Book - The Pineal Organ (1940) 33
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Gladstone RJ. and Wakeley C. The Pineal Organ. (1940) Bailliere, Tindall & Cox, London. PDF
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The Pineal Organ
Chapter 33 General Conclusions
The surgery of the pineal organ, although yet in its infancy, may be said to be advancing rapidly owing to the fact that neurological diagnosis becomes more established and more accurate each year.
The symptomatology tends to be more definite : there is usually a severe degree of raised intracranial pressure, associated with headache, vomiting, papilledema, epileptiform fits, and some cranial nerve paralysis. The eye signs are definite, with loss of pupillary reaction and failure of upward movement of the eyes.
Operations for the removal of pineal tumours have become standardized ; and even if the complete removal cannot be undertaken, a postoperative course of deep X-ray therapy will complete the cure, as the majority of pineal tumours are radio-sensitive.
Morphology
1. The pineal system, including the parietal eye, its nerves, and the related cerebral ganglia is one of the most ancient sensory systems of the vertebrate phylum. The existence of a parietal sense-organ being plainly indicated in certain of the primitive ostracoderm fishes by the presence of a pineal plate, showing either a complete pineal canal or a pineal pit on the inner surface of the plate. The canal and plate are well seen in the examples of Anaspida and Cephalaspida, which are found in strata ranging from the lower Silurian 1 to the Devonian eras and in specimens of Pterichthys and Bothriolepis belonging to the Order Antiarchi, found in upper Devonian strata.
2. In these fishes there is definite evidence that the parietal eye coexisted with other sensory organs of the head, namely : the lateral eyes, the olfactory organs, and the vestibular or static organs ; and also that these had approximately the same relative positions to each other and the parietal foramen or pit that they have in the heads of living cyclostomes and other vertebrates.
3. The closure of the outer or superficial end of the parietal canal by a thin plate of bone in certain examples indicates that in these specimens regression of the organ had already commenced, and that it had ceased to function as a visual organ.
1 The Silurian Epoch has been estimated by Barrell to embrace a period from 390,000,000 to 460,000,000 years ago.
4. In some palaeozoic fishes, e.g. Pholidosteus, Rhinosteus, and Titanichthys, bilateral pineal impressions are visible, either (a) on the dorsal or outer aspect of the pineal plate, or (b) on its inner or intracranial surface. Moreover, evidence of the bilateral nature of the pineal system is also present in existing species. Thus in some species in which two separate parietal eyes are present, e.g. Petromyzon or Geotria, each eye is connected by its own nerve with the habenular ganglion of the same side ; and when the two parietal organs differ in size there is a corresponding difference in size of the habenular ganglion and also of the fasciculus retroflexus of Meynert of the two sides.
5. In those animals in which there is normally only one parietal sense-organ or an unpaired epiphysis, the normal connections of the basal part of the stalk of the parietal organ or of the epiphysis with the right and left habenular ganglia and posterior commissure are bilateral. Moreover, the occasional occurrence of accessory parietal sense-organs and indications of coalescence of two retinal placodes, or of two lenses in a single eye, may also be regarded as evidence pointing to a primary bilateral origin of the system. Bifurcation of a single pineal stalk into two terminal vesicles has also been observed as a variation in different classes of vertebrates, more particularly in fishes (Cattie) ; in amphibia (Cameron) ; in reptiles (Spencer, Klinckowstroem) ; in birds, e.g. Emys europea (Nowikoff ) ; and among mammals several instances in human embryos.
6. The development of two separate pineal diverticula, in the median plane and in the interval between the habenular commissure and the posterior commissure, seems to be a rare occurrence, although two terminal vesicles which have arisen from a common stalk may lie one behind the other. If one parietal vesicle only is developed and it is later cut off from its stalk of origin, the latter is usually displaced backwards so that the epiphysis lies behind the parietal eye. Apart from the paraphysis, which originates anterior to the velum transversum, diverticula arising from the roof of the third ventricle in front of the habenular commissure are developed from the dorsal sac or postvelar arch, and give rise to the suprapineal recess or are an outgrowth from the choroid plexus. Neither the paraphysis nor diverticula originating from the postvelar arch are epiphyseal in nature.
