Talk:Book - Quain's Elements of Anatomy

II. DEVELOPMENT OP PARTICULAR SYSTEMS AND ORGANS OF THE BODY

THE SKELETON AND ORGANS OF VOLUNTARY MOTION.

The morphological development of the skeleton and organs of voluntary motion is closely in accordance with the general plan of development which belongs to the whole vertebrate body. The first steps are connected with the formation of the strictly axial part, consisting of the enclosing walls of the cranio-vertebral cavity for containing the rudiments of the brain and spinal marrow, and for the issue of the successive pairs of nerves arising from them. These are succeeded by the formation of the walls of the great visceral cavities of the head and trunk, in which the facial and costal arches are to be distinguished ; and lastly, the appendicular parts, or the limbs and limb-arches, are developed. The permanent forms of these parts are only produced in the process of ossification ; but the rudiments of most of them are already to be distinguished in the masses of cartilage or formative tissue which precede the ossifying change.

As the mode of ossification of the several bones has been described in the osteological part of the work, and the histological view of theprocess of formation of bone has been given in the part on General Anatomy, the morphological view of the development will alone be referred to in this place, in which will be included the more important phenomena of the preparation of the matrix or formative material for the various parts of the skeleton.

Fig. 525. — Embryo of the Doo seen from above, Fig. 525.

wIth a Portion of the Blastoderm attached.

The medullary canal is not yet closed, but shows the dilatation at the cephalic extremity with a partial division into the three primary cex'ebral vesicles ; the posterior extremity shows a rhomboidal enlargement. The cephalic fold crosses below the middle cerebral vesicle. Six primordial vertebral divisions are visible ; so, the upper division of the blastoderm ; sp, the lower division.

1. VERTEBRAL COLUMN AND TRUNK

Relation of Vertebral Rudiments to the Notochord

It has already been shown (General Phenomena of Development, p. 692), that all the parts of the skeleton ow^e their primitive formative material to mesoblastic elements, and that the bodies and arches of the vertebras, and the adjacent part of the cranial walls are formed from continuous blastodermic substance lying below and around the primitive medullary canal. A part also of the basis of the cranium has this in common witli the vertebral axis, that its formative substance surrounds the notochord, extending forward from the column of the vertebral bodies into the occipito-sphenoid part of the cranial basis, which is there composed of the formative substance termed the invesiing mass of Ratlike.

It is to be remembered, however, that closely as the formative tissue of the bone elements appear to surround the notochord, that structure does not itself, nor by its sheath, contribute to the formation of the vertebral or basi-cranial bones, but merely lies within them ; and the formative material, out of which the bones are produced, is derived from mesoblastic substance which passes inwards from the primordial vertebral plates, and envelopes the chorda external to its sheath. The formation of the notochord, therefore, precedes that of the formative bone-elements which afterwards envelope it, and the remains of the notochord, unaffected directly by any ossifying change, are found in the interior of the commencing bones, and may be traced even for a long time throughout the whole length of the column of the bodies of the vertebrre.

This important fact was first demonstrated by H. Miiller, of Wm-tzbrn-g-, who showed fui-ther that the notochord did not pass through the anterior arch of the atlas, but was traceable directly from the body of the axis vertebra through its odontoid process, and thence into the basi- occipital and basi-sphenoid bones, reaching as far as the pituitary fossa. (Heinrich Miiller, " Ub. d. Vorkommen von Eesten des Chorda Dorsalis b. Menschen nach der Geburt," in " Zeitsch. fiir Rat. Med.," von Henle u. Pfeifer, 1858, b. ii. See also Gegenbaur, " Untersuch. ub. Vergleich. Anat. Das Kopfskelet der Selachier," Leipzig, 1872. W. Miiller, " Bau der Chorda dorsalis," in '■ Jenasch. Zeitsch." b. vi. E. Dursy, " Zur Entwick. des Kopfes," 1869, and Mihalkovics, '• On the Chorda and Pituitary Body," in "Archiv filr Miki-oscop. Anat.," b. xi., 1875.)

It may be mentioned further, as the result of H. Miiller's observations, that though in general the chorda passes through the middle of the vertebral bodies, the position was found subject to variation in the caudal portion of the column, where it sometimes pa,?sed above, and at other times below, the vei-tebral bodies.

The notochord itself has been generally held to be produced from an intruded central column of mesoblastic cells, and this seems to be the nrode of origin in birds ; but it may be doubtful whether it is the same in all animals. In sharks Balfour finds that ihere is no median column derived from the mesoblast, and attributes the origin of the notochord to the hj^joblast (Quart. Joum. of IMicroscop. Sc, Oct. 1874). The same origin is ascribed to it in mammals by Hensen, who finds that the notochord is late of being formed in the rabbit, — an observation confii-med by Kolliker : Mihalkovics, on the other hand, is inclined to refer it in all animals to the epiblast. However this may be, the tendency of recent research appears to be to show that the notochord may be more nearly alUed to epithelial structiu'es than to cartilage with which it has generally been previously associated. It is at all events important to note that it is in many respects different from the parts ascertained to proceed from the mesoblast, that it never combines with their elements, and that there is no penetration of its substance by connective tissue or blood-vessels, as happens in all other parts derived from the mesoblast.

The interesting observations of Kowalevsky on the existence of a chorda dorsalis in Ascidia (Mem. de TAcad. de St. Petersboiu'g, torn. x. and xi., 1867 and 1868), would appear to show that this structure, and the tj^je of development which accompanies it, are not confined to vertebrate animals, and that in them the notochord may present more of a merely vestigial character than constitute an important element in the formation of the skeleton. The constancy of its position and relations, however, is an important fact regai-ding its history.

The notochord does not undergo transverse segmentation in the same manner as the protovertebral plate does. It remains undivided to the last, but in the course of vertebral ossification it shows alternate diminutions and enlargements of its diameter, corresponding in number and position with the vertebral divisions. One of these enlargements is found between the odontoid process and the basi-occipital bone, and another has been observed by Mihalkovies in the interval l>etwecn the basi- occipital and the basi-sphenoid bones or their cartila[,nnous matrices.

Fig. 526. — View from above of the Embryo-Chick in the Fig. 525.

FIRST HALF OF THE SeCOND DaY.

1, 2, tbe three primary enceplialic vesicles ; iu front and to the sides the cephalic fold ; crossing at 2 the fovea cardiaca ; 3, the caudal extremity of the medullary canal dilated into a rhomboid form ; 4, 4, six primordial vertebral divisions.

In front of this last swelling, the chorda is beiit down below the base of the sknll, and tapering to a fine filament, ends or is lost in the floor of the pituitary fossa. The enlargements now mentioned have some interest in connection with the question of the vertebral constitution of the sknll

Segmentation of the Protovertebrae

The transverse vertebrate segmentation which occurs in the primary vertebral plates affects only that part of these plates which is formed of mesoblast. It begins at a very early period, as already stated, even before the close of the primary medullary canal, in the form of one or two, or it may be three short transverse transparent lines which separate a corresponding number of dark or condensed quadrilateral masses of the primitive vertebral plates. These quadrilateral masses, the so-called primordial vertehrcc (Urwirbel of the Germans) (fig. 526, -4, 4), do not, however, correspond merely to the vertebrae of the skeleton, nor are they directly converted into theii- rudiments, but they are rather divisions equivalent

Fig. 527. — Cervical part of the Piujiitive Vertebral COLDMX AND ADJACENT PARTS OF AN EmBRYO of the Sixth Day, showing the division of the Primitive Vertebral Segments (from Kolliker after Remak).

