vesicle show on the sides
of the head. There is also the beginning of rnandibnlar head cavity El. c)
and the branchia1 pouch. The original Inetameric divisions are still very plain.
 
FIG. Ja. Embryo just after the closure of the neural groove in the anterior
end. The posterior part of the neural canal is not cornpletely closed. Note the
metarneric divisions indicated by numbers J, e, F, etc. ·
 
Fritz. 33. Embryo after complete closure of the neural groove and before the
appearance of the ear vesicle.
 
FI(;. 34. Embryo after the differentiation of the ear saucen The five anterior
metarneric divisions are no longer distinguishable, those of the hind-brain are
prominent and are approximatecl in the middle plane. 0ne gilbcleft has broken
through. The nasal pit has started. ·
 
Flci 35. slightly older embryo showing several characteristic changea The—
line of neural segments are being forced apart by lateralgrowth of the roof of the
hind-brain. The fifth nerve is plainly visib1e. Over the gillsclefts runs a continuous nodulated thiclcening containing the branchial Sense-Organs and the radirnent of the lateral 1ine.
588 Lock:
 
Flcsp 36. Some-what older embryo differing from the preceding mainly in show
ing the rudiments of the seventh, eighth ninth, and tenth next-es. Note the
lens and choroid Hssure in the eye vesic1e.
 
Eis. 37. Slightly older than the precedjng The line of neural segments are
undergoing some ehanges whereby the concavity on the lower margin is made to
correspond vvith a crest on the upper margitx In embryos of about the age represented in Fig. 36 or a very little older the epiphysial outgrowth arises from the
roof of the tha1amencepha1on.
lot-Aal of· Jløæzøleologg Ist-I.
590 LOCJT
 
EXPLANATION 0F PLATE XXVIIL
 
A series of partial dissections of embryos of syst-III« in such a way that the
brainswalls have been laid bare. .Figs. 46—49 X 45 diameters Figs. »so-so X io
diameters.
 
Fig. 38. Embryo with open neural groove. The epidermal layer and the
mesoderm have been removed from the sides of the brain-Wall; behind the optic
vesicle is seen a bilobed protuberance—t·he combined mid-brain vesicle and the
anterior accessory optic vesicle. ·
 
Fig. 39. 0lder embryo with neural canal partly formed. The specimen shovvs
the same condition of mid-brain vesicle and that of the anterior accessory optic
vesicle »
 
Fig. 4o. The braiwwalls of an embryo of the same age as that shown in Fig. Ja.
 
Fig. 4I. Embryo before dissectiom showing especially well the contours of the
brain-wal1s.
 
Fig. 42. The same embryo after exposure of the braiwwalls by dissection
The thalamencephalon is well exhibited · The mid-brain is bilobed.
 
Fig. 43. slcetch of partially dissected embryo just after the appearance of the
auditory vesicle.
 
Fig. 44. The exposed brainwvalls of an embryo slightly older than that represented in Fig. 34. ..
 
Fig. 45. Brain of embryo about same age as that in Fig. 35. The auditory
vesicle has been Ieft in Position. The midbrain is now indistinctly trilobed.
 
Fig. 46. Brainwvalls of embryo with the optic vesicle removed. About the
same age as the preceding
 
Fig. 47. Dissection of the brain of the embryo of which Fig. 36 is an external
view. The mid—brain is distinctly tri1obed. There are eight clearly marked segments in the hind-brain.
 
Fig. 48. Dissection of embryo slightly older than the one represented in
Fig. 37. The thalamencephalon is now dekinitely marked out by furrows ; it
bears upon its summit two rounded confluent pr0tuberances.
 
Fig. 49. Dissection of brain of embryo somewhat older than the preceding
The thalamencephalon is clearly defined. The posterior protuberance from its
roof has grown much faster than the anterior one. The former is the beginning
of epiphysis. There are nine neural segments in the hind-brain.
 
The embryos are now too large to represent advantageously on the same scale,
and in the following figures the scale of magniiication is reduced from 45 to to—
diameters
 
Fig. so. Shows embryo of same age as that represented in Fig. Ja.
 
Fig. Hi. Embryo of nearly the same age as that represented in Fig. 34.
 
Fig. se. Partly dissected embryo of about the same age as that represented in
Fig. 36 and again in Fig. 47.
 
Fig. II. Braiiikwalls of an embryo just older than that shown in Fig. 49.
 
Flgs. 54 and IF. Successively older embryos to show especially the changes in
the tha1amencephalon and the outgrowth from its roof of the epiphysis.
 
Fig. so. The same brain shown in Fig. IF, with the cerebral lobesreinoved
 
and turned so as to vievv directly against the epiphysia
»Jetzt-seit? of JXGXZJJZAZCJYV list-IT
J  THE-I»
 
si
!
 
« «« -..«
592 zociz
 
EXPLANATION OF PLATE XXDL
 
Figs 57—62 a continuation of the series of dissections shown on the two previous plates. All X io diametera Figs. 65—87 X 45 characters.
 
Fig. 57. Side view of brain of an older embryo than that slcetched in the fore—
going ügure The neural segments have now become ob1iterated, and the three
lobes of the mid—brain (secondary«divisions) are no Ionger distinguishable The
epiphysis is well developed ; in front of it is a semicircular fold of the braiwwallz
this structure is the remnant of the elevation which started in front of the epiphs
ysis on the roof of the thalamencephalon (see Figs. 48 and 49). It has become
rednced by compression between the rapidly growing adjacent brain regions. The
paraphysis is also visible in this drawing
 
Fig. 58. View of the same brain from above looking into the cavity of the
fonrth ventriclia
 
Fig. so. The same brain after removal of the cerebra1 lobes and arranged so
as to show the epiphysis in front view.
 
Fig. Ho. Brain of older embryo showing substantially the same features as the
preceding
 
Fig. 6I. The cerebral lobes of the same brain after removal. Note the
paraphysis arising from the posterior part of the roof of the prosencephaloiic
 
Fig. Ha. The same brain after removal of prosencephalon and arranged so as
to show to best advantage the epiphysis with its Stall-r.
 
Fig. 63. Horizontal Section of ernbryo shown in Fig. 25 to show the general
appearance of the metameres in Section. X so.
 
Fig. 64. Horizontal Section of embryo sketched in Fig. 26 showing metameres
in Section, and adjacent layer of mesodernn X Ho.
 
Fig. 6I. Section of heacl of Tasse-ob ocelzaro through the optic vesicles at the
time of their first appearance compare this with Zieglers’ Pl. IV, Fig. i93.
 
Fig-s. 66, 67, 68, 69, 7o. successive sagittal Sections of embryo just after
closure of the neural groove and prior to the appearance of the ear vesicle show
the primary for-obtain, the inid— and hind—brains, and three very prominent neural
segments of the hindsbrain The segments are the seventh, eighth, and ninth
respectivelyn When the ear is first differentiated it arises opposite the ninth
Segment.
 
Fig. 71. Deeper section of the same embryo showing a curved line of mesoss
derni over the mandibular cavity.
 
Fig. Je. Sagittal Section of ernbryo near the median plane after the formation
of the auditory saucer and the complete closure of the neural groove. The five
neural segments of the fore- and mickbrain are exhibited The second neural
Segment nearly coincides in Position with the neuropore
 
Fuss. 73, 74, 7 H. Three successive sagittal Sections of an embryo of about the
age of that drawn in Fig. 34. It is slightly older, shows well the neuromeres of
 
· the hind-brain. The ear capsule is in the space of the tenth neuromere. In front
 
of it in Figs. 73 and· 74 are seen the roots of the seventh and. eighth nett-es.
 
Fig-s. 76-87. Sagittal Sections of the specimen photographed as Fig. 17, Pl.
XXVL Giving evidence of a series of cupälike depressions on the neural plate of
headand trank. The series of these cupped areas is terrninated in front by the
optic vesicles 0n the cephalic plate they are relativelzss large and resemble the
primary optic vesicles in mode of origin and in structure How far the series
extends into the trank I have not been able to determine.
ji«-IS« Z  »M«k2.syzsgckssfx-jejy , «;
 
«. .-.«—-,x7s.«k.MPOMPO4
 
se«  ·- -.-. -.
H 94 LOC P.
 
EXPLANATION OF PLATE XXX.
 
Flog. 88—1 la. Twentystive transverse Sections of an embryo very slightly older
than that represented in Fig. I6. The Sections 88-1oo 1ie in the region of the
cephalic p1ate. The following Sections los-I 12 lie in the neck and trunlc regions
 
These Sections are remarlcable in bringing to Iight serial depressions along the
walls of the neural folds. They show that the serial cup—like differentiations
extend back of the cephalic plate. I have not been able to determine the number
of serial differentiations of this Character in the embryo, but it is clear there are
several pairs behind the cephalic plate upon which I have noted in surface study
four pairs in addition to the eyes. X 45 diameters.
 