7. The parietal eye, which seems to have attained its maximum development in certain extinct amphibia, reptiles, and mammal-like reptiles, and the epiphysis or pineal body usually show signs of regression in specimens of mature living species. The most important of these indications are : (1) the frequent absence or disappearance during the later stages of development of the nerve or nerves connecting the parietal eye or epiphysis with the central nervous system ; (2) excessive development of pigment in or around the retinal cells, or development of pigment in the lens or cornea ; (3) degeneration of the retinal epithelium of the parietal organ ; in the epiphysis of anamniota degeneration of the lining epithelium of the pineal stalk ; or in the pineal organ of adult birds, degeneration of the epithelium lining the follicles, accompanied in some cases by obliteration of the lumen of the follicles ; and in the pineal organ of adult mammals frequent degeneration of the parenchyma cells. The degree of degeneration of the parenchyma cells in adult mammals varies both in different individuals and in different parts of the organ in the same individual. In the latter case it is common to find areas in which the parenchyma cells have disappeared altogether and been replaced by neuroglial plaques or bands. These often break down in the centre to form cysts, and deposits of calcareous salts are frequently seen in the walls of the cysts or in the trabecular or capsule. See Figs. 221, A, B, C, 2 %5> 317? and 318.
8. In addition to the evidence in some extinct and living vertebrates of a single pair of pineal organs which are united, either partially in the stem of a Y-shaped organ bearing two terminal vesicles, or completely fusion having taken place throughout the whole length of the stalk which terminates in a single composite vesicle, there are indications, according to certain authors, of the existence of two pairs of parietal organs arranged serially, one pair lying in front of the other. Thus in the Palaeozoic fish Bothriolepis (Fig. 319), Patten describes, in addition to the median eye tubercle situated on the pineal plate between the two orbital cavities, a pair of bilateral impressions which are visible only on the internal aspect, and are present on the deep surface of the post-orbital plate (Fig. 320). These he believed lodged a pair of posterior median or parietal eyes. The three impressions or pits form a triangular group disposed in a similar manner to the median eyes of many invertebrates, and, more particularly, the triplacodal entomostracan eye which is found in certain Branchiopods, e.g. Apus and Branchipus (Figs. 248, 250), and in the " carp louse " Argulus foliaceus, which is typical of many other crustaceans. Another interpretation of the meaning of these two impressions is given on p. 472 by Stensio, who suggests that they are produced by the attachment of paired muscles of the lateral eyes. The existence and exact position of two pairs of retinal placodes which will give rise to the median eyes of vertebrates and which lie one in front of the other on each side of the open medullary plate, has not, we believe, been definitely established, nor is there agreement with respect to their exact position relative to the pair of placodes which give origin to the lateral eyes. Thus, Patten assumes that two pairs of retinal placodes which become incorporated in the roof of the third ventricle and give rise to the parietal eyes of vertebrates lie in front of those for the lateral eyes in a position which he describes as typical, in the development of Arachnids (Figs. 257, 258, 259), whereas Locy in his account of two pairs of
p pi , n h " accessor y " or pineal eyes in
Acanthias, figures these as lying behind the placodal pits, which will develop into the optic vesicles of the lateral eyes (Fig. 143). Moreover, the intermediate stages between the first appearance of the two pairs of rudiments for the accessory eyes and the outgrowth of the pineal diverticulum in the later stages do not appear to have been definitely established by Locy. The appearance, however, of symmetically arranged sensory placodes or pigment spots formed in series around the margin of the medullary plate (Figs. 257, 258, 259), or head region (Fig. 19) in invertebrates suggests the possibility that one pair of a series of simple eyes being more favourably placed for the reception of visual impressions than the others — e.g. at the
Fig. 319. — Dorsal Aspect of Bothriolepis canadensis, showing the nasohypophyseal Opening, Lateral Orbits, Pineal and Postorbital Plate, and the Cephalic Appendages WHICH HAVE RECENTLY BEEN SHOWN TO
be True Pectoral Fins. (After Patten.)
n.h. op. : naso-hypophyseal opening.
orb. : orbital cavity.
p.f. : pectoral fin.
p. orb. pi. : postorbital plate.
p. pi. : pineal plate.
antero-lateral margins of the head on each side — becomes more highly evolved than those in front of or behind this pair The more favourably situated pair, it may be assumed, gains the ascendancy over the others and becomes the principal pair, whereas the less favourably situated ocelli retain their primitive simple character and tend to degenerate. If this is the case, and if as is commonly believed both the lateral and median eyes of vertebrates have been evolved from the simple eyes of a lowly organized type of invertebrate, the discrepancy which exists with regard to the position of the median or accessory placodes relative to the optic pits for the lateral eyes in vertebrates may be readily explained.
