Book - An Introduction to the Study of Embryology 5

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Haddon An Introduction to the Study of Embryology. (1887) P. Blakiston, Son & Co., Philadelphia.

   Introduction to Embryology 1887: Chapter I. Maturation and Fertilisation of Ovum | Chapter II. Segmentation and Gastrulation | Chapter III. Formation of Mesoblast | Chapter IV. General Formation of the Body and Appendages | Chapter V. Organs from Epiblast | Chapter VI Organs from Hypoblast | Chapter VII. Organs from Mesoblast | Chapter VIII. General Considerations | Appendix A | Appendix B
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Chapter V. Organs Derived from the Epiblast

As the epiblast constitutes the external skin of the embryo, it naturally has a protective function ; and it gives rise in the adult to the epidermis, together with those portions of the organs of active or passive defence which arise from the epidermis. It further gives origin to numerous glands, and also to the nervous system and to the sensory portion of the sense-organs. The functions of this layer may be summed up as protective, secretory, respiratory, and sensory.

Protective Structures


When the outer skin (epidermis) of an animal consists of a single layer of cells, it is usually protected by a more or less continuous and structureless membrane or cuticle.

The cuticle may become cornified, as in most compound Hydrozoa and some Polyzoa; or calcified, as in the Hydrocorallinae and other Polyzoa; or chitinised, as in the majority of Arthropoda. In most Crustacea the cuticle is both calcified and chitinised. Such an indurated cuticle (exoskeleton) may be produced into spines and other weapons of attack or defence.

The horny axial skeleton (coenenchyma) of the Antipathidse (and possibly of the Gorgoniidse) has been shown by Yon Koch to be the secretion of an invaginated ectodermal epithelium. It has been recently stated by Klaatsch that in some Hydrozoa (Clytia) the perisarc is produced by the outer layer of the ectoderm itself becoming chitinised. According to Yon Koch and Fowler, the hard parts of the Hexacoralla are also probably secreted by the ectoderm. The cells which secrete the spicules of Alcyonaria are also of epiblastic origin.

Iu many cases the outer layer (ectosarc) of the protoplasm of the epiblast cells gives rise to one or numerous delicate contractile protoplasmic hair-like processes (flagella or cilia), which penetrate the cuticle, when present, and have a lashing movement. They serve for the progression of the embryo or adult, or to set up a current in the surrounding medium for the procuring of food, aeration of the tissues, discharge of waste matter, and other purposes.

The epiblast cells (usually termed ectoderm) may in the Coelenterata develop within themselves, by a modification of their own protoplasm, sacs containing a long coiled thread, - the thread -cells or nematocysts, - which can be suddenly projected and form powerful stinging organs. In the Turbellarian worms analogous short rods often occur.

The shells of Brachiopods are secreted by the outer surface of the delicate pallial membrane, and therefore may be regarded as a special form of cuticle. The shells of adult Mollusca are composed of three layers, of which the cuticle or epiostracum (“ epidermis -) and the prismatic layer are secreted by the thickened edge of the mantle, while the general upper surface of the mantle secretes the nacreous layer.

Fig. 84. - Veliger Larvae of Mollusca.

A. Side view of veliger of Purple-snail (Ianthina). B. Longitudinal vertical optical section of early veliger of the Pond-snail (Lymnseus stagnalis), [After Hoices .] C. Optical section of primitive kidney of embryo Murex. D. Section of shell-gland of the same.

a. archenteron ; an. ciliated patch in position of future anus; bl. blastocoel (archicoel) ; blp. blastopore ; c. tuft of cilia above thickened epiblast at the apex of the head ; ep. epiblast ; /. foot ; hy. hypoblast ; mn. mantle-fold ; ms. mesoblast ; pig. spot of violet pigment ; p.k. primitive kidney ; r. invagination to form the sac of the radula ; sh. shell; sh.g. shell-gland; st. stomodseum ; v. velum ; y. yolk-cells, forming the liver in B.

In all Molluscan embryos an invagination of columnar epiblast (figs. 84 and 18) takes place on the dorsal side behind the velum. This is known as the shell-gland, and is of invariable occurrence : later it flattens out; the surface thus formed, the mantle, secretes the larval shell. In the Lamellibranchs the axial line of the shell-area remains uncalcified, and persists as the ligament and hinge-line of the adult. The primitive shell of Mollusca at first forms the apex of the permanent one, but it usually disappears in time.

A pit-like depression of the mantle occurs in all embryo Cephalopoda, which may be termed the shell-sac. This soon atrophies in Octopus, while that of the Squid and Cuttlefish secretes the 11 £ pen - and “ cuttle-bone - respectively. The shell-sac is often regarded as the equivalent of the shell-gland of other Mollusca, but Lancaster has shown that it cannot have the simple significance which it appears to possess : the student is referred to his paper for a statement of the argument. The conclusion arrived at is, in brief, that the shell-sac of embryo Cephalopoda is not only equivalent to the shell-gland of other Mollusca, but in addition corresponds with an upgrowth of mantle-folds over the original external shell, much in the same manner as the shell of Aplysia is concealed.

In all cases the shells of the Mollusca are entirely epiblastic in origin, and are consequently, morphologically speaking, always external, or exoskeletal, structures.

Fig. 85. Sections of Skin of Embryo Birds. [After Jeffries.']

A. Section of epidermis of hi hours - Fowl embryo. B. Of 134 hours - Fowl. C. Of 17 days - Duckling.

e. epitrichial layer ; m. mucous layer ; t. transitional cells.


In the majority of Chordata embryos the epiblast consists at first of a single layer (figs. 23, 26, 29-32, 43); but in the Anura (figs. 24 and 62) two layers are present. Of these two layers, the lower alone is the active layer, and from it are developed the glandular and nervous structures. In the Urodela the primitively single layer (figs. 58, 59) early becomes double, the lower one of which behaving as in the Anura.

The epidermis of Amphioxus permanently remains as a single layer.

In all other embryo Vertebrates, the epiblast, from being single, becomes double layered, owing to the primitive epiblast giving rise to a layer of flattened epithelial cells, the epitrichial layer (fig. 85). This may be regarded as the primitive horny or protective layer of the epidermis. The lower layer is the mucous or Malpighian epithelium, and persists throughout life as the active and regenerative layer of the epidermis. Later the mucous epithelium gives rise to cells of irregular shape which eventually become more or less spindle-shaped (transitional cells of Jeffries). The epitrichial layer is always shed, and the oldest transitional cells, by a process of drying and consequent shrinkage, become the horn-cells of the adult.

The horny layer is present in all the purely terrestrial Vertebrates ( e.g ., Mammalia, Aves, and Eeptilia), but not in other forms. In these latter there are parenchymatous cells very similar to an early stage in the development of the horn-cells.

Fig. 86. - Six Stages in the Development of Haik.

[From Wiedersheim .]

C. derma ; CZ. central zone of hairgerm, which forms the hair-shaft with its medulla or pith and its sheaths ; Dr. sebaceous gland; F. mesoblastic sheath or follicle ; HK. hair-knob ; P. commencement of formation of hair-papilla ; P\ the same at a later stage when it has become vascular; PZ. peripheral zone of hairgerm, later giving rise to the outer rootsheath ; Sc. stratum comeum of epidermis ; SM. stratum Malpighii.

The horn-cells are doubtless an adaptation to, or result of, an aerial life, and consequent drying of the surface of the body. Such protected surfaces as cavities of the ear and nostrils do not develop horn-cells, although the evanescent embryonic epitrichial layer is present. It is thus the effete epiblastic cells themselves which constitute the protective layer.

A transverse section of the epidermis of Man, which may be taken as being typical of Mammalia generally, shows a superficial horny layer ( stratum comeum ), and a deeper-seated Malpighian layer or rete viucosum. The latter has a basal layer of columnar cells, from which the whole epidermis is derived, and there is a complete transition between this layer and the flattened scales, which are thrown off the surface by desquamation. Anatomists usually distinguish several layers in the epidermis, but the three layers already referred to, the mucous epithelium, the transitional cells ( = mucous layer), and the horny layer, are alone of morphological importance.

Nails, claws, hoofs, horny beaks, the horny sheaths of the horns of the Bovidse, are merely local condensations of the horny layer of the epidermis, while hairs are similar linear extensions.

A hair commences as a minute solid ingrowth of the columnar layer of the epidermis into the derma (fig. 86). A small bulb, the hair papilla, containing nutritive capillaries, grows up from below

Fig. 87. Development of Feathers. [After Jeffries.]

A. Transverse section of a feather papilla near the tip, twenty days - Duckling.

B. The same lower down of an eighteen days - Duckling. C. Longitudinal section of the same as A ; there is a capillary within the pulp full of blood-corpuscles.

D. Transverse section of a pin-feather of an embryo Robin.

b. primitive barbs ; bl. incipient barbules ; c. capillary ; e. epitrichial layer : h. horn cells; m. mucous or Malpighian layer; m.b. mucous layer of barb; p. pulp; p.s. pith of shaft; s. shaft; t. transitional cells; v. vane; * point of division between the two vanes.'

into the hair follicle. The outer cells of this papilla elongate, become cornified, and thus form a hair, which soon forces its way to the exterior through the follicle.

Down-feathers arise from large papillae, which contain a central vascular mesohlastic pulp ; as the papillae grow in length, they tend to sink below the surface, more especially at the posterior side, thus producing the backward slant of most feathers ; the depressions are known as feather-follicles. Two thickenings of the epidermis appear on the upper surface of the papilla and encroach on the pulp, starting from the top, and slowly extending downwards (fig. 87, A, b ). Whilst these first two barbs are growing, the epitrichial layer becomes more compact and the transitional cells horny, thus forming a protective case for the incipient feather. As the papilla grows, more barb folds appear (fig. 87, b, b). The barbs are formed by the cells at the angle of the thickenings, as seen in section, while the cells on the sides arrange themselves in columns (fig. 87, c, bl), which bend slightly towards the tip of the papilla, and ultimately form the barbules. The walls of the cells of the barbs and barbules finally become converted into a kind of horn, and the protoplasmic contents dry up.

The contour feathers of adult Birds are developed upon the same plan as the down-feathers, by a renewed growth of the primitive papilla. Two primary barbfolds appear as before, and are very shortly followed by numerous others. The two primaries unite to form the two halves of the shaft, and are joined later by those on the sides. At the side of the papilla, opposite to that where the shaft is formed, is a slight inversion of the mucous epithelium ; it is here that the separation will occur which results in the two vanes of the feather. As the barbs are set at an angle of about forty-five degrees, the portions farthest from the shaft in a transverse section are sections of the tips of lower barbs. In feathers with a hollow shaft, the two sides bend in and enclose a column of the pulp (fig. 87, d), which subsequently dries up ; in solid shafted feathers the sides are simply flattened together. When the feather is matured, the covering falls off and the pulp withers away, and the barbs separate into the two vanes. Thus it comes about that the upper surface of the shaft and barbs of a feather (together with the whole of the barbules) is formed from horn cells or modified transitional cells, whereas the lower surface is composed of degraded mucous epithelium. The quill is produced by a cornification of the walls of the lower portion of the papilla.

As hairs consist of the horny layer of the epidermis only, it is evident that they can scarcely he regarded as strictly homologous with feathers; the latter are never found out of the group of Birds, and the former are equally peculiar to Mammals.

The scutse which occur on the legs of Birds are mere folds of skin with a horny layer, a mucous epithelium, and a mesodermal core. They occasionally bear feathers.

The scales of Snakes, of Chelonia (tortoise-shell), and also of some Lizards are purely epidermal structures ; but those of other Lizards (Anguis, Cyclodus, Scincus) and the scutes of Crocodiles, and of the Armadillos amongst Mammals, are partly derived from the epidermis, but chiefly from the corium; in other words, they are mainly of mesoblastic origin (see p. 193). The scales of the Manis, like the horn of the Rhinoceros, are formed of hairs agglutinated together.

Teeth are not purely epiblastic organs, hut they may be conveniently dealt with here. There is little doubt that teeth were primitively structures similar to the placoid scales of Elasmobranchs which have been retained and emphasised in the jaws.

A placoid scale arises as an ingrowth from the derma into the epidermis, the basal columnar cells of the latter are pushed up and thus form a kind of sheath to the papilla. The basement membrane, which is a product of the epidermis, becomes thickened and calcified at the apex of the papilla, and constitutes an enamel cap, the papilla becoming converted into dentine, bone, and pulp. The point of the scale eventually forces its way to the exterior.

In the development of a milk (deciduous) tooth a prolongation from the epidermis arises which passes into the derma (fig. 88) ; the inferior end becomes dome-shaped, forming the “ enamelorgan - (fig. 89). A papilla of the derma projects into the hollow of the dome, and soon becomes vascular ; the papilla produces the dentine and cement of the tooth, while the columnar layer of the enamel-organ or germ, which overlies the papilla, is stated to secrete the enamel layer (fig. 90). The permanent teeth are similarly developed, but the enamel germ arises as a bud (fig. 89, e ) from that of the deciduous tooth. Huxley, and after him Miss Nunn, asserts that the enamel, like the dentine of teeth and scales, owes its origin to odontoblasts, and is therefore mesoblastic ; and that the cuticula dentis is formed by the metamorphosis, either in whole or in part, of the enamel cells, which have nothing whatever to do directly with the formation of the enamel. However this may be, the large size and the invariable presence of the enamel organ prove that it has, or has had, an important function in the production of teeth.


Fig. 88. Fig. 89.

Early Stages in the Development of Milk-Teeth.

[From Landois and Stirling.']

Fig. 88. - a. dental ridge; b. commencement of the enamel organic, dentine germ, first trace of the pulp ; d. first indication of the mesoblastic investment or tooth-sac.

Fig. 89. - a. dental ridge; 1 upper, 3 lower or secreting layer of the enamel organ (b), 2 intermediate epiblast cells ; c. dental papilla, with capillary ; d. commencement of dental sac; e. enamel germ of the corresponding permanent tooth.

In Elasmobranchs all the teeth of each jaw are developed from a common rod of tissue, which is derived from a ridge-like proliferation of the mesoblast into the epidermis of its jaw. The enamel cap of each tooth is formed in the same manner as in the placoid scales.

Scott finds that the horny teeth of the metamorphosing Lamprey are developed from the deeper layer of the epiblast which rises in a cap-like manner over a mesoblastic papilla : this appears to be the representative of the enamel organ. A second tooth is developed vertically below the first. The original papilla and enamel organ are functional throughout the life of the animal.

Horny teeth and a horny sheath to the jaws occur in larval Anura.

Epiblastic Glands

The epidermis is the seat of origin of many and varied glands. The simplest cases are where certain cells become enlarged and secretive, forming unicellular glands. These

Fig. 90. - Later Stage in the Development op a Tooth. [From Wiedersheim.]

Bg. connective tissue which forms the dental sac: D.S. dentine; E.M. epithelium of mouth; Ma. membrana adamantina (cuticula dentis); O. odontoblasts; SK. enamel germ ; ZK. tooth germ.

alone occur in the Ccelenterata ; in higher forms they often coexist with multicellular glands.

Multicellular glands may be simple (figs. 18, 84) or compound (figs. 91, C-E ; 93, A, ; 141, f). The development of such a very complex gland as a salivary or mammary gland is as follows : - A simple solid process from the epidermis sinks into the derma; branches sprout out from its blind end ; these acquire a central cavity, elongate, and greatly increase in number, until a muchbranching tubular organ is developed. The ultimate ramifications in the above-mentioned glands expand into secretory pouches or alveoli.

Although there is a solid ingrowth of the epidermis, it is the Malpighian layer alone which forms the secretory tissue of the gland; the central epidermal cells eventually disappear. The solid ingrowth of the incipient gland is clearly a secondary process.

Other glands may always remain simple tubes, or at most become slightly branched.

Thus the most complex type of gland reproduces in its own development those simpler conditions which it must have passed through in the course of its evolution, and which are severally the permanent states of other glands. In the simplest glands all the cells are secretory, but as complication arises the stem and main branches lose this function and constitute ducts to convey the fluid secreted by the terminal portions.

Fig. 91. - Diagrams to Illustrate the Evolution oe Complex Glands. [ After Huxley. ]

A. Section of an ideally simple skin, showing the mucous and horny layer of the epidermis (ep), and a capillary (c) within the derma or cutis ( d ). B. A simple gland, with its capillary network. C, D, E. Glands of increased complexity. The vascular supply is omitted in these figures.

All the glands opening on the general surface of the body are of epibiastic origin ; such are the sweat, scent, anal, poison, adhesive, byssus, slime, spinning and mammary glands. The salivary glands of Insects develop as paired invaginations from the ventral plate of the mouth, behind the stomodaeum, and on the inner side of the mandibles.

According to Klaatsch, the mammary glands develop from a shallow depression, the glandular area or areolar epithelium, the margin of which is slightly raised. This condition is permanent in Monotremes (fig. 92). In adult Man, the glandular area is raised to form the nipple ; the same occurs in the Mouse, but the glands have a single duct. The nipple of Carnivores, Pigs, Horses, and especially that of Ruminants, is formed by the upward growth of the raised margin in such a manner that the glaudular area forms a pit, at the bottom of which the glands open.

It seems probable that the mammary glands are greatly enlarged and modified sebaceous glands, the hairs to which they belong having disappeared in course of

Fig. 92. - Diagrams of the Arrangements of the Ducts of the Mammary Glands in Various Mammals. [ From Bell after KLaatsch.]

Tlie glandular area of the epidermis is indicated by the thicker line.

A. A dult Echidna. B. Human embryo. C. Human adult. D. Adult Mouse.

E. Embryo Cow. F. Adult Cow.

time (see figs. 86, e and f, Dr; 93, b, h) ; but Rein denies this, and believes that they are organs sui generis. Gegenbaur has lately shown that the so-called mammary

Fig. 93. - Development of Mammary Glands of Marsupials.

A. Embryo of Phalangista vulpina (9.5 cm.) B. The same of Perameles gunii (8.6 cm.) Vertical section through rudiment of the mammary depression. [After Klaatsch.~\

d. derma; ep. horny layer of epidermis; h. hair; m. Malpighian layer of epidermis ; milk-glands ; p. processes of Malpighian layer, indistinguishable from hair or gland rudiments ; sebaceous gland.

glands of the Monotremes are phylogenetically distinct from those of other Mammals. They consist of tubular glands, modified derivatives of the sudoriferous type.