1, 1, chorda dorsal is iu its sheath, pointed at its nipper end ; 2, points by three lines to the original intervals of the primitive vertebrffi ; 3, in a similar manner indicates the places of new division into permanent bodies of vertebrte ; c indicates the body of the first cervical vertebra ; in this and the next the pirimitive division has disappeared, as also in the two lowest represented, viz. , d and the one above ; in those intermediate the line of division is shown : 4, points in three places to the vertebral arches ; and 5, similarly to three commencing ganglia of the spinal nerves : the dotted segments outside these i^arts are the muscular plates.

in number and position to the vertebral segments of the body (somatomes of Goodsir) ; each one comprising superficially a segment of the muscular plate, and more deeply a pair of intervertebral ganglia and nerves, as well as the parts of the skeleton which lie before and behind them.

The more obvious protovertebral segmentation docs not extend into the mesoblastic tissue beyond the commencement of the basis of the cranium, the mass of blastema which there surrounds the prolongation of

Fig. 528. — Sections op the Vertebral Column of a Human Fcetus of eight ■WEEKS (from Kolliker).

A, transverse longitudinal section of several vertebrre. 1, 1, cliortla dorsalis, its remains thicker opposite the intervertebral discs ; 2, is placed on one of the bodies of the permanent vertebrce ; 3, on one of the intervertebral discs.

B, transverse horizontal section through a part of one dorsal vertebra. 1, remains of the chorda dorsalis in the middle of the body ; 2, arch of the vertebra ; 3, head of a rib.

the notochord (the investing mass of Ratlike) remaininoj one and undivided, or being devoid at least of the marked cleavage which occurs in the strictly vertebral part.

It is fi'om this protovertebral plate on each side, whether in its entire primitive condition, or in its later and divided state, that the material is derived for the formation of the bodies and laminae of the vertebra? and the muscles which cover them. This is effected by the rapid increase of the mesoblast, and by the extension of that structure beyond the immediate confines of the vertebral laminae in an inward and downward direction, so as to throw a quantity of new mesoblastic material round the notochord, and inwards and upwards, so as to pass in between the primary medullary canal and the enveloping layer of epiblast.

The muscular plate

Shortly after this extension of the mesoblast in the two directions before mentioned, another separation, or rather differentiation, is observed to take place in the direction of its length, in the formation along the dorsal surface, and below the epiblast, of a series of circumscribed plates which form the foundation of the erector muscles of the spine, and the great dorsal muscles of the trunk. These constitute together the muscular, or rather the musculo-cutancovs plate, for it appears also to include the formative rudiment of the true skin.

There is thus deposited the formative material for the vertebral bodies, the vertebral arches, and the muscles which immediately surround them, together with the general integument.

Meanwhile the vertebral segmentation goes on progressing from before backwards, extending through the dorsal, lumbar, sacral and coccygeal vertebrfB, till the process is complete ; but this is accompanied by other changes having reference to the separation of the nerveroots and ganglia from their formative tissue, and the development of the elements of the permanent vertebra?.

In the outer portion of each protovertebral mass a transverse partition arises which separates the anterior part, as ganglion and nerve root, from the posterior, as matrix or forerunner of the bone and other structures which belong to the vertebral column. Each nerve then comes thus to be placed in front of the future permanent vertebra with the proto Fig. 529.

Fig. 529. — Transverse section through the Dorsal region of an Embryo-Chick, END of Third Day (from Foster and Balfour).

Am, amnion ; mp, muscle plate ; c v, cardinal vein ; Ao, dorsal aorta at the point where its two roots begin to join ; Ch, notochord ; Wd, Wolffian duct ; Wh, commencement of formation of Wolffian body ; tip, epiblast ; so, somatopleure ; liy, hypoblast. The section passes through the place where the alimentary canal Qiy) communicates with the yolk-sac.

vertebral division of which it was originally connected. In the inner or central part of the primordial vertebrae a different kind of change occurs, first, by the amalgamation or fusion of the protovertebral masses, and subsequently by their subdivision in such a manner that the intervertebral disc arises on a level with or opposite the centre of each protovertebral mass, and the blastema, out of which a permanent vertebral body is formed, is made up by the union of two parts, an anterior and a posterior, the first of these being derived from the hinder part of the preceding protovertebral mass of the same number, the other part being supplied by the anterior section of the protovertebral mass immediately following.

This is a somewhat complicated change ; and the more difficult to be followed that it would appear that the original division between the protovertebral masses disappears previous to a new segmentation taking place. Thus it results that, as respecta the centrum or body part, the posterior half of one protovertebra is thrown into connection w4th the anterior half of the one next following, and tlius each permanent body is formed from imrts of two protovertebral masses ; ■while in respect to the arches, each one proceeds from the hinder segment of the anterior of the two protovertebra3 concerned, the spinal gangiion and root being thrown into connection with the hinder part of the permanent vertebra immediately in front of the protovertebra of which they originally formed a part.

Formation of Vertelbral Matrices

While the material for the vertebral bodies is laid dowu round, the notochord, a further extension of mesoblastic substance from the primordial vertebral plates takes place at the sides and round the medullary cavity for the matrix of the vertebral arches, and in due course, by differentiation of the formative cells, chondrification of the substance occurs in the form of the strips which constitute the first rudiments of the vertebral arches, and the accompanying transverse and other processes. The first ossification of these bones is from cartilage, but doubtless in them, as in other bones, much of the subsequent growth and extension of the bone substance proceeds from sub-periosteal deposit. It is also to be remarked that in some bones originating in membrane, cartilage may subsequently contribute to the growth and extension of the bone, as ajspears to occur in the lower jaw and clavicle.

The chondrification of the formative matrix of the bones in the human embryo takes place chiefly during the fifth and sixth weeks of foetal life, and in the seventh and eighth, ossification has begun in several of the long bones. But even before this time an ossific deposit shows itself in the fibrous matrix of the clavicle and lower jaw. By the ninth week the greater number of the bones have begun to ossify.

The formation of cartilages for the arches of the vertebrae begins first in those of the dorsal region, and spreads from these forwards into the cervical vertebrae and basis of the skull, and backwards * into the lumbar and sacral vertebrae : but the extension of the matrix upwards ceases in the lower sacral and coccygeal region where the arches are deficient.

A small cartilaginous band forms the matrix of the subcentral portion or anterior arch of the atlas vertebra, quite distinct from that of the body of the axis, and out of the line of prolongation of the notochord.

In the lateral plates the cartilaginous matrices of the ribs are formed in connection with those of the transverse processes, and in the vertebral part of the ribs themselves ossification is comparatively early ; but a considerable part remains unossified in the sternal portion, or costal cartilages, in connection with their special use in the mechanism of the respiratory movements.

Certain portions of the transverse parts of the cervical and lumbar vertebrae are undoubtedly homologous with ribs ; but we give the name only to those costal bars which are separately articulated to the vertebra, and the first of the vertebras with which a rib reaching the sternum is articulated is reckoned as dorsal. Among the thoracic ribs a certain number, as elsewhere stated, of the cartilaginous matrices behind the first, are in the commencement united together at their ventral extre Hci'fi aud elsewhere, unless otherwise explained, the terms used to indicate position npply to the primitive proue position of the embryo as it lies in the blastoderm, the dorsal aspect upwards aud the veutral dowuwiu-ds.

mities into a strip of cartilage on each side, and thus the matrix of the sternum is at first cleft in two behind the pre-sterual portion. Subsequent fusion of these two lateral strips unites them into one ; and the transverse division of the bone only appears from the result of ossification in successive distinct centres. This fact possesses an interest in connection with the tendency of the meso-sternuui and xiphi-sternum to divide and to produce various degrees of the malformation termed fissura sterni.