[Figs. II 3——1 I s were drawn from nature by Miss Tanetta Gilleland Figs. 117
and 118 are talcen from Kupffer’s «Studien zur vergleichenden Entwickelungsgeschichte des Kopfes der KraniotenX and Fig. I 16 is after FroriepJ
 
FIG. Ir3. View of the head-end of embryo of xlmzhxxoma in the open neural
groove stage, X about Io diameters. shows segmental folds in the neural folds
 
and a srnooth neural plate.
 
Eis. rr4. View on the caudal extremity of the sarne embryo.
 
Fig. II s. Embryo of Kam: Pelz-sinks- showing large obvious segmental folds
in· the median plate and also srnaller fainter folds in the neural ridges. The latter
correspond to the segments in the neural ridges in -4-zäJ·J-.kzo»-a.
 
FIG. 116. Embryo of THE» cyisiazms after Froriep. showing large obvious
folds in the median plate with unsegmented neural ridges These median folds
 
probably correspond to those in Reiz« Feier-Iris and nof to the Inetameric divi—
 
isions in the neural folds.
 
Eis. I17. View on the head-end of Falæwamlsa »Im, according to Kupffen
showing segmental folds in the median plate but none in the neural ridges com
pare vvith Fig. II s.
 
Ho. Its. caudal end of same embryo.
 
Figs I 19-124 are sagittal Sections of syst-il« arg-Esset. X about 45 diameters.
 
Eis. I19. Section of head of embryo about same age as those represented in
Figs 37 and 48, showing the roof of the thalamencephalon raised into two eleva-tions. · ·
 
Fig. I2o. somewhat older ernbryo showing the tubulaklilre grovvth of the
.epiphysis. ·


pro. I21. still older stage showing reduction of the anterior part of roof of
thalamencephalon and great increase in depth of the furrow in front of the
thalamencephalotn


Fu; r22. 0lder stage in which the paraphysis has made its appearance from
{{Locy1895 TOC}}
sthe roof of the prosencephalotn The choroid plexus has also started.


Flcs 123 and 124. Two older stages showing the increase in length of the
{{Footer}}
epiphysis ; its distal end is somewhat enlarged and is inserted into a concavity in
the cranial roof. The paraphysis is indicated at pp. — The choroid plexus is considerably increased in extent and hangs into the cavity of the fore-brain.
III-«. .I7. Pf, . Akt.

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Locy WA.Contribution to the structure and development of the vertebrate head. (1895) J. Morphol. 11(3): 497-595.

Contents: General Introduction | Part I - Metamerism of the Head | Part II - The Sense-Organs | Figures
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This historic 1895 paper by Locy is an historic description of embryo head development.



Modern Notes: head | neural | sensory | hearing | vision | pineal



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Historic Embryology: 1906 Eye Embryology | 1907 Development Atlas | 1912 Eye Development | 1912 Nasolacrimal Duct | 1917 Extraocular Muscle | 1918 Grays Anatomy | 1921 Eye Development | 1922 Optic Primordia | 1925 Eyeball and optic nerve | 1925 Iris | 1927 Oculomotor | 1928 Human Retina | 1928 Retina | 1928 Hyaloid Canal | Historic Disclaimer


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Historic Embryology - Hearing 
Historic Embryology: 1880 Platypus cochlea | 1892 Vertebrate Ear | 1902 Development of Hearing | 1906 Membranous Labyrinth | 1910 Auditory Nerve | 1913 Tectorial Membrane | 1918 Human Embryo Otic Capsule | 1918 Cochlea | 1918 Grays Anatomy | 1922 Human Auricle | 1922 Otic Primordia | 1931 Internal Ear Scalae | 1932 Otic Capsule 1 | 1933 Otic Capsule 2 | 1936 Otic Capsule 3 | 1933 Endolymphatic Sac | 1934 Otic Vesicle | 1934 Membranous Labyrinth | 1934 External Ear | 1938 Stapes - 7 to 21 weeks | 1938 Stapes - Term to Adult | 1940 Stapes | 1942 Stapes - Embryo 6.7 to 50 mm | 1943 Stapes - Fetus 75 to 150 mm | 1946 Aquaductus cochleae and periotic (perilymphatic) duct | 1946 aquaeductus cochleae | 1948 Fissula ante fenestram | 1948 Stapes - Fetus 160 mm to term | 1959 Auditory Ossicles | 1963 Human Otocyst | Historic Disclaimer
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Part II - The Sense-Organs (1895)

The Sense-Organs of Vertebrates embrace all those structures that are endowed with Special sensibility They differ from one another mainly in degree of differentiation, and form a series, at the lower end of which are simple sensory papillæ, and at the upper end are complex organs like the eye and ear. We recognize two orders of Sense-Organs — simple generalized ones, the ganglionic Sense-Organs, and more specialized ones» belonging to the so—ca1led five senses.


The question of the origin and relationship of these sense-— Organs is full of interest. From the combined results of investigations on both Invertebrates and Vertebrates it seemsaltogether probable that the higher· Sense-Organs have been derived from those of a lower order, and, indeed, that they have all been differentiated from a common sensory basis, and are therefore related in a direct way.


This view has been entertained by morpho1ogists for upwards of ten years, and has a firm foothold in philosophical discussions. The evidence favoring such an interpretation has been accumulating, and although still incomplete, the hypothesis was never so well supported as at present.


Professor Whitmarks researches were among the first to bring out this conception. Ten years ago he demonstrated in 1eeches the genetic relation between eyes and tactile papil1ae, and since that time it has no longer been a matter of pure assumption to say that certain end-organs are modifications of a common sensory basis. He shows that the sensory papillæ located on each bodycking in the leeches have a very interesting relation. In the hinder part of the body the papillæ are purely tactile, but passing headward they become gradually modified, and are at first mixed visual and tactile, and tinally the anterior ten pairs are purely visual. We have thus had for upwards of ten years a welbauthenticated case of the derivation of Sense-Organs of a higher grade from those of a lower order.


In a more recent publication, The Metamerism of Clepsine", 1892, Professor Whitman has added to the def1niteness of his earlier discoveries by observations on a new species, ask-r» Clepsine hol1ensis. He shows by a comprehensive study of the elements of the nervous System, the nerve distribution, and the sensillæ the complete homodynamy of all the segments.

Regarding the relationship of the eyes to other metameric sense-organs, he says: "One more peculiarity of this species may be noticed The median rows of sensillæ are quite well developed as eyes, at least on segments IV, V, and VI. The visual elements in Segment IV are quite numerous, and they are placed in a pigment cup quite like that of the principal eyes, only thinner and sma1ler, and directed backward instead of forward. The sensi1la of Segment V is a little sma1ler, but still presents the same general features. In Segment VI the organ is still eye-lilce, butiits visual elements are fewer andthe pigment cup imperfectly represented by loose pigment. In Segment VII we find one or two visual eells and a little scat . teredpigment In the following segments we usually find the sensillae in about the. same condition. In no other species hitherto described do we find the sensillæ passing by such gradations into the principal eyes. The serial homology of these organs with the eyes is, then, a fact demonstrated not only by the embryological development, but also by the structural gradations exhibited in the adult animal" (pp. 39I, 392).


The existence of a series of rudimentary sense—organs in Vertebratos was brought to 1ight in 1885 by Froriep and Beard. While it is by no means clear that they are the homo1ogues of the sensory papillæ of anne1ids, it is nevertheless probab1e. It is now generally admitted that they constitute the basis from which the higher Sense-Organs of Vertebrates are derived. These organs are arranged in 1ongitudinal series. The series has become rudimentary or lost in the adult forms of higher existing Vertebrates, but they are present in the embryos Kupffer and others have shown that there are at least two series of these Organs, the upper one corresponding to the lateral line of comparative anatomy, and the 1ower one embrac— ing the so-ca11ed branchia1 Sense-Organs of Beard. From the upper series the ears and nose are probab1y derived, and possibly the eyes.


Allis (’88), in his well-known memoir on the lateral line of Amia, gives Hgures that show a connection between the epithelium of the nasal pit and that of the1ateral1ine. His Fig. I, P1. XXX, representing a larval form one day old, shows the nasal epithelium forming part of the lateral line. In subsequent growth it follows the same course that the surface organs of the lateral System do when they become canal Organs. Allis says, p. 537: The nasal pits are inclosed in the same way that the lateral cana1s are, and the short canal so formed is at first continuous with the canal inclosing organ 5 infra-orbita1.


There is now genera1 agreement that the ears be1ong to the lateral line series, but there has been much dissent on the part of Some morphologists to admitting the eyes to the same category; but the grounds for the Opposition seem to me to be largely removed. It must be said, moreover, that the Organs of taste and touch have not as yet been shown to have any genetic relation to the ganglionic Sense-Organs.


I am g1ad to acknowledge my indebtedness to Minot’s introductory remarks to his chapter on the Sense-Organs. He says: "Very suggestive in this connection are the observations of H. V. Wilson («91), of a thickening of the nervous layer of the epiderrnis on either side of the head in the bass embryo (Serranus atrarius). This thickening forms a long, shallow furrow,

which subsequently divides into three parts, of which the first

becomes a Sense-Organ over the gil1-cleft, the second, the auditory invagination, and the third, the Anlage of the sense-organs of the lateral line. This peculiar development confirms the notion that all these organs belong in one series, but the appearance of a continuous thickening as the Anlage of them all has, as yet, been observed only in this fish, and may not indicate a corresponding ancestral condition Unfortunately, Wilson was unable to make out anything as to the connection of the sensory plate with the ganglia. The sense-organ above the gi1lcleft, although differentiated, is a larval structure only, and disappears in the adult."