Those glands derived from the stomodaeum (p. no) and proctodaeum are also epiblastic in origin.

In the “Veliger- larval stage of marine Prosobranch Gasteropoda a group of epiblast cells, on each side of the body behind the velum enlarge, become vacuolate, and constitute what are generally regarded as provisional renal organs (fig. 84, A, p.k, c). The red or violet pigment spots occurring in the Veligers of Opisthobranchs and a few other Molluscs may be of a similar nature (fig. 84, A, pgt).

De Meuron describes the primitive renal organs of Helix as arising from epiblastic invaginations, and not as being mesoblastic in origin, as are, according to Eabl, the kidneys of the aquatic Pulmonata. This organ is a tube with a ciliated internal orifice as in other Pulmonata. Pol had previously described the provisional excretory organs of the terrestrial Pulmonates as a pair of nonciliated epiblastic pits, with no internal orifice. The permanent kidney appears to be formed as an epiblastic invagination supplemented by mesoblastic tissue.

Muscular Elements. - The root-like prolongations of the large ectodermal cells of Coelenterates are contractile, and practically form an external muscular sheath to the body. The brothers Hertwig have demonstrated distinct ectodermal muscle-cells in the Actiniae, and Hubrecht has found similar cells in the Nemertea.

The non-striated muscle-fibres which surround some sweat glands are stated by Eanvier to be derived from the epidermis in Man.

Respiratory Organs.- - Prom the nature of the case, the external skin of the body must always act as a respiratory surface, except when it is surrounded with an impervious cuticle or exoskeleton ; but certain areas are usually more especially devoted to the interchange of those gases which constitute what is known as respiration.

Invertebrates. - The external organs for aquatic respiration are mostly delicate filaments or plates (branchiae or gills), within which the blood freely circulates.

The manner in which such organs are developed is so selfevident as to need no special comment. In certain cases, as, for example, in the gills of some Lamellibranchs (e.g. } Anodonta, Dreissena), the primitively simple bent gill-filaments form the perforated plate-like gills of the adult by concresence between their two limbs, and by the union of the filaments with each other.

It may be here noted that the respiratory plumes of Serpulaceae amongst the Cbtetopoda are supported by (probably mesoblastic) cartilaginous bars.

Hartog and others have shown that anal respiration occurs in probably all larval Crustacea, and also in some adults. Leaf-like respiratory organs occur in the rectum of larval Dragon-flies. It will be shown that in the Arthropoda the rectum is derived from the proctodseum (see p. 1 1 1).

Aerial respiration has supplemented branchial respiration in some Mollusca and Crustacea by the upper portion of the branchial chamber becoming vascular and functioning as a lung ( e.g ., Ampullaria, Birgus). In the Pulmonate Mollusca the gills have entirely disappeared. Lankester has suggested the probable evolution of the pulmonary sacs of the aerobranchiate Arachnida (Scorpions and Spiders) from the lamellate gills of their Limulus-like ancestors (Haematobranchiata).

The trachese of the tracheate Arthropoda appear to have been derived from simple diffused cutaneous glands, which have evolved into delicate branching respiratory tubes.

Hubrecht has shown that the ciliated pits which penetrate the posterior brainlobes of the higher Nemertean worms arise solely from the epiblast, and not partially from the oesophagus, as previous observers had stated. It is probable that these pits have a sensory as well as a respiratory function.

Chordata. - The epidermis is undoubtedly respiratory in some Chordata, especially amongst the Amphibia.

The characteristic respiratory organs of the Chordata are of hypoblastic origin (see p. 177).

External or epiblastic gills are, however, developed in a few forms. These are the larval external gills of the Ganoid Polypterus, the Teleost Cobitis, and the permanent external gills of the Dipnoid Protopterus, and of some Urodeles. In other Amphibia they are purely larval organs. The so-called external gills of embryo Elasmobranchs are merely the extremely long filaments of the internal gills of the posterior lamellae, only, of each arch, which protrude beyond the clefts.

In certain Teleosts {e.g., Anabas, some Siluroids) accessory respiratory organs, which are supported by very delicate contorted bony plates, may grow out from the upper portion of the gill-arches into the branchial chamber (p. 179). They are thus necessarily invested by the epidermis, and constitute organs for aerial respiration. In Saccobranchus and a few other Fishes, air is respired by means of membranous sacs which evaginate from the branchial chamber and push their way along the lateral muscles of the body. These Teleosts have thus acquired new epiblastic organs for breathing air direct ; but the problem of aerial respiration has been more satisfactorily solved by the utilisation of the air-bladder of Fishes and the development of lungs in the ancestral Amniota.

Stomodaeum. - The invagination of the epiblast, which in most animals forms the month of the adult, is known as the stomodseum. There is reason to believe, as has already been stated (p. 75), that in the Invertebrates the stomodseum corresponds with the anterior extremity of the primitive mouth (blastopore). In the Chordata the same interpretation is held by some embryologists, but Dohrn and his school believe that the stomodseum is a new formation possibly corresponding to a pair of fused gill-slits.

Whatever theory may be held concerning its nature, the fact remains that in a large number of animals the mouth arises as an epiblastic invagination which subsequently unites with the blind anterior end of the mesenteron (archenteron). The following types will serve as examples: - Asterias (fig. 52), Lymnseus (fig. 84, b), Astacus (fig. 140), and Petromyzon and Bombinator (fig. 94).

Usually the stomodseum forms but an insignificant portion of the alimentary canal; but in the Arthropoda, especially in the Crustacea (fig. 140, f.g), it is of considerable size. In the latter group the stomodseum forms the large crop or masticatory “ stomach,- which in the Decapoda is complicated by the development of the gastric mill and the filtering apparatus. In Insects it forms the oesophagus, crop, and proventriculus or gizzard, when such are present. The mouth, oesophagus, and masticatory apparatus of Botifers are also derived from the stomodseum.

All the structures and glands developed from the stomodseal epithelium are necessarily of epiblastic origin, amongst which may be mentioned the radula sac of Mollusca (fig. 84 B, r ), with its contained odontophore, the enamel organ of teeth, and various mucous and salivary glands, but not the salivary glands of Insects.

The Pituitary Body (Hypophysis Cerebri). - The pituitary body arises in most Vertebrates as a tubular invagination of the roof of the mouth (stomodseum) approaching the infundibulum (see p. 127). The upper end becomes swollen, and the stem gradually atrophies. The enlarged distal portion, which is surrounded by vascular tissue, becomes lobed, and the central lumen may or may not persist; it ultimately enters into close union with the infundibulum (figs. 107, hph ; 108, pit, 109, H; 112, hp; n 6, hph ) ; but it is only in the Mammalia that the two structures fuse with one another.

According to Scott, the pituitary body arises in the Lamprey as an epiblastic invagination between the olfactory epithelium and the stomodseum (fig. 94, a). In the Frog it appears before the invagination of the stomodseum; but owing to the large size of the latter, and the rapid growth of the cerebral hemispheres, the pituitary body is carried into the mouth (fig. 94, c). In other forms the early development of the cerebral lobes, probably combined with a later appearance of this now function less organ, causes it to be apparently derived from the roof of the stomodaeum itself.

Miss Johnson and Miss Sheldon state that in the Frog and Newt the stomodaeum is at first a solid ingrowth of the deeper layer of the epiblast ; the lower part of the ingrowth fuses with the fore-gut, while the upper part projects freely and forms the pituitary body.

In Teleosts, according to Dohrn, the hypophysis arises as a pair of hypoblastic e vagin ations in front of the mouth, and Hoffmann finds that the earliest rudiment of the hypophysis is developed in the common Snake from the anterior end of the archenteron ; the same apparently occurs in the human embryo (fig. 143, H.p).

The organ has probably a pre -vertebrate, and possibly a pre-chordate, significance.

Proctodaeum. - The arguments in favour of the stomodaeum corresponding to the anterior end of the primitive blastopore of

Fig. 94. - Diagrams to Illustrate the Relation of the Pituitary Invagination to the Stomodaeum.

A. Longitudinal vertical section through the head of an embryo Lamprey just before hatching \from Scott], B, C. Similar sections through the head of an embryo, and of a young Tadpole, respectively, of a Toad (Bombinator) [ from Scott, after Gotte],

ep. epiblast ; hy. hypoblast ; inf. infundibulum ; ncli. notochord ; olf. olfactory epithelium ; ph. pharynx ; pt. pituitary invagination ; st. stomodseum ; th. thyroid invagination.

Invertebrates apply to the proctodaeum with regard to its posterior extremity. The blastopore, or a portion of it, however, often persists as the anus, or the anus shortly appears at the spot where it has closed up.

Any invagination of epiblast at the anus constitutes a proctodaeum. In most Invertebrates the proctodaeum is small, but the long rectum of Crustacea (fig. 140) is derived from this invagination ; it is also large in other Arthropods.

The Malpighian tubules of the Arachnida and Insecta arise as a single pair of evaginations from the anterior portion of the proctodaeum ; but these usually increase in number.

The proctodaeum forms the cloaca of many of the lower vertebrates, or at all events its outer portion, the anterior section being formed by the dilated end of the alimentary canal, into which the urogenital organs open (figs. 73, 143, c). The epiblastic section of the cloaca is sometimes marked off from the hypoblastic portion by a small fold.

Cloaca of Amniota. - In a recent paper Gadow states that “ the cloaca of the Amniota consists originally, either permanently or in the embryo only, of three successive chambers. I. The Proctodceum [Lankester]. The outermost anal chamber of epiblastic origin, with its derivatives : (i.) bursa Fabricii in Birds, (2.) various hedonic glands in most Amniota, (3.) the copulatory organs, the, at least partly, epiblastic nature of which is indicated by the frequently developed horny armament of the glans, by the various sebaceous glands, and by development. II. The Urodceum [Gadow]. Hypoblastic, this is the middle chamber or primitive cloaca, into which open the urinogenital ducts and through which pass the faeces. With its differentiations : (i.) urinary bladder, ventral ; (2.) anal sacs in Tortoises, dorsal. III. The Coprodceum. This is the innermost cloacal chamber.

The urodaeum is the oldest portion of the whole cloaca, then follows the proctodaeum, and, lastly, the coprodaeum has secondarily assumed cloacal functions.-

Nervous System. - The nervous system and the sensory surfaces of the sense-organs are, as has been stated, derived from the epiblast. In scarcely any other section of Embryology is more light thrown upon the significance of the facts of development by a comparative study of the adult condition of these structures in the lower animals. For the sake of convenience the development of the central nervous system will be first considered, and afterwards that of the sense-organs.

Invertebrates. - In the majority of Invertebrates the central nervous system originates from certain areas of the epiblast. The cells of these areas are usually more or less columnar, and undergo rapid cell-division (proliferation). The mass of cells thus formed sinks into the underlying mesoblast, and eventually differentiates into nerve-cells or ganglion-cells, and into nerve-fibres. Outgrowths from the incipient nerve-centres (ganglia) form nerves and commissures.

Nerve-cells and nerve-fibres occur in all the higher or more active Coelenterata. They are undoubtedly modified ectodermal cells which have assumed a deeper position, and in the case of nerve-fibres have become greatly elongated. As all the ectodermal cells are connected with one another by means of their basal rootlike processes, the nervous system is from the first connected with the superficial ectoderm cells on the one hand, and the deeper seated muscle-cells on the other, that is, of course, when the latter are present.

This undifferentiated nervous system is generally diffused over certain areas, chiefly the oral surface, or it may be restricted to a circum-oral ring, as in certain Hydroids.

Even in adult Starfish the nervous system is scarcely separated from the epidermis ; and it has recently been shown that in most Echinoderms a nervous network surrounds the whole animal. There is, however, a more concentrated nervous tract round the mouth and along the ambulacral areas in all Echinoderms. The Carpenters and Marshall have proved the existence of an additional aboral nervous system in Crinoids.

A diffused nervous system lying immediately below the epidermis occurs, according to Hubrecht, in the lower Neinertean worms, in addition to the lateral cords of the higher forms (see also p. 165).

Fig. 95. - Sections to Illustrate the Development of the Nervous System in an Earthworm (Lumbricus [After Kleinenberg.

A. Through the head. c.c. cephalic portion of the body-cavity ; c.g. cephalic ganglion ; ce. oesophagus.

B. Through the ventral wall of the trunk, c. body-cavity ; ep. epiblast ; hy. hypoblast ; m. longitudinal muscles ; n. ventral nerve cord ; so. somatic mesoblast ; sp. splanchnic mesoblast ; v.g. ventral groove ; v.v. ventral blood-vessel.

Hatschek describes the nervous system of Criodrilus as first arising as an anterior ectodermal thickening which extends backwards as a cord on either side of the mouth forming the oesophageal commissures. The process of thickening continually extends backwards, resulting in the formation of the double ventral nerve-cord. The nervous system of the Earthworm (Lumbricus), according to Kleinenberg, develops from the epiblast as two long cords on each side of a shallow ciliated median ventral groove (fig. 95). The two cords early unite, and segmental ganglionic enlargements are soon indicated.- The cephalic ganglion is apparently at first quite independent of the ventral cords. Hatschek states that the ventral groove invaginates, and takes part in the formation of the nerve-cord.

In the Mollusca the nervous system is usually developed in the ordinary manner by proliferation of the epiblast. This occurs in two regions. In the early Veliger larva of Gasteropods, or at the corresponding stage of other Molluscs, a pair of cephalic plates is formed on the pre-oral lobe within the velum by the rapid celldivision of the locally thickened epiblast. These give rise to the cephalic ganglia. The pedal ganglia arise from a pair of similar areas in the foot. Tig. 96, A, shows the proliferating areas which are giving rise to the cephalic and pedal ganglia in a Prosobranch Gasteropod (Purpura) ; these are seen in section in fig. 96, B.

The nerve-cords of Chitons have been shown by Kowalevsky to arise throughout their whole length from the epiblast in the region corresponding to that which they occupy in the adult. They form, in fact, a double nervous ring surrounding the latero-ventral aspect of the bodjn On recalling the relationships of the mouth and anus witli the primitive blastopore (p. 76), it will be found that the nervous system

A. Side view of early veliger. B. Transverse section of the same. c.g. cephalic ganglion; /. foot; m. mesoblast ; mtl. pedal ganglion ; s. shell ; v. velum ; y. yolk.

of these primitive Mollusca constitutes a double circum-oral ring, in other words, a nervous system comparable with that of many Coelenterates.

Kowalevsky also finds that in Dentalium the cephalic ganglia are derived from pitlike invaginations of the cephalic plates. The depressions soon lose their connection with the external epiblast and later their central cavity disappears. The pedal ganglia at first arise from an unpaired area ; this divides, and each ganglion increases at the expense of the epiblast of the foot.

Lankester states that in the Cephalopoda, the white body originates from the epiblast of the head in the same manner and in the same position as the cephalic ganglia in other Mollusca, but that the true ganglia are of mesoblastic origin, the white body becoming an apparently functionless structure.

Bobretzky also derives the nerve-ganglia of Cephalopoda and of the Prosobranch Gasteropod Fusus from the mesoblast. Two explanations suggest themselves. 1. That the earliest stage of these structures has not yet been observed. 2. That if the observations are correct, it is a secondary phenomenon due to precocious segregation (see p. 165).

The formation of the nervous system in the Lamellibranchiata is, so far as is known, quite normal.

The ventral nerve-cord in the Crayfish arises in the median ventral line on each side of a central groove ; this thickened epiblast is continued along anteriorly round the stomodaeum, and passes into the incipient cerebral ganglia, which are formed in the centre of the pro-cephalic lobes.

According to Eeichenbach, the development of the nervous system is somewhat more complicated, but the above account is probably substantially correct.

The development of the nervous system is very uniform throughout the Arthropoda ; the ventral cord may arise as a single or a double thickening of the ventral epiblast ; the median groove may be shallow, deep, or absent : it is stated to sometimes take part in the formation of the nerve-cord. The cerebral ganglia are apparently always continuous with the ventral cord.

The series of ganglia and the commissures connecting them, which together constitute the central nervous system of Invertebrates, is thus developed directly from the epiblast. These commissures are usually composed of nerve-cells as well as of nervefibres ; in fact, the ganglia are merely local thickenings of the commissures with a preponderance of the nerve-cells.

The nerves proper develop as prolongations from the central nervous system, and may give rise to other ganglionic enlargements.

Nature of the Invertebrate Brain. - The portion of the central nervous system situated in front of the mouth (pre-oral) is always associated with the eyes, and constitutes the primitive brain. Lankester has appropriately termed this the archicerebrum. All the nerves which originate from it supply the pre-oral region of the head.

The brain of most, if not of all, Worms is an archi- cerebrum, as is also the preoral nervous system of the Amphineurous Mollusca (Neomenia, Chiton).

There is a tendency in the Arthropoda for the anterior appendages with their ganglia to shift forwards. In this manner a composite brain (syn-cerebrum) is formed. As the nervous system is composed of two lateral halves, there is no antecedent improbability in the migration forward of the ganglia.

All the appendages of the Nauplius larva of Crustacea are post-oral; and Pelseneer has recently shown that in Apus the ganglia of the first pair of antennae have migrated to the brain, although their nerves apparently arise from the (esophageal commissure. The concentration is still greater in other Crustacea ; thus in this group the brain is always a syn-cerebrum.

Balfour has shown that in the Spider the ganglia of the Chelieerae are post-oral, but they soon become fused with the pre-oral ganglia.

The antennae of Insects and Myriapods develop from the pro-cephalic lobes, and are always innervated by the pre-oral ganglia. Therefore the antennae of these forms are probably not homologous with those of the Crustacea, and their brain is an archicerebrum. Hatschek states that the ganglia of the mandibular segment disappears in the oesophageal commissures, and that the sub-cesophageal ganglion is formed by the ganglia of the two maxillary segments.

An analogous concentration occurs in the brain of the higher Mollusca.

There is in the embryos of Arthropoda a pair of ganglia for each segment of the body, but a fusion of ganglia often occurs in the thoracic region of the body, notably in the case of the Brachyura and Spiders ; in the former case the concentration occurs around the sternal artery.