In the lumbar region there is reason also to look upon part of the transverse processes as representing costal elements, but it is only in cases of abnormal formation that they are found distinct from the rest of the vertebra. (See the Descriptive x\natomy, Vol. I., p. 22.)

The sacrum is peculiar in presenting, thi'ust in and compressed between its strictly vertebral elements and the iliac bone with which it is united, several bony pieces which may be regarded as interposed ribs. The ossification of two of these occurs as early as the fifth or sixth month of fostal life.

2. THE HEAD.

The head of the embryo consists at first, as already stated, of the cranial part alone, the face, nose, and mouth being absent. Below the cranium, and extending as far forward as the point of junction of the anterior with the middle encephalic vesicle, is situated the pharyngeal portion of the primitive alimentary canal, closed in anteriorly by the inflection of the blastodermic layers. It is at this place that subsequently the opening of the alimentary canal to the exterior takes place in what constitutes ultimately the isthmus of the fauces ; and in front of this the buccal cavity, not yet existing, is afterwards formed.

In the progressive development of the head the principal changes by which its fundamental parts come into shape may be enumerated shortly as follows, viz., First, increase of deposit and textural differentiation of the mesoblastic substance for the formation of the cranial walls in their basilar, lateral, and upper portions ; second, the interpolation of the sense-capsules as connected with the formation of the rudiments of the nose, eye, and ear ; third, the development of the cerebral hemispheres and other parts of the brain from the three primary encephalic vesicles; fovrtli, the occurrence of the several cranial inflections ; and fifth, the new formation of outgrowths for the development of the parts of the face.

1. The Cranium

The basal portion of the cranium consists primarily of two fundamental parts. Of these the i^osterior is distinguished by the presence of the prolongation of the notochord within it as far forward as the part of the skull which afterwards becomes the pituitary fossa. This portion, which may be named occipito-sphenoid, is originally formed by the undivided investing mass of Ratlike, which surrounds the anterior extremity of the notochord, and contains the matrix of the future basi-occipital and basi-sphenoid cartilages. By its later extension to the sides, it forms the matrix of the exoccipitals and the periotic mass of cartilage which surrounds the primary auditory vesicles. The main part extends forward below the posterior and middle primary encephalic vesicles, ending at the pituitary fossa.

The anterior portion of the basis cranii may be called sphcno-ethmoid, as containing the matrix of the pre-sphenoid, and the ethmoid cartilages. It is mainly produced in connection with the trabeculae cranii, which contain between their separated limbs the pituitary fossa. This part of the cranial basis contains no prolongation of the noto

Fig. 500.

Fig. 530. — The Lower or Cartilaginous part op THE Cranium op a Chick op thk Sixth Dat (from Huxley).

1, 1, chorda dorsalis ; 2, the shaded portion here and forwards is the cartilage of the base of the skull ; at 2 the occipital part ; at 3 the prolongations of cartilage into the anterior part of the skull called traheculce cranii ; 4, the pituitary space ; 5, parts of the labyrinth.

chord ; it lies below the anterior encephalic vesicle (thalamencephalon), and becomes greatly modified in connection with the expansion of the cerebral hemispheres and primary ocular vesicles, and the development of the nasal fossae and mouth, together with the other parts of the face.

The primary parts of the three principal sense organs, it may here be stated, the nose, eye, and ear, formed in connection respectively with the cerebral hemispheres, the thalamencephalon, and the third primary vesicle, are interpolated between the rudimentary parts of the head as follows, viz., 1. The nose between the frontal, intermaxillary and ethmoid ; the eye between the frontal, sphenoid, ethmoid and maxillary ; and the ear between the basi-occipital, exoccipital and alisphenoid. "While the base of the cranium, to the extent already mentioned, is

Fig. 531. Fig. 53L — View from below op the Cartilaginous Base of the Cranium with its Ossific Centres in a Human Foetus op about Five Months (from Huxley, slightly altered).

The bone is dotted to distinguish it from the cartilage, which is shaded with lines. 1, the basilar part, 2, the condyloid or lateral pvarts, and 3, 4, the tabular or superior part of the occipital surrounding the foramen magnum ; 5, centres of the pre-sphenoid on the inside of the optic foramen ; 6, centres of the post-sphenoid ; 7, centres of the lesser wings or orbito-sphenoid ; 8, septal cartilage of the nose ; 9 & 10, parts of the labyrinth.

cartilaginous in its origin, the lateral and upper walls are chiefly of membranous formation, as in the squama occipitis, the squamo-zygomatic of the temporal, the parietal and the frontal bones.

The trabeculai stretch forward to the anterior extremity of the head, and maintain the foremost place as the seat of the nasal cartilages and external apertures of the nose. Behind these the coalesced trabeculae form a narrow cthmo-vomerine cartilage, the nasal septum, round the back of which the vomer is formed as a bony splent covering ; while in the hinder lyre-shaped interval of the separated trabeculge is placed the infundibulum in connection with the pituitary body.

Fig. 532.

Fig. 532. — Basilar Part op the Primordial Cranium OF A Human Fcetus of three months seen from ABOVE (from Kolliker).

a, iipi)cr half of the squama occipitis ; h, lower half of the same ; c, cai-tilaginous plate extending into it ; d, (in the foramen magnum) the exoccipital ; c, basi-occipital ; /, peti'ous, with the meatus anditorius interuus ; g, dorsum selliB, with two nuclei belonging to the basi-sphenoid bone ; /;, nuclei in the anterior clinoid processes ; i, great wing nearly entirely ossified ; I; small wings ; I, crista galli ; m, cribrethmoid ; n, cartilaginous nose ; o, strip of cartilage between the sphenoid and the parietal ; p, osseous plate between the lesser wings and the cribriform plate.

From the side of the presphenoid cartilage the matrix of the orbitosphenoids or lesser wings, containing the optic foramina, is developed ; and from the sides of the basi-sphenoid proceeds the matrix of the greater wings, which are also cartilaginous in their origin.

In the periotic or cartilaginous rudiment of the temporal bone three centres of formation are distinguished by Huxley, viz., 1. OpisthoUc, or that surrounding the fenestra rotunda and cochlea ; 2, prootic, or that which encloses the superior semicircular canal ; and 3, epiotic, or that which surrounds the posterior semicircular canal and extends into the mastoid portion. They soon unite into one so as to form the petromastoid bone.

Fig. 533. — Longitudinal Section through the Head op an Embryo op Four Weeks (from Kolliker). -j" V, anterior encephalic vesicle, cerebral portion ; z, interbrain ; m, midbrain ; h, cerebellum ; n, medulla oblongata ; no and a, optic vesicle ; o, auditory depression ; t, centre of basi-cranial flexure ; t', lateral and hinder parts of tentorium ; p, the fold of epiblast which forms the hypophysis cerebri.

The styloid process and the auditory ossicles are of cartilaginous origin.

The squamo-zygomatic and tympanic are produced from membrane.

