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559 


1886.] On the Median Eye in Lizards. 

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XIV. Preliminary Communication on tlie Structure and 
Presence in Sphenodon and other Lizards of the Median 
Eye, described by von Graaf in Anguis fragilis.” By W. 
Baldwin Spencer, B.iL, Demonstrator of Comparative 
x4natomy in University of Oxford, Fellow of Lincoln College. 
Communicated by Prof. H. N. Moseley, F.R.S. Received 
June 10, 1886. 

In 1872 Leydig* described a structure in Lacerta agilis , L . muralis, 
L. vivipara , and Anguis fragilis , to which he gave the name of “ frontal 
organ.” 

In the embryo, owing to its being deeply pigmented, it forms a 
prominent feature on the roof of the original forebrain in connexion 
with the pineal gland; in the adult it lies immediately beneath the 
skin, and, according to him and subsequent observers, completely sepa¬ 
rated from the brain. 

In Anguis fragilis the organ is seen microscopically to consist of long 
cells like those of a cylindrical epithelium, which are so arranged that 
together they form a shallow pit with a circular outline. The edge of 
the pit is directed downwards, and has a thick black girdle of pigment. 
It corresponds in position to that occupied by the parietal foramen 
in the adult. 

Leydig regarded our knowledge of the organ as insufficient to allow r 
of any statement being made with regard to its function. 

Rabl-Riickhard,t in 1882, describing the development of the pineal 
gland in the trout, pointed out the resemblance between its develop¬ 
ment as a hollow outgrowth of the brain and that of the optic 
vesicles. 

Granted such secondary developments from the epiblast and meso- 
blast as combine to produce the eye, and which are absent in the case 
of the pineal gland, though the distal extremity of the latter lies in a 

# “ Die in Deutschland lebenden Arten der Saurier,” p. 72, taf. 12. 

f <( Zur Deutung und Entwickelung des G-ehirns der I^noehenfische. , ’ Arch, 
fur Auat. u. Phys., Jahrg. 1882, p. 111. 


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560 


Mr. W. B. Spencer. [June 10, 

favourable position immediately beneath the epiblast, and, he states, 
there is no difficulty in the way of the idea that an unpaired sense 
organ similar to the eye would be developed out of the pineal gland. 

Ahlborn^ also, in 1882, independently arrived at the conclusion 
that the epiphysis is to be regarded as the remains of an unpaired 
median eye, founding this conclusion upon general considerations, 
such as the agreement in origin of the eye vesicles and the epiphysis 
as hollow outgrowths of the brain, the connexion of the epiphysis 
with the eye region of the brain (especially the optic thalami), and 
the peripherally directed position of the structure in Selachian Ganoids 
and Petromyzon, and the completely peripheral position in Amphibia 
on the outside of the skull. He even goes so far as to suggest a 
comparison of this structure with the unpaired eye of Amphioxus and 
Tunicates. 

More lately Henri de Graaff has published an outline of his 
results in studying the development of the epiphysis in Amphibia, and 
its structure in the adult Lacerta agilis and Anguis fragilis. 

He agrees with Strahl and Hoffmann in stating that the “frontal 
organ 51 of Leydig is the distal part of the epiphysis completely sepa¬ 
rated off from the proximal. 

He describes in detail the structure of the organ in Anguis, where 
it develops, he says, into a structure very similar to a highly 
organised invertebrate eye, as that of Cephalopods, Pteropods, and 
Heteropods. 

The following is a preliminary notice of results obtained recently 
by studying the structure of the organ in various forms of lizards, at 
the suggestion of Prof. Moseley, and by means of materials procured 
for the purpose with great kindness by him from various sources.J 

The forms investigated at present are the following, though only 
the more important results obtained from a few are given in th 
abstract:— 

Hatteria punctata. 

Lacerta ocellata. 

' Lacerta vivipara. 

Iguana (2 sp.). 

Galotes ophiomaca. 

Galotes versicola. 

Leiodera nitida. 

Plica umbra. 


Anolius (sp ?). 
Grammatopleora barbata. 
Chameleo vulgaris. 

Stellio cordyUna. 

Varanus bengalensis. 
Varanus giganteus. 
Gyclodus gigas (?). 

Seps chalcidica. 


* “ TJeber die Bedeutung der Zirbeldriise.” “ Zeit. fiir Wiss. Zool.,” vol. 40 
(1884), p. 336. 

f “ Zur Anat. u. Entwick. der Epiphyse bei Amphibien u. Beptilien.” Zool. 
Anzeig., Jahrg. 9 (1886), p. 191. 

X I am especially indebted to Professor Gunter, through whose kindness I have 



1886.] 


561 


On the Median Eye in Lizards . 