Central Nervous System of the Chordata. - Throughout the Chordata the central nervous system appears very early, usually as a more or less well-defined plate of columnar epiblast (neural or medullary plate) in the median dorsal line of the embryo (figs. 59, 61, 9 7, 100). A central shallow longitudinal groove (neural or medullary groove) appears in this plate ; it is often widely open at both ends. The neural plate extends from the dorsal rim of the blastopore to what will he the anterior extremity of the embryo.

Fig. 97. - Embryo of Frog, with Split-like Blastopore and well-developed Neural Folds. [After 0. Her twig. ~\

bl. blastopore ; d.f. dorsal furrow : n. neural folds.

The walls of the neural groove bend over, and, fusing in the median line, convert the groove into a canal, the neural or medullary canal (figs. 63, 64, 102). The enlarged anterior portion of the neural tube is the incipient brain; the remainder will develop into the spinal cord. Before closing over the canal becomes ciliated in Amphioxus and the Fowl.

Neurenteric Canal. - In those Vertebrate embryos which have but little food-yolk, the blastopore occurs as an opening from the archenteron to the exterior, and the neural groove arises immediately dorsal and anterior to it ; the neural folds, as a matter of fact, extend round each side of the blastopore (fig. 97).

The supposed relation of the blastopore of such embryos to the primitive blastopore, and the position of the latter with regard to the nervous system, has already been briefly mentioned (p. 76).

When the neural folds unite in the median line to form the neural canal, their posterior portion which surrounds the blastopore may also close over. ;By this means the blastopore would be shut off from the exterior by this overgrowth, and would necessarily open into the posterior extremity of the neural canal. The short tube connecting the cavities of the nervous system and archenteron is known as the neurenteric canal (fig. 99, ne). The ventral portion of the canal is also termed the post-anal gut.

It was till quite recently supposed that this occurred in the Cyclostomi and Amphibia ; but in these groups it appears that the blastopore persists as the anus, consequently what was termed the post-anal gut (solid in the Newt), which was imagined to extend between the closed-over blastopore and the new anus, is merely a ventral extension of the neural canal, owing to the growth of the tail taking place above the blastopore (figs. 98, 99).

Fig. 98. - Diagrammatic Longitudinal Section THROUGH THE EMBRYO OF A FROG. [From Balfour after Gotte.]

al. alimentary canal (archenteron) ; m. mesoblast ; nc. neural canal ; yk. yolk-cells ; x. point of junction of epiblast and hypoblast at the dorsal lip of the blastopore. For the sake of simplicity the epiblast is represented as if composed of a single row instead of two layers of cells.

In those forms in which the blastopore, as such, is obsolete, being partially represented by the primitive streak (see p. 41), the neurenteric canal may be lost, but in many ( e.g ., Lizard, Goose, Duck, Parrot, Mole) it still occurs and occupies the same relative position. In the Fowl and other Amniota the canal is lost, but traces of it may occur.

The closure of the neural groove takes place from behind forwards in Tunicates and Amnhioxus. but usuallv in Vertebrates it

Fig. 99. - Longitudinal Section

THROUGH AN ADVANCED EMBRYO of a Toad (Bombinator). [ From Balfour after Gotte.]

an. anus, this should be represented as an opening into the alimentary canal ; ch. notochord ; l. liver ; m. mouth (stomodseum) ; me. neural (medullary) canal ; ne. neurenteric canal, - between this and an is the so-called postanal gut ; pn. pineal gland.

first closes in the region of the neck or hind-brain (fig. 100). The closure in some cases takes place more rapidly backwards, but in others the brain is the first to close over (fig. 10 1).

It is important to note that in the Tunicates and Amphioxus an anterior pore (neural pore) persists for some time after the rest of the canal is completed. At this stage (fig. 57, oe) the cavity of the archenteron can only communicate with the exterior through this pore. For suggestions concerning a possible significance of this arrangement, the reader is referred to papers by Sedgwick and Van Wijhe.

In the Teleostei, Lepidosteus, and Lamprey (fig. 61, b), the central nervous system arises as a solid axis of epiblast cells ; the epidermal layer may, however, be carried down into this keel to line the subsequently acquired central lumen ; but Shipley denies that this occurs in the Lamprey. This variation has only a secondary significance.

Fig. ioo. - Embryo Fowl, 3 mm. long, of about twenty-four hours, seen from above, magnified thirty-nine diameters. [From Kolliker].

Mn. union of the medullary folds in the region of the hind-brain ; Pr. primitive streak ; Pz. parietal zone ; Bf. posterior portion of widely open neural groove ; Rf'. anterior part of neural groove ; Rw. neural ridge ; Stz. trunk zone ; vAf. anterior amniotic fold ; vD. anterior umbilical sinus showing through the blastoderm.

His divides the embryonic rudiment into a central trunk zone, and a pair of lateral or parietal zones.

In those forms in which the epiblast is early separable into an epidermic and nervous or mucous layer (some Ganoids and Anura) (fig. 24, e), the nervous tract is entirely formed at the expense of the latter, while the epidermal layer of the medullary plate persists as the epithelium of the central canal of the nervous system.

It will be convenient first to trace the further history of the spinal cord and its nerves, and afterwards that of the brain and the cranial nerves. The nervous system at this stage consists of a tube of epiblast several cells thick, with an anterior enlargement (fig. 98). This is practically the adult condition in Amphioxus, except that in this form there is no increase in size of the neural canal anteriorly.

Fig. ioi. - Embryo Fowl, 4.2 mm. long, of the second day, seen from above, magnified a little over fifty diameters. [From Kolliker.]

Ao. area opaca or vasculosa, bounded by tbe rudiment of the terminal vessel ; the more external portion of this area has not been shaded, and the blood-vessels are not represented ; Ap. area pellucida ; Hh. hind-brain ; Mh. mid-brain ; Vh. fore-brain ; om. rudiments of omphalo-mesenteric veins ; omr. point where the closure of the neural groove is travelling backwards ; Uw. muscle-plates ; other lettering as in fig. 100.

Spinal Nerves. - Immediately after the neural tube has become quite disconnected from the epidermis, paired outgrowths from the dorsal portion of the nervous wall arise at definite intervals (fig. 102). These grow ventral-wards, and are the dorsal (afferent, sensory, or posterior) roots of the spinal nerves. An enlargement, which is apparent very early, is the rudiment of the ganglion. A short time after the appearance of the dorsal roots, the ventral (efferent, motor, or anterior) roots sprout from the inferior angle of the spinal cord ; eventually they fuse with the former.

In Amphioxus there are large nerves with dorsal roots, and the ventral roots are epresented by a few loose nerve-fibres which do not unite with the former. The ventral roots form distinct nerves in the Marsipobranchs, but in Myxine alone are they united with the dorsal into a common trunk.

In a fully developed spinal nerve (fig. 103) a dorsal branch (ramus dorsalis) passes off to the dorsal region immediately below the ganglion ; below the latter a branch (ramus intestinalis) passes to the sympathetic system, and finally the main trunk (ramus ventralis) divides into its peripheral branches.

The dorsal roots of the spinal nerves are generally stated to arise from a median dorsal ridge of cells, termed by Marshall the “neural crest.- Later, they emerge more from the sides of the spinal cord ; and, in some forms, all or some of the

Fig. 102. - Transverse Section through the Trunk of an Embryo Dog-Fish (Pristiurus). [ From Baljour.']

al. alimentary canal ; ao. aorta ; mp. muscle-plate ; mp'. portion of muscle-plate converted into muscle ; nc. neural canal ; pr. dorsal root of spinal nerve arising from the neural crest ; sc. somatic mesoblast ; sp. splanchnic mesoblast ; Vv. portion of the vertebral plate which will give rise to the vertebral bodies ; x. subnotochordal rod.

roots on each side are temporarily connected together by a longitudinal commissure (fig. 104). It is possible that the lateral attachment is not, as some investigators believe, an entirely new formation, but that it is due to the upward growth of the dorsal portion of the spinal cord, and the commissures may be each lateral half of the neural crest.

It is, however, conceivable that while the apparent shifting of the attachment of the dorsal roots may primitively be due to the dorsal growth of the spinal cord itself, in some cases, at all events, a second connection due to concrescence may have originated lower down on the sides of the spinal cord.

Sympathetic Nervous System

The sympathetic ganglia arise, according to Balfour, as enlargements of the main branches of the spinal nerves. Later they are removed from their nerves, but are still connected by short nerves (fig. 103).

Schenck and Birdsell state that in Mammals the main portion of the sympathetic system arises from the lower portion of the spinal ganglia, and that especially in the neck the sympathetic cords arise as a continuous ganglionated chain.

Histogenesis of the Spinal Cord

When the neural canal is completed, its walls are several cells deep ; the thickness increases, and gradually differentiation occurs.

Fig. 103. - Transverse Section through the Anterior Part of the Trunk of an Embryo Dog-Fish (Scyllium). [ From Balfour. J

As a matter of fact, the ventral nerve roots do not arise immediately below the dorsal but half-way between two dorsal roots.

ao. aorta; ar. ventral root ; ca.v. cardinal vein ; ch. notochord; du. duodenum ; dn. ramus dorsalis ; hp.d. point of junction of hepatic duct with duodenum ; mp. muscle-plate ; mp'. part of muscle-plate already converted into muscles ; mp. 1. part of muscle-plate which gives rise to the muscles of the limbs ; nl. nervus lateralis ; pan. pancreas ; sd. segmental duct; sp.c. spinal cord; sp.g. ganglion of dorsal root ; sp.n. spinal nerve ; st. segmental tube ; sp.g. sympathetic ganglion ; umc. umbilical canal.

The peripheral cells lose their cellular appearance, become much elongated in a longitudinal direction forming nerve-fibres. The nerve-fibres are at first non medullated, and occur in greatest profusion in certain definite tracts (white matter), usually ventral or lateral, but soon extending all round the cord. The remaining primitive cells metamorphose into the nerve-cells of the grey matter, with the exception of those cells which line the central canal, and which always retain their epithelial character.

Fig. 104. - Vertical Longitudinal Section through Part of the Trunk of a Young Scyllium Embryo. [From Balfour.]

ar. ventral (anterior) roots of spinal nerves ; com. commissure uniting the dorsal ends of the dorsal nerveroots ; ge. epithelium lining the body-cavity in the region of the future germinal epithelium ; pr. ganglia of the dorsal (posterior) roots ; sd. segmental duct ; st. segmental tubes.

The nerve-cells are at first rounded and apolar. His states that in the human embryo radial processes arise very early, and that the majority of the cells are at first bipolar.

The central canal retains its primitive slit-like appearance in transverse sections for a long time, but the exact form of the canal in section varies according to the region of the body and age of the embryo. Ultimately it becomes reduced by closure from above downwards to the minute round canal of the adult, which therefore represents the ventral portion of the primitive canal.

The ventral (anterior) fissure is produced by lateral downgrowths of the cord, while the dorsal (posterior) fissure has in the Pig, according to Barnes, the following origin. After the dorsal (posterior) columns of white matter nearly meet one another in the median dorsal line, they grow downwards as two horns (Burdach -s tract) ; in the narrow space between them are wedged two masses of cells (Goll -s tract), which are either derived from the cord, or more probably are of mixed origin, i.e ., partly mesoblastic (fig. 105). They are separated below by “horn fibres,- derived

Fig. 105. - Diagrams Illustrating the Formation of the Anterior and Posterior Fissures of the Lumbar Region of the Sfinal Cord in a Pig. [After Barnes.]

A. From an embryo 43 mm. in length.

B. ,, ,, 65 „ ,,

C. ,, „ 97 >>

af anterior (ventral) fissure ; b. Burdach -s column ; c.c. central canal ; o. Goll -s column ; g.m. grey matter ; p.h.f. posterior horn fibres ; w.m. white matter.

from the degraded epithelial cells of the retreating central canal. The dorsal fissure is thus produced by ingrowths of the dorsal columns of white matter, and the atrophy of the tissue lying between them. The downgrowth appears to be independent of the reduction of the canal, as the latter may be reduced to nearly its minimum length before the former commences (fig. 105, c).

Development of the Vertebrate Brain. - The enlarged anterior portion of the neural canal early exhibits definite dilatations ; of these, three primary brain vesicles are usually recognised, the fore-, mid-, and hind- vesicles (fig. 106, Vh, Mh, Eh), but these must not be regarded as having equal morphological value.

The middle-brain vesicle is apparently simple in character, but the last is undoubtedly compound, being formed of several imperfect dilatations, each of which is comparable with the mid-vesicle. The anterior one of these (fig. 106, Hh) is always well marked, and dorsally gives rise to the cerebellum.

A noticeable feature in the embryonic brain is the downward curvature of its anterior portion. The flexure is slight in those forms which have small cerebral hemispheres (Cyclostomi, Ganoidei, Teleostei, Amphibia), but well marked in the remaining groups. The “ cranial flexure,- as it is termed, is apparently rectified as development proceeds, but this is merely due to the increased size of the cerebral hemispheres, thing, becomes more pronounced.

Fig. 106. - Dorsal View or Anterior Portion of Embryo Fowl at the End of the Second Day, 4.27 mm. long. Magnified 40 diameters. [From Kolliker .]

Abl. optic vesicle ; H. heart ; Hh. cerebellar dilatation of the primitive brain ; Mil. mid-brain ; Mr. neural canal ; Mr', wall of mid-brain ; Uw. muscle-plates ; Vh. anterior primary brain vesicle ; Venn, omphalo-mcsenteric vein.

The primitive flexure, if anyYh

Before describing the development of the brain, it will be advisable to give a brief account of the structure of such an unspecialised type of brain as that of the Frog.

The posterior region of the Frog -s brain, the medulla oblongata, gradually passes behind into the spinal cord or myelon. It is triangular in shape, with thick sidewalls and floor, but the roof is very thin, and richly supplied with blood-vessels forming the choroid plexus. The central canal of the spinal cord expands in the medulla to form the fourth ventricle.

The dorsal anterior wall of this region of the brain is thickened and dorsally produced (fig. 107, cbl), and is known as the cerebellum.

The roof of the brain in front of the cerebellum is produced into two thick-walled hollow vesicles, the optic lobes. The cavity of the region of the brain, into which the optic lobes open, is the iter a tertio ad quartum ventriculum (or passage between the third and fourth ventricle), or more shortly, the iter. The anterior end of the iter is narrowed ; in the dorsal wall of this neck lies a transverse bundle of nervefibres, the posterior commissure.

The cavity of the brain again expands to form the third ventricle ; this brain region is the thalamencephalon. The anterior portion of its roof is prolonged to form the pineal gland, and the posterior portion of its floor forms the sac-like infundibulum, to the extremity of which the pituitary body is attached. A fan- shaped bundle of nerve-fibres passes down the side walls of the thalamencephalon, and decussating on its ventral wall, forms the optic-chiasma (fig. 107, The median anterior wall of the thalamencephalon is called the lamina terminalis ; about half-way up is situated the “ anterior commissure - of authors, but this latter is really composed of a separated upper and lower bundle. Osborn has recently shown that the upper bundle (which occurs in all Amphibia and Reptiles) is a rudimentary corpus callosum, as it contains the fibres of the dorso -medial moiety of the hemispheres. The lower bundle (Reptiles, Amphibia, Fishes) represents the anterior commissure of Mammals (fig. 109, Ca). Two regions are discernible in the lower bundle, the pars olfactoria and the pars temporalis ; the latter, feebly developed in the Amphibia, increases with the progressive development of the temporal lobe.

A. Dorsal view. B. Ventral view. C. Horizontal section. D. Side view [ after Howes]. E. Longitudinal section [after Osborn].

a.c'‘". anterior commissure (pars olfactoria and pars temporalis); chi cerebellum ; cc. corpus callosum ; c.h. cerebral hemisphere ; 3 and 4. choroid plexus of the third and fourth ventricles respectively ; f.m. foramen of Munro ; hph. hypophysis (pituitary body); inf. infundibulum; iter, aqueduct of Sylvius; lamina terminalis; my. myelon ; op. optic lobe; optic chiasma; optic thalamus ; s.c. superior commissure ; 1. olfactory nerve ; 11. optic nerve ; iv. fourth cranial nerve ; 3 and 4. third and fourth ventricles.

The antero-lateral angles of the thalamencephalon are produced into a pair of elongated lobes, the cerebral hemispheres. They gradually narrow in front, but again slightly enlarge to form the olfactory lobes ; from their anterior extremities the olfactory nerves (fig. 107, 1) pass off to the nose. The olfactory lobes are fused together in the middle line. The common cavity, lateral ventricle, of each hemisphere and olfactory lobe communicates with the third ventricle through the foramen of Munro.

A diagram of a section of the brain of an embryo Fowl (fig. 108) may be advantageously compared with the Frog -s brain. It will be at once noticed that the

Fig. 108. - Diagrammatic Outline of a Longitudinal Section THROUGH THE BRAIN OF A FOWL

Embryo of Ten Days. [From Quain after Mihalkovics .]

ac. anterior commissure ; amv. anterior medullary velum ; below this are the aqueduct of Sylvius and the crura cerebri ; ba. basilar artery ; bg. corpora bigemina ; cai. internal carotid artery ; cbl. cerebellum ; ch$, ch i . choroid plexus of the third and fourth ventricles respectively ; h. cerebral hemisphere ; inf. infundibulum ; It. lamina terminalis ; Iv. lateral ventricle ; obi. medulla oblongata ; olf. olfactory lobe and nerve ; opc. optic commissure ; pin. pineal gland ; pit. pituitary body; ps. pons Varolii ; r. roof of fourth ventricle; st. corpus striatum ; v$. third ventricle ; v*. fourth ventricle.

thalamencephalon with the hemispheres and the cerebellum are in this case relatively much larger, and the optic lobes smaller. This is increasingly the case as development proceeds.

A figure of a vertical section through the human brain is given (fig. 109) to illustrate the disproportionate increase in size of the cerebral hemispheres over the rest of the brain, and other Mammalian characteristics.

Fig. 109. - Longitudinal Section of an Adult Human Brain.

[ From Wiedersheim after Reichert .]

Aq. aqueduct of Sylvius ; B. corpus callosum ; Ca. anterior commissure ; Cm. middle commissure ; Col. lamina terminalis ; Cp. posterior commissure ; FM. foramen of Munro ; G. fornix ; H. pituitary body ; HH. cerebellum ; MH. corpora quadrigemina ;

NH. medulla oblongata ; P. pons Yarolii ; R. spinal cord ; Sp. septum lucidum ; T. infundibulum ; Tch. tela choroidea ; To. optic thalamus ; VH. cerebrum ; Z. pineal gland ; I. olfactory lobe and nerve ; II. optic nerve.