The Cranial Flexures

The earliest and the most important of the cranial flexures is that which takes place at the anterior extremity of the notochord and in the region of the mid-brain or middle encephalic vesicle. Here, as previously stated, the notochord extends into the substance of the basis of the cranium as it is prolonged forwards in the line of the vertebral bodies. At this place the medullary tube, and the substance forming the wall of the cranium especially, undergoes a sudden bending downwards and forwards, so as to cause the projection of the thickened cranial base in a marked manner upwards. This coincides with the place where the investing mass and the trabeculae meet, and where inferior]? the })ituitary body, and superiorly the infnndibnlum are afterwards formed. The investing mass of blastema, in which the anterior extremity of the notochord is enclosed, and the notochord itself, terminate here behind the pituitary fossa, or what afterwards becomes that part, in a place corresponding to the dorsum selliB of human anatomy. Above and behind this, the middle cerebral vesicle forms the most prominent part of the cranium, which remains a characteristic feature of this part of the embryo head for a consideral^le time. Another early flexure of the cranium accompanies the development of the cerebellum from the third primary vesicle, a cleft now appearing behind and below the rudimentary cerebellum, in the region of the fourth ventricle, and above the medulla oblongata, and this flexure is necessarily attended with a convexity forwards, or another flexure in the place of the pons Varolii.

Fig. 534. Longitudinal Section of the Human Embryo at the sixth or seventh week

1, cerebral hemispheres

2, vesicle of the third venti'icle ; 3, mid-hrain ; 4, cerebellum ; 5, medulla oblongata ; c7i, notochord passing up through the bodies of the vertebrae into the basis cranii and terminating in the head between the infundibulum and the sac o£ the hypophysis cerebri ; s, the vertebral spines ; n, the spinal cord ; p, the phannix ; 7i, the heart ; I, the liver ; ?', the stomach and intestine; cl, the cloaca ; r, the urinary bladder and pedicle of the allantois ; it, u', the umbilicus containing the vitellointestinal duct, urachus and vessels ; between i, and /, superiorly, the Wolffian body is shown.

The great cranial flexure thus marks the division between the strictly basi-cranial, or occipito-sphenoidal, and the basi-facial, or sphenoethmoidal part, the chorda terminating between those two portions of the cranial base, with a conical and sharp point. Here the chorda is itself lient downwards and forwards, and terminates in a s]3ot which corresponds to the post-sphenoid body, or dorsum sellas. According to Mihalkovics, who has recently investigated the subject with care (see Archiv filr j\Iikroskop. Anat., vol. xi., 1875,) in connection with the formation of the pituitary gland in mammals and birds, the chorda tapers off to a fine point in front of this spot, but presents a slight swelling just at the place of the future occipitosphenoidal suture.

The formation of the mouth, and its opening by the fauces into the pharyngeal or first part of the primitive alimentary canal, are phenomena of development intimately connected with the formation of the central part of the cranium and sella turcica, but they are also associated with the development of the face, which is next to be considered.

Formation of the Moutli and Hypophysis cerebri. — Along with the changes which accompany the formation of the principal cranial flexui-e, is associated in a remarkable manner the origin of a body (the pituitary gland or hypophysis cerebri) the nature and uses of which in the adult are entii'cly

Fig. 535.— Vertical Section of the Head in Early Ejibryoes of the Rabbit, Magnified {from Mihalkovics).

A. From an embryo of five millimetres long.

B. From an embryo of six millimetres long,

C. Vertical section of the anterior end of the notochord and i^ituitary body, &c., from an embryo sixteen millimetres long.

In A, the faucial opening is still closed ; in R, it is formed ; c. anterior cerebral vesicle ; mc, meso-cerebrum ; mo, medulla oblongata ; co, corneous layer ; to, medullary layer ; if, infundibuhim ; avi, amnion ; spe, spheno-ethmoidal, be, central (dorsum sellte), and spo, sj^heno-occipital parts of the basis cranii ; /;, heart ; /, anterior extremity of primitive alimentary canal and opening (later) of the fauces ; i, cephalic jrortion of primitive intestine ; tha, thalamus ; p', closed opening of the involuted part of the pituitary body (py) ; ch, notochord ; pk, pharynx.

imkno-mi, but the constancy of whose presence, and the imiformity of its connections in the whole series of vertebrate animals, points to some important morphological relation.

The general nature of this body, in its joint connection with the infundibulum of the brain on the one hand, and a diverticulum of the alimentary canal on the other, was first pointed out by Eatlike (Miiller's Archiv, 1838. p. 482), although he afterwards abandoned the view there set forth. It was, however, fully confirmed by others ; and, among recent observers, we owe more especially to William Miiller an elaborate investigation of the whole subject (Jenaische Zeitschr., vol. vi., 1871), who traced most carefully the nei-vous and diverticular elements in their development, and their union with mesoblastic elements in the formation of the gland, Goette next ascertained that the diverticulum from below is connected with the buccal cavity and epiblast, and not with the pharynx and hypoblast, as was previously supposed (Archiv fur Miki-oscop. Anat., vol. ix., p. 397). The observations of Mihalkovics on Mammals complete the history of this point in development, and will be mahily employed in the following description.

The formation of this body may be shortly described as consisting in the meeting and combination of two outgrowths from very different fundamental parts ; one cerebral or medullary from above, and the other corneous or epiblastic (glandular), from below, in a recess of the cranial basis which afterwards becomes the pituitary fossa (fig. 53.5, B, <f,'py). The cerebral outgrowth, the posteiior of the two parts, takes place by the fonnation of a pointed projection do\\Tawards of a portion of the lower medullaiy waU of the vesicle of the third ventricle, and its firm adhesion to the base of the cranium. This is the commencement of the infundibulum. Meanwhile, a little in front of the same place, there is projected upwards from below a part of the basilar surface of the cranium, so as to form a deep recess lined by the corneous layer from the back and upper part of the future mouth. This recess is the commencement of the hypophysis or pituitary- body in its glandular portion, which is not, as has been supposed, a recess from the pharynx, seeing that it is in front of the opening which is afterwards formed for the fauces. The depressed and sharpened out anterior part of the notochord is directed downwards and forwards, while the sac of the hypophysis is can-ied upwards and backwards, and, according to Mihalkovics, the attenuated end of the chorda soon disappears from between the infundibulum and the hypophysis, previous to the occurrence of the intimate union which follows between these two bodies. The anterior extremity of the chorda, therefore, is lost in the floor of the pituitary fossa, and the swollen or dilated portion of the chorda which succeeds, and which comes then to form the apparent termination, occupies the interval between the basi-occipital and the basi-sphenoidal cartilages. The chorda traced back from this point, presents another swelling at the junction of the basi-occipital cartilage with that of the odontoid process, into which last it passes. The third swelling of the chorda lies between the odontoid cartilage, and that of the body of the axis vertebra.

Fig. 536. — Cranium and Human Embryo seen (from Ecker).

Face of the from before

A, from an embryo of about three weeks : 1, anterior cerebral vesicles and cerebral hemispheres ; 2, interbrain ; 3, middle or fronto-nasal process ; 4, superior maxillary plate ; 5, the eye ; 6, inferior maxillary or mandibular plate (first postoral) ; 7, second plate ; 8, third ; 9, fourth, and behind each of these four plates their respective pharyngeal clefts. B, from an embryo of five weeks : 1, 2, 3, and 5, the same as in A ; 4, the external nasal or lateral frontal process ; 6, the superior maxillary plate ; 7, the mandibular ; x , the tongue ; 8, the first phaiyngeal cleft, which becomes the meatus auditorius extenius.