A quite similar condition is now known to obtain in some elasmobranch forms. Mitrophanom in 1890, published a preliminary report of his observations on the lateral line of Acanthias and other Elasmobranchs In 1893 he published a full report of the same, illustrated by many iigures He describes a continw ous thickening of the epidermis along the sides of the head embracing the territory of the roots of the seventh to tenth nerves. From this thiclcening there is separated the material for the auditory saucer, the branchia1 sense-organs, and the beginning of the 1ateral line. My observations on this region in Squalus agree with those of Mitrophanow.


In looking over the literature on the sense-organs one cannot fai1 to be struck by the remarkable uniformity of the sensescells in the different kinds of sense-organs. , They are easily reducible to one type, and this, of course, favors the view that they have been derived from a common form.

I. The Lateral Eyes

Squalus acanthias is an especially favorab1e form for observing the beginnings of the optic vesic1es. They make their appearance in this animal at a very early stage, while the neura1 p1ate is broadly expanded, and before the neural folds have arched upwards in any part of the embryo. I should estimate their appearance in this animal to be as early as in any other anima1 in which they have been described. Previous to I889 it had been current opinion that the optic vesic1es appeared very early only in the class of Mammala Bischoff, Kö11iker, His, Van Beneden, Heape, and others had recorded their early appearance in that group ; but it was regarded as a precocious deve1opment for some reason confined to mamma1ian forms.


More careful observations on the earlier stages have shown that the optic vesic1es customarily arise in different anima1s much earlier than was supposed. In I889 Whitman called attention to the very early appearance of the optic vesicles in Necturus: « Its basis being discernib1e as a circular area—-— after treatment with osmic acid, followed by MerkePs fluid —- long before the closure of the neura1 folds of the brain." Dura1noted the early appearance in birds. In 1893 Eyc1eshymer gave notes upon their appearance and structure in Necturus and other Amphibia His most noteworthy observations are upon Rana pa1ustris, a form hitherto unstudied, in which he ftnds a remarkably early deve1opment of the primary optic vesicIes. They are very evident from surface study in this form, on account of the distribution of pigment granules in them, just after the neural ridges have begun to form. cross-sections- in these early stages show a considerable amount of histological differentiation from the surrounding ce11s.


The early differentiation of the optic vesicles had not been recorded in any e1asmobranch form unti1 the publication of my preliminary papers in 1893 and I894, where the optic vesic1es are described and their serial relation to other structures on the cepha1ic plate is show-vix. Since then I have had opportunity to coniirm my observations on Squa1us, and to extend them to some other forms ; for instance, in Diemycty1us and Amblystoma the optic vesicles are well deve1oped in very early stages In Torpedo oce1lata they show fairly well, and in the chick they may be readily made out while the groove is wide1y open.


In Squalus acanthias the optic vesicles are the first ruchments of Sense-Organs to appear. counting out the gastricular cavity (and the neural segments) they are the Hrst organs of any kind to be established. Strangely enough, their early history in these animals has been entirely overlooked. Balfour’s statement that « The eye does not present in its early development any very especial features of interest," seems to have withdrawn attention from that organ in the elasmobranch Hshes.


In order to iix clearly the time when the optic vesicles first appear in Squa1us, we must glance over the early steps in the formation of the head-plate. Pl. XXVL Figs. I, 2, Z, represent three stages that immediately precede the formation of the eyes. In Fig. I the embryo is well established; it is a stage slightly older than Balfoufs stage B. The front part of the embryo is not broader than the rest, and the curve of the anterior margin is uniform and unbrokerr In Fig. 2 we note two changes, vie» that the front end is broader than the part just behind it, which is apparently constricted, and the anterior margin is drawn out into a rounded median cusp. The headend continues to expand laterally (Figs. 3 andr4) until we may (with some appropriateness) speak of it distinctively as a headplate. In the meantime the trunk region has grown longer, and is relatively narrower, so that the entire axial embryo has a Characteristik: appearance of a narrow body terminated by a rounded plate-like head. When this stage has been reached the first satisfactory traces of the eyes become visible ; when first formed they present the appearance that might be produced by pressing down lightly upon a plastic surface with two rounded dies. They are circular areas slightly concave upwards, and occupy a position far forwards on the cephalic p1ate. The early stage of these vesicles may easily escape observation, as they become evident only when the light strikes them properly, and when they are in a favorable position for the observer.


Fig. 5 is engraved from a photograph in swhich the optic vesicles show very well. The specimen from which the photograph was made showed about six mesoblastic somites, and was ZHF mm. in length The circu1ar areas may be made out satisfactorily in slightly earlier stages, in which only three mesoblastic somites are differentiated, and in which the neural fo1ds of both head and trunk are not only broadly expanded, but are even ventrally —curved. The circular depressions are at first separated from one another by a raised welt. This has already been spoken of in Part I as atonguedike process extending from the mediananterior tip bacliwards to two—thirds the length of the cephalic plate. It continues to be a prominent feature of the cephalic plate for some time. I can offer no suggestion as to its signifis cance, outside of the obvious suspicion that it may represent a proboscis of Some lcind, or that it may be related to the 1arge notochord of this region. On this point 1 have no conjectures to make. The depression to form the infundibulum starts a little after the first appearance of the optic vesicles, and cuts the median process in two, so that the anterior tip is separated from the rest, and the depression for optic vesicles and infundibulum combined reaches across the median plane of the cephalic plate (Figs. 6, J, 8, 9, ers-J. The optic vesicles, however, do not reach to the 1ateral margins of the neural folds ; the latter are much expanded beyond them.


It is evident that we cannot, in a strict Sense, speak of such vesicles as "diverticula of the fore-brain." The "diverticula" are found before the fore-brain. The depressions once started grow deeper and press outward laterally ; when fully formed they are cupped, almost rounded in out1ine, concave from within, and they form rounded elevations where they come in contact with the outer layer of epiblast. Figs. H, J, 8, 9, Io, show very well the appearance presented by these primary optic vesicles when viewed from above. Fig. 7 is magnified higher than the others, and is, therefore, larger, but the others are all uniformly en1arged. These figures all show the interior of the optic pits. They represent a range of stages only slightly older than that shown in Fig. 5. In all of them the depression in which the optic vesicle lies extended across the median p1ane, and in its median portion, the depression develops into the infundibulum. The lateral depressions, which form the optic vesicle, sink downwards more rapidly than the median part of the depression, and as a resu1t there is left a ridge or keel separating the optic vesicle and connecting the anteriopmedian tip of the headp1ate with the Central tonguelike process. Miss P1att, in her studies on the head of this animaL says: "It may be here noticed that the downward growth in the Hoor of the brain,y which gives rise to the infundibulum, is prirnitively bilateraL and not median," and I think, therefore, that she saw these circular optic pits in an early stage, but interpreted them as 1atera1 pockets of the infundibu1um. There might be reason to doubt whether they are the optic vesicles, if they could not be traced with perfect continuity into the eyes, and if there were not evidence of histological differentiation in their areas.


Cut 8. - Eight transverse Sections of the embryo shown in Fig. s, Pl. XXVL x about zo diameters The nnmbers below the Sections refer to their Position in the series.


By the time the neural folds of the head begin to rise the optic vesicles are very prominent, and may be observed from the outside and from within. Fig. I 3 shows an embryo viewed obliquely from the 1eft side, and the optic vesicle of that side is seen from the external surface, where it presents the appearance of a rounded eminence 0n the opposite side the optic vesicle is seen from within. cut 9 shows transverse Sections of the head of the same embryo. Figs. 16 and 17 show embryos in which the optic vesicle Ho. A) shows from without, and also from within («:)P.i-). 1n Figs. I9 and 22 the optic vesicle shows clearly from the outside. Transverse Sections of the embryo photographed in Fig. 23, after partial closure of the neural groove, are illustrated in cut I0.


Cut 9.-— Eight transverse Sections of the embryo shown in Pl. XXVL Fig. is. I( about so characters. The accessory optic vesicles H. Ha) have been formed.