The presence of the two impressions on the deep aspect of the posteromedian plate of Bothriolepis described by Patten has recently (1929-1930) been confirmed by Stensio, who gives an illustration (Fig. 321) of the same two pits in Aster olepis, an allied genus. He suggests that the pits are produced by the attachment of one or several of the recti muscles of
Fig. 320. — Dorsal Aspect of the Ocular and Olfactory Plates of Bothriolepis ENLARGED. (AFTER PATTEN.)
A part of the olfactory and rostral plates has been removed on the left in order to expose the deeper-lying sclerotic plates. Between the lateral eyes is the quadrangular parietal plate, nearly perforated by a deep conical pit opening inward and covered externally by a thin, lens-like tubercle, beneath which was the parietal eye. On the deep aspect of the post-orbital (post median) plate are two similar pits, which Patten believed were occupied by a pair of posterior parietal eyes.
a.s. pi. : anterior sclerotic plate. le. : lateral ethmoid. Is. pi. : lateral sclerotic plate. me. : mesethmoid.
0. : corneal opening.
01. : site of primitive olfactory organ. p.e.t. : parietal eye tubercle.
po. pi. : postorbital plate.
p.p. : position of paired pits on inner
aspect of po. pi. p.s. pi. : posterior sclerotic plate. r. : rostrum. rs. : shelf plate on inner surface of
rostrum.
the lateral eye on each side ; a supposition which appears much more probable than Patten's hypothesis ; more especially since the investigations of Stensio and others into the general anatomy of these fishes have definitely proved that the cephalic appendages of Bothriolepis, Asterolepis ornata, and allied genera — which were at one time thought to closely resemble the cephalic appendages of the Merostomata, e.g. Eurypterus — are true pectoral fins, consisting of two segments, each of which contains inside the dermal bony exoskeleton, an axial cartilaginous endoskeleton, which in Bothriolepis was provided with a perichondral layer
of lime-bearing tissue, intermediate between true bone and calcified cartilage. The endoskeleton in the specimen described did not participate either in the axial articulation or in the articulation between the two segments of the fin, since both these articulations were formed solely by the dermal bones. In the proximal articulation the inner ends of the dermal bones embraced the neck, of the processus brachialis of the anterior ventro-lateral plate ; the opening in the dermal bones of the appendage which surrounded the process is called the axial foramen, and besides enclosing the head or condyle of the processus brachialis transmitted vessels and nerves to the appendage. The intermediate position in a direct line of descent between fishes and the invertebrate Merostomata, which was claimed for the Antiarchi (Bothriolepis,
Pterichthys), is thus not confirmed by recent work. This Order being now considered to belong definitely to the fishes, their appendages being true pectoral fins and their resemblance to the large paddle-like appendages of the Merostomata (Eurypterus or Pterygotus) being functional rather than structural.
9. The theory that a higher race of animals which was " predominant " arose directly from a lower race in the geological period which immediately preceded it, must, in the light of modern knowledge, be radically modified, since it is evident that the common ancestor of two highly differentiated and in many respects divergent classes must have been of a much simpler type than either of the two classes under consideration. Moreover, the divergence of the two classes must have taken place long before dominance of one class over the other could have existed as a factor in their evolution. Certain points of similarity in particular organs or systems seem to have been preserved in the two divergent classes, although even these when critically examined are found to present modifications in detail ; and the modifications or divergences are in general more pronounced in the phylogenetically older races and in adult animals as compared with their larval or embryonic stages. To take a concrete instance, the difference between the compound faceted eye of an arthropod and the inverted eye of an adult vertebrate is very great, and since the upright faceted eyes of certain arthropods were already highly evolved in some trilobites which were living in the Upper Cambrian period and since median eye tubercles have been found in both larval and adult specimens of Trinucleus and
Fig. 321. — Intracranial Surface of postmedian plate of asterolepis ornata. (after Stensio.)
other closely related forms of trilobites, it is evident that the distinction between median eyes and lateral eyes had occurred at a very early date and that the time required to produce the differentiation of the complex faceted eyes must place the actual origin of the lateral eyes of arthropods at a still earlier period.
The degree of differentiation of the lateral eyes of invertebrates varies greatly in different classes, and the divergence from the simpler types is greater in the adult animal than in the larva and in the more highly organized types of animal than in the more primitive.