The base of the skull, therefore, consists of two parts, one the posterior, in which the chorda is imbedded, and corresponding to the futui-e basi-occipital and basi-sphenoidal parts, the other in front of this, into which the chorda does not penetrate, the sphcno-ethmoidal, and which, according to the researchee of Parker and Gegenbaur, is of a later fonnation, and is more immediately related to the development of the face.

The flask-like outgrowth of the buccal epiblast which gives rise to the hypophysis cerebri, is now gradually shut off from the corneous layer and cavity of the mouth, iirst by the constriction, and subsequently by the closure of its place of communication. There remains however, for a considerable time, a longish.

Fig. 537. — OcTLiXE Plan ViKW OF THE Upper Part of the Body of an Embryo Pig, two-thirds

OF AN INCH IN LeNGTH.

Magnified seven diameters (from Parker).

Fig. 538. — Plan op the Skull, &c., of the same Ejibryo seen from below. Magnified ten diameters (from Parker).

In this a7id the preceding figure the letters, where j)resent, indicate the following parts : —

c' to 0^ , the five primary divisions of the brain ; a, the eye ; n, the nose ; m, the mouth ; <?•, cartilage of the trabeculEe ; ctr, cornua trabecularum ; pn, prenasal cartilage ; 2}p[/> pterygo-palatine cartilage ; mn, the mandibular arch with Meckel's cartilage ; te, first visceral cleft which becomes the tympanoeustachian passage ; au, the auditory vesicle ; hi/, the cerato-hyoid arch ; Ir, the branchial bars and clefts, 1 to 4; tItJi, the thyro-hyoid ; py, the pituitary fossa ; ch, the notochord in the cranial basis, .surrounded by the investing mass (ii') ; vii, facial nerve ; ix, glosso-pharyngeal ; X, pneumog;istric ; xii, hypoglossal nerve.

thread of union between the two (fig. 535, C, 7/). The epithelium of the enclosed portion subsequently undergoes development into glandular coeca and cell-cords, and its internal cavity becomes gradually obliterated. This fonns the anterior part or lobe of the pituitary body. The posterior part owes its origin to the combination with mesoblastic tissue of a. widened extension of the infundibular process of the brain, which is thrust in between the sac of the pituitary body and the dorsum sella;. The nervous structure of this posterior lobe afterwards disappears in the higher animals, but in the lower it retains its place as a part of the brain.

vol.. ir. 3 B

2. Subcranial, Facial, or Pharyngeal Plates or Arches. —

In man, and all vertebrates, there are developed below and on the sides of the cranial part of the head, a series of processes or bars in pairs, which contribute to the formation of the subcranial structures constituting the face and jaws, and the hyoid and other parts intervening between the head and trunk. These bars first received attention

Fig. 539.

Fig. 539. — Outlines showing the early changes in the form of the Head op THE Human Embryo.

A, profile view of the head and fore i)art of the body of an embryo of about four weeks (from nature, '^^) : the five primary divisions of the brain are shown, together with the primai-y olfactory and optic depressions, and a, the auditory vesicle ; 1, marks the mandibular plate, and behind this are seen the three following plates with the corresponding pharyngeal clefts. B, from an embryo of about six weeks (from Ecker, f ) : the cerebral hemispheres have become enlarged and begin to spread laterally ; 1, the lower jaw ; 1', the first pharyngeal cleft, now widening at the dorsal end, where it forms the meatus externus ; the second cleft is still visible, but the third and fourth clefts are closed and *he corresponding plates have nearly disappeared. C, from a human fojtus of nine weeks (from nature, \) ; the features of the face are now roughly formed ; the first pharyngeal ■cleft is now undergoing conversion into the meatus, and the auricle is beginning to rise at its outer border.

from their discovery by Eatlike in 182G, published in the Isis of that year, and were named by him the branchial arches, from the relation ■which some of them bear to the gill bars of branchiate vertebrates. Their nature and transformations were fully investigated by Eeichert in 1837 (Miiller's Archiv, 1837). From later researches it appears that other processes, with somewhat similar relations to the cranium, occurring further forward, may be associated with those described as branchial by Eatlike, and it will be expedient therefore to describe the whole of the subcranial outgrowths together at this place. In this the views of Huxley and Parker will be chiefly followed. (See "On the Stracture and Development of the Skull in the Pig," by W. K, Parker, in Trans. Eoy, Soc. 1873, p. 289 ; Huxley, in Elements of Compai\ Anat., 1864, and Manual of Compar. Anat., 1871 ; also Gegenbaur, Das Kopfskelet, &c., 1872). According to these views the parts of the head situated in fi*ont of and above the future mouth, are formed from two pairs of plates, w^hich may thence be called preoral, in one pair of which the bars are the same "with the trahecukc cranii of Eatlike surrounding the pituitary gland, and are the basis of formation of the pre-sphenoid, ethmoid, nasal, and pre-maxillary portions of the skull, while in the other pair, consisting in each of a deeper and a superficial part, the bars form the foundation

SUBCRANIAL AND FACIAL STRUCTURES. 7.VJ

■of the pterygo-palatine wall of the nose and mouth, and the superior maxillary bone. The nasal pits or primary nasal depressions, which extend themselves afterwards into the nasal fossae, and remain permanently open to the exterior, are formed by a depression of the surface of the epiblast in the anterior prolongation of the head as constituted by the ends of the trabeculas, and the harder structures of the septum and walls of the nasal cavities, as well as the turbinated structures on which the olfactory nerves are distributed, are all derived from the anterior parts of these trabecule ; — a mesial union giving rise to the nasal septum, while lateral parts circling round the nasal pits, form the alar enclosures of these depressions (see figs. 537 and 538).

The second preoral subcranial plates have received the name pterygopalatine from the nature of their deeper connection with the bar in which the pterygoid and palate-bones are afterwards formed. These enclose the posterior nasal apertures, and advancing from the two sides, at last meet each other in the palate, and in front meet the pre-maxillary process to complete the palate and upper jaw.

But these are only the deeper parts of the structure out of which the upper jaw is formed, there being on the surface of the head, and behind the depression of the eye on each side, a bulging process known as the superior maxillary process, in which the upper jaw and malar bone are formed, and which has externally the appearance of bending round the angle of the mouth in continuity with the mandibular or inferior maxillary.

The formation of the superficial parts of the face, as seen from before, may be described as follows, viz. : — In the middle, there descends in what now forms the region of the forehead, a mesial portion, the fronto-nasal plate, which forms the integument of the nose, as far as the inside of the nostrils, and the columella of the nose, together Avith the mesial part or lunula of the upper lip, that is, all the part lying inside the depression of the nostrils. On the outside of these depressions, a short lappet surrounds the orifice of the nostrils, as wings forming the external nasal processes. It is towards the outside of these last plates that the ocular depression is situated, that depression being thus interposed between the lateral or external nasal plate,- and the maxillary plate, and forming the fissure which has been tailed the ocular fissure, but which afterwards becomes more fetrictly the lachrymal in its anterior part (see figs. 536 and 539).

The great buccal aperture now passes across the face, having the middle and external nasal with the superior maxillary plates above and in front, and the inferior maxillary plates below and behind. It will be remembered, however, that the cavity of the mouth is thrown forward by the outward development of the subcranial plates, which deepen more and more the buccal cavity as they grow outwards from the primitive cranium.