These optic vesicles are so well developed at an early period in Squa1us, that it seemed to me probab1e that similar early formed vesicles might exist in Torpedo oce11ata, and have been overlooked by the Zieglers, who have studied the early stages of that form in 1892; accordingly I procured embryos of Torpedo oce1lata from the Zoölogical Station at Naples, and studied the head region with some care. The first thing noted was that the embryos of Torpedo ocellata aire not nearly as favorable objects for observations as those of SquaIus. They are smaller and the beginnings of Sense-Organs, branchiæ, and other structures about the head are by no means so clear as they are in Squa1us. As far as I am ab1e to discern, there is no very distinct differentiation of the optic vesicles, in the nearly stages of this form; nevertheless, they are present at the stage designated C, by the Zieglers, and perhaps a little earlien In studying the actual specimens, I had occasion to note that the Zieglers’ models are admirable representations of the embryos of the form they are intended to represent The optic vesicles are much fainter than they are in Squa1us, but net-ertheless are simi1ar to them. I have studied them in Torpedo both in surface views and in Sections. One noteworthy fact is that they have been sectioned and actua11y Hgured by Ziegler in his Pl. IV, Fig. 19I. It will be noted in this iigure, that the brain-wa11s bulge outwards on either side and these depressions mark the interior of the optic vesicles P1. XXIX, Fig. 65, is a drawing of one of my Sections of Torpedo s1ight1y younger than the one of Zieg1er’s just referred to. The optic vesicles are indicated at pp.



Cut 10. — Twe1ve transverse Sections of the embryo shown in Pl. XXVL Fig. 23. x about zo diameters The number-s below the Sections reker to their positions in the series. In Section 14 the optic vesicles show below and the depressions for the mid-brain vesicles above.


The external appearance of the optic vesicles in later stages is shown in Pl. XXVII, Figs 34, 35, 36, 37. When the stage represented in Fig. 35 has been reached the lens shows externally ; it is forrned in the usual vertebrate way. The choroid fissure shows in Figs. 36 and 37. My studies have not been made to include the later stages of development of the optic vesic1e, but are coniined to its earliest differentiation.


Sections of the earliestckormed circular areas, show something in the direction of histological differentiation Mitoses are more frequent and more of the cells are e1ongated and pear-shaped than in the other parts of the cepha1ic plate. The differentiation is by no means as marked as in the Rana pa1ustris Hgured by Eycleshymer —- nevertheless indications of change are not wholly lacking My Sections are too thick (I0«- to I In) for satisfactory histological study, but one can see in them that the middle of the wa11s of the optic cups are areas of differentiation The differentiation does not progress far until later, but the frequency of dividing cells, and their change of form continue, in these early stages, to be marks for distinguishing visual epithelium from that of the surrounding brain—wal1s. The dividing cells are neurob1asts, and these are known to be points from which thenervedibres spring. Their presence in any considerable number would, therefore, indicate a differentiation in the direction of increase of sensibi1ity.


The eyes are the highest developed of the Sense-Organs, and in that particular stand at the head of the series. But, do the eyes belong to the same series with the other Sense-Organs, or do they occupy a position by themse1ves, can they be homo1ogized with the restk This is a puzzling question, over which there has been much controversy, and upon which there is still much difference of opinion among anatomists The stock objection to their being classed with the other Sense-Organs, is this:—— they have apparently been derived from a different basis, and their nerves are developed in a different way. The eyes are formed out of neural-plate material as diverticula of the fore-brain, while the other Sense-Organs arise outside the neural plate ; they have an independent epiblast origin.


It is questionable, whether any particular stress should be put upon that argument, as the neural plate is at best only a part of the modified epiblast ; moreover, the suggestion that the neural plate is, undoubtedly, very much widened and expanded from its ancestral condition, and that certain sensory area, originally lying outside, may have been included in it, does much to 0ffset that objection. The neural plate has been a prodigiously long time in forming, and the eyes have been brought into closer relationship with it. The plate is broadest in front, and the eyes are the most anterior sense-organs, and have become included in this expansion While there is not sufficient data to give a wholly satisfactory answer to the question propoundech the assumption that the eyes are closely related to all the others is not without foundation, and we are now in the attitude of awaiting further facts.

II. Accessory Optic Vesicles

The neura.1 plate, while still in very young stages, and while the neural grooves are widely open, becomes the seat of some accessory differentiations that resemble the optic vesicles. So far as I know no observations upon these structures have been recorded except those in my preliminary account in the JOURNAL OF« MORPHOLOGL I quote from that paper: But, more interesting than the fact of their (the optic vesicles’) very early appearance in Elasmobranchs, is their apparent relation— ship to other depressions that are formed upon the cephalic plate, behind the already established optic vesicles. The new involutions referred to, make their appearance upon the cephalic plate just baclc of the optic vesic1e. Two of them (Fig. 13, as. v) and Fig. U, as. Eil, ask. IN) take precedence of all others in deve1opment, and are so distinctly formed as to afford a good basis for cornparison with the optic vesicles. They are circular depressions formed in front of the latter, and they produce upon the exterior corresponding rounded e1evations.


The optic vesicles are formed first, and when, at a very litt1e later stage, the others arise behind them, it appears as if the process of eye-formation were repeating itself serial1y.


These circular pockets not only arise in a simi1ar way, but structura1ly resemb1e the optic vesic1es. In cross and 1ongitudina1 Sections the ce11s of the sunken patches are similar to those in the eye pockets. They may be designated accessory optic vesicles.


The assumption that they are primitively visual in character may be going too far, but the basis upon which it rests will be shown directly. The 1east that can be said of them (from their structure, the place and manner of their appearance), I think, is that they are segmental sensory patches.


These structures begin to appear soon after the eye vesic1es. There are several pairs of them stretching in rows baclc of the eyes, as stated above, the two anterior pair are the most prom— inent. They gradually grow fainterz the anterior one is the best differentiated; the second one is not so clear, and the succeeding ones grow fainter as we pass backwards I have counted as many as four pairs in surface studyz but Sections show that the series is more extensive and contains not less than eight pairs of these circular pits. The two anterior ones are well shown in Figs. 9 and ro. These are the earliest surface indications. In Fig. V, which is slightly more advanced, the accessory structures are clearly developedz the ftgure shows four pairs behind the eyes. In longitudinal Sections of the same specimen, Pl. XXIX, Figs. y6——87, eight pairs may be counted. The form of the depression, as seen at first from the surface view, is ovate; they soon become circular patches, and form themselves into concave, shallow cups. I have seen them in many specimens.


It will be noted that they make their Erst appearance while the neural plate is broadly expanded, and spring into prominence while the neural folds are growing upwards It is during the rise of the neural folds that similar, but fainteky vesicles can be made out, in surface study, behind the two anterior pairs.


Pl. XXX, Figs. 88 to 112 show twentyckive transverse sections of an embryo very slightly older than that represented in 5 58 LOCPI IJVo1.. XI,


Pl. XXVL Fig. 16. The Sections 88 to 100 lie in the region of the cepha1ic plate. The following Sections, 105 to 1I2, lie in the neck and trunk region. These Sections are remarkable in bringing to light serial depressions a1ong the walls of the neural folds. They show that the serial cup-like differentia— tions extend back of the cephalic plate. I have not been able to determine the number of serial differentiations of this character in the embryo, but it is clear that there are several pairs behind the cephalic plate upon which I have noted, in surface study, four pairs in addition to the primary optic vesicles.


My sections are not favorable for a critical study of the histological conditions, but it is clear, from them, that« a differentiation starts in the anterior patches simi1ar to that mentioned above, for the true eye vesicles. There is a greater frequency of dividing cells and many of the cells become elongated and pear—shaped.

comparisons of the two anterior pairs with the eye vesicles give the following points of resemblance They are formed in precisely the same way, and present the same general appearancez viewed from within they are all similar concave cups; they produce corresponding elevations on the outer surface, and, if observed from the« outside, they form a series of rounded knobs serially arranged, the eye being the anterior terminations of the row. The marked differentiation in the eye takes place in later stages. There are, however, the histological features spoken of above that serve to distinguish it in early stages, and the accessory vesicle shows the same characteristics

These resemblances, coupled with the existence of serial eyes in Some of the Invertebrates, has led to the assumption that in Vertebrates these rudirnents represent accessory optic vesicles. The fate of the anterior pair would also favor this view ; they pass into the pineal sense—organ, a structure whose visual character is acknowledged.

If the view expressed above is true, we have a multiple-eyed condition in the embryos of these animals. This is common enough in Invertebrates, but has not been previously noticed in Vertebrates.


These accessory structures are present for a brief time only. That region of the head, behind the optic vesicles and in front of the ears, becomes the seat of great modifications, and, as the neural groove closes, the structures described give way to later formed ones. They are, therefore, not only embryonic structures, but are also transitory, It is a truism in develop— mental history that the more primitive characteristics appear first, and the secondary modiiications come in later. The structures described are among the most primitive that have been preserved in this very ancient group of Vertebrates, and should be of signiiicance in indicating the ancestral relations of the eye. I have spoken of the disappearance of the Organs, but I have been able to trace the anterior pair further along, an account of which will be given in the next Section on The Pineal Sense-Organs.

I have also traced out simi1ar rudimentary organs in the embryo chick, at about the conventional 24-hour stage ; there is a series of seven vesicles behind the optic vesicles These structures are rudimentary, and very quickly fade away.

All this acquires new interest when taken in connectiion with the researches of Whitman on the eyes of leeches. As indicated above, he showed that the eyes of the leeches are derived from segmental sensory papillæ. It now appears that we have traces of a somewhat simi1ar line of segmental sensory organs in Vertebrates It is essential to say Drecke-F, for these structures are transitory, and do, not rise to a high grade of differentiation, except in the case of the pineal Sense-Organs. They may never, in the vertebrate group, have been raised to the rank of eyes, but it is certain that in Annelids simi1ar segmental sensory patches have been so raised. It is, however, reasonable to suppose that members of this series have been functionaL even in Vertebrates, when we recollect the highly deve1oped condition of the pineal eye in Some forms.