The Posterior Primary Brain Vesicle. - At first the walls of the hind- vesicle have a fairly uniform thickness (figs. 159, 160), hut a noticeable change occurs when the above-mentioned anterior thickening (cerebellum) increases in size. The side walls of the posterior multiple division, medulla oblongata, become much thickened and grow away from each other dorsally, leaving a very thin roof which possesses but little nervous tissue (figs. 125, 126). In transverse sections the medulla at this stage has a very characteristic triangular outline (figs. 1 12, 126).

The side walls and floor of the medulla become greatly thickened, and local enlargements form the olivary bodies and pyramids. The thin roof of the cavity of the medulla, fourth ventricle, soon becomes very vascular, and is known as the choroid plexus of the fourth ventricle.

The minor enlargements of this region of the brain alluded to above disappear very early and leave no trace.

The Cerebellum at first appears as a thickened anterior dorsal border to the medulla ; in many types this undifferentiated condition is practically retained throughout life (Marsipobranchs, some Ganoids. Dipnoi, Amphibia, and some Eeptiles). In other forms the roof becomes greatly enlarged ; in Elasmobranchs the cerebellum is relatively very large, and at an early stage appears to be composed of two lateral halves. In Birds a central lobe appears and grows to a very large size ; the walls being much folded, constitute what is termed an abor- vitae ; there are two small lateral lobes or flocculi. In the development of the higher Mammals the central lobe (vermis) is the first to appear, and remains relatively large for some time, but the lateral lobes (hemispheres) usually eventually dwarf the former. In connection with this it is interesting to note that the cerebellum in the Monotremes consists almost entirely of the median lobe, and that in the Marsupials the lateral lobes are still small. The cerebellar fissures at first appear on the vermis and then extend to the hemispheres.

The Pons Yarolii, being the ventral commissure connecting the two hemispheres of the cerebellum, has a proportionate development with them, and appears rather late. In the Monotremes it is scarcely more developed than in many Sauropsida.

The Middle Primary Brain Vesicle. - The mid- vesicle, or, as it is usually termed, the mid-brain, has a much simpler history than the other regions of the brain. The cavity always remains small, and is known as the Aqueductus Sylvii or iter a tertio ad quartum ventriculum. In most of the lower Vertebrates the roof is produced into two vesicles, the optic lobes or corpora bigemina (fig. 107, ojj). In Birds these assume a lateral position, and the roof of the mid-brain is thin. In Mammals the roof gives rise to the solid corpora quadrigemina (fig. 109).

In an early stage of their development in Mammals the corpora quadrigemina are said to appear as an indistinct pair of lobes, a phase comparable with the optic lobes (corpora bigemina) of the lower Vertebrates. But Kolliker states that the anterior pair are at first separated from one another by a short longitudinal groove and only partially from the posterior undivided mass. Later the posterior bodies are completed by a meeting of the lateral grooves and a posterior extension of the median groove. In the Monotremes the anterior bodies are well marked, the posterior being inconspicuous, and, according to Owen, not separated by a median groove.

The floor of the mid- vesicle is greatly thickened, and forms the crura cerebri. The relative size of this section of the brain is very much greater in the embryo than in the adult.

The Anterior Primary Brain Vesicle. - The primitively single cavity of the fore-vesicle is very early produced into a pair of lateral vesicles, the optic vesicles (figs. 106, 1 10), the further history of

Fig. no. - Horizontal Section of the Brain of a Rabbit of Ten Days. Magnified 40 diameters. [From Kolliker.'] ab. mesoderm ; as. peduncle of optic vesicle (83 fj. diam.); ch. notochord ; g. thickening of the epiblast in the region of the future olfactory pits ; i. infundibulum ; m. mid-brain ; mes. optic vesicle (26 mm. high) ; v. anterior brain vesicle ; v. veins.

which is connected with the development of the eye (pp. 1 57-167). The fore vesicle grows anteriorly, and a small downgrowth from the roof indicates the distinction between the anterior and posterior divisions of the fore-brain. The posterior division is the thalamencephalon (figs, m-115); the anterior will give rise to the cerebral hemispheres and olfactory lobes.

The anterior portion of the floor of the thalamencephalon thickens to form the optic chiasma, while the posterior part is produced into a blind backwardly directed pouch, the infundibulum (figs, no, 115, 135).

In the lower Vertebrates the infundibulum is usually relatively large, but in the higher forms it is much reduced. In Teleostei ventral-lateral swellings of the infundibulum constitute the lobi inferiores ; the single tuber cinereum of Mammals occupies a similar position. The corpus albicans, which is single in the lower Mammals, but double in Man and the higher Apes, though single when first developed, arises behind the infundibulum.

The pituitary body (figs. 107, hph; 109, h; 112, hp; 116, hph) (hypophysis cerebri) becomes more or less intimately connected with the fundus of the infundibulum, but it is in nowise a nervous structure (see p. 100).

The walls of the thalamencephalon greatly increase in thickness, and form the optic thalami (fig. 111). The middle or soft commissure of Mammals unites these structures anteriorly across the cavity of this region of the brain (third ventricle). It is probably homologous with a commissure described by Balfour in Elasmobranchs, and by Osborn in Amphibia (supra-commissura) (fig. 107, s.c.), which crosses the roof of the third ventricle immediately in front of the pineal gland.

Fig. hi. - Horizontal Section of Anterior Portion of the Brain of an Embryo Sheep, 15 mm. long. Magnified 5 diameters. [From Kolliker.]

h. cerebral hemispheres ; m. position of the future foramen of Munro ; 0. recess which, deeper down, passes into the optic nerve ; t. third ventricle ; t'. central portion of thalamencephalon, in front is the lamina terminalis ; th. optic thalamus.

Fig. 112. - Horizontal Section of the Head of an Embryo Sheep, 15 mm. long. Magnified 50 diameters. [From Kdlliker.]

d. thin roof of fourth ventricle q ; g. Gasserian ganglion ; gr. nerve-cells in floor of fourth ventricle; h. cerebral hemisphere ; hp. hypophysis (pituitary body) ; l. lateral ventricle ; to. position of future foramen of Munro ; ms. axial portion of skull ; 0. cavity of optic stalk ; p. nervefibres of pyramid ; s. lamina terminalis ; t. posterior and deeper portion of third ventricle ; t'. anterior portion of the same.

The pineal gland, or epiphysis cerebri, develops as a diverticulum from the roof of the third ventricle (figs. 107-109, 116). It usually becomes a long narrow tube, the lumen of which may persist throughout life, but usually the proximal end atrophies to a thread-like stalk, while the distal portion is enlarged, and becomes lobular or branched. The enlarged termination may remain outside the cranium (Raja and Anura) or become imbedded within it (Acantliias and some Lizards), but in most cases it lies beneath the roof of the skull. In Elasmobranchs and some Urodela the pineal gland retains its sac-like character (fig. 138*, b).

Ahlborn regards the pineal gland as the rudiment of a primitive unpaired eye, from its position, origin, and mode of development, and compares it with the unpaired eye of Amphioxus and larval Ascidians. This view has since been confirmed by De Graaf, who has shown that in Anguis the epiphysis has the structui'e of an eye constructed on the invertebrate plan. Spencer has still more recently extended this discovery to Hatteria and other Lizards (fig. 138*, c-e). This organ is lodged within the parietal foramen. A similar foramen is found in the skulls of Labyrinthodonta and certain extinct Reptilia, and also, as Osborn has pointed out, in the Mesozoic Mammal Tritylodon (see also p. 162).

Behind the pineal gland the optic thalami are further connected across the roof of the brain in the Elasmobranchii, Amphibia (fig. 107, p.c), Sauropsida, and Mammalia (figs. 109, Cp; 11 6, by a transverse commissure, the posterior commissure. This is always situated at the base of the posterior peduncle of the pineal gland.

In front of the pineal gland the greatly thinned roof of the third ventricle, velum interpositum, becomes very vascular, and forms the choroid plexus of the third ventricle or tela choroidea (figs. 107-109, 1 16, ch.p 3).

The cerebral hemispheres usually arise as a pair of lobes from the roof of the anterior or cerebral portion of the fore-brain, each containing a cavity, lateral ventricle, which is continuous with that of the central nervous system (figs. 107, 108, hi, 112).

That portion of the fore-brain lying in the median line between the cerebral hemispheres is the lamina terminalis (figs. 108, 116, l.t ), and it extends from the roof of the thalamencephalon to the optic chiasma.

The Y-shaped passage connecting the lateral ventricles with the third ventricle is the primitive foramen of Munro. Though at first wide (fig. in, m), it is ultimately greatly narrowed (fig. 109, F, m).

There is throughout the Vertebrate series considerable diversity in the size and structure of the cerebral hemispheres. Their condition in the Amphibia has already been described.

The cerebral hemispheres show a marked increase in size in the Sauropsida, and reach their culminating point in the Birds ; but even here they attain a low stage of evolution as compared with the hemispheres of the Mammalia.

Not only do the cerebral hemispheres in Mammals grow forward, but they extend backward so as to hide the thalamencephalon and the mesencephalon in a dorsal view, and even project beyond the cerebellum in Man (fig. 109) and the higher Apes. The com Fig. 113. - Lateral View of the Brain of an Embryo Calf of 5 cm. [From Balfour after Mihalkovics.\

The outer wall of the hemisphere is removed, so as to give a view of the interior of the left lateral ventricle.

am. hippocampus major (cornu ammonis) ; cb. cerebellum ; d. choroid plexus of lateral ventricle ; fm. foramen of Munro ; hs. cut wall of cerebral hemisphere ; in. infundibulum ; mb. mid-brain ; op. optic tract ; ps. pons Varolii, close to which is the fifth nerve with the Gasserian ganglion ; st. corpus striatum ; iv. v. roof of fourth ventricle.

plexity of this region of the adult brain is due to local thickening, reduction, infolding, and fusion.

The external walls of the primitively simple cerebral hemispheres become greatly thickened, while the inner walls - i.e., those in contact with one another in the median line - are extremely thin.

The mesoblastic sheath surrounding the developing brain grows downwards as a lamina into the longitudinal fissure between the

Fig. i 14. -Brain of a Human Embryo of Six Months. Natural Size. [From Kolliker.]

c. cerebellum ; /. flocculus ; fs. fossa Sylvii ; 0. y s \ olivary body ; ol. olfactory bulb ; p. pons Varolii.

o'l " ' F


hemispheres. From this will be derived the falx cerebri and the choroid plexus (fig. 1 1 5, / and pi).

The floors of the hemispheres become much thickened and constitute the corpora striata. These protrude so much into the lateral ventricles as to cause them to assume a curved appearance in a longitudinal vertical section (fig. 1 1 3, st), thus constituting the anterior and posterior cornua of the lateral ventricles.

The position of the corpus striatum is indicated in an external side view of a cerebral hemisphere by the fossa Sylvii (fig. 1 14,/s), which demarcates the frontal and temporal lobes.

Owing to their backward extension, the corpora striata become increasingly connected with the optic thalami (fig. 1 1 5, st } th ), with which they ultimateh fuse so completely that the line of separation cannot be recognised.

The corpora striata are connected together by the anterior commissure which traverses the anterior wall of the third ven

Fig. 115. - Transverse Section of'The Brain of an Embryo Sheep, 2.7 cm. long.

Magnified 10 diameters. [From Kolliker.]

a. cartilage of orbito-sphenoid ; c. peduncular fibres ; ch. optic chiasma ; /. median cerebral fissure ; h. cerebral hemispheres, with a convolution upon their inner wall projecting into the lateral ventricles, l ; m. foramen of Munro ; 0. optic nerve ; p. pharynx ; pi. lateral plexus ; s. termination of the median fissure which forms the root of the third ventricle ; sa. body of the anterior sphenoid ; st. corpus striatum ; t. third ventricle ; th. anterior deep portion of the optic thalamus.

tricle. This is the earliest developed commissure which connects the cerebral hemispheres, and is found, though of smaller size, in the Sauropsida and Ichthyopsida. It lies in the substance of the lamina terminalis (figs. 107- 109, 116 a.c).

The inner wall of each hemisphere projects into its lateral ventricle as two longitudinal ridges extending from the foramen of Munro to nearly the posterior end of the descending cornua. The upper one, hippocampus major or cornu ammonis (figs. 1 13, am; 1 1 5, h), is a solid nervous structure, while the lower ridge is very thin and folded, and by the ingrowth into it of a large number of blood-vessels from the falx forms the choroid plexus of the lateral ventricles (figs. 1 1 3, d ; 1 1 5, pi).

The cerebral hemispheres of Mammals unite with one another in front of and above the lamina terminalis ; the fused internal walls being very thin, are termed the septum lucidum or septum pellucidum (figs. 109, Sp; 116, s.l). In Man the two walls enclose a slit-like cavity, the so-called fifth ventricle. As this space is really only a portion of the longitudinal fissure between the hemispheres enclosed by overgrowth, it, morphologically speaking, lies outside the brain, and consequently is not lined by an epithelium, like the true ventricles.

The fornix (fig. 1 1 6, fx) is a band of nerve-fibres which unites the hemispheres along the inferior border of the septum. In front it divides into two anterior pillars or columns, each of which, passing in front of the foramen of Munro and behind the anterior

Fig. 116. - Longitudinal Vertical Section


Brain of an Embryo Babbit of 4 cm. [After Mihalkovics.] anterior commissure ; c.h. cerebral hemisphere'; c.p. cerebral peduncles ; cal. corpus callosum; ch.p. 3. chloroid plexus of the third ventricle ; f.m. foramen of Munro ; fx. fornix ; hph. hypophysis (pituitary body) ; inf. infundibulum ; iter, acqueductus; l.t. lamina terminalis; to . b. midbrain ; olf. olfactory lobe ; op. ch. optic chiasma; posterior commissure ; pin. pineal gland ; p. V. pons Varolii ; s.l. septum lucidum ; F.3. third ventricle.

commissure, terminates in the corpus albicans (or in each of the two corpora in Man). Behind, the fornix also divides into two posterior pillars or crura, each of which eventually passes into the hippocampus major in the descending cornu of the lateral ventricle of its side.

The characteristic commissure of the Mammalia, the corpus callosum, arises last of all in the upper portion of the septum lucidum, and serves to directly connect the two cerebral hemispheres. The curved anterior section (genu) is the first portion to develop, and this alone occurs in the Monotremata and Marsupials ; in these groups the anterior commissure is relatively very large. The corpus callosum keeps pace with the hemispheres as they increase in size and extend backwards. As was stated on p. 124 a rudiment of the corpus callosum is found in Amphibia and Reptiles.

In the lower Vertebrates the cerebral hemispheres are smooth throughout life, but in the higher Mammals the surface of the hemispheres is thrown into a number of folds (convolutions) with deep grooves, or sulci between them.

Kolliker was the first to distinguish two kinds of cerebral convolutions and sulci, which he now terms primitive and secondary. The former appear early, and all but disappear long before birth. The sulci are the expression of actual infoldings of the walls of the hemispheres, and correspond with those local thickenings which constitute such structures as the corpus striatum, hippocampus major, &c. The sulcus of the first of these (fig. 114,/s) is the only one which markedly persists throughout life.

The secondary convolutions begin to appear about the middle of foetal life in Man. They affect only the more superficial portion of the cerebral walls, and probably originate by arrest of growth in the sulci, accompanied by active growth in the convolutions ; the arrest of growth may be partly induced by the pressure of the main blood-vessels of the hemispheres.

In many of the lower Mammals the cerebral hemispheres are smooth, i.e., free, or nearly so, from the secondary convolutions. The order of the appearance of the convolutions is too special a subject to be dealt with here ; but, speaking in general terms, the cerebral convolutions of the brains of certain adult Lemurs and Monkeys correspond with stages observed in the development of the human brain.

The olfactory lobes (Bhinencephala) usually arise as hollow prolongations from the antero-ventral end of the cerebral hemispheres (figs. 107, 108, 1 16). According to Marshall, they arise in Elasmobranchs (fig. 120, ol.v ) and Birds after the appearance of the olfactory nerves. They are relatively large in the adults of low forms, and in the embryos of the higher Mammals.

In all Mammals the olfactory lobes are at first hollow, the cavities being prolongations of the lateral ventricles ; in Man the lobes become solid and quite small (figs. 109, 1; 114, ol). In the lower Mammals they constitute the anterior extremity of the brain ; but owing to the forward growth of the cerebral hemispheres in the higher Mammals, they eventually occupy a ventral position.

The envelopes of the brain are entirely of mesoblastic origin.

Summary of the History of the Mammalian Brain

The primitive neural tube dilates to form certain vesicles, all of which have not the same morphological value. They may be thus tabulated : -

Primary Vesicles.

Secondary Vesicles.


Wilder, “Quain.-

/ Fore -brain*



Anterior or Fore- Vesicle

< Inter-brain or )


( Thalamencephalon or

1 -Tween-brain )

( Diencephalon

Middle or Mid- Vesicle

j- Mid-brain



Posterior or




Hind- Vesicle




The greater portion of the walls of these primitive vesicles become enormously thickened, thus the anterior portion of the roof of the hind-vesicle (hind-brain) forms the cerebellum, and the floor and sides develop the olivary bodies, pyramids, &c., and anteriorly the pons Yarolii.

The corpora bigemina (or quadrigemina) are developed from the roof of the middle vesicle, and the crura cerebri from the floor.

In the anterior vesicle, the floor of the thalamencephalon develops the corpus albicans and optic chiasma, and the walls of the optic thalami. The floor of each half of the prosencephalon (cerebral hemispheres) develops the corpora striata, and the inner walls the hippocampus major ; the external walls are greatly thickened.

But portions of the primitive vesicles remain thin and develop vascular plexi ; these are : - The roofs of the myelencephalon (medulla) and thalamencephalon, and part of the inner walls of the prosencephalon.

The cerebral hemispheres grow backward, and their lateral vesicles are considerably altered in shape and their cavities reduced by the ingrowth of the walls and floor ; as, for example, the hippocampi and corpora striata.