Now the earliest known fossil vertebrates, the ostracoderms, agree with the invertebrate Eurypteridae in possessing both lateral and median eyes, and they were contemporary with each other, living in the sea under much the same conditions and in the same geological period. Comparisons were therefore made between the ostracoderms and the eurypterids, and between the living representatives of these two extinct classes, the cyclostomes, which are the direct descendants of the ostracoderms and certain of the more primitive types of cartilaginous fishes on the one hand, and the land scorpions, spiders, Limulus, and certain of the Crustacea on the other ; all of which resemble each other in possessing lateral and median eyes in the same relative positions with regard to each other and other organs in the head.
It will be unnecessary to refer to more than two or three of the more salient points which have recently been settled by a critical examination of the alleged similarities between the ostracoderm fishes and the eurypterids. One of these apparent similarities was the possession in Cephalaspid fishes of an exoskeleton which seemed to closely resemble the chitinous exoskeleton of eurypterids and Xiphosura. Now the exoskeleton of the fishes, whether it consists of denticles, scales, scutes, or " armour plating," consists of an outer layer of epidermal bone or of enamel, which covers a dermal bony stratum, or osteodentine ; and it will be recalled that in the development of a tooth the formation of the enamel is at the inner or deep end of the enamel cells or ameloblasts ; further, the increase in thickness of the enamel is by the laying down of new layers on the superficial surface of those which have already been deposited ; and also that the dentine which is formed on the surface of the dermal papilla by the odontoblasts is layed down in the reverse direction to the enamel, namely, from without inwards, the increase in thickness of the dentine being due to its formation at the outer or superficial ends of the odontoblasts. The shields or plates forming the armour plating of the ostracoderm fishes are of the nature of a vaso-dentine, and the rhombic scales on the posterior part of the body, in some examples, e.g. Pteraspis (Fig. 322), were coated on their superficial aspect by an enamellike layer. In the formation of the chitinous exoskeleton of an arthropod, however, there is a secretion of a cuticular nature from the outer ends of the columnar hypoblast cells or deric epithelium, this becoming condensed forms a hard chitinous shell on the surface of the hypoblast. The shell thus consists of a thickened and hardened cuticle and differs both structurally and chemically from enamel and osteodentine. Chitin is a nitrogenous and carbohydrate substance allied in its composition to horn ; it may be impregnated with lime salts, but no true Haversian systems, such as those present in bone, are found in it. Increase in size of the animal including its appendages is obtained by a series of moults (ectdyses) in which the hardened cuticle undergoes softening and is cast off; the
Fig. 322. — Lateral Aspect of Pteraspis rostrata, an Ostracoderm Fish characterized by the absence of pectoral or pelvic flns, a hypocercal Tail, Large Plates or Scutes covering the Head and Anterior Part of the Body, and Rhombic Scales covering the Remaining Part of the Body and Tail.
The pineal plate is not perforated in P. rostrata, but in some specimens a pit is present on its internal surface. In P. monmouthensis a complete perforation is found. (E. Ivor White.)
growth of the animal taking place chiefly in the intervals between the moults ; whereas the increase in size of vertebrates which possess an exoskeleton is similar to that of the skull, a continuous process, taking place partly along the lines of suture between the plates and in the case of dermal bones which have sunk beneath the surface of the skin also by deposit of new bone on the surface of the old, and absorption of bone on the internal surface.
Another important distinction between the fishes and the palaeostracan arthropods is the existence of median dorsal and caudal fins in the former, as compared with the long, tapering caudal spine of the Xiphosura, as well as the presence in some, e.g. Asterolepis, Remigolepis, Bothriolepis, of a cartilaginous endoskeleton, having the structure of a true pectoral fin inside the bony plates forming the exoskeleton. Finally the existence of a notochord (Fig. 238, p. 341), and the vertebrate position of the heart and main blood-vessels relative to the alimentary canal are fundamental differences which serve to place the ostracoderms definitely among the fishes, and not, as was formerly supposed, in an intermediate position within the direct line of descent of the vertebrates from a highly differentiated ancestral arthropod, such as Limulus, or a species resembling any other of the living arachnids.
Summary of Observation on the Development and Structure of the Human Pineal Organ
1. The pineal diverticulum first appears in human embryos of approximately 15 mm. length.
2. The apex of the diverticulum is primarily directed forwards.
3. The pineal outgrowth lies a short distance in front of the posterior commissure, and sometimes presents a constriction subdividing it into an anterior and posterior segment.