The POSTOKAL pairs of pharyngeal visceral plates are four in number ; the first being that already mentioned as following immediately behind the mouth, and forming the mandibular or inferior maxillary.

At an early period of foetal life in the human foetus, and in that of all vertebrates, plates of this description are found, but in the lower vertebrates a greater number exists than in the higher. The number of four pairs belongs to man in common with all the non-branchiate vertebrates. Behind each of these plates there are formed from within, in the course of development, clefts which penetrate the wall of the pharynx ; these clefts, or so-called branchial apertures, running completely through the wall of the pliarynx and the external wall of the body of the embryo (see figs. 537 and 538).

The auditory pit, or primary depression from the epiblastic surface which forms the rudiment of the labyrinth of the ear, is situated immediately above the upper or proximal end of the two first postoral plates, and consequently on a level with the first postoral cleft. And this proximity of position is connected with the intimate relation in which two sets of parts stand to each other : for the part called the first branchial cleft is afterwards converted into the external and middle passages of the ear, (meatus, tympanum, and Eustachian canal), the membrana tympani being at a later period thrown across the passage. It forms thus the tympauo-eustachian cleft or canal. The tympanic bono is of membranous origin and is formed round the first cleft. The external auricle is of integumental origin, and is formed in the second postoral bar posteriorly and externally to the aperture of the first cleft.

The second postoral cleft is the first true water passage, or the first of those which serves as a gill aperture in branchiate vertebrates, and which may in the lower classes be increased to a greater number.

Although the description of some of the changes which the several pharyngeal plates or branchial arches undergo in the further process of development, belongs to a different part of the subject, yet it may be useful to describe sliortly the more important of them in this place.

In the first or mandibular arch a strong cartilaginous bar is formed known as the cartilage of Mcclcel, on the exterior of which, but not in its own substance, throughout a considerable part of its distal length, the lower jaw-bone is afterwards developed. The proximal part next the cranium, which comes later to be connected with the auditory capsule, becomes in mannnalia the malleus, in birds and reptiles chiefiy the OS quadratum. (Seefigures in connection with development of the ear.)

In the second or hyoid arcli are developed the styloid process, the stylo-hyoid ligament, the lesser or upper coruua of the hyoid bone, the series of parts which connect them with the basis of the skull, being united like the first to the auditory capsule : but the proximal part of this arch would appear also to have the incus formed in it, and to be connected with the stapes and stapedius muscle.

The third arch' is the thyro-hyoid, and is related to the formation of the lower or great cornua, and the body of the hyoid bone. It corresponds with the first true branchial arch of amphibia and fishes, in which animals the clefts and bars behind this arch become more numerous than in the higher vertebrata.

The fourth arch, which has no special name, but might be called suhhyoid or cervical, does not seem to form the basis of any particular organ, but is situated exactly at that part of the body which becomes elongated as the neck, — a part Avhich may be considered as absent in the foetus, and the formation of which by a simple process of elongation gives ^""^e to some peculiar features in the anatomy of the parts composing it.

Relations of Cranial Nerves

The rudiments of four cranial nerves, besides the optic and auditory afterwards to be referred to, are found at a very early period in connection with the plates now under consideration, and the following is the relation in which, according to Parker, they stand to these plates in all vertebrate animals. These nerves are the fifth pair or trip-eminus, the facial, the glosso-pharyngeal, and the pneumo-gastric. The two first are situated iu front, and the two latter behind the auditory sac. These nerves all divide or fork above a visceral cleft, one division going to the posterior face of the arch in front of the cleft, the other to the anterior face of the arch behind it.

Fig. 540. — Embryo of the Ciiioii at tub end of the Fouhth Day (from Foster and Bcalfour).

The amnion has been removed ; Al, allantois ; Clf, cerebral hemispheres ; FS, thalamencephalon, with Pn, the pineal gland projecting from its summit; MB, midbrain ; Cb, cerebellum ; IV, V, fourth ventricle ; Z, lens ; chs, choroid slit ; C'en V, iiuditory vesicle ; sm, superior maxillary process ; IF, 2F, &c., first, second, third and. fourth visceral folds ; V, fifth nerve in two divisions, one to the eye, and the other to the first visceral arch ; VII, seventh nerve passing to the second visceral arch ; Gph, glosso-pharjmgeal nerve passing to the third visceral arch ; Pg, pneumo -gastric nerve passing to the fourth arch ; / V, investing mass ; ch, notochord ; Mt, the heart ; HIP, muscle plates ; W, wing ; HL, hind limb.

The orbito-nasal and the palatine divisions of the trigeminus belong to the trabecular arch, the former above, the latter below the optic nerve. Of the other division, one part (the superior maxillary nerve) follows the palato-pterygoid arch, the other (inferior maxillary nerve) accompanies the mandibular arch.

The facial nerve (portio dura of seventh pair) divides above the tympano-eustachian passage, its anterior part (chorda tympani) going to the posterior side of the mandibular arch, and its posterior part (descending branch of facial) to the outer or anterior side of the hyoid arch.

The glosso-pharyngeal nerve, by a similar division, goes by its inner or anterior branch (lingual) to the inner or posterior side of the hyoid arch, and by its other division (pharyngeal) to the front of the first branchial or thyro-hyoid arch.

In the higher animals the pneumo-gastric nerve shows no close relation to the clefts, but in branchiate vertebrates it is continued past the gills, and sends forked branches to the gill arches in front and behind each of the clefts.

3. ORIGIN AND FORMATION OF THE LIMBS.

The close connection of the limb-arches with certain vertebral segments of the trunk has been previously referred to in the morpholoo-ical remarks, given under the description of the bones and muscles m the first volume ; and although the vertebral homology of the parts of the limb proper is not so apparent, at least in the proximal segments, yet in the quinquifid division of the more remote parts, in the preaxial and postaxial arrangement of these divisions, and in their relation to the nerves and some other circumstances, we can scarcely fail to perceive some very near relationship between the structure of the limb as a whole, and a certain number of the vertebral segments of the trunk.

Fig. 541. — Human Embryo of about FOUR WEEKS (froiu Kijllikei', after A. Thomson). | , the anterior limb rising as a semicircular plate from the lateral ridge. (The figure is elsewhere described. )

The limbs do not exist from the earliest time of the formation of the cranio-vertebral part of the trunk, but only begin to be formed when the development of the axial part of the body has made some advance, as in the first half of the fourth day of incubation in the chick, and at the commencement of the fourth week m the human embryo.

They first make their appearance as two pan-s of buds from the side of the vertebral part of the trunk, in the form of flattish lateral elevations with curved free margins projecting from the exterior of the body, outside the thickened ridge (sometimes called the Wolffian ridge) where the division of the mesoblast into somatopleure and splanchnopleure take place, and near the outer margins of the muscular plates. The anterior pair of limbs appears earlier than the posterior, and for a long time is always more advanced in the development of its parts.

The place of formation of the anterior and posterior limbs does not vary to any great extent throughout the vertebrate animals,— and this fact may be looked upon as one of the most marked features ot vertebrate organisation. .

The thickened plate which forms the commencing limb, by its increased growth, projects still more from the side, so as to take the form of a flattened lappet with a semicircular free margin ; presenting then two surfaces which may be named dorsal and ventral with reterence to their correspondence to the like surfaces of the trunk, constituting respectively the primitive extensor and flexor surfaces of the limb ; while the anterior margin of the semicircular lappet corresponds to the preaxial and the posterior margin to the postaxial borders of the future limbs.