(1.) External Appearance, 

The organ is situated upon the dorsal surface in the median line, 
and at varying distances posterior to the level of the paired eyes; the 
presence or absence of an external indication of the organ can be by 
no means relied upon as indicating the existence or non-existence of the 
structure in a highly developed state. In many cases, as Varanus Ben - 
galensis, the various species of Calotes and Lacerta ocellata , the organ is 
marked externally by the presence of a specially modified scale, usually 
considerably larger than the surrounding ones, and with a circular 
patch of pigment behind the whole resembling a cornea. . Being 
transparent, and forming the anterior boundary of a capsule con¬ 
taining the organ, the appearance of a dark pupil surrounded by a 
light circle is produced. On the other hand, as in Plica umbra or 
Cyelodus (sp.), a more or less highly specialised scale may be present, 
but the organ beneath be not highly developed; or again, as in 
Hat.teria, there may be no special scale, but only a general trans¬ 
parency in the median line immediately above the organ, which may 
nevertheless be in a highly developed state. 

(2.) Position of the Organ . 

The organ may lie at different levels imbedded more or less deeply 
in connective tissue beneath the skin, or even within the skull cavity, 
but is always placed external to the dura mater. It always has a 
definite relationship to the parietal foramen usually lying within this. 
In Calotes it is placed immediately beneath the specially modified scale; 
in most forms, such as Varanus, Seps, Anolius, Leiodera, &o., it lies 
within the foramen, and separated by specially modified connective 
tissue from the skin. In Hatteria it lies on the inner side of the 
airamen, which is filled up by a plug of connective tissue, and in 
Lacerta ocellata, the bone around the foramen is modelled to fit closely 
to the outline of the organ and the connective tissue surrounding 
this. 

(3.) Structure of the Organ . 

It may be said at once that Leydig’s “frontal organ ” resembles in 
essential structure an invertebrate eye. 

This resemblance has lately been clearly pointed out by Gfraaf, in the 
case of Anguis fragilis, and is found to hold good for many others. 
He in common with all previous observers regards the organ as the 

been allowed to examine duplicate specimens of ten species from the British 
Museum ; they are not all described in this communication, but will be dealt with 
more fully subsequently. By Professor C. Stewart's kindness also I have been able 
to examine duplicate specimens of Iguana and Yaranus from the museum of the 
.Royal College of Surgeons. 

VOL. XL. 2 P 



562 


Mr. W. B. Spencer. [June 10, 

distal portion of the epiphysis, which becomes completely separated 
off from the proximal portion of the same, and lies completely sur¬ 
rounded by connective tissue in the parietal foramen. If in contact 
with a nerve, as frequently happens according to Graaf in Amphibia, 
then the nerve in question is a subcutaneous branch of the ramus 
supramaxillaris of the trigeminal. 

It is difficult to imagine why a single medianly placed organ should 
be supplied in any way by a branch from one only of two paired 
lateral nerves. 

The two most important facts established by the present series of 
observations are— 

(1.) That Leydig’s “ frontal organ” exists as a structure comparable 
to an invertebrate eye, widely distributed amongst Lacertilians. 1 fc 
may, in reference to its position and structure, be perhaps best called 
the 'pineal eye. 

(2.) That the eye is connected by a medianly placed nerve with the 
proximal portion of the epiphysis, and thus with the dorsal surface of 
the brain in the median line. 

There can be further little doubt that this nerve is the remains of 
the part connecting the distal with the proximal part of the epiphysis, 
that it is in other words formed as the optic nerve from a hollow 
outgrowth of the brain, which subsequently becomes solid. 

The structure of the eye in two or three typical cases is as 
follows :— 

(a.) Hatteria .-—In this form the organ is well developed, and being 
through Prof. Moseley’s kindness enabled to procure a fresh specimen, 
it has been possible to determine the elements comprising the optic 
vesicle. 

In all the eyes yet examined a lens is present. Yon Graaf figures 
it in Anguis as separated from the hinder part of the vesicle, but this 
does not hold good for any of those examined during the course of this 
work. The lens, on the contrary, appears to be only the modified 
anterior portion of the optic vesicle with the hinder walls of which 
it is directly continuous. In Hatteria it is somewhat cone-shaped, 
with a broad base corresponding to the anterior surface of the vesicle ; 
it is distinctly cellular, the nuclei being well marked, and the cells 
having a definite arrangement. 