4. The whole thickness of the neural wall participates in the formation of the pineal evagination.
5. In some specimens there is an indication of the anterior segment being subdivided into right and left lobes.
6. The " anterior lobe " first described by Krabbe appears in embryos of about 22 mm. length as several neuro-epithelial buds which grow forward into the surrounding connective tissue.
7. A well-marked supra-pineal recess (dorsal sac) is present at the 22-mm. stage.
8. Transverse grooves, which are produced by folding of the roof of the aqueductus cerebri in the region of the posterior commissure, represent temporary infrapineal recesses.
9. Between the third and fourth months of foetal life there occurs an active proliferation of cells derived from the inner or ependymal zone of the pineal diverticulum. These grow outward in the form of cords, the component cells of which are arranged radially round a central axis which is destitute of nuclei. This is accompanied by a simultaneous ingrowth of vascular processes of mesenchyme.
10. A special mass of proliferating cells growing from the anterior wall of the main diverticulum gives rise to the solid anterior lobe of Krabbe, whereas the cords which grow from the fundus of the diverticulum form the principal solid part of the posterior lobe. The cavities at the base of the stalk and that of the posterior diverticulum appear to open out, and their lumina thus become incorporated in the cavity of the third ventricle, whereas the cavity of the main or anterior diverticulum, which may be cut off as the " cavum pineale," usually disappears.
11. The neuro-epithelial cells give rise to (1) the glia lining the fibrous capsule and covering the trabecular, (2) the parenchyma cells, and (3) the neuroglial cells (astrocytes). The surrounding connective tissue and ingrowing vascular mesenchyme form the fibrous capsule and the connective tissue basis of the septa and finer trabecular, including the contained vessels. Many of the sinusoidal vessels in the central part of the pale vascular areas, which are seen in the earlier stages of development, disappear, leaving only a very fine capillary plexus in the parenchymatous tissue of the lobules.
12. The parenchymatous tissue in the adult consists of a reticulum of branched pineal cells, among which are a few neuroglial cells, chiefly of the astrocyte type. The " alveolar " appearance which is sometimes seen in adult specimens is due to the persistence of primary neuroepithelial cords, cross-sections of which appear as rosettes.
13. Cells and nerve-fibres belonging to the sympathetic system accompany the vessels entering and leaving the pineal organ ; and medullated nerve-fibres connect the habenular and posterior commissures with the parenchymatous tissue, but the exact mode of termination of their axons with regard to the pineal cells is not certain. True ganglion cells belonging to the central nervous system and having an axis cylinder process, although described by some authors, appear to be very rarely seen in the human pineal gland, but transitional forms exist, which are intermediate between true nerve ceils and parenchymatous cells. These are described as " neuronoid."
The experimental and clinical evidence with respect to function of the mammalian pineal body is at the present time too conflicting to allow of any definite conclusions being drawn. We know that before the age of puberty, more especially in boys, pineal tumours have sometimes been associated with premature growth in size of the body, precocious development of the genital organs, and the early appearance of secondary sexual characters. But it seems probable that certain other factors have been involved in the production of these symptoms — more particularly pressure, either direct or indirect, of the pineal tumour on neighbouring parts of the brain, such as the hypothalamus and pituitary region, combined with the irritative reflex effects produced by increased intracranial tension — and that the symptoms are not directly attributable to disturbance of any special function possessed by the pineal body itself. Moreover, a considerable number of cases have been reported in which sexual precocity and macrogenitosomia have been present but there has been no pineal tumour, and the reverse condition in which a pineal tumour has been present in young boys but unaccompanied by the Pellizzi syndrome. Further, that although some cases of premature development of the breasts in girls and gynecomastia in males have been reported, these conditions appear to have been absent in the majority of cases of pineal tumour.
An accurate knowledge of the immediate anatomical relations of the pineal body, is essential in order to clearly distinguish the symptoms due to implication of neighbouring parts and those due to a supposed special function of the pineal gland. Some of the structures in close relation with the pineal body are : the aqueduct of Sylvius, the quadrigeminal plate, the geniculate bodies, the nuclei and nerve tracts of the ventral part of the midbrain, the thalamencephalon, the hypothalamus and " portal system " of vessels supplying the pituitary gland, the cerebellum, and the related intracranial nerves and blood-vessels. The mere enumeration of these parts which are liable to be involved in a growth of the pineal body will indicate that when the pressure symptoms are eliminated from the total " symptom complex " accompanying the growth of such tumours there is little left in support of the contention that the human pineal gland has a regulating influence on the normal development of the body and the genital organs, and more especially in the direction of inhibiting or retarding their growth.