DEVELOPMENT OF THE LIMBS.

743

The whole thickness of the somatopleure division of the mesoblast is involved in this primary limb-bud, and it is of course also covered with the epiblast or cuticular layer, in the substance of which there is considerable increase of thickness at the most prominent part of the margin.

As the limb-buds increase in size, the lateral limb-plate, or Wolffian ridge, which is at first very prominent in its whole length, becomes less, and gradually flattens down into the more even surface of the wall of the trunk.

The part of the limb which appears first, corresponds more immediately with the hand or foot than with the other divisions of the limb. Along with this, however, at a very early period there is an indication of the formation of the limb-girdles as folds passing off from the side

Fig. 542. — Diagrammatic Outline of the Profile VIEW OF the Human Embryo OF ABOUT seven weeks, TO SHOW THE PRIMItive relations of the Limbs to the Trunk. (Allen Thomson.) f

r, the radial (preaxial), and u, the iilnar (postaxial) border of the hand and forearm ; t, the tiljial (preaxial), and /, the fibular (postaxial) border of the foot and lower leg. (The foot is represented at a somewhat more advanced stage than the rest of the embryo).

of the trunk. As the projection of the limb increases from the side of the body, the distal or terminal segment becomes slightly notched off from the part next the trunk. This terminal part, forming nearly three-fourths of a somewhat circular flattened plate, contains the rudiments of the hand or foot. The next change which takes place is in the division of the proximal part, or rather the preaxial border and ventral surface, by a notch which separates the fore-arm and lower leg from the upper arm and thigh at the elbow and knee joint respectively. In the third stage the notched division of the free lateral curved margin, with intermediate slightly tubercular projections of the substance, shows the commencement of digital development, in which it soon becomes apparent that the pollex and hallux occupy the preaxial position in their respective limbs, and are followed by the series of other fiugers to the fifth, which is placed on the postaxial border. From these it is easy to trace, by reference to the simple original position of the limbs, the preaxial position afterwards held by the radius in the one and the tibia in the other, and the postaxial position of the ulna or fibula. In the meantime the internal differentiation of texture takes place, by which is brought out the more complete distinction of the segments of the limbs, and the various component parts of each, which gradually appear in the cartilages for the bones, muscular plates extended from the general muscular sheath of the trunk, prolongation of the cutaneous layer of the integument, the formation of nerves, blood-vessels, &c., the consideration of all which belongs to the history of more advanced development.

In order also to complete the history of the formation of the limbs, it is necessary to take account of the changes of attitude the anterior and posterior respectively undergo, as compared with the primary embryonic position. In this the elbow comes first to be turned outwards and then directed backwards, bringing the flexor surface of the upper arm forwards, while the position of the flexor surface of the fore-arm and hand, though generally and naturally inwards, may, by supination, be brought forwards, and by pronation backwards, the "latter being the permanent position given to the manus in most animals. In the hinder limb, again, the thigh is turned inwards, so that in the higher animals the flexor surface looks backwards, and in all animals the lower division of the limb is turned inwards and the sole of the foot downwards, so that the extensor surface and dorsum look forwards. (See vol. i., p. 122.)

4. DEVELOPMENT ^ OP THE MUSCLES.

The muscles of the trunk derive their origin from the muscular plates previously referred to as lieing separated by difierentiation of the

Fig. 543.

WJ?

Fig. 543. — Section through the Lumbar IIkgion of an EiiBRYO-CnicK of Four Days (from Foster and Balfour).

nc, neural canal ; jir, posterior root and ganglion of a spinal nerve ; ar, anterior root ; ■»«/>, muscle-plate; cfi, notochord ; WB, Wolffian ridge; AO, aorta; Vca, cardinal vein ; Wd, Wolffian duct ; Wb, Wolffian body with glomeruli ; ge, germinal epithelium ; Md, depression forming the commencement of the Miillcrian duct ; d, alimentary canal ; M, mesentery ; >S'0, somatopleure ; SP, splanchnopleure ; 1', blood-vessels ; P2h pleuro-peritoneal space.

FORIvIATIOX OF THE JOINTS. 7J5

formative cells in the outer or superficial part of the protovertel)raI masses. Some difference of o]iinion exists, however, among embryologists, as to how far the hypaxial (hyposkeletal of Huxlc}') as well as the epaxial muscles, proceed from this source alone, or whether only the latter are traceable to the muscular plate formed by the protovertebral differentiation, and the hyj^axial may be supposed to proceed from a deeper source.

Eeccnt observations seem to show that a downward extension of the mesoblast from the protovertebrge may also give rise to the hypaxial muscles.

Being developed from the segmented protovertebral elements, the muscular plate shows at first the same division into segments, which ai-e separated for a time by intermuscular septa (myotomes) as occurs during life in a considerable number of them in fishes and amphibia.

The formation of the longer muscles of the trunk proceeds from the disappearance of the septa, and the longitudinal union of the fasciculi of successive myotomes. In the trunk the direction of these remains for the most part chiefly longitudinal, but those connected with the limb-girdles change their direction with the development of the limb.

The formation of the muscles of the limbs themselves has not been traced in detail. The greater number of these muscles appear rather to arise independently in the blastodermic tissue of the limb-bud, than to be prolonged from the sheets of trunk-muscles (Kiilliker).

The facial muscles and the platysma, belong to the subcutaneous system, and are developed along with the skin.

The diaphragm is at first wanting. It arises soon after the formation of the lungs, fi'om two parts which spring from above and the sides, and which divide the pleural and peritoneal cavities, which were jDreviously in one, from each other.

The muscles begin to be formed in the human embryo in the sixth and seventh week.

Formation of the Joints

With regard to the formation of the joints, very little is known. It would appear that the cavities of the synovial joints are not yet formed at the time when chondrification has taken place in the matrix of the bones. It is therefore by a secondary process of solution of continuity that these cavities are produced. The articular cartilages remain as the coverings of the opposed surfaces of the bones, and the various ligamentous and other parts belonging to the joints arise by processes of textural differentiation which it is unnecessary to particularise here.

Distinction of Bones according to their Cartilaginous or Membranous Origin. — There is here appended for the assistance of the reader a note of the distinction as regards their origin from cartilage or fibrous membrane of the .several permanent bones of the skeleton.

1. Boiu'.s- (irisiiif/from Cartilage: —

fl, III the Head.

Basi-occipital. ex-occipital, and part of the supra-occipital or squama occipitis.

The whole sphenoid except the cornua sphenoidalia.

The p.etro-mastoid or periotic portion of the temporal bone.

The mes-ethmoid and ethmo-turbinal.

The pterygo-palatine.

The malleus (quadi-ate of animals) with Meckel's cartilage.

The incus and stapes, with the stylo-hyoid.

The thyro-hyoid.

b. Ill the Trnnh.

The bodies, arches, and processes of the vertebraa. The ribs and sternum.

c. In the Limbs.

The scapula and coracoid. The clavicle in part, and all the other bones of the Tipper limbs (excepting sesamoid).

The ilium, ischium, and pubis, and all the other bones of the lower limbs, including the patella, but excepting sesamoid of toe.

2. BoJies arittiiig from Fihvcms Memirane : —

a. In the Head.

Pai-t of the squama occipitis.

The frontal.

The parietal.