The lateral elements of the brain are co-ordinated by the development of transverse commissures, of which the following are the- most important : - Pons Yarolii for the cerebellum, posterior commissure, anterior portion of the roof of the mesencephalon, middle commissure across the third ventricle, and the anterior commissure in its front wall. This, with the fornix at the base of the septum lucidum and the corpus callosum above it, serve to directly connect the cerebral hemispheres with each other. The decussation of the fibres of the optic chiasma, strictly speaking, come under this head.

The Cranial Nerves. - The dorsal roots of the cranial nerves, like those of the spinal nerves, arise from the dorsal portion of the cerebro-spinal axis. A neural crest, continuous with that of the spinal cord, is probably always present.

Most of the cranial nerves are usually regarded as homologous with the spinal nerves, and as having a segmental significance, but considerably modified, owing to the great changes which have taken place in the cephalic region.

Corresponding to the German Yorderhirn, Zwischenhirn, Mittelhirn, Hinterhirn, and Nachhirn.

The following is a brief summary of what is known concerning the development of the cranial nerves. The numeration and terminology is that which is usually adopted by anatomists.

XII. and XI. The Twelfth or Hypoglossal, and the Eleventh or Spinal Accessory Nerves

Neither of these nerves is constant as a cranial nerve throughout the vertebrate series. For the present they may be dismissed, as they are regarded by some as belonging to the spinal series (see p. 14 1). Their development is not well known.

X. The Tenth or Vagus Nerve

The tenth nerve arises from the neural ridge in the myelencephalon (medulla) behind the auditory involution ; it soon develops a large ganglion, beyond which it is produced as the intestinal branch. Later several anterior roots arise from the ventral surface of the brain and join the vagus. This nerve sends a pair of branches to supply the two sides of the posterior branchial (visceral) clefts (see p. 177). In the Marsipobranchii, and in Notidanus, the last six of the seven branchial clefts are supplied by thir nerve; in other Vertebrates the number is less. Thus the tenth nerve is usually regarded as equivalent to at least six segmental nerves, the single origin of the tenth nerve being supposed to be of secondary significance. For several reasons Amphioxus cannot be utilised for comparison, one being that there is no correspondence between the number of the body segments and branchial clefts in that form.

Fig. 117. - Diagram Illustrating the General Distribution of the Cranial Nerves.

[Modified from Beard,.]

A-C. The three anterior head-cavities. I.-X. The cranial nerves (ordinary numeration).

au. auditory vesicle; bi -by. seven branchial clefts; ci. ciliary ganglion; h. hyoid cleft; int. intestinal branch of vagus nerve; m. mouth; olf. olfactory pit ; oph. v. and vn. ophthalmic division of the trigeminal and facial nerves respectively ; oph. prof, ophthalmicus profundus ; pal. palatine branch of the facial nerve. The radix longa unites the ciliary with the Gasserian ganglion (v.).

IX. The Ninth or Glosso-Pharyngeal Nerve

The ninth nerve usually has a common origin with the tenth nerve, but it very soon becomes distinct, and, like the latter, it acquires numerous roots. This nerve passes immediately behind the auditory capsule and expands above the first branchial cleft into a ganglion. From the latter a thick posterior branch is distributed to the anterior border of the first branchial arch, and a thinner branch to the posterior border of the hyoid arch.

VIII. The Eighth or Auditory Nerve

The eighth nerve (fig. 126, E, viii) arises in such close contiguity with the seventh that it is usually stated to be a branch of it ; but Beard maintains that it is a true segmental nerve. It is a short thick nerve with a large ganglion, and is solely the sensory nerve of the ear.

VII. The Seventh or Facial Nerve

The seventh nerve early develops as an outgrowth from the neural crest on the dorsal surface of the myelencephalon just in front of the auditory capsule. At an early stage it acquires a new or secondary attachment to the side of the brain ; but, unlike any other nerve, cranial or spinal, the original or primary root is retained as well as the secondary [Marshall]. The main branch of this nerve passes down the anterior side of the hyoid arch (p. 178); a smaller branch (praespiracular) forks over the hyomandibular cleft (spiracle) ; in Mammals it joins the mandibular division of the fifth nerve, and is known as the chorda tympani. The seventh nerve also gives rise very early to two anterior branches, the upper (portio facialis of the ophthalmicus superficialis) passes to the front end of the head along with the ophthalmic division of the fifth nerve. The lower or palatine (superficial petrosal of Mammals) runs superficially to the superior maxillary division of the fifth.

VI. The Sixth or Abducent Nerve

The sixth nerve arises from the median ventral line of the brain below the seventh nerve, and never develops ganglion cells. It is an exclusively motor nerve, which supplies the rectus externus muscle of the eyeball, and also in some forms the retractor muscle of the bulb of the eye and the nictitating membrane.

V. The Fifth or Trigeminal Nerve

The fifth nerve develops from the neural ridge in front of the seventh nerve. After expanding into a large ganglion (Gasserian ganglion), it arches over the mouth, the main trunk (mandibular or inferior maxillary) beingdistributed over the lower jawq and the smaller (superior maxillary) over the upper jaw. The dorsal division of the fifth nerve emerges anteriorly from the Gasserian ganglion, and follows the ophthalmic division of the seventh nerve to its distribution at the anterior end of the head ; it is known as the portio profunda or minor of the ophthalmicus superficialis. A nerve connecting the Gasserian with the ciliary ganglion is usually termed the ophthalmic division of the fifth nerve ; it appears not to be a branch of that nerve.

IV. The Fourth, Pathetic or Trochlear Nerve

In its earliest recognised condition the fourth nerve has the same position that it occupies in the adult, viz., the dorsal surface of the extreme hinder border of the mid-brain. It invariably innervates the superior oblique eye-muscle, and in many Vertebrates sends sensory branches to the conjunctiva and the skin of the upper eyelid.

III. The Third or Oculomotor Nerve

Marshall thinks it is probable that the third nerve grows from the neural crest on the top of the mid-brain ; but as in the adult it arises very near the mid-ventral line, it must undergo the maximum amount of change of position. But Beard states that the nerve described by Marshall is really the radix longa, and believes, though he has no direct evidence to give, that the oculomotor does not arise from the neural crest. This nerve is associated with the ciliary or ophthalmic ganglion, and is distributed to all the muscles of the eyeball except those supplied by the fourth and sixth nerves, as well as to the levator palpebrge superioris and the circular muscles of the iris.

II. The Second or Optic Nerve

The second nerve is merely a degenerate portion of the brain itself, being the stalk of the optic vesicle (p. 160).

I. The First or Olfactory Nerve

The first nerve arises from the dorsal part of the sides of the anterior cerebral vesicle before the cerebral hemispheres have commenced to develop. Owing to the enormous development of the latter in the higher Vertebrates, the nerve comes to occupy a ventral position. It is exclusively distributed to the nasal fossse (figs. 117, 120, 1).

Hypotheses concerning the Segmental Value of the Cranial Nerves

Recently both Spencer and Beard have shown that after the (dorsal) roots of the cranial nerves arise from the neural ridge, they fuse with the epiblast at the level of the notochord. The epiblast cells at these spots proliferate the masses of cells thus developed, forming the cranial ganglia ; and at the same time a rudimentary structure is formed, termed by Beard the branchial sense organ, and by Spencer the sense organs of the lateral line in the head. As these organs at first only appear in the gill-bearing region of the body, the former term is perhaps the preferable.

Miss Johnson and Miss Sheldon, who have still more recently investigated the development of the cranial nerves in the Newt, admit the fusion of the cranial nerves with the incipient serial sense organs (mucous canals or lateral line organs of the head). They deny that the ganglion is derived from this fusion, but state that it takes its origin from the original outgrowth from the neural ridge, as Redot has also shown for the spinal nerves in Triton.

Beard states that the dorsal root of a cranial nerve develops in the following manner. The nerve grows downwards from the neural ridge below, hut unconnected with the epiblast. About the level of the notochord it fuses with the epiblast, but part of the nerve passes on to the lateral muscle-plates of the segment (fig. 118); this main or posterior branch (post-branchial nerve) of Beard, chiefly innervates the gill-muscles. Proliferation at the junction of the nerve with the epiblast gives rise to the ganglion of the dorsal root, and externally to the rudiment of the primitive branchial sense organ of that root. As the ganglion separates from the skin a nervous tract is left, the so-called dorsal branch (supra-branchial nerve). The anterior or praebranchial nerve, and probably the pharyngeal branch, are also derived from this proliferation.

Following up this discovery, Beard has attempted a re-enumeration of the segments of the head, and a review of the nature of the nerves themselves. The following is briefly his position.

Fig. i 18. - Diagrammatic Transverse Section THROUGH THE GILL-BEARING REGION OF AN ELASMOBRANCH OR OTHER ICHTHY opsid. [After Beard. ]

Neural canal not yet closed over. On the left side the gill muscle-plate is shown, and on the right tue gill cleft.

br.g. branchial ganglion ; br.o. branchial sense-organ; cl. visceral cleft; d.n. dorsal (posterior) root of segmental nerve ; h.c. head-cavity; l.m.p. lateral muscle-plate; n.c. neural canal ; nch. notochord ; ph. pharynx ; p.n. post-branchial nerve.

The ganglia known to human anatomists as the olfactory bulbs, ciliary, Gasserian, geniculate, auditory, petrous, and pneumogastric ganglia, all belong to the same series, and are associated with primitive sense organs. The table given on p. 140, when compared with fig. 117, will elucidate their relationships, and but few remarks will be necessary.

On this hypothesis the nerves arising from the cranial neural crest and uniting with the primitive sense organ of its segment correspond to some extent with the dorsal branches of the spinal nerves. The nerves and their ganglia are: (1) the olfactory nerve and ganglion ; (2) the radix longa (or the nerve uniting the ciliary with the Gasserian ganglion) and the ciliary ganglion ^ (3) the trigeminal nerve and the Gasserian ganglion ; (4) the facial nerve with its ganglion ; (5) the auditory nerve and ganglion ; (6) the glosso-pharyngeal nerve and its ganglion ; (7+ ) the vagus nerve with its segmental branches and their associated ganglia.

The association of the dorsal root of the ciliary nerve (radix longa) with the Gasserian ganglion, instead of its directly arising from the brain, is explained by Beard as being due to the primitive outgrowths being very close together.

The oculomotor (III.), trochlear (IV.), and abducent (VI) nerves are regarded as the anterior roots of the radix longa (ciliary), trigeminal (V.), and facial (VII.) nerves respectively. They all supply the eye-muscles, the latter being developed from the first two (? three) head cavities.

The fact that there are two anterior branches (ophthalmic and palatine) of the seventh nerve, is one reason for supposing that there may be a missing head segment between the third and fourth of the above enumeration. Independently of this, there are two pre-oral segments ; and counting the auditory as a true segment, there are nine post-oral in the Fish, with the greatest number of gill-clefts (Notidanus). This makes a total of at least twelve segments in the Vertebrate head.

Little need be added concerning the segmental sense organs, as they usually at first appear as patches of columnar cells lining a slight depression of the epidermis.

Serial Cranial Sense Organs. - The organs of the lateral line consist of a series of mucous canals containing groups of sense-cells which are segmentally disposed in the trunk (see p. 148). The canals are variously distributed in the head, but in the body they almost invariably extend along the middle line of each side, as far as the tail. This system of sense organs is only found in Fishes, Urodele Amphibia, and the larvae of the Anura.

In the head the canals are innervated by cranial nerves, the lateral line proper being supplied by the lateral branch of the vagus.

The lateral line itself is developed from a backward growth of the epiblastic proliferation, which gives rise to the sense organ of the vagus. This ploughs its way along the superficial epiblast and the indifferent epiblast cells, which are thus thrust aside are probably lost [Beard] (fig. 103).

As in other cases, the nerve of the sense-organ is formed from the deeper layer of the sensory thickening.

The extension of these (primitively branchial) sense organs to the hinder end of the body is supposed by Beard to be of only secondary significance. Some authors, however, believe that the connection of the (segmental) organs of the lateral line with the vagus is itself secondary.

Of the primitive segmental sense organs, the first has become retained and modified as the olfactory organ. In most Ichthyopsida the organs of the lateral line of the head are still innervated by certain cranial nerves (ciliary, trigeminal, facial, and glosso-pharyngeal). The auditory organ may possibly be a highly specialised segmental sense organ, its histological structure also lending support to this view. The posterior organs persist as the organs of the lateral line of the body in the Ichthyopsida.

The presence of primitive branchial sense organs is not confined to the Ichthyopsida. Froriep has discovered rudiments of them for the facial, glosso-pharyngeal, and vagus segments in Cow and Sheep embryos ; and Beard finds them in the Fowl for the ciliary and trigeminal, in addition to the above segments. In all cases they disappear very soon.

Thymus Gland. - The paired serial rudiments of the thymus gland arise in a manner which is very suggestive of their having possessed a primitive branchial sensory function. For the sake of convenience the development of this composite gland will be described in another section (p. 184).

The primitive branchial clefts suffer great reduction. The more or less rudimentary hyoidean cleft (spiracle) is lost in the Teleosts. Most Fishes have but five true branchial clefts. The absolute extinction of the branchial clefts is well exhibited in the higher adult Urodele Amphibia ; but in these and in all higher animals the hypoblastic evagination concerned in the hyoid cleft more or less persists as the Eustachian tube or recess (p. 180).

Table of Cranial Segments and their Nerves and Sense-Organs

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This table will be found to differ from fig. 117 in having two hyoid segments, and consequently in accounting for a total of thirteen segments. The first segment corresponds with the fore-brain vesicle, the second with the mid-vesicle, and the remainder with the hind region of the brain.

Froriep divides the Mammalian head into three regions: - (1) prepituitary or trabecular, with the nose and eyes ; (2) pseudo-vertebral, with the trigeminal, facial, glosso-pharyngeal, and composite vagus nerves, which supply the pharyngeal clefts ; (3! vertebral, consisting of the occipital bone and hypoglossal nerve. He has found that in the embryos of Ruminants there are rudiments of three distinct protovertebrae in front of the first cervical (spinal) nerve and behind the vagus. In front of each of these rudiments ventral nerve roots arise, which all unite in a single trunk, the hypoglossus. A single dorsal ganglionated root unites with this composite nerve. Thus the hypoglossus is a fusion of at least three segmental nerves, and the occipital region corresponds to as many vertebrae. (This view has been independently arrived at by M c Murrich on purely anatomical grounds.) It must further be admitted that the occipital region of the cranium is not identical throughout the vertebrate series.

Ahlborn, from his studies on Petromyzon and Anura, has also arrived at the view that the hinder portion of the skull and the anterior cervical vertebrae may not respectively be homologous in different Craniates. He has come to the conclusion, mainly from a consideration of the cephalic mesodermic segments, that there were primitively nine pairs of spinal nerves in the hind-brain, of which Nerves III., IV., and VI. , had only motor roots ; but as neural segmentation (neuro-merism) is secondary, the spinal-like cerebral nerves of the craniota cannot be compared with the segmental spinal nerves.

An endeavour has been made to give a brief account of some of the views which are held respecting the significance of the cranial nerves, and of a few of the attempts which have been made to utilise the nerves in solving the problem of the segmentation of the Vertebrate head. It must, however, be borne in mind that there are very good reasons for regarding the apparent segmentation of the cephalic region as an arrangement perfectly distinct from the metamerisation of the trunk.

Sense Organs. - The simplest organs of sense are epiblastic cells, which, haying a stiff hair-like process, are excited by vibrations in the external medium (fig. 119). These sense-cells are usually collected into groups or series, and constitute definite sense organs.

Sense organs may be roughly grouped into those which appreciate vibrations of air or matter, and those which are stimulated by light.

It is usually possible to distinguish between sense organs which have a tactile, olfactory, gustatory, and auditory function; but in the lower animals it is probable that other kinds of vibrations may be appreciable which give rise to sensations of a less distinct, or even of a different character. These various senses are doubtless differentiations of a primitive tactile sense ; this is rendered more probable from the similarity in their development and their fundamental similarity of structure.

Tactile Organs

Tactile organs are direct modifications of epidermal cells ; they may either be the simplest of sense-cells, or may be more or less differentiated. Numerous kinds of tactile organs are described in works on comparative anatomy and histology. They may be generally diffused or restricted to certain prolongations of the body, more especially of the anterior end, such as tentacles, palpi, and antennae.

Olfactory Organs

The higher invertebrate Metazoa alone possess any organs which can be recognised as olfactory. In the

Fig. i 19. - Sense-Cells of Ccelenterates.

A. Isolated sense-cells from dorsal nerve-ring in connection with two multipolar ganglion cells (from iEginura myosura). [After Haeckel.']

B-E. Isolated elements from the upper nerve-ring of Carmarina hastata. [After 0. and R. Hertwig. ]

B. Ordinary small sense-cell. C. Large sense-cell. D. Large ganglion cell. E. Ordinary ganglion cells and nerve-fibrills.

F. Three supporting cells and one sense-cell from tentacle of Anthea cereus.

G. Isolated sense-cell from the same. [After 0. and R. Hertwig.]

Arthropoda these are minute bristles which are connected with nerve-fibrils. The olfactory organ of Mollusca (osphradium of Lankester) consists of a patch of sense-cells which is situated over each gill.

A pit or papilla behind or above each eye is stated to be the olfactory organ of the Cephalopoda.

In Amphioxus a single ciliated pit, situated on the left side at the anterior end of the neural canal, is usually spoken of as an olfactory organ ; but Hatscliek has shown that it is of hypoblastic origin (p. 185).

An undoubted olfactory organ is present in all higher Chordata. It first appears as a pair of tracts of columnar epiblast at the anterior end of the body, immediately in front of the stomodaeum (fig. 94, A, olf). The sensory epithelium invaginates as two shallow pits (fig. 1 1 7, 120), which soon deepen. Although the internal epithelium (Schneiderian membrane) is thrown into folds to increase the sensory surface, or the surface may be further increased by the projection of coiled, and sometimes very complicated, cartilages and bones (turbinal bones), yet the sac-like character and the primitive opening of the nasal pits are always retained.

The single nasal sac of the Cyclostomi has probably no phylo

Fig. 120. - Sections through Two Stages in the Development of the Olfactory Organ of an Embryo Dog-Fish (Scyllium). [After A. M. Marshall.']

A. Early, B. Later stage.

c.h. cerebral hemisphere ; f.b. fore-brain ; olf. olfactory pit ; ol.v. olfactory vesicle or lobe ; jpn. pineal gland ; sch. Schneiderian folds ; i. olfactory nerve.

genetic significance, as in the younger stages there are distinct evidences of a double nature. In all other Vertebrates the nose is paired from the first.