There is really very little direct evidence to support the proposition that the Vertebrates are derived from multiple—eyed ancestors, although it is aconception that has been in the minds of morphologists for Some time. Besides the indications of such a condition afforded by the facts of this paper there has recently come more evidence bearing in the same direction. This consists in the discovery of multip1e pineal eyes, in several distinct forms, and, in one case, the existence of three distinct nerves to the pineal organs (see Section on The Pineal Sense-Organs)


Whitman, in his paper just cited, says: ssAlthough the evidence appears to me conclusive that the eyes and the segmenta1 papillæ were, originally, morpho1ogical as well as physiological equivalents, it does not, of course, follow necessarily that both Organs now have the same fnnctional signifk cance. The original papillæ may have represented Sense-Organs of a more or less indifferent order, among which, inthe course of the historical development of the leech, a division of Iabor was introduced, a few at the anterior end becoming specialized as lighbperceiving Organs, the rest either remaining in their early indifferent condition or becoming specialized in Some other direction.


"The discovery that these papillæ are Sense-Organs might lead us to speculate on aHinities of a distant and somewhat uncertain nature, such as are supposed by the writerz in common with many others, to exist between annelid worms and Vertebrates At all events, the existence of such organs in the leech furnishes a broader basis for the discussion of the question whether the Vertebrates and Annelids have been derived from a common form possessing metameric Senseorgans, as was first argued by Dr. Eisig of the Naples station. Assuming that the Sense-Organs of the Vertebrate and the segmental papillæ of the leech may be traced to a common origin in some remote ancestral form, it does not follow that they should now present close structural resernblances It is far more important to show that they possess certain general features in common. The most important of their common features is undoubtedly their metameric origin. The nervesupply forms another feature of fundamental importance, in which, according to the interesting observations of Mr. Beard, on the segmental Sense-Organs of the lateral line (Z·:)x)Z. des» VIL Nos. 161 and I62) of the Vertebrate there is essential agreement. The developmenta1 history of these latera1 organs in the Hsh, where they make their first appearance as segmental papillae in the strictest sense of these words, cannot be explained on a more satisfactory hypothesis."


III. The Pineal Sense-Organs

1. Growth of Knowledge regarding the Pineal Sense-Organs

The pineal outgrowth has attracted much attention since the discovery, in I886, of its eye-like structure. Since then it has been accepted by morphologists as a rudimentary Sense-organ. The earlier 1iterature bearing on the subject has been theroughly reported on by Spencer («86) and Francotte («89). several recent1y published papers taken together put the subject in a new light, and, as Ritter («94) has said, «At no time since the epiphysis and pineal eye have been the topic of investigation has it been a more interesting or a more inviting one than it is just now." The growth of our knowledge regarding this remarkable sense-organ, scattered through the various publications of Spencer, Beraneclg Francotte, Strahl and Martin, Leydig, and others, may be reduced to the following summary : i First came the demonstration of its eye-lil(e structure in Amphibia, by De Graaf, and in Lacertilia by Spencer. De Graaf’s publication is much briefer, and was published only a few months before Spencer’s extensive study. Both authors investigated the structure in adult forms. Spencer studied its condition in twenty-eight different species of Lacertilia, and demonstrated very clearly that even in the adult forms of these animals, this organ has an eye-like Struc ture, with a 1ens, a pigmented retinal layer, and a nerve. It seems doubtful, in the light of recent work, whether the kibrous Strand connecting the eye-capsu1e with the epiphysial Stall-i, and designated the nerve by Spencer, is really nerve or not.


Following the study of the adult structure came investigations of the embryonic conditions The first studies dealingv with the embryonic development of this organ, and with its minute structure in the early stages, were those of Strahl and Martin. Along this same line were the studies of Båraneclg Francotte, Leydig, and others. By these investigators the organ was traced backwards to a certain point in its history, that is, to the time when it springs from the roof of. the thalamencephalon as a tubular outgrowth, like the tip of a finger of a glove. To this period also belongs the discovery of a nerve of transitory existence The investigations were coniined almost entire1y to two groups of animals, Amphibia and Reptilia. In the embryos of these animals many points of minute structure were worked out, such as the distribution of the nerve within the eye—like capsule, the division of the retinal part into distinct layers, the fact that the pineal eye is higher deve1oped in the embryonic periods than later, etc.


The next step was the discovery that there is more than one epiphysial outgrowth. Se1enka, Leydig, Eycleshymen Hill, successive1y ca11ed attention to the existence of two distinct outgrowths, or vesicles, from the roof of the thalamencephalon in Anguis, Amblystoma, and Teleosts.


Hi11’s studies on these two vesicles are the most complete He has shown the existence in Teleosts and Amia of two vesicles, anterior and posterior, and has traced them through the stages of deve1opment. The posterior develops into the epiphysis and the anterior degenerates. The dista1 portion of the former is histologically very complex, and receives a nerve from the posterior commissure. Hill made out the distribution of nervedibres in this part of the epiphysis. He traced quite clearly the history of the vesic1es.


Two vesic1es, an upper and a lower, have long been known to exist in Petromyzon, in both embryos and adu1ts. It is much to be regretted that we do not know their embryonic history, for much depends on this. The most recent paper on the epiphysial Organs in Petromyzon—that of studniöka—— does not clear up the question of the origin of the two vesic1es. Histological material was 1acking in just the stage required. The two outgrowths in embryos outside the Cyclostomes have been designated epiphysis, for the hinder, and pineal eye for the anterior. Studniöka designates them pineal and parapineal, respectively, in the Cyclostomes As I shall show latet, the so-ca11ed paraphysis is a different structure.


Beraneck showed that the pineal eye and epiphysis have been confused, and, on account of the existence of the nerve from outside sources, he argued for the complete independence of the pineal eye.


Klinckowstrom has p1aced himse1f in Opposition to this opinion, showing the eye vesicle is formed as an outgrowth from the epiphysis.


Next to the discovery of two or more epiphysial outgrowths may be mentioned the existence of accessory pineal eyes The ear1iest publication containing an account of such Structur-es is that of Duva1 and Kalt, in I889. In a brief note (Des Yeux Pinåaux Multiples chez 1’Orvet) these authors record the fjnding of two or three accessory pineal vesic1es in Anguis. They do not say whether adu1t or embryonic forms were studied.


Carriere ('89), in the same year, noted the occurrence in embryos of Anguis of a very rudimentary accessory pineal vesic1e.


Leydig ('90) observed these structures independently in embryos of the same anirnals He showed that there is much variabi1ity, both as to the presence and the histological Structure of these accessory capsu1es. In Some individuals they are lacking, and in others of the same age present and well deve1oped. He records the occurrence, in Some cases, of two accessory Organs, a. larger and a smaller one. The larger one resembles the pineal eye in structure and arrangement of pigment. The smaller one is very rudimentary Leydig com— pared them to the ocelli of insects. It is c1ear from his account that variabi1ity and inconstancy are characteristics of these accessory Organs.

Prenanh in the latter part of I893, again records the occurrence of one accessory pineal eye in the embry0s of Anguis He directs attention to the fact that up to that time they had been discovered only in a single species, VIII» Anguis fragilis, and further, that they were all in ernbryos, and not in adults. Ritter (-94) has recently recorded the, occurrence of such an» accessory org-an in the adult Phrynosoma.


The above observations, taken together, give suflicient positive data to establish the fact that there appears, from time to time, in some animals the vestiges of accessory pineal Organs, and that they are variab1e and inconstant in their occurrence.

The most recent advances consist in carrying the history of these organs baclcwards, and showing them to be connected with patches of sensory epithelium that arise on the cephalic plate in very young stages, and in the discovery, in Iguana, of three distinct nerves connected with the epiphysial outgrowths. The former work was done by the present writer, and the latter by Klinckowström.

As indicated in the preceding Section, there are several pairs of cups on the cephalic plate back of the optic vesicle, formed while the neural groove is widely open. I have designated them accessory optic vesicles In the process of closure of the neural groove the anterior pair are brought together, and form part of the thalamencephalon, from the roof of which the pinea1 outgrowth is derived. This process will be described more in detai1.


Klinckowström ('98) has recent1y shown in Iguana the presence of two nerves connected with the anterior outgrowth or pineal eye, and sometimes a third connected with the posterior soutgrowth or epiphysis.


These observations suggest new interpretationsz still it is not an auspicious moment to indulge in speculation. It is clearly evident that we need more data regarding these organs and their relationships But the trend of these discoveries is to strengthen the suggestion already made in this paper, that, primitively, the Vertebrates were multiple-eyed. The presence of accessory pineal eyes, the discovery of serial sensory areas on the cephalic plate from which epiphysial outgrowths arise, and the presence of two or three pinea1 nerves, are all consistent with this interpretation so far as the evidence goes, there is more than one epiphysial outgrowth, and therefore I have headed this Section, The Pineal Sense-Organs. In what way it will be necessary to qualify this proposition will depend on subsequent discoveries.