The walls of the vesicle posterior to the lens consist of the following 
elements :—(1.) A layer of rods bordering the vesicle internally, 
deeply imbedded in dark pigment, arranged as seen when the rods are 
separated so as to give the latter a clearly marked striated appearance. 
(2.) External to these is a layer composed of rounded nucleated 
elements, twm, and in fact possibly three, rows deep. (3.) External to 
this what may be called a molecular layer, consisting of finely punc¬ 
tated material, through which seems to run a supporting structure; 



563 


1886 .] On the Median Eye in Lizards . 

processes from the structures on both sides of the layer seem to run 
into its substance. (4.) External to the molecular layer an outer part, 
in which three kinds of elements may be distinguished: ( a ) round 
nucleated cells, somewhat larger than those of the inner layer; ( b ) 
rod-like structures, somewhat conically shaped, with their broad ends 
external; (c) small nucleated spindle-shaped elements, placed between 
the latter at their bases. (5.) Though difficult to trace, a fine layer 
of nerve fibres appear to spread round the vesicle from the nerve 
which enters it at the surface nearest the epiphysis. The elements 
are connected serially, though processes from the rods may be seen 
passing at times directly into the molecular layer, or the rod elements 
externally, or even right through to the external surface. Such pro¬ 
cesses are accompanied by pigment, and may in some cases merely 
indicate supporting structures. 

In Hatteria, as in other forms, a special bundle of rods lying in the 
optic axis is highly developed, being much lengthened out and 
running down into the nerve, their outer extremities being in connexion 
with a particular group of nucleated cells. 

Yon Grraaf describes in Anguis a layer of small rod-like structures, 
similar apparently to those found in many invertebrate eyes, though he 
is not certain as to their nature. In Hatteria and other forms examined, 
the vesicle appears to have been filled during life by a fluid material, 
and this in coagulating adheres to the wall. The coagulation often 
apparently sets in from definite points, and these being the ends of 
rods, gives the appearance, under certain conditions of light, of 
refracted processes attached to these structures. 

The nerve enters the vesicle posteriorly, certain fibres appear to enter 
into connexion with the cells connected with the specialised rods, the 
remainder spread out around the external surface of the vesicle, and 
here enter into connexion with the elements, that is, the rods 
bound the internal surface of the vesicle, and the nerve-fibres the 
external. 

The nerve, whilst differing in appearance from an ordinary one, 
yet resembles more closely than anything else the developing optic 
nerve, being formed of long spindle-shaped elements, which recall the 
stage passed through when the at first round cellular elements of the 
optic stalk are gradually lengthening out. The nerve in both cases 
developes in a similar manner. 

The whole eye lies in a special capsule of connective tissue into which ■ 
enters and breaks up a blood-vessel, this vessel being present in con¬ 
nexion with the eye in all Lacertilia examined, even in those in which 
a nerve could not be distinctly traced. 

(b.) Laeerta ocellata .•—In this form the organ lies considerably below 
the surface, and so shut in by bone that it may be said to lie within 
the skull. The dura mater which surrounds it is deeply pigmented, 

2 r 2 



5fi4 Mr. W. B. Spencer. [June 10, 

and the presence of the branched pigment cells renders the examina¬ 
tion of its structure very difficult. 

A well developed cellular lens is present, formed from the anterior 
part of the vesicle apparently. The retinal elements are imbedded 
in pigment, and, save the rods, are difficult to detect, though by care¬ 
ful examination two rows of round nucleated cells may be detected. 
The pigment obscures the nerve, which is nothing like so clearly 
marked as in Hatteria, due largely to the dura mater encasing the 
eye so closely that no capsule is formed. 

The nerve enters posteriorly, and a slight differentiation of the rods 
at two points may be noticed, the nerve appearing to divide into two 
just before entering the eye ; it passes down, lying in the dura mater 
to join the proximal part of the epiphysis, which is itself deeply pig¬ 
mented. 

The blood-vessel accompanying the nerve is well developed. 

(c.) Iguana .—The structure agrees in the main with that of Hatteria, 
though, owing to the eye being not so well preserved as in the latter, 
the elements cannot be so clearly differentiated. The lens is cellular, 
and somewhat similarly shaped to that of Hatteria; the rods are as 
usual deeply pigmented, and external to them may be detected (1) a 
row of round nucleated cells ; (2) a well-marked molecular layer, in 
which the nucleated cells are often embedded,* and (3) an outermost 
layer of cone-shaped bodies, similar to those of Hatteria. 

The rods in the optic axis are again lengthened out and prominent, 
running down into the nerve. 

The eye lies in a capsule of connective tissue within the foramen, 
and into the same space passes also a hollow process from the 
epiphysis, into which the nerve enters. 

In another Iguana examined the process appears not to be hollow, 
and the eye, instead of lying in a capsule, is closely invested by con¬ 
nective tissue. 

In both cases the vesicle is filled with a coagulation, indicating the 
presence of a fluid material in life. 

( d .) Anolius (sp. P).—The eye lies close beneath the skin, and almost 
entirely fills up the parietal foramen, the remainder being occupied 
by vacuolated tissue, in which large nucleated cells are present at 
intervals, together with branching pigment cells. 