In Elasmobranchs the orifice of the olfactory pit is ventrally situated. In the Ganoids and Teleosts a distinct and often wide bridge of tissue divides the orifice of the nasal sac into an afferent and an efferent orifice, which always come to be situated on the dorsal aspect of the snout.

A groove extends in many Elasmobranchs from each nasal sac to the mouth ; the central flap of skin between the grooves is the nasal valve or fronto-nasal process (fig. 121), The lateral folds of the fronto-nasal process sometimes fuse with the cephalic integument across the nasal groove, in this way forming two apertures to the nasal sac.

The walls of this groove grow over and coalesce in the middle in Dipnoi and all higher animals, thus forming a canal which opens in front by the anterior nares or nostrils, and behind as the posterior nares. The latter are situated just behind the upper lip in Dipnoi and Urodela. In Anura and higher forms they lie somewhat farther back, but they are, in all, morphologically in

Fig. i 2 i. - U nder Surface of Head of Dog-Fish.

/. nasal flap, reflected on the left side of the fig. ; g. nasal groove ; to. mouth ; na. opening of olfactory organ.

front of the palatine bones. With the formation of the palate, the mouth cavity becomes subdivided into two, a lower buccal cavity and an upper nasal passage. The secondary posterior nares thus established may be carried back, as in the Crocodilia, Myrmecophaga, and in some Cetacea, even to the extreme hinder end of the mouth.

The development of the nasal passage in the Fowl is briefly as follows. The edge of the nasal pit develops a thickened border, except towards the mouth, thus

Fig. i22 . - Ventral Views of the Heads of Embryo Fowls, (i) At the end of the fourth day of incubation. (2) At the commencement of the fifth day. [From Kolliker. ]

an. outer nasal process ; in. inner nasal process ; V. second visceral arch (hyoid) ; to. mouth ; n. nasal or olfactory pit ; nf. nasal groove ; o. superior, and u. inferior, maxillary process of the first (mandibular) visceral arch ; s. cavity of pharynx ; sp. choroidal fissure of the eye ; st. fronto-nasal process.

leaving a shallow groove, the nasal groove. The central portion of this groove is converted into a canal by the lower angle of the fronto-nasal process overlapping, and ultimately fusing, with the superior maxillary process. The nasal canal thus formed opens well within the mouth by the posterior nares.

The adult condition of the nasal groove in some Elasmobranchs (fig. 1 21) corresponds with a transient stage (fig. 122) in the embryos of those Vertebrates which have posterior nares.

The organ of Jacobson is primitively developed as a pair of: diverticula from the nasal sac. These are at first large, but their subsequent development is less rapid than that of the olfactory sacs. Eventually they give rise to comparatively small organs, which usually open directly into the mouth independently of the posterior nares.

A shallow depression, which extends from the eye to the nasal pit while the nasal groove is still open, separating the outer nasal process (as the outer raised border of the nasal pit is termed) from the superior maxillary process, is known as the lachrymal groove.

The lachrymal duct is formed from a solid cord of epiblast cells which separates from the floor of the groove. It subsequently becomes hollow, and places the orbit in communication with the nasal chamber.

Gustatory Organs. - The gustatory organs always retain so simple a condition that they require no special mention.

Auditory Organs. - The so-called auditory organs of the invertebrate Metazoa are very varied in origin and position, but, except in the case of a few Medusae, they are all epiblastic structures.

Some of these organs appear to possess a truly auditor} 7- function. Balfour has suggested that in some cases their function may be to enable the animals provided with them to detect the presence of other animals in their neighbourhood, through the undulatory movements in the water caused by the swimming of the latter. In the case of the Medusae, however, the vibrations of waves reflected from the shore and rocks would affect these organs, and may possibly warn the Medusae of danger.

Two forms of auditory organ are found amongst the Medusae, tlie first alone being purely epiblastic, and consisting of an open sac, which may be converted into a complete cup. These occur along the base of the velum in the Vesiculate Hydromedusae. Some of the cells form a concretion (otolith) within their walls, and others are sense-cells with auditory hairs, which lie close to the former (fig. 123, A, b).

The second form is found in the Trachymedusae and Acraspeda, and consists of a modified tentacle, the terminal endodermal cells of which secrete otoliths, but the auditory hairs are solely ectodermal. The whole structure is usually more or less enclosed within a reduplicature of the ectoderm, sometimes forming a vesicle which entirely surrounds the auditory tentacle. In all cases the auditory cells of the Medusae are connected with the peripheral nerve-ring (fig. 123, u.n.r).

Paired otocysts containing several otoliths, rarely one, occur in some Nemerteans, Nematodes, and a few Annelids. Practically nothing is known of their structure, and their origin is also unknown; this also applies to the unpaired otocyst of Planarians.

The otocysts of Mollusca develop as epiblastic pits (fig. 124) close to the proliferating areas which form the pedal ganglia. Very rarely they are at first solid. The pits are converted into rounded vesicles, from which a small ciliated canal (ductus Kollikeri)

Fig. 123. - Auditory Organs of Various Medusae. [After 0. and R. Hertwig.']

A. Open auditory pit of Mitrocoma annae. B. Closed auditory sac of iEquorea forskalea. C. Endodermal otoliths in a modified tentacle of Cunina lativentris.

a.h. auditory hairs; c.c. circular canal; ec. ectoderm; en. endoderm ; l.n.r. lower nerve-ring ; m. muscle-fibres ; m.v. muscle of velum ; ot. otolith; s.c. sensecells ; s.l.v. supporting lamina of velum ; u.n.r. upper nerve-ring.

often projects, this being the remnant of the tube which for a time connects the vesicle with the orifice of the primitive invagination. At first a single small concretion is secreted by one of the cells of the vesicle; this may increase in size, and persist as a single otolith ; in other cases it remains small, and a large number of minute concretions are added (Pteropods, Dentalium, Nautilus, most Gasteropods). Earely the numerous otoliths fuse to form a single large one (Paludina, Decapods). The interior of the vesicle is clothed with cilia; but in the specialised otocysts of Heteropods there is a patch of definite auditory cells (macula acustica), and a similar ridge (crista acustica) occurs in Decapods. The otocyst often shifts its position anteriorly, and usually comes to be innervated from the cephalic ganglion.

Fig. 124. - Two Stages in the Development of the Otocyst in Murex.

A. Open pib. B. Closed vesicle, with small otolith.

ep. epiblast ; m. mesoblast.

The Arthropoda never possess otocysts comparable with those of other Invertebrates. Unicellular hairs, or setse on various parts of the body, especially on the antennae of Crustacea, are generally regarded as auditory; they are usually lodged within cuticular depressions.

In the Candida! (Shrimps and Prawns) the auditory hairs usually occur on the basal joints of the antennules and on the tail; auditory pits may occur at both ends of the body. In the Schizopods a large otolith is present, which is secreted by the walls of the sac, and is renewed after moult. The auditory sac is situated in the caudal endopodite. The auditory hairs are restricted in Decapods to the basal joint of the antennules ; they are usually feathered, and often bent. The otocvst in these forms may be widely open (Palinurus), but the opening is usually reduced to a narrow fissure In Hippolyte the sac is completely closed. Only in the Crabs does the otocyst become

Fig. 125. - Transverse Section through the Auditory Involutions of an Embryo Fowl of the Second Half of the Second Day. Magnified 84 diameters. [From Kolliker.]

a. descending aortse ; am. amnion, with, its two layers ; am', amniotic suture, situated on the right side and not drawn in its whole extent ; c. root of the inferior cerebral vein ; dfp. splanchnic mesoblast (fibro-intestinal layer) of the pharynx, continuous with the external envelope of the heart and forming an inferior cardiac mesentery; H. heart; hp. somatopleur passing into the amnion ; ihh. endothelium of the heart ; ph. pharynx ; va. widely open auditory sacs.

at all complicated. The otoliths are entirely foreign particles, and appear to be introduced by the animal itself.

A remarkable sense organ, usually stated to be acoustic, is found in certain Hexapoda, and is situated either on the thorax or at the base of the legs. It consists essentially of a series of nerve-fibres, each of which passes into a nerve- cell, from which arises a multicellular elongated structure, usually containing a stiff rod. The multicellular fibre is usually attached to a tympanum, supported by a chitinous ring. The whole structure is always situated over an air sac.

In Appendicularia there is a single otocyst on the left side of the ganglion, consisting of a spherical sac enclosing a spherical otolith which is supported by delicate isolated hairs. In other pelagic Tunicates there are two symmetrically placed otocysts ; their development is not known. In fixed Ascidians an otolith is developed from a single cell on the dorsal and right side of the brain. This cell projects into the cavity of the brain, and its free end is pigmented. Eventually the cell becomes stalked, and travels round the right side of the brain until it reaches the summit of a patch of cylindrical sense-cells, the crista acustica. The adult organ thus consists of a crista acustica on the floor of the anterior region of the brain and projecting into its cavity, upon which is perched an oval otolith, the lower part of which is clear and refractive, while the upper half is pigmented. This is the only known example of a cerebral auditory organ.

The Organs of the Lateral Line -In Teleostei the sense organs of the lateral line appear in segmental patches of simple sense-cells ; each area is then invaginated to form a short groove, which partially closes over. The fusion of these channels forms the canal of the lateral line, but numerous external openings are left. The lateral line of Elasmobranchs is at first a solid cord of cells,

Fig. 126.- - Early Stages in the Development of the Vertebrate Ear.

A-D. Four stages in the development of the labyrinth of a Fowl. [After Meissner.'] E. Transverse section through the auditory pit of a Fowl -s embryo of fifty hours. [After Marshall.] F. Transverse section through the head of a foetal Sheep (16 mm. in length) in the region of the hind-brain. [ After Bottcher.] a.c.v. anterior cardinal (jugular) vein ; am. amnion ; ao. aortic arch ; c.g. cochlear ganglion ; d.c. ductus cochlearis; h.b. hind-brain ; nch. notochord ; ph. pharynx ; r.v. recessus (aqueductus) vestibuli; v. vestibulum ; v.c. vertical semicircular canal ; vm. auditory nerve.

but this is probably an abbreviated process. In Chimaera the lateral line persists in the adult as an open groove. (See also p. 139.)

The Vertebrate Ear. - The auditory organ of Vertebrates may possibly prove to be a highly specialised organ of the lateral line series. The auditory sac first appears as a shallow depression of the epiblast in the region of the posterior brain vesicle above the first (hyoid) visceral cleft (figs. 125, 126). It soon becomes a flask- shaped vesicle which is separated from the skin, although in some Elasmobranchs the primitive opening to the exterior is retained throughout life.

The stalk of invagination persists as the aqueductus vestibuli, and its blind swollen distal extremity is the saccus endo-lymphaticus or recessus vestibuli (figs. 126 and 127, r.v).

The swollen portion of the primary auditory vesicle is modified to form the utriculus and the semicircular canals, while a Ventral diverticulum gives rise to the cochlea and the sacculus hernisphericus.

The rudiments of the anterior and posterior semicircular canals grow out from the lateral wall of the vesicle as two flattened processes. Their central walls become applied together, obliterating the cavity, except at the circumference, and eventually the centre is absorbed, leaving two ring-like canals. The horizontal semicircular canal is developed somewhat later in a similar manner.

The Cylostomi possess two imperfect vertical canals, which, with the utriculus, form a ring-shaped membranous labyrinth. All other Vertebrates have the three semicircular canals.

The body of the primitive vesicle persists as the vestibule or utriculus.

Fig. 127. - Transverse Section of Auditory Labyrinth of an Embryo Cow, lines in length. Magnified 30 diameters. [From Kolliker .]

a. boundary of the cavity in the cranial wall containing the epithelial labyrinth (6), which does not everywhere fill up the cavity ; c. mouth of cochlea ; c'. lagena of cochlea ; ch. notochord ; rv. recessus vestibuli ; se. horizontal (external) semicircular canal ; sh. cranial cavity ; sr. mouth of sacculus hemisphericus (?) ; ss. vertical semicircular canal ; v. vestibulum.

The cochlea of Mammals higher than the Monotremes consists of a helicoid spiral tube, connected with the utriculus by a narrow canalis reuniens. It develops as a simple process from the inferior end of the auditory vesicle. The various stages in its development in the higher forms are permanently retained in the adults of various lower animals.

The sacculus hemisphericus is a round vesicle which is evaginated from the base of the cochlea shortly after the appearance of the horizontal canal. A constriction opposite the mouth of the aqueductus causes the passage between the utriculus and the sacculus to diverge slightly up the aqueductus instead of pursuing a straight course (fig. 128).

The simple epiblastic aural invagination becomes in this manner a complicated labyrinth. The sense-cells are restricted to certain tracts, and, with the exception of the organ of Corti, they retain a very simple character. The auditory hairs project into the fluid (endolymph) contained within the labyrinth. The otoliths or otoconia are masses of carbonate of lime secreted by the lining epithelium.

The neighbouring mesoblast enters into relation with the auditory apparatus, the cells immediately surrounding the labyrinth being converted into a connective tissue investment (the membranous labyrinth). The whole being protected by a cartilaginous, and, in most animals, a subsequently osseous capsule, which is known as the osseous labyrinth. The latter is undeveloped at one spot, the fenestra ovalis in Elasmobranchs, Amphibia, and higher animals. A second foramen occurs in Mammalia, the fenestra rotunda.

Between the membranous and osseous labyrinths imperfect lymph spaces are found in the Sauropsida ; these are well developed in the Mammalia.

In the cochlea of the latter two longitudinal lymph-spaces are formed, the dorsal of

Fig. 128. - Diagram of the Auditory Labyrinth : A. of a Fish ; B. of a Bird ; C. of a Mammal. [From Bell after Waldeyer .]

6. lagena ; c. cochlea ; cr. canalis reuniens; k. coil (helix) of the cochlea ; r. recessus vestibuli ; s. sacculas ; u. utriculus or vestibulum with the three semicircular canals ; v. csecal sac.

which (scala vestibuli) communicates with the cavity round the membranous labyrinth, and at the apex of the cochlea is continuous with the ventral space (scala tympani). The latter terminates blindly at the fenestra rotunda. The fluid contained within these lymph spaces is the perilymph.

It must not be forgotten that the cavity (scala media or canalis cochleae) lying between the two scalae is the sensory portion of the cochlea, and is alone lined by epiblast. The scalae and the bony labyrinth are protective structures.

In most Fish the labyrinth or internal ear is more or less enclosed within the ear capsule, and is quite cut off from the outer world, the sound vibrations passing through the skull to the ear. But in some Teleosts the fenestra ovalis or its equivalent is in connection with the air-bladder through the intervention of a chain of ossicles (e.g., Cyprinoids and Siluroids). (See p. 181.)

Howes calls attention to a fenestra in the roof of the chondrocranium of many Elasmobranchs situated behind the orifice of the aqueductus vestibuli, the covering of which evidently functions as a tympanic membrane.

The hypoblastic diverticulum of the pharynx, which forms the hyoid cleft of Eishes (see p. 178), may acquire an external opening in some Amphibia which soon closes over. In all higher Vertebrates it persists as a blind recess, the Eustachian tube, dilating distally into a chamber (tympanic cavity) which partially surrounds the utriculus.

The external auditory meatus corresponds to the lower section of the outer or epiblastic portion of the original hyoid cleft. The meatus is formed principally, if not entirely, by the growth of the surrounding tissue in such a manner as to leave a deep tube. A pit (Hunt -s depression), corresponding to the upper section of the cleft, soon disappears. The external ear, concha or auricle, appears early (in the Pig) as a small triangular flap arising from the anterior border of the hyoid arch opposite the meatus ; it corresponds in position with the operculum of Fishes.

The tympanum in Mammals is at first a vertical thick wall of tissue separating the Eustachian tube from the shallow external depression, much as in Amphibia. By the subsequent extension of the two tubes the tympanum is reduced to a thin membrane, and is situated in a plane perpendicular (instead of parallel) to the surface of the head. The outer epithelium of the tympanum is clearly of epiblastic origin, while the inner epithelium is hypoblastic.

There is in Amphibia and Sauropsida a bony rod, the columella auris, extending from the fenestra ovalis to the tympanum. The greater portion, according to Parker, is a dismembered section of J the hyoid arch ; the base (stapes) being a plug of cartilage severed j from the auditory capsule.

A chain of three ossicles, the stapes, incus and malleus, connects the tympanum with the fenestra ovalis in Mammals ; the first of these is homologous with the Reptilian stapes, but there has been a good deal of discussion concerning the nature of the last two bones. Huxley and Parker -s original view was, that the incus is the proximal portion of the hyoid arch and the malleus is the arrested quadrate ; the processus gracilis of the malleus representing the primitive continuation into Meckel -s cartilage. The current view in Germany is that both the incus and the malleus belong to the mandibular arch (in which case the former might represent the quadrate and the latter the articular element of the lower 1 jaw). This homology, which was independently arrived at by Salensky and Eraser, now receives Parker -s unqualified support. According to Reichert, the stapes is part of the hyoid arch, but Salensky and Fraser hold that it arises from a mesoblastic blastema |j which surrounds the mandibular artery, hence the perforation of the stapes.

Albrecht maintains, however, that the quadrate cannot form part of the chain of auditory ossicles of Mammalia, and that the zygomatic portion of the squamosal is the homologue of the quadrate of Sauropsida. Dollo supports this conclusion, and adds that he has found an element in Lacertilia which he homologises with the malleus of Mammalia. He slightly modifies Albrecht -s series of homologies in the following manner. The symplectic + hyomandibular of Teleosts or the suspensorium of Fishes ! generally equals the columella of Urodeles and the four ossicles of Anura. These, again, are equivalent to the malleus + columella of Sauropsida and the malleus + incus + os lenticulare + stapes of Mammalia.

Visual Organs

The more or less definite appreciation of those vibrations of ether which result in the sensation of sight is a faculty which is readily acquired by the outer cells of the body, hence what are termed eyes have appeared perfectly independently in numerous groups of the animal kingdom. Even in the same order of animals eyes of quite dissimilar morphological value may occur, as, for example, the eyes in the shells of certain Chitons [Moseley], on the back of Onchidium [Semper], on the edge of the mantle, and on the siphon of numerous Lamellibranchs ; but it is almost certain that the cephalic eyes of the Odontophora, when present, including even the transient eyes of larval Chitons, are homologous all through the group.