The results of all the embryo1ogica1 studies on the epiphysia1 outgrowth between 1887 and 1893 were to settle on one point of common agreement regarding its origin. Each investigator successiveiy found it to arise as a tubular outgrowth from the roof of the tha1amencepha1on comparative1y late in ontogeny, after the parts of the brain are estab1ished. No earlier trace of it had been found; but there was a previous undiscovered history, and it is now time1y to make the inquiry, What is the very beginning of the pinea1 Sense-Organs?

2. Remote Origins of the Pineal Outgrowth

As already indicated, there are upon the cephalic plate accessory eye vesicles which have made their appearance while the neural groove is widely open; and in tracing their fate I shall attempt to f111 up that gap in the history of the pineal outgrowth from the time these cup—1ike structures appear upon the cepha1ic p1ate to the time the tubu1ar outgrowth begins from the tha1amencephalon.


As the wa11s of the neural groove grow upwards these Cuplike structures are carried up vvith them; and there comes a time (Figs. I3, 3I) when they may be seen from the outside as rounded eminences, and from the inside as concave cups.

When the edges of the neural groove meet in the middle line the cups are approximatech and come to form part of the thalamencepha1on.


While these changes have been going on, further differentiations have been taking place in the wa11s of the brain. The bulging of the wa11s to form the mid-brain vesicle has come on insidious1y, and has taken a position behind the vesicles of the paired eyes, in apparent1y the same position previously ssoccupied by the accessory vesicles. These transformations are confusing, as the wa11s of the mid-brain resemble the accessory optic vesicles grown largerx 1n an ear1ier published paper I made the mistake of identifying the mid-brain with the accessory optic vesicles; but it was merely an error of identification, and did not affect the main contentionof that artie1e.


In working out the details of the formation of pineal outgrowth in Squa1us, it soon becomes apparent that the surface contours of the head are considerably altered by the distribution of mesoblastic Hcells lying between the external layer of epiblast and the brain-waIIs. The mesoblast forms a pad of varying thickness in close contact with the brain—wa1ls; the depressions are tilled, and in some places thick patches of mesoblast give rise to external rnarkings that render the surface appearances untrustworthy. There is, for instance, a deep pad of mesoblast tilling up a depression at the sides of the cerebellum, and also extending forwards on to the midbrain; at the external surface the pad assumes a circular form, and it is that pad of mesoblast which is seen in Fig. 32. The true mid-brain vesicle in that Hgure is the eminence marked wo. The accessory optic vesicles are present, but are not well marked externally. They are in front of the protuberance marked »O.


In order to get a view of the brain-walls I have found it necessary to remove the mesoblast and its coverings of epib1ast, and thus to complete1y expose the brain-wal1s. The brain thus laid bare, enables one to see with complete satisfac— tion its different parts and their relation to one another. A very interesting relation comes to light through the dissections. Somewhere between the stage with an open neural groove (Fig. 3I) and the completely closed groove (Fig. 33) the midbrain vesicle insidious1y takes the former position of the accessory optic vesicle while the latter is carried forward by the pr0cess of cranial flexure During the formation of the midbrain vesicle the two structures become incorporated into one faintly bi1obed protuberance (PI. XXVIIL Figs. 38 and 39). The anterior part is the accessory optic vesicle, and the posterior one the mid—brain. The separation soon becomes com— p1ete, and by the stage represented in Fig. 40 the two are completely separatedz but owing to the arrangernent of the mesoderm the accessory optic vesicle is rendered indistinguishable from the outside. When the mesoderm is entirely removed it may be seen. All this takes place in very brief intervals of time, and a complete series of embryos representing the different phases of the closure of the neural groove is required to see it all. I must say also that the changes are so perplexing that there exists in my mind Some doubt as to the adequacy of the above account. 1t may have to be modified, but I have given the facts as they now appear to me.


Figs. 38-54 show a series of embryos that have been partly dissected in the way just indicated They show characteristic changes in the brain-wa1ls as seen from the exterior.


In Fig. 38 the vvalls of the neural groove have not yet met in any part of their course, and the thalamencephalon cannot be distinguished from the rest of the brain. The first pair of accessory vesic1es are visible; they soon become inc0rporated in the walls of the tha1amencephalon. I have not been able to determine whether it is the epithe1ium of the first pair of accessory vesicles only, or whether epithe1ium from the second pair is also included. Either the epithe1ium of the two pairs is incorporated into the walls of the tha1amencepha1on by being carried together, or the epithe1ium of the second pair fades into the surrounding substance of the brain-wa11, which almost immediately grows into the vesicle of the mid-brain. Although I am doubtful as to whether the second pair of accessory vesicles pass into the inter——brain, I am sure that the epithe1ium of the anterior pair is so included.


Fig. 39 shows the first accessory optic vesicle and the midbrain vesicle forming a common protuberance, but they are very quiclcly separated, and the anterior vesicle goes into the tha1amencepha1on.


In Fig. 40, which is somewhat o1der, this has occurred The lateral wa11s of the thalamencephalon now consist of two shallow cups, approximated so that the structure looks lenticular when viewed from above. As a usual thing, the thalamencephalon is not evident at this stage before the removal of the overlying tissues. Fig. 4I shows, however, a specimen in which it could be seen before any dissection. Compare this with Fig. 32, which is the more usual appearance of an embryo of this age.


Fig. 42 is a dissection of the embryo shown in Fig. 41. The mid-brain, which, from external view, appears like a single rounded eminence, is shown after exposure to be bilobed.


Figs. 43, 44, and 45, which are successively o1der, show an increase in the size of the thalamencephalon, and marked changes in the mid-brain.


In Figs. 46 and 47 a faint furrow has appeared in front that serves to mark the boundary between the thalamencephalon proper and the cerebral lobes. This furrow is clearly deiined in Figs. 48 and 49, so that we have now the thalamencepha1on distinctly marked off from the rest of the fore-brain.


In Fig. 48 the optic vesicle and all the mesob1ast have been removed, and the view is talcen from the right side. The brain— walls show very clearly. The tha1amencephalon is now bounded anterior1y and posterior1y by furrows. The two furrows run nearly parallel to one another, and they serve to mark oft« the inter-brain very distinct1y.


The roof of the thalamencephalon is at this stage raised into two rounded eminences of nearly equal dimensions, an anterior and a posterior one. The posterior is, from the beginning, somewhat in advance of the other. It becomes the pineal out— growth, while the posterior eminence goes on deve1oping. The anterior one becomes greatly reduced by compression from the rapidly growing adjacent brain-walls. It very soon loses its rounded Character, and becomes pressed into a semicircular fo1d lying in front of the epiphysis. It is, I thinl(, equivalent to the Zirbelpolster of German author-s. If longitudinal sections be made at the stage represented in Fig. 48, the two e1evations show as in Minot’s Fig. 3I9, p. 5y2, of his Hasen« EmZ--;1-·c)!og·j-. In that Hgure they look like two equal vesicles springing from a single elevation of the brain roof, and«opening by a common wide passage into the brain vesicle No especial signif1cance, I think, is to be attached to the fact that there are at first two equal embossrnents from the roof of the tha1amencephalon. The anterior one is converted into a fold of the roof of the ’tween—brain that has long been recognized in different anima1s. It is designated Zirbelpolster by Burckhardt It does not in any sense correspond to the anterior epiphysial vesicle discovered by Hill in Teleosts, but rather to the fold of ’tween-brain that lies in front of both epiphyses. Studniölca has, however, marked the fold in diagramrnatic figures of Teleosts, the anterior vesicle of Hi1l, but this identification is not right.


We have in this specimen the first appearance of the tubular outgrowth, which, accorciing to previous authors, marks the beginning of the epiphysia It is somewhat important to fix the stage at which this outgrowth begins. Its very first appearance as an elevation of the roof of the thalamencephalon is in the stage represented in Fig. 36 after the saucersstage of the ear is past.


In Fig. 49 the posterior protuberance of the roof of the thalamencephalon is assuming a tubular form. It becomes the epiphysis The surface of the mid-b1«ain is now considerab1y changed. Two furrows have made their appearance, which divide the 1ateralwalls into three nearly equal 1obes. These do not show at all from the exterior before the removal of the epidermis and the mesoblast.


The gradual reduction of the thalamencephalon by compression between the Central hemispheres and the mid-brain is shown consecutively in Figs. 53, 54, II, If, and 6o. In Fig. 54, and 1ater, the epiphysis appears pear—shaped from the side, but from in front the enlargement on the distal end is seen to be broader than thick. It is borne upon a hol1ow sta1k.


Fig. 57 represents the brain of an embryo considerably older than in Fig. 56. The thalamencephalon is now very much compressed. The anterior half of it, which originally had a rounded protuberance growing from its roof, is now reduced to a small semicircular fold in front of the epiphysis.