The eye is elongated in the direction of the optic axis, and provided 
with a nerve running back through the vacuolate tissue and entering 
the proximal part of the epiphysis. 

Pigment is largely developed, ensheathing all the elements of the 
retina; the rods may be traced into rounded elements ; these again 
externally into cone-shaped elements. Tue rods in the optic axis are 
again modified and prominent, and their ends facing into the vesicle 
appear striated. 



565 


1886 .] On the Median Eye in Lizards . 

The lens is cellular, and has a slight development of pigment in some 
of its cells in the optic axis. 

Special connective tissue fibres pass, as in some other forms, from 
the capsule to the edge of the lens. 

(e.) Leiodera nitida. —This may be taken as the type of several 
forms, such as Galotes ophiomaca, and versicola and Seps chalcidica , 
in which the eye is lengthened out in a direction at right angles to the 
optic axis. The lens is distinctly cellular, and continuous with the 
hinder walls of the vesicle. The retinal elements consist of (1) rods; 
(2) a layer of round nucleated cells ; (3) a well marked clear space, 
corresponding in position to the molecular layer; and (4) an external 
layer of cone-shaped elements. 

In these forms a nerve cannot be detected with certainty, though 
very possibly with freshly killed specimens its presence might be 
demonstrated. 

In Leiodera the scale above the eye is beautifully modified, and a 
transparent dome-shaped cornea.developed, sections showing that the 
pigment is absent from the scale in this region, though very abun¬ 
dantly developed elsewhere. 

(/.) Varanus Bengalensis and giganteus. —The eye in these forms 
will be dealt with fully on a subsequent occasion ; at present one point 
only will be mentioned. In two specimens examined (perhaps of dif¬ 
ferent species of Varanus) the connexion with the proximal part of 
the epiphysis was of an importantly different nature; in one it was 
in the form of a hollow process , in the other of a solid stalk, much 
as in Hatteria. 

There can be no doubt that the connecting parts in the two instances 
are equivalent to each other. 

(g.) Gyclodus gigas (?).—This may be taken as the type of those 
forms, in which no structure comparable to an eye is at present found. 
The distal extremity of the epiphysis is swollen out, the cells of its 
walls, which are thrown into folds, become shaped like those of cylin¬ 
drical epithelium, and amongst them pigment is deposited, but no 
true retina is formed, or any structure comparable to a lens. 

The distal swollen part of the epiphysis is enveloped in pigment in 
the dura mater, some distance in front of the proximal part, with 
which it is connected by a hollow process. The whole structure lies 
on the inner side of the cranium, closely fitted to the bone, as in 
Lacerta> ocellata. 

The scale on the surface of the head is imperfectly modified to form 
a cornea, and has the appearance of degenerating. 



566 


Titles of Papers deferred . 


[June 10, 


XV. “ Star Photography; the Effects of Long and Short 
Exposures on Star Magnitudes/’ By ISAAC Roberts, 
F.R.A.S. Communicated by the Rev. S. J. Perry, E.R.S. 
Received May 21, 1886. 

[Publication deferred.] 

XVI. “ An Instrument for the speedy Volumetric Determination 
of Carbonic Acid.” By W. Marcet, M.D., F.R.S. Received 
June 9, 1886. 

[Publication deferred.] 


XVII. “ On the Practical Measurements of Temperature; 
Experiments made at the Cavendish Laboratory, Cam¬ 
bridge.” By H. L. Callendar, B.A., Scholar of Trinity 
College, Cambridge. Communicated by J. J. Thomson, 
F.R.S., Professor of Experimental Physics at the Cavendish 
Laboratory. Received June 9, 1886. 

[Publication deferred.] 

XVIII. “• The Determination of Organic Matter in Air.” By 
Professor T. Carnelley and William Mackie. Commu¬ 
nicated by Sir H. E. Roscoe, F.R.S. Received June 10, 
1886. 

[Publication deferred.] 

XIX. “The Carbonic Acid, Organic Matter, and Micro¬ 
organisms in Air, more especially of Dwellings and 
Schools.” By Professor T. Carnelley, J. S. Haldane, and 
Dr. A. M. Anderson. Communicated by Sir H. E. ROSCOE, 
F.R.S. Received June 10, 1886. 

[Publication deferred.] 

XX. “Preliminary Report on the Pathology of Cholera Asiatica 
(as observed in Spain, 1885).” By C. S. Roy, F.R.S., 
J. Graham Brown, M.D., &c., and C. S. Sherrington, 
M.B. Received June 10, 1886. 

[Publication deferred.] 

The Society adjourned over the Long Vacation to Thursday, 

November 18fch.