It is probable that the power of distinguishing light from darkness is a primary characteristic of protoplasm ; if this be so, it would necessarily be readily retained by epiblastic cells, especially if pigment is present. Semper has suggested that a simple rounded tubercle covered with a transparent cuticle, or a mere local thickening of the cuticle, would serve to concentrate rays of radiant energy and would stimulate the adjacent cells ; but eyes appear to have been derived from the much more elementary condition of a small patch of pigmented epithelium. From such a simple beginning almost any kind of eye can be derived without special difficulty.

Eyes of Invertebrates

Eyes consisting of but slightly modified epithelial cells covered by a thickened cuticle occur in nearly all the lower Metazoa. It is characteristic of the eyes of the Invertebrates that the light falls directly on the sensory (retinal) cells, their inferior extremities being connected with nerve-fibrils which transmit the stimulus to the nerve centres. The dorsal eyes of Onchidium and the pallial eyes of Pecten and Spondylus offer a remarkable exception to this rule, as in these Molluscs the rays of light, after passing through the cornea and lens, have to penetrate a layer of nerve-fibres before impinging upon the sense-cells. Patten has shown that in Pecten this is due to the primitive optic cup being converted into a vesicle, of which the lower (inner) wall becomes aborted, the retina being formed of the upper (outer) wall. The sensory surface of the latter would necessarily be internal to the cup, and the nerve layer external. The same general arrangement also occurs in the eye of the Chordata.

The simplest eyes in the Arthropoda are those of the larvae of certain Insects ; in these the hypodermis forms a slight depression (fig. 129), the lowermost cells of which form the retina, and are connected with the fibres of the optic nerve ; a biconvex thickening of the cuticle forms the lens.

Lankester, working on the lines of Grenacher, has suggested the following stages of evolution as occurring in the Arthropod eye : -

Instead of remaining distinct (non-retinulate), the retinal cells may he aggregated together to form what is termed a retinula, as in the lateral eyes of Scorpions and Limulus, and the eyes of Myriapoda.

A higher stage of differentiation consists in the division of the retinal cells into an outer vitreus and an inner retinal layer. These double-layered eyes (diplostichous, as opposed to the above-mentioned single-layered or monostichous eyes) may either be composed of separate cells (non-retinulate), as in the dorsal eyes of Spiders and the simple eyes of adult Insects, or the sensory cells may be grouped into retinulse.

The retinulate diplostichous eyes may either be provided with a single lens (monomeniscous), as in the central eyes of Scorpions and Limulus, or the cornea may become divided into a number of lenses or facets (polymeniseous), as in the compound eye of Insects and Crustacea.

Fig. 129. - Section of Eye of Larva of a Water- Beetle (Dytiscus). [From Bell after Grenacher .]

An example of a non-retinulate, monostichous, monomeniscus eye.

g-p. optic cup ; h. hypodermis (epidermis) ; l. lens ; 0. optic nerve ; r. retina.

It seems that a non-retinulate eye cannot be polymeniseous, since the segregation of retinulse is the developmental antecedent of the segregation of the lens. Hence we may have monostichous polymeniseous eyes (lateral eyes of Limulus) as well as diplostichous polymeniseous eyes, but all non-retinulate eyes are monomeniscus. The compound (poly meniscus) eye is formed, not by the gradual concrescence of a number of simple eyes, but by the segregation of the elements of a simple eye, which affects first the retina and then the lens.

All these structures are modifications of the epiblast.

It is stated that in Astacus the corneal lenses and the crystalline cones are directly developed from the epiblast of the optic pit which very early makes its appearance on the procephalic lobes of the embryo ; while the retinulse with their rhabdoms, together with the optic ganglion and nerve, are developed from the cephalic ganglion. But, it will be remembered, the latter also arises from a proliferation of the epiblast of the same area. The pigment is stated to be derived from neighbouring mesoblast cells, but the visual pigment is probably epiblastic.

Patten believes the development of the Decapod eye to be as follows : - The cephalic epithelium (hypodermis) gives rise, by cell proliferation, to two layers - an inner one, the brain ; and an outer one, the permanent epidermis. That part of the brain arising from the seat of the future eye gives rise to the optic ganglion, which is never entirely separated from the seat of its origin. That part of the epidermis from which the optic ganglion originated again thickens and divides into two layers, an outer corneal hypodermis and an inner ommateal layer, consisting of retinophorae surrounded by their circles of retinulae (see p. 156).

Kingsley has very recently found that in Crangon, the cephalic pits, which Reichenbach formerly believed to be concerned in the development of the cephalic ganglia, are the rudiments of the eyes. Each optic pit is converted into a vesicle which sinks below the epidermis. The outer portion of the optic vesicle develops into the retina, while the inner portion forms the ganglionic layer. Later mesoblastic cells migrate between the retina and the ganglionic layer ; these subsequently become pigmented. Nerves grow from the ganglionic layers into the retinal elements. The eyes are only connected with the cephalic ganglia at about the time of hatching.

Fig. 130. - Ocellus of Larval Insect. [After Patten.']

ax.n. axial nerve; c.c. corneal cuticula; c.liy. corneal epidermis (hypodermis ) ; rtf. retinophorae. Each retinophora (retinal cell of Grenacher) consists of a group of four cells round an axial nerve. The cuticular portion or rod of each retinophora is provided witli a plexus of nerve-fibrils (not shown in fig.), and projects into the optic vesicle ; rtn. retinulae or pigmented cells ; v. b. vitreous body.

A section of a retinophora showing the peripheral and axial nerves is placed by the side of the figure.

A-B. Gasteropod (Murex). C-D. Cephalopod (Loligo). [The latter after LanJcester .]

c.g. proliferation to form cephalic ganglion ; m. mesoblast ; op. optic pit ; p. pigment ; r. retina.

According to Patten, the primitive optic pit (fig. 1 29) is converted into an optic vesicle (fig. 130), the anterior wall of which atrophies, while the posterior is greatly thickened to form the retina. This view differs fundamentally from Grenacher -s.

The cephalic eyes of the Mollusca arise as a single pit of the epiblast from the area from which the cephalic ganglia proliferate, and at the base of the tentacles (fig. 131, a).

Fraisse first demonstrated that the eyes of the Limpet (Patella) never advance beyond this stage of development (fig. 132), and that Haliotis is intermediate between this larval eye and the eyes of such Gasteropods as Fissurella (fig. 132, c) or Helix (fig • 133 . B).

In the last two forms, as in most other Odontophora, the embryonic pit is converted into a vesicle, the inner wall of which constitutes the retina. The lens is a cuticular deposit. The outer wall of the vesicle, together with the overlying epidermis, form the cornea. The eyes of Chaetopoda and Peripatus are very similar to this.

The stalked eyes of the Nautilus (fig. 133, a) always persist as

Fig. 132 - Diagrams Illustrating Three Stages in the Evolution of Eye of Gasteropods. [A and C. after Fraisse ; B. after Patten.} A. Patella. B. Haliotis. C. Fissurella.

In A. the eye persists as a simple optic cup. In B. the lower or retinulate layer of the cuticle is converted into the retinal rods ; the corneal layer is divided into a semi-fluid inner portion ( v.b ) and a harder outer portion (i). In C. the optic cup is converted into a vesicle, and the epidermis is continued under the cornea.

c. cornea; c.c. corneal cuticula; ep. epidermis; l. lens; op.n. optic nerve; r. retina; r.r. rods of retinophorse ; v.b. vitreous body.

a simple optic pit, although considerable differentiation occurs in the retinal cells.

The most complex type of eye occurring amongst the Invertehrata is found in the Dibran chiate Cephalopoda. In these forms the two stages just mentioned are passed through, hut a second smaller lens is secreted by the corneal epiblast immediately in front of the former, and an annular pigmented fold of skin (fig. 133) which develops round the front of the eyeball functions as an iris. Later a circular fold surrounds the eye ; it may either grow completely over, or leave a smaller or larger central aperture. This fold becomes transparent and forms a secondary cornea ; the space between it and the lens is known as the anterior optic chamber. An eyelid is usually superadded. The secondary cornea passes below into a tough mesoblastic sheath or “ sclerotic,- which is further protected externally by a cartilaginous capsule. The optic cavity is bounded behind by the several layered retina, and in front by the lens ; a ciliary body is developed where the retina joins the lens. The outer wall of the eyeball contained within the anterior optic chamber is sometimes termed the choroid.

The complexity of this type of eye is “merely the result of secondary folds of the external skin (iris, cornea, eyelid), more or less enclosing the typical Molluscan eye. The white body is a problematical structure which is situated at the side of the optic ganglion (see p. 114). Although the eye of these Cephalopoda strangely simulates that of Vertebrates, there is a profound morphological dissimilarity, which is readily apparent when their development is compared together.

Fig. 133. - Three Diagrammatic Sections of the Eyes of Mollusca. [After Grenacher.]

A. Nautilus. B. Gasteropod (Limax or Helix). C. Dibranchiate Cephalopod. ci. epithelium of ciliary body; co. cornea; e.l. eyelid; ep. epidermis; i.l.r. inner layer of retina; ir. iris; l. lens; l'. outer segment of lens; n.s.r. nervous stratum of retina ; op.g. optic ganglion ; op. n. optic nerve ; R. retina.

The nature and evolution of eyes of certain Invertebrates has most recently been studied by Patten ; his views briefly are that the structural element (ommatidium) of all eyes consists of from two to four colourless cells (retinophorse) surrounded by a circle of pigmented ones (retinulse). The external cuticle consists of two layers, an outer structureless one (corneal cuticula), and an inner layer (retinidial cuticula),

Fig. 134. - Diagram Representing the Transformation of Epidermal, Cells into Sense- and NerveCells in Mollusca. [After Patten .]

a. neuro-epithelial cell with its nervous prolongation, transformed in c to a bipolar and in d to a multipolar nerve-cell (g ) ; d. a myo-epithelial cell with its radiating fibres forming a basal membrane, two hypodermic nerves (n) are shown, the fibrils of which form a network ( nt . retia terminalia) on the upper portion of the cell and in the lower layer of the cornea ; at e the essential portion (ommatidium) of an invaginate eye is figured : the central retinophora (rtf) is composed of two cells, whose nuclei persist, enclosing an axial nerve (ax. n) which supplies its retinal rod ; the two lateral pigment cells, retinulse (rtn), have also retinal rods (rtn), which, however, disappear in more specialised eyes ; their nerves (n) form a network on the rods ; c.c. corneal cuticula ; r.c. retinidial cuticula.

filled with the retia terminalia or ultimate ramifications of the hypodermic nerves. The cuticular secretion of each cell forms a rod containing a specialised part of the retia terminalia (retinidium).

In the more specialised ommatidia the rods of the retinulse disappear, leaving the double (ex. Molluscs, Worms) (fig. 134) or quadruple (crystalline cone of Arthropoda) (fig. 130) rods of the retinophorse.

The apposed walls of the retinopliorse disappear to a greater or less extent, so that the nerve-fibres between the cells come to lie in the centre of the group, and constitute the axial nerve (fig. 134, ax.n) According to Patten, the epidermis of Molluscs consists mainly of columnar cells, the inferior expansions of which form the basal membrane. The cuticle secreted by these cells consists of two layers, an outer corneal layer (fig. 134, c.c) and an inner retinidial layer ( r.c ). The nerve-fibres of the skin ramify into an extremely delicate fibrillar network on the upper portion of these cells, and into the lower (retinidial) layer of their corresponding euticular areas or rods (fig. 134, n.t). An eye is initiated by the appearance of (red) pigment in one or more of these cells, the red pigment (ommerythrine) being peculiarly sensitive to light vibrations. An optic element or ommatidium consists of a group of such pigmented cells (retinulse) round one or more colourless nervous cells (retinophorse). Although at first all the cells of an ommatidium are sensitive, the retinophorse persists as the truly sensitive cells, while the retinulse take on secondary functions. It must be distinctly understood that Patten alone is responsible for the above conclusions.

Lankester draws attention to the fact that “ it is difficult to make out what precisely is the situation and the limit of the pigment in all Arthropod eyes.- Pigment granules are often very freely developed in the protoplasm of the ordinary hypodermis (epidermis) cells and of the indifferent cells (both perineural and interneural) of the ommateum. Should the nerve-end cells be pigmented, the pigment granules are confined to the surface of the cell, leaving the axis transparent.

“ The relation of pigment to the optical apparatus cannot be said to be at present properly understood. It is perfectly certain that in some eyes, and possibly in all, pigment does not play a primary part in the physiological process set going by light. Light acts with full effect upon transparent protoplasm, and no pigment is necessary, converting the energy of light into the energy of heat, in order that the protoplasm of cells may constitute an apparatus sensitive to light. The function of pigment in an eye is a secondary one, as we learn from the sight of albino varieties. What precisely the significance of pigment may be in relation to the cells in which the optic nerve ends, is not yet agreed upon by physiologists.-

Eyes of Vertebrates

The eyes of the Vertehrata are of a compound nature, part being developed from the brain and part from the outer skin of the head ; both these elements are therefore of epiblastic origin, and they are protected by mesodermal structures.

The first rudiment of the eye to appear is a pair of diverticula, which bud out from the sides of the anterior cerebral vesicle (figs. 106, abl, and no, mes), and which are known as the primary, optic vesicles. They usually arise as soon as the primitive brain shows traces of serial dilatations (cerebral vesicles) ; but in some Mammals, at all events, the optic vesicles are recognisable before the cerebral neural groove is converted into a canal.

The optic vesicles at first have a wide opening into the brain, but they are soon partially constricted off, and their narrowing stalks will develop into the optic nerve. The constriction which separates the optic vesicle from the brain extends from above and from the front, so that the stalk of the vesicle is situated at the

base of the brain, and arises from the posterior region (thalamencephalon) of the anterior cerebral vesicle.

The external wall of the optic vesicle invaginates until it is completely inverted (fig. 135), recalling the manner in which a blastula is typically converted into a gastrula.

The epiblast of the head, which lies immediately external to the optic vesicles, becomes columnar, and invaginates as a rounded vesicle at the same time that the optic vesicle is introverted. The sac thus formed is the rudiment of the lens (fig. 1 1 2). As this becomes constricted off, the outer skin again becomes continuous, and is eventually transformed into the cornea.

Fig. 135. - Horizontal Section through the Head of an Embryo Fowl, Illustrating the Development of the Eye.

A. Embryo of fifty-four hours - incubation. [ After Marshall .] The section is oblique ; on one side it passes through the optic stalk.

B. Section of about the same age, thi-ough another plane. C. Later stage.

a. a. aortic arches ; a.c.v. anterior cardinal (jugular) vein ; au. auditory vesicle ; c.h. cerebral hemispheres ; /. 6. fore-brain ; h.b. hind-brain; inf. infundibulum ; l. lens ; l.t. lamina terminalis ; nch. notochord ; o.c. optic cup ; p. pigment layer of the retina ; ph. pharynx ; pit. pituitary body ; r. retina; v.c. visceral clefts.

The eye at this stage consists of a stalked double-layered cup, containing a hollow sphere, and bounded externally by the skin (figs. 1 1 2, 1 35 a). The cavity within which the lens lies is known as the secondary optic vesicle, or, more correctly, as the optic cup. The lens does not grow so rapidly as the optic cup, and consequently is soon relatively much smaller, and comes to be embraced by the rim of the mouth of the cup (figs. 135, c, 136).

The various elements of the eye will now be described separately, but previously certain points concerning the mode of the invagination of the optic vesicle require consideration.

The invagination does not occur solely on the outer face of the optic vesicle, but also, in a linear manner, along its ventral line. The cup thus has a wide mouth, plugged by the rudiment of the lens, and a ventral slit (choroidal fissure) which opens into the cavity of the eyeball (fig. 136, ch.f).

To again borrow a simile, the orifice of invagination of the optic cup may be said to resemble a linear blastopore with an anterior enlargement. The latter persists, but the former ultimately becomes closed.

It is at present an open question how far the invagination to form the optic cup is primitively the result of the pressure of the lens.

Fig. 136. - Diagram Illustrating the Position of the Choroidal Fissure.

A. Surface view, from the side. B. Skeletal view, the greater portion of the optic cup being supposed to be cut away.

ch.f. choroidal fissure ; l. lens ; o.n. optic nerve ; p. pigment layer ; r. retina.

From the first, the inner or anterior layer of the optic cup is thicker than the outer or posterior, and it becomes increasingly so. The former is the rudiment of the retina, while the latter persists as the pigment layer within which the retinal rods are imbedded (the so-called pigmented epithelium of the choroid) (figs. 13 7 ,; 138,3)).

The retina soon becomes several cells deep, but it is probable that for some time, at least, each cell extends throughout its whole thickness. The histogenesis of the retina is still obscure. It however appears to be unquestionable that the layer of rods and cones is developed from the epithelial layer of the central nervous system (fig. 139) ; and that the main portion of the retina, with its nerve fibres and nuclear layers, together with the inner (anterior) layer of nerve-fibres and nerve-cells, is formed from the more specially nervous portion of the cerebral epiblast.

The optic nerve is, as has already been stated, derived from the stalk of evagination of the optic vesicle. The reduction of the cavity of the canal by the thickening of its internal walls takes place centripetally, i.e., from the eye to the brain.

Fig. 137. -Section of the Eye of a Fowl at the Fourth Day.

[From Balfour .]

e.p. superficial epiblast of the side of the head ; R. true retina, anterior wall of the optic cup; p.Ch. pigment epithelium of the choroid, posterior wall of the optic cup ; b. is placed at the extreme lip of the optic cup, at what will become the margin of the iris ; l. lens, - the hind-wall, the nuclei of whose elongated cells are shown at n.l, now forms nearly the whole mass of the lens, the front wall being reduced to a layer of flattened cells, el ; m. the mesoblast surrounding the optic cup, and about to form the choroid and sclerotic, - it is seen to pass forward between the lip of the optic cup and the superficial epiblast.

Filling up a large part of the hollow of the optic cup is seen a hyaline mass, the rudiment of the hyaloid membrane and of the coagulum of the vitreous humour (»/).

In the neighbourhood of the lens it seems to be continuous, as at cl, with the tissue a, which appears to be the rudiment of the capsule of the lens and suspensory ligament.