In Fig. 323 we have tried to show in a diagrammatic manner the general distribution of the different types of median and lateral eyes in the animal kingdom. We have not attempted to include in this scheme any of the aberrant forms of eye such as those met with on the back of the Chitons, or " coat-of-mail shells," or invertebrate eyes with inverted retina; such as those on the back of Oncidium or at the edge of the mantle in Pecten, since these are not specially concerned in the phylogeny of either the paired median or paired lateral eyes of vertebrates, and although of great interest in showing how special organs are sometimes evolved in anomalous situations in adaption to special needs, they do not assist in tracing the general evolution of the eyes of vertebrates. We hope that the diagram will be of some assistance in showing graphically how very far removed the more highly organized classes of living vertebrates are from the highly organized living invertebrates ; and, although the form and dimensions of the " tree " are not intended to accurately represent the periods of time which have elapsed since the divergence of the various classes took place in the course of evolution, that it will give some indication of the way in which certain of the simple types have persisted to the present day without, it may be presumed, having undergone marked modifications in general form and structure, while others have diverged from the primary simple type, but have nevertheless retained some of their older traits, which appear either in a simple form in the early larval condition, or may be present in the adult, in a modified and highly differentiated form. We have limited the term " parietal eye " to the parietal sense-organ of vertebrates, and designated the median eyes of invertebrates as such, or as frontal, triplacodal, or entomostracan eyes. We do not, however, wish it to be inferred that we consider the parietal senseorgan of vertebrates has arisen quite independently of the median eyes of invertebrates. Further, we have used the term " parietal eye " in the singular although, as explained elsewhere, it may represent in some cases one member of a pair of median eyes or in other cases be formed by the fusion of the right and left members of a pair of primarily bilateral organs.
Fig. 323. — Scheme indicating the General Distribution of Different Types of Median and Lateral Eyes in the Animal Kingdom.
CLASSES OF ANIMALS E- OIFFERENT SEQUENCE OF GEOLOGICAL PERIODS b ESTIMATED NUMBER OF YEARS TYPES OF EYE
iRY, PLEISTOCENE 6 PLEIOCENE
TERTIARYqliGOCENE & EOCENE —
GREAT MAMMALS
SECONDARY or MEZOZOIC
$5,000,000 TO 240,000,000
JURASSIC
155. OOO.OOO TO 195,000.000
TRIASSIC
190,000,000 TO 240,000,000
PE RM I AM
PRIMARY
or
PALEOZOIC
2/5,000,000 TO 700000 000
215. OOO, OOO TO 280, OOO, OOO
CARBONIFEROUS
2 SO OOO.OOO To 33U.OOOOOO
370, 000,000
DEVONIAN
360. OOO, OOO TO 420 OOO. OOO
SILURIAN
3 90.000 OOO TO 46OOOOO00
ORDOVICI AN
480,000,000 TO 590 OOO.OOO
CAMBRIAN
550,000 OOO TO 700,000,000
ARCHAEAN
MAMMALS
BIRDS
GREAT REPTILES
LARGE PARIETAL FORAMEN or IMPRESSION IN SOME REPTILES & AMPHIBIANS, MAMMAL-LIKE REPTILES LAST TRILOBITES
GREAT AMPHIBIA
LAND SCORPIONS
STEGOCEPHALIA MAILED FISHES
MARINE SCORPIONS
EXISTENCE OF PAIREO-EYES OF VERTEBRATES PRESUMED
FIRST FISHES & INSECTS
VERTEBRATES APPEAR
INDICATIONS OF MEO'AIN &-LATERAL PAIRED-EYES OF INVERTEBRATES
FIRST TRILOBITES
EXISTENCE OF PAIRED-EYES PRESUMEO (WORMS. CRUSTACEANS. MOLLUSCS)
EVOLUTION OF PHOTO-RECEPTIVE ORGANS
INVERTEBRATES
Fig. 324 t — Geological Chart indicating the Order in which Organs Sensitive to Light and various Types of Eye have been evolved, and also the Estimated Age in which the Different Classes of Animals have been found. (Modified from Scheme and Data published by Gaskell and
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