Fig. 59 is a view upon the epiphysis of the same embryo from directly in front. It was obtained by removing the cerebral 1obes, which lie in front of, and partly hide the epiphysia The epiphysis is seen to be composed of a Stall( and an en1arged dista1 extremity ; leading from the Stall( on either side are the epiphysialpedunc1es. In this hgure we are looking through the ventricle of the thalamencephalon into that of the midbrain. On the inside of the lateral walls of the opening are two enlargements, the beginnings of the thala1ni optici. These contain the ganglia hebenulae.

In this specigmen the paraphysis had made its appearance as an outgrowth from the cerebrum. It is relatively late in arising, and is entirely distinct from any outgrowth from the thalamencephalon The lateral wal1s of the mid-brain are no 1onger10bed as in earlier stages. Fig. 58 is the same brain viewed from above ; we are looking directly into the cavity of the fourth ventricle.


Fig. 6o shows the brain of a still older embryo. It is the largest embryo of squalus in my collection, measuring 35 mm. in length. The cerebral lobes are divided into right and left halves by a sha11ow furrow. From the back of the cerebrum rises the paraphysis (P); just behind this is seen the semicircu1ar fo1d, belonging to the ’tween-brain, and behind that is the epiphysis, rising above and resting upon the anterior wal1 of the mid-brain. Fig. 6I shows a front view of the cerebral lobes after being removed from the rest of the brain ; the paraphysis is also brought into view. Fig. 62 is the same brain with the cerebral lobes removed, and p1aced so as to give a ful1-face view on the epiphysisz the stalk is considerably longer than in Fig. 59.


I have not had materia1 to work out the subsequent history of the epiphysis In specimens about six inches long it is situated in the wall of the cranium, directly over the cerebral lobes, and is connected to the roof of the ’tween-brain by a long threadJilce stallc The enlarged distal end is about the same size as in Fig. 62. cattie shows it in the adult with several capsules on the end of the stalk.


Sections of the stages just studied serve to coniirm the observations made from the outside. They do« not add very much to its features, speaking strictly morphologically The surface views have been prepared in such a way that they may stand for reconstruction, but they are more accurate than any actual reconstruction can be, as they show the relations entirely undisturbed.


Figs. 119—124 show a series of sagittal Sections covering the period of the first appearance of the epiphysis, the reduction of the anterior part of the thalamencepha1on, and the beginning of the paraphysis and the choroid plexus.


Fig. I19 is a specimen of the same age as that shown in Fig. 48. It shows that the roof of the thalarnencephalon is raised into two nearly equa1 protuberances.

Fig. I20 shows the beginning of the reduction of the anterior of these elevations, and the increased growth of the posterior one. In Fig. 121 the posterior e1evation, or epiphysis, has become a tubular outgrowth, while the anterior one is reduced and depressed.

In Fig. 122 the tubular Stall( of the epiphysis is considerably elongatech while that part of the roof of the thalamencephalon in front of it has become reduced to a narrowfo1d that has been carried ventracL The paraphysis has now arisen from the roof of the prosencepha1on. The beginning of the choroid plexus extends from the structure down into the ventric1e of the fort-brain, and its posterior part is connected with the fold in front of the epiphysis The anterior and posterior Commissures are now distinguishab1e.


Figs. 123 and 124 exhibit snbstantially the same relations ; the choroid plexus has considerably increased in extent, and is rapidly becoming much convolutedz the brain vesicles have become reduced. In Fig. 124 the dista1 end of the epiphysis is inserted into a cavity in the roof of the cranial Wall, that is, the mesenchyme forms a cup over its rounded end.

3. Comparisons between Epiphysial Outgrowths

The recent studies have done much towards establishing a basis for comparison between the epiphysial outgrowths in different groups of anima1s. Beraneclg Francotte, and others claim that the anterior epiphysial vesic1e—common1y designated the pineal eye — is formed independently, having no generic connection with the posterior one, or epiphysis Hill’s observations on the Teleosts coincide with this view. Klinclcowström, on the other hand, claims that the anterior one is formed at the expense of the posterior one, and Substantiates his conclusions with figures of Sections of 1guana, showing the two vesiclesin union with one anothetc It is not slifficult to conceive how the condition represented by Klinckowström might be brought about, and still not be inharmonious with the other observations. The two vesicles might, in ear1ier stages than he represents, have been formed indepencL ently, side by side, and thrown into communication by an upward growth of the brain roof involving them both. Or, indeed, there may be variations in the details of the formas tions of these two vesic1es.


However this may be, the resu1ts of the different observers all go to show conclusively that there are two (not counting the paraphysis) distinct outgrossvths from the roof of the thalas mencephalon of Petromyzon, Teleosts, and Lacerti1ia. The fate of the anterior vesicle varies in the different groups, while the posterior one persists in all as the epiphysis The anterior one may be formed as a rudimentary organ of transitory existence, as in Teleostsz or it may be developed into the pineal eye, in front of the epiphysis, as in Lacertilia.


The sources of nervesupply to the two vesic1es have been made known in Cyc1ostomes, Teleosts, and Lacerti1ia, and these facts are important in establishing homologies. It has been shown (studniåka, Gaskelh KlinckowströrrO that the upper capsule of Petromyzon receives its nerves from the posterior commissure, and that the 1ower vesicle is innervated from the left ganglion hebenu1a. Hi1l, in embryos of Te1eosts, has traced a nerve from the posterior commissure into the epipipzysis and has studied its distribution in detail in that structure The anterior vesicle of Te1eosts disappears before it has formed any nerve connection with the brain. Klinckowström («9s) has observed very interesting conditions in embryos of Iguana. In this anima1 he found in one case three nerves distributed to the parietwpineal Organs: one coming from behind and entering the epiphysis, and two coming from right and left ganglion hebenula, respective1y, and entering the pinea1 eye. He found the right and left nerves in three different embryos of Iguana eighteen days old. In one case the nerve from the left ganglion hebenula was nearly the same size as that from the right, but in the other two individuals the left nerves were much reduced l It should be added that Klinckowstrom has found the nerve to the pineal eye in this animal to come normally from the right ganglion hebenu1a, while in Petromyzon it comes from the ganglion of the left Ziele. But, his discovery of the occasiona1 presence of two nerves in Iguana, the left one being smallen affords an interesting transition between the two. It also enables us to account for the marked asymmetry in the ganglia hebenulæ in Petromyzon, on the hypothesis that the right ganglion, in Cyclostomes has, for some reason, ceased to have any nerve connection with the pineal Organ, while the left ganglion has retained its connection.


As Beraneck has shown, a nerve leading to the pineal eye arises in front of the epiphysis, and has but a transitory existence.

Putting the facts together and basing relationships on thenervesupply, and comparative study of the vesic1es, we may express the probable homologies in the following diagrams and tab1e :


I. Uppser vcsicle in Petkomyzon, epiphysis in Teleost and Lacerti1ia. II. Lower vesicle in Petrotny2on, HiIPs anterior vesicle in Teleosts, pineal eye in laccrtilia IV. Nerve to posted-im« seminis-sure. XI. Nerve from lower vesicle in Petromyzon to Ganglion hebenuliy embryonic nerve of transitory existence in Lacertiliix

Z.P. «Zirbelpolster."

+++Reformat table below+++

s I PETRoMYzoN. THE-USE. . LACERTILIA. I H . . . « Posterior epiphysial Is the superior Is the epiphysis Is the epiphysis vesicle of I-Ii1l. vesicle. Nerve- with nerve con- net-ve- s u p pl y

niislca. « posterior com— posterior com- commissure. Epiphysis of others. [ missure. missure.

Pinealorgan of stucl- I. 1 supply from necting it to from posterior

Anterior epiphysial f "Is the inferior Is formed as in Becomes pineal

vesicle of Hill. ] l l vesicle. the two other eye supplied

Parapineal Organ of Ell. groups, but with a nerve

studniislca. j l aborts. which early cle[ generates


If these comparisons are well taken, we have in the Fette— myzon, the epiphysis in its highest state of development Its distal end is enlarged into an eye—like capsu1e that is more differentiated than their inferior vesicle which represents the pinea1 eye. This is the reverse of what is true in most Lacertilia, and, on this account the upper capsule of Petrornyzon has usually been eompared with the pineal eye of Reptilia and Amphibia But the fact that the superior capsule in Petrornyzon receives its nervesupply from the posterior commissure, corresponding in this particular with the epiphysis of other animals, has much more weight in determining its homo1ogies than any purely structural resemblances We are on a better foundation, therefore, in comparing the upper capsule of Cyclostomes to the epiphysis of other animals, than in taking the other Position. It may be noted, in passing, that the superior capsu1e is in the same relative position as the epiphysis, and the inferior one corresponds in position with the pineal eye.


This view would make the epiphysis eye—like in Character, and there are many facts that weigh in that direction. Hil1’s studies speak strongly for the visual nature of the epiphysis He shows a high degree of differentiation in the nerve (:ells to which the nerve from the posterior commissure is distributed. There is also formed in Iguana, Plica, and Phrynosorna an eyelike enlargement at the distal extremity of the epiphysis.

Between the condition in Petrornyzon and that in the Lacertilia many gradations have already been brought to light.