While the optic stalk is still partially open, fibres appear in its proximal portion, some of which pass over into the root of the other, and thus initiate the optic chiasma. The nerve-fibres later extend along the optic stalk.

The optic nerve is at first continuous with both layers of the optic cup (fig. 136), but in process of time the connection with the outer or pigment layer is lost.

In Mammals the distal portion of the optic stalk is flattened and its cavity obliterated whilst the optic cup is forming ; and since the stalk itself partakes in the invagination, the choroidal fissure may be said to extend for some distance along the nerve. The central blood-vessels of the retina (fig. 138) enter this groove, and are subsequently surrounded by the overgrowth of the nerve. The retinal circulation is entirely confined to these vessels and their capillaries. Kolliker suggests that the invagination of the optic stalk is due to the pressure of the mesoblast, which develops into the blood-vessels.

The retina is unprovided with true retinal blood-vessels in animals lower than the

Fig. 138. - Horizontal Section of the Eye of a Rabbit of Eighteen Bays.

Magnified 30 diameters. [ From Kolliker .]

ap. orbito-sphenoid (lesser wings of the sphenoid) ; c. cornea, with its anterior epithelium, e ; f. rudiment of the choroid ; g. vitreous body detached from the retina by shrinkage, except behind, where the central artery of the retina passes into it ; i. iris ; l. crystalline lens ; l'. epithelium on the anterior face of the lens ; m, m. rectus superior, and r. inferior muscles ; mp. membrana pupillaris ; o. optic nerve ; p. outer pigmented layer of the retina ; p'. anterior border of secondary optic cup, where the retina proper passes into the pigmented layer ; pa. upper eyelid ; pp. lower eyelid ; r. retina ; re. pars ciliaris retinae.

Mammals, but their place is possibly to some extent taken by the vascular structures which penetrate the cavity of the eyeball through the choroidal fissure. These are known as th e processus falciformis in Ichthyopsida, and the pecten in Sauropsida.

The lens was left as an oval vesicle, with uniformly thick walls. Very soon the cells of the front wall become thinner and flattened, while those of the inner wall elongate and entirely obliterate the cavity of the vesicle (fig. 137). The latter cells early become strap-shaped and acquire their final disposition (fig. 138). At no time is the wall of the lens more than one cell deep.

The lens capsule is a cuticular membrane probably secreted by the epithelial cells of the lens.

The vitreous humour appears to be derived from a fluid transudation from the vascular ingrowth, which enters the retinal chamber through the choroidal fissure. In some cases a few embryonic mesoblast cells occur.

The anterior epithelium of the cornea is formed by the growing together of the epiblast after the formation of the lens. Its deeper or proper substance is of mesoblastic origin, and is derived from an ingrowth of the neighbouring mesoblast. A similar but shorter inferior fold constitutes the iris. The mesoblast cells of the incipient cornea occupy a space which lies between the epithelium of the cornea and a flattened epithelium (membrane of Descemet), which is also of mesoblastic origin.

The aqueous humour is a watery fluid which occupies the cavity between the lens and the cornea.

Eyelids are developed as simple folds of the skin ; their inner surface is lined by a mucous membrane, the conjunctiva, which also covers part of the sclerotic and the exposed surface of the cornea. There may be three eyelids, a dorsal, a ventral, and an anterior, the nictitating membrane, arising from the inner angle of the eye.

The eyelids are rudimentary or absent in Eishes, except in some Elasmobranchs. All three eyelids are present in most Amphibia and Sauropsida, but the nictitating membrane is rudimentary in Mammals.

In many Mammals the two eyelids meet together and unite during a period of embryonic life. A similar condition is permanent throughout life in Snakes and some Lizards, the lachrymal ducts opening into the space thus formed between the fused lids and the cornea.

Lachrymal glands occur in the Amniota. Their character varies greatly in the different groups, but they always arise as solid ingrowths of the conjunctiva.

The sclerotic and choroid coats of the eye are protective envelopes developed from the mesoblast.

Epiphysial Eye. - The possession of a rudimentary median eye, lodged in the parietal foramen and developed from the pineal gland, in several Lizards has already been alluded to (p. 129). The lens of this eye is a direct derivative of the optic cup, and what light reaches it impinges directly on the retina without first penetratingthrough the retinal layer of the fibres of the optic nerve (fig. 138*, d). Thus, as in the Pectinidae and Onehidium, certain Invertebrates have accessory eyes constructed on the vertebrate plan, so some Lacertilia amongst the Vertebrates possess a typically invertebrate unpaired eye. A radical distinction between the pineal eye of Lizards and the eyes of Invertebrates consists in the fact that the essential constituents (retina and lens) of the former are entirely differentiated from a diverticulum of the brain (fig. 138*, c-e), -whereas in the latter they are invariably epidermal structures.

Fig. 138*. - Diagrams Illustrating the Evolution of the Epiphysis (Pineal Gland). [After Spencer .]

A. Early stage of epiphysis in Bufo cinerea ; this corresponds with the early stage in larval Tunicates and the probable condition in the ancestral Chordate. B. Early stage in all higher Chordata ; permanent in Elasmobranchs and Cyclodus. C. Later stage in Anura and Sauropsida ; permanent in Chameleo. D. Adult stage in certain living Lacertilia, e.g., Hatteria, Varanus ; probable condition in Labyrinthodonta, and in ancestors of Reptilia and Aves. Final stage in many Lacertilia, e.g., Calotes, Seps, Leiodera. F. Anura, adult. G. Aves, adult. H. Mammalia, adult.

It will be seen from the above figure that the epiphysial or pineal eye of certain living Lizards is differentiated from the distal vesicular portion of the pineal gland. The central section being converted into an optic nerve, the proximal practically forms an optic lobe. A-D. illustrate the development of the organ to its most specialised condition. - E-H. indicate various phases of degeneration. The shaded portion indicates the parietal bone. In D. the anterior portion of the vesicle is modified to form a lens, the posterior wall differentiating into an inner pigmented retinal layer and an outer layer of nerve-cells.

Hypothetical Evolution of the Vertebrate Eye. - The fact that the optic cup is developed from the anterior brain vesicle is at first sight very anomalous. The following considerations, however, may tend to throw some light upon it.

It will be remembered that an ancestral form of the Chordata was assumed (p. 76) to possess a nervous system but little differentiated from the epiblast extending along the primitive oral aspect of the body, and expanding in front of the mouth. Upon this region a pair of cuplike eyes was supposed to be situated, the eyes having essentially the same structure as in Patella (fig. 132). This condition is diagrammatically represented in fig. 139, a, b, the latter being a supposed transverse section through the pre-oral region of a. It will be seen that the eye-pits are connected with the pre-oral neural epiblast, much in the same manner as the eye-pits of Mollusca (fig. 1 31) are developed in connection with the proliferations which form the cephalic ganglia (fig. 96, c.g).

The involution of the nervous area to form the neural canal also implicated the optic pits (fig. 139, c). Since this figure was drawn, Heape has shown that in the Mole the optic vesicles appear as depressions of the cephalic neural plate even before the neural groove is established. Heape figures a section which very closely resembles the diagram given in c, fig. 139. On the closure of the neural tube the pits would appear as vesicles (optic vesicles) opening into the anterior cephalic enlargement.

A local thickening of the overlying lateral epiblast to form a lens might be a mechanical cause for the invagination of the optic vesicle to form the optic or retinal cup. Every subsequent stage of evolution, being an optical improvement, could be accounted for once the retinal cup was established.

Fig. 139 also illustrates that the visual sense-cells (rods and cones) are derived from the epithelial layer of the central nervous system, in other words, from the

Fig. 139. - Diagrams Illustrating a Hypothetical Evolution of the Vertebrate Eye.

A. Surface view of head of a hypothetical type. B. Vertical section of same across the optic pits. C. Invagination of the pre-oral neural plate and optic pits. D. The process completed. E. Formation of lens and optic cup.

ep. epidermal layer of epiblast of head; ep.b. epithelium of brain; l. lens; m. mouth ; n. nervous layer of epiblast of head ; n.b. nervous layer of brain ; n.p. neural plate ; n.r. nervous layer of retina ; o.p. optic pit ; op.v. optic vesicle; p.b.v. primary brain vesicle ; p.o. pre-oral neural plate ; r.r. layer of rods of retina.

external epiblastic epithelium ; that is to say, from precisely the same layer which gives rise to the similar elements in Invertebrates. The deeper or nervous layer of the epiblast is concerned in the formation of the layer of nerve-fibres and nerve-cells of the retina.

The transparency of the body of the primitive Chordata, assumed by Lankester, would enable light to reach the optic pits, although the latter were situated within the brain. But as the animal became more opaque, it may be assumed that the visual apparatus (optic vesicles) would grow out towards the sides of the head through which most light would penetrate. The lens is clearly a secondary structure. On this hypothesis the eye could be functional whilst it was undergoing this unique metamorphosis.

Observations on the Evolution of the Nervous System and Sense Organs

The origin of the nervous system and sense organs from the epiblast is one of the best attested of embryological discoveries, and from the foregoing brief account they would appear to be universally so derived. The only general statement, however, that can be made is that nerve and sense cells have arisen in response to a stimulus, or, more correctly, as the result of a stimulus.

As a matter of fact, such a stimulus would most readily and frequently act upon the exterior of the body, and therefore upon epiblastic tissue ; hence the almost universal origin of these structures from that layer; but there are a few exceptions which are of considerable interest.

The brothers Hertwig have demonstrated that in addition to the diffused ectodermal nervous system present in the Actinise, there is a distinct layer of nerve fibres and cells, and in some cases of sense-cells, which can only be derived from the endoderm. The occurrence of the latter may possibly receive an explanation from the fact that the mesenteric chambers open widely into the digestive cavity of the body in these animals. As the wide mouth and oesophagus are so generally open, there is really considerable facility for stimuli, such as vibrations in the external medium, to act upon the internal tissues. In both cases, therefore, the differentiation occurs in tissues directly exposed to the surrounding medium.

Quite recently Hubrecht has discovered that the nervous system, i.e., the brain and lateral nerve cords of the Nemertean Worm Lineus obscurus are derived from the mesenchyme. Certain of these wandering cells (mesamoeboids) apply themselves to the interior of the body- wall in definite areas, and there differentiate into the nervous system of the adult. The mesenchyme has a double origin, being partly derived from the epiblast and partly from the hypoblast (fig. 49). Although direct proof is not attainable, it is fair to assume that the nervous system is developed out of the epiblastic rather than from the hypoblastic mesenchyme. If this be the case, it is probably another example of “ precocious segregation.-

As has been already mentioned (p. 114), Bobretzky states that the nervous system of the Prosobranch Gasteropod Fusus is derived from the mesoblast, and that the wandering cells apply themselves to certain areas of the epiblast, as in the case of Lineus, but in all the other Gasteropods which have been examined, and even in the allied forms of Purpura (fig. 96) and Murex, the nerve centres have an epiblastic origin. Bobretzky -s statement must therefore be received with caution. The same applies to Fol -s account of the origin of the pedal ganglia from the mesoblast of the foot of Limax, while the cephalic ganglia are developed from the epiblast of the velum.

Lastly, the origin of the sense-cells and nerve-cells of Sponges, which have been described by Stewart, Yon Lendenfeld, and Sollas, is still somewhat uncertain. They have been stated to be mesodermal (mesenchyme) elements, from the fact that the ectoderm of Sponges always occurs as a delicate flattened epithelium and never exhibits any transitional stages into sense-cells, in this respect offering a marked contrast to that of Coelenterates. Whereas the position and appearance of the nerve and sense-cells irresistibly suggest a mesodermal origin.

One important point should not be lost sight of in these considerations. It is that protoplasm from its very nature is what has been termed “ irritable,- that is to say, it responds to stimuli. This irritability is inherent to all cells, and probably is never lost while the cell lives ; certain cells have this function greatly developed, while in others it is more or less diminished. It is probable that stimuli may readily pass from one cell to another in most tissues, as animal cells are usually in close contiguity when not in actual continuity. In many adult animals, and usually in embryos, different tissues may be connected together by branched mesoblastic cells (indifferent connective tissue), which may also be amoeboid. If these latter cells retain their irritability, there is probably nothing to prevent their transmitting as well as receiving stimuli. They may thus serve as incipient nerve-fibres ; and it is further possible that this function may be sufficiently pronounced to cause the formation of a definite nervous tissue which is purely mesoblastic in origin. This secondary nervous system might be developed in adults as well as in embryos. The observations of Yon Lendenfeld on Sponges tend to support this hypothesis.

From numerous researches on the nervous system of the lower Metazoa, it is not difficult to trace the stages by which ectodermic (epiblastic) cells are gradually modified into nerve-cells.

In the primitive Metazoon most of the external cells of the body were probably ciliated, and had very similar functions. In process of time certain cells would gradually acquire a greater degree of sensitiveness, while others would become more protective in function. If, for instance, a cilium-like prolongation of a cell lost its power of contractility and became rigid, it would then, as a mechanical necessity, vibrate in response to the vibrations of the surrounding medium. These induced vibrations would act as stimuli to the cell and excite a manifestation of irritability, which might expend itself in various ways. Most sense-cells are constructed on this plan ; they are, in fact, epidermal cells with a stiff projecting hair or rod-like process, and are interiorly continuous with other cells.

Chatin has recently found that all intermediate stages can be found between the auditory rods and ciliated cells of the auditory epithelium of the labyrinth in Batrachia.

It is now demonstrated that the cells of the tissues of the Coelenterata are connected with each other by means of very delicate, usually branching, root-like processes, which serve for the contraction and general co-ordination of the parts or whole of the organism or colony. The sense-cells form no exception, and in some of them the upper sensory portion appears to be gradually becoming smaller, while the lower portion, which contains the nucleus, is swollen (fig. 119, c, g). As the nucleus is mainly the centre of the activity of the cell, it may be assumed that in these cells general irritability is preponderating over special sensibility, and that it only needs a slight further specialisation to constitute a cell wholly given over to irritability ; in other words, a nerve-cell. The same process also occurs in the skin of Molluscs. In fig. 134, a, b, c, d, diagrammatically represent the gradual transformation of a sense-cell, a, into a multipolar nerve-cell, g.

The nerve-cell retains connection with the neighbouring cells by its root-like processes, and thus may be united with a sense-cell on the one hand, and with a glandular or muscular cell on the other. By this double connection the nerve-cell may receive a stimulus from a sense-cell, and by the excitation of its own irritability may transmit the stimulus in an intensified form to the distal cell, and the latter will be stimulated to perform its special function.

The foundation of a distinct nervous system will thus be laid, and the multiplication and localisation of sense-cells and nerve-cells has probably been effected to a large extent independently in the different groups.

This suggestion concerning the evolution of the nervous system seems to be warranted from a consideration of the histology of adult Coelenterates (fig. 119) and Molluscs (fig. 134) ; but even if it be a correct interpretation of the facts in these groups, it is possible that in other forms the history may be somewhat different. For example, nerve-cells may originate by the division of certain epidermal cells into an outer protective portion and an inner more irritable or nervous moiety, the latter always retaining connection with the former by means of protoplasmic threads.

In the embryos of the lower Chordata the epiblast primitively consists of a single layer ; in Amphioxus alone is this condition retained in the adult. In the Urodele Amphibia the epiblast is single layered till the completion of the gastrula stage; but in the Anura the epiblast is several layers thick in the blastula stage.

In all cases the distinctly nervous elements of the central nervous system and sense organs is formed entirely from the deeper layer of the epiblast. Thus there is in the Anura and some other groups, Ganoids and Teleosts, an early separation of the epiblast into the epithelial and the mucous or nervous layer.

Spencer has recently stated that the segmental nerves and ganglia in the Frog arise in situ by a local persistence of this deeper layer ; thus there is, as he points out, in Amphibia a primitive nervous sheath to the body, the nervous tracts being local retentions of this diffused nervous system. Later still Misses Johnson and Sheldon, from their studies on the Newt and Frog, support the generally received view of the outgrowth of the nerves from the neural ridge.

In this connection it is interesting to notice that Bateson has shown that in Balanoglossus (the lowest known member of the Chordate series) the central nervous system arises as a delamination of a solid cord of epiblast in the dorsal middle line of the middle third of the body of the embryo ; this, by invagination of its two ends, is afterwards extended as a tube in both directions. Other collections of nerve-fibres are afterwards deposited in various parts of the body, and finally a general network of nerve-fibres occurs on the under surface of the skin of the body, especially in the line of the gill-slits. The tail-like processes of the epiblast cells run into the different superficial nervous tissue, and many fibres pass into the subjacent mesoblastic tissues. The fibres entering this nerve-substance on its outer side are plainly sensory, or at all events afferent, and the fibres passing from it on its inner side are presumably motor, or at least efferent, seeing that they innervate the mesoblast.

li It is clear, then (as Bateson points out), that on the separation from the skin of a cord thus composed the relations of the efferent fibres will not be changed, as they still remain in contact with the mesoblast. But, on the other hand, if this nervecord be entirely separated from the skin, the supply of outer or afferent fibres is cut off from it, unless cords of epiblast remain to connect it with the skin. Applying this reasoning to the particular case of the separation of the dorsal cord, we see that the afferent fibres are entering it on its dorsal side, and that the efferent fibres are leaving it on its ventral side. If the nervous system arose in this way, the dorsal roots were from the first sensory, and did not arise as differentiations of roots of mixed function, as has often been supposed.-

The epithelium lining the cavity of the central nervous system and the sensory epithelium of the sense organs are derived from, or from what corresponds to, the external layer of the epiblast. Exceptions occur in the auditory sacs of Ganoids and Teleosts, which are solely developed from the deeper layer of the epiblast, and in the optic vesicles of Teleosts, which are formed as solid buds from the solid nervous keel which will develop into the brain. In this, as in many other respects, the development of the Teleosts is extremely modified.

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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)
   Introduction to Embryology 1887: Chapter I. Maturation and Fertilisation of Ovum | Chapter II. Segmentation and Gastrulation | Chapter III. Formation of Mesoblast | Chapter IV. General Formation of the Body and Appendages | Chapter V. Organs from Epiblast | Chapter VI Organs from Hypoblast | Chapter VII. Organs from Mesoblast | Chapter VIII. General Considerations | Appendix A | Appendix B

Cite this page: Hill, M.A. (2024, April 20) Embryology Book - An Introduction to the Study of Embryology 5. Retrieved from

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