In Teleosts, the anterior vesicle arises, as in the other groups, but it is transitory, ancl soon degenerates It would appear from a recent paper by Ritter, that the anterior vesicle in Phrynosoma is more persistent than in Teleosts, but does not reach the grade of differentiation exhibited in the Lacertilia. He gives a Hgure showing the rare occurrence of the anterior vesicle in an adult Phrynosoma, and although large this capsule is not so highly differentiated as the capsule behind it, but to compensate for this the distal end of the epiphysis is highly developed.t· ln other korms of Lacertilia, the anterior vesicle reaches a high grade of perfection, while the epiphysis is not well developed. In most cases, however, there is a deposit of pigment in the distal Position of the epiphysis. The nerve distribution in that organ is known only in the Teleosts.


We might carry our comparisons further and inquire whether the structure present in selachians is to be homo1ogized with the epiphysis or the pineal eye of other forms. The nerve relations are not known in the Se1achians and, theref0re, we have not, as yet, a satisfactory basis for comparison. In its structure and persistent attachment to the brain roof by a sta1k, the outgrowth in Se1achians resembles the epiphysis and one would be inc1ined to say that only the epiphysis is represented in the Elasmobranchs, and that the pineal eye is lacking. Klinckowström has offered the ingenious suggestion that in Se1achians and birds the second or anterior epiphysis is never ful1y separated from the posterior, and this is worth thinking about. There is no positive evidence to support it.


However the above question may be settled, it seems to me that the fundamental features of the morphology of the epiphysis are tolerably clear, and that future work on this subject will be main1y in the direction of ampliiication and discovery of details.


The invertebrate homologies of the pineal eye are not known. Spencer supposed it to be homologous with the median eye of Tunicates, but that relation is a strained one.


Various authors have compared the pineal outgrowths to the ocelli of Arthropods Leydig compares them to the stemmata of Insects, and shows that there are many resemblances to support the comparison.


Patten has recent1y studied the deve1opment of the eyes of Limulus, and argues for a homo1ogy between the median eyes of these Arthropods and the pineal eye of Vertebrates He shows the median eye of Limulus to. arise by the fusion of paired eye-stallcs, giving us a case of undoubted union of originally paired Sense-Organs. I have c1aimed a similar occurrence in the epiphysis of Squa1us. But it seems to me that the evidence is too circumstantial at present to bear the interpretation that we already know the invertebrate homologue of the vertebrate pinea1 Organ.

4. The Double Nature of the Epiphysis

Receives support from two sources. It is diflicult to interpret Klinckowströmk diScovery of a double nerve in Iguana on any other hypothesis That interpretation is also strengthened by the claim I have made of tracing the epiphysis in Se1achians back to a pair of epithelial cups on the cepha1ic plate.


Hill ('94) regards the two vesicles he has discovered in Teleosts as having been primitively side by side; and Ritter («94) has recently suggested that the epiphysis and the parapineal organ of Studniälca are right and left mates rather than independent eyes of double origin. It will be remembered, in this connectiorn that Klinckowström found three nerves in one individual of Iguana, two from the ganglia hebenulae, and one from the posterior commissure; that Leydig discovered two accessory pineal organs in Anguis ; Duval and Kalt found as many as three of these Organs, and I have found several pairs of epithelial patches back of the eyes on the cephalic plate, and have traced one pair into the epiphysis It seems to me that all this evidence is more favorable to the idea that the pineal Sense-Organs are multip1e, and individually of paired origin, than to the idea expressed by Ritter.

5. Paraphysis

Considerab1e confusion has arisen in identifying the paraphysis in different forrns. As soon as it was made known that there are two outgrowths from the roof of the fore-brain, it was at once assumed that the most anterior one is the paraphysis, and the posterior one the epiphysis But there is now evidence that there are, at times, more than two outgrowths Hill has shown in Amia the presence at one and the same time of three tubular outgrowths from the roof of the fore-brain. Two of these come from the thalamencephalom andthe anterior one arises from the prosencephalon He points out that the 1atter is the paraphysis proper, and that in Amia we have to deal with two other outgrowths from the roof of the thalamencepha1on. These he makes homo1ogous with the two vesicles he has described in Teleosts. Hill concludes that in the 1atter group the paraphysis is lacking on account of the nomdevelopment of the choroid plexus This identification is in harmony with the original description of the paraphysis Francotte, who described it in 188 J, and Selenka (-90), to whom the credit of its discovery is usually accorded, both state that it arises from the prosencephal0n. Selenka says: « Wie das Zwischenhirn seine Epiphyse, so hat das Vorderhirn seine Paraphyse.« Francotte suggested that it represents the beginning of the choroid plexus.


Minot ('92), p. 69o, designates the anterior vesicle discovered by Hill (-91) in Coregonus the «paraphysis," but the Structure in question arises from the roof of the thalamencepha1on, and between it and the prosencephalon is a marked downward fold in the brain-wal1, which in many other forms is present behind the paraphysis. It is more probable, as Hill suggests in his1ater paper («94), that the paraphysis is lacking in the Te1eosts.


It is to be understood that in some forms there are two epiphysia1 outgrowths from the thalamencepha1on that are entirely independent of the paraphysis When the latter Structure is present in conjunction with the former two, as in Amia, there are then three separate outgrowths from the roof of the fore-brain.


As already indicated in this paper, the paraphysis is present in Squalus as an outgrowth from the prosencephalon, and is connected with the choroid plexus.

IV. The Beginning of the Auditory Organ

The formation of the auditory saucer is preceded by a thickening of the epidermis along the sides of the head in the auditory region. The thickening portion occupies a larger area than that used by the ear vesicle when it is Erst formed, and it is coniiuent with the epidermal thickening just above the branchiae As Mitrophanow has pointed out, this thickened patch of epithelium is composite in character; it embraces not only the epithelium for the formation of the auditory plate, but that to form the so—called branchial Sense-organs, and also the beginning of the lateral line. I had noted this relationship in Squa1us before seeing Mitrophanoufs paper, and 1 lind no com— parison that my resu1ts correspond —— so far as the earlier Stages are concerned—very closely with his. He has, however, carried his studies considerably farther, and has shown how the different branches of the lateral-line System arise.


The general relationship noted between the auditory plate, the branchia1 Sense-organs, and the lateral line is very similar to what Wilson found in sea bass. In that form the Connection between ear, branchial Organs, and lateral line is evidently more c1ear on account of the presence of a distinct sensory furrow and the way in which the separation into three parts takes place.


The general thickening in the sharks is not so well circumscribed. The fact that these different organs proceed from a common epithelial thickening indicates relationship of a fundamental kind. After separation the ear gives further evidence of its relationship with the lateral organs by preserving its canal (endolyrnphatic duct) connection with the exterior and developing in a manner that is characteristic of the canal organs of the lateral line.


Mitraphanow departs from the usual point of view that the organs of the lateral line are metarnericz and in that particular, I think, I should be inclined to follow him.


The auditory plate is at first differentiated from the general thickening, and its epithelium then becomes gradually rounded up into a circu1ar area, that is depressed in the centre—the auditory saucer. This structure sometimes covers the space of three neuromeres, and I have one surface preparation in which the outer epithelium shows three bars running vertically across it; these bars correspond with the underlying neuromeres.


We may interpret these superficial bars either as being moulded upon the underlying neural segments, or as being due to the same general cause as the neura1 segments I am of the opinion that the latter alternative is the correct one.


When iirst formed, the auditory saucer is opposite the ninth neuromerez but subsequent1y, while it is still in the saucer condition, it shifts backwards, and cornes to lie at the side of the tenth neural segrnenh and later still, as a capsule, it lies opposite the eleventh neuromere.


The history of the auditory vesicle in sharks has been worked out beyond this period by Ayers (-92), and it is very clear from its mode of growth that it is directly related to the canal organs of the lateral 1ine.


A consideration of the so—ca11ed branchia1 Sense-Organs and their ganglia is reserved to be published later with the part on the Nerves.

Note. — I have been indebted to the persons mentioned below for material that has helped jill up the gaps in rny collection of embryos, and I desire here to express Iny appreciation of their Kindness.
To Miss Julia B. platt, for the loan of Sections of Acanthias ; to Dr. E. L. Mark, for young stages of Squalus ; to Mr. F rank smith, for a considerable number of embryos ; to Professor J. E. Reighard for the loan and free use of his entire collection of elasmobranch embryos; to Professor A. D. Morrill, for early stages of Squalus.

Lake Forest, Illino1s,

January, I895.

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VI. ZIMMERMADL Ueber die Metarnerie des Wirbe1thierlcopfes. Prslxxrmiä

rä XIV-ex. Gereizt-Ä. F. VersamtnL München. S. 1o7-—It3. 584 » Lock: kvop XI.


Locy 1895 Contents: General Introduction | Part I - Metamerism of the Head | Part II - The Sense-Organs | Figures

Cite this page: Hill, M.A. (2024, May 8) Embryology Paper - Contribution to the structure and development of the vertebrate head 2. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_Contribution_to_the_structure_and_development_of_the_vertebrate_head_2

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