The squamo-zygomatic and tympanic of the temporal.

The nasal and lachrjTual.

The maxillaries and i3re-maxillaries.

The vomer and comua sphenoidalia.

The inferior or maxillo-turbinal.

The malar or jugal.

The inferior maxillary or mandibular.

i. Ill the Trunh. None.

c. In the Limbs.

The clavicle in part.

(The marsupial bone of animals.)

The smaller sesamoid bones of tendons.

DEVELOPiUENT OF THE NERVOUS SYSTEM.

The Cerebro-spinal Centre

From what has been previously stated it will have been seen that the rudiment of the cerebro-spinal the medullary canal, not yet closed, shows at the cephalic extremity a partial division into the three jirimary cerebral vesicles ; and at the posterior extremity a rhoniboidal enlargement. Six proto-vertebral divisions are visible ; so, the upper division of the blastoderm, sp, the lower division.

Fig. 544. — Embryo of the Dog seen from above, with a portion of the Blastodekm attached (from Bischoff).

Fig-. 545. — Embryo of the Dog more advanced, seen from above (after Bischoff).

The medullaiy canal is now closed in ; c, the anterior encephalic vesicle ; o, the primitive optic vesicle in communication with the anterior encephalic ; au, the primitive auditory vesicle opposite the third encephalic vesicle ; am, the cephalic fold of the amnion enclosing the anterior third of the embryo ; ov, the omphalo-mesenteric vein entering the heart posteriorly ; in-, the proto- vertebral divisions, now become numerous.

nervous centre is formed more immediately from the thickened medullary plates of the involuted epiblast, the ridges of which, rising from the surface of the blastoderm, become united dorsally along the middle line into a hollow medullary tube of a cylindrical form. This tube becomes dilated at its anterior or cephalic extremity, and this dilated portion becomes divided by two partial constrictions into the three primary cerebral or encephalic vesicles, which, as representing fundamentaf portions of the brain, have been termed the fore-brain, mid-brain and hind-brain. The spinal portion retains its more uniform cylindrical shape, escepting towards the caudal extremity, where it is longer of

Fi<r. ZiPj.

Fig. 546. — Transverse Section THROUGH THE EjlBRYO OF THE ChICK, AND

A, Blastoderm at the end

Of THE First Day. Magnified FROM 90 TO 100 TIMES (from Kolliker).

d'/> tUuh d d i,a-p c/i /i, epiblast ; (?c?, hypoblast ;

sp, mesoblast ; Pv, medullary

groove ; m, medullary plates ;

c7i, chorda dorsalis ; utrp, proto-vertebral plate ; uw7i, commencement of division of mesoblast into its upper and lower laminte ; between i?/ and h the dorsal laminte or ridges which by their approximation close in the medullary canal closing, and forms for a time a flat open rhomboidal dilatation. The continuous cavity enclosed within the primitive medullary tube is the same with that which, variously modified, afterwards constitutes the central ventricles of the brain and canal of the spinal cord.

The formative cells composing the medullary substance are at first spherical, but they afterwards become elongated and spindle-shaped, and increase rapidly by multiplication. They represent at first the grey substance, or the nerve-cells and non-medullated fibres. The cylindrical cells which, from the first, line the whole canal, remain permanently in the part of it which forms the central canal of the spinal marrow, and frequently present the ciliated structure.

Fig. 547. — Transverse Section of the Embryo Chick on the Second Day. Magnified FROM 90 to 100 times (from Kolliker).

The explanation of the letters is in part the same as in the foregoing figure, mr, the medullary tube now closed along the dorsal line and covered in by continuous epibkist ; nwh, hollow of the proto-vertebral mass ; mp, mesoblast external to the protovertebroe dividing into hpl, somatopleure, and df, splanchnopleure ; ao, one of the primitive aortas ; iinff, intermediate mass connected with the origin of the Wolffian body.

The internal grey substance of the spinal marrow is first formed; the white substance is produced later on the exterior. The sides acquire considerable increased thickness, while the dorsal and ventral parts remain comparatively thin, so that the cavity assumes the appearance in section of a slit, which becomes gradually narrower as the lateral thickening increases ; and at last the opposite surfaces uniting in the middle divide the primary central canal into an anterior or lower and posterior or upper part (see figs. 547 and 548).

The lower of these divisions becomes the permanent central canal, the upper or dorsal is afterwards so far obliterated that it is filled with a septum of connective tissue belonging to the pia mater, and becomes the posterior fissure of the cord (in human anatomy). (Lockliart Clarke, Phil. Trans. 1862.)

In birds and mammals there is no distinction to be seen at first between the outer or corneous layer of the involuted epiblast and the cells which by their increase more immediately constitute the medullary plates. In batrachia, however, the dark colour of the corneous layer shows it to be distinct from the more strictly nervous layers. In osseous fishes there is no medullary groove or canal at first, but an involution of a solid column of cells, which is subsequently hollowed out for the formation of a ventricular cavity.

The masses of grey matter first formed in the spinal marrow correspond chiefly to the anterior columns ; these are succeeded by lateral masses or columns, and somewhat later by small posterior columns. There are at first no commissures except by the passage of the deepest

Fig. 548. — Transverse Section of the CervicaIi Part of the Spinal Cord of a Human Ejibryoop Six Weeks (from Kolliker). 'f

Tins and the following figure are only sketched, tJie white matter and a part of the gre.y not being shaded in. r, central canal ; e, its epithelial lining, at e (inferiorly), the i)art which becomes the anterior comniissure ; at c (superiorly), the original place of closure of the canal ; a, the white substance of the anterior columns, beginning to be separated from the grey matter of the interior, and extending round into the lateral column, where it is crossed by the line from [/, which points to the grey substance ; p, posterior column ; ar, anterior roots ; j5r, posterior roots.

layer of cells across the middle line, but the fibres from the roots of the nerves when formed are traceable into the grey substance of their rcs])cctive anterior and posterior columns.

The white substance is formed external to or on the surface of the deeper grey substance j but it is not yet determined whether it is developed out of the cells composing the grey matter or from separate blastema to which the mesoblast may in part contribute. It is combined with connective tissue elements, and its structure is <lifferent from that of the grey substance, which is undoubtedly produced by multiplication and differentiation of the involuted epiblastic cells.

Fig. 549.— Transverse Section of Half THE Cartilaginous Vertebral Column AND THE Spinal Cord in the Cervical Part of a Human Embryo op from NINE to ten weeks (from Kolliker) '~

c, central canal lined with epithelium ; a, anterior column ; ^j, posterior column ; p', band of Goll ; [/, ganglion of the posterior root ; p r, posterior root ; a r, anterior root passing over the ganglion ; d m, dura-matral sheath, omitted near p r, to show the posterior roots ; b, body of the vertebra ; cl), chorda dorsalis ; n a, neural arch of the vertebra.

On the fifth and sixth days in the chick, according to Foster and Balfour, the white columns increase rapidly in size, and the anterior median fissure begins to be formed between the anterior columns by their swelling outwards and leaving its interval between them. It is at first wide and shallow and soon receives a lining of vascular connective tissue or pia mater. The commissures are now also formed ; the anterior grey commissure first, then the posterior grey, and somewhat later the anterior white commissure.

In the further increase of the anterior and lateral white columns as they thicken, they become more united together on each side, so that they can only be arbitrarily distinguished ; the fibres of the roots of the nerves are traced through them into the grey matter ; the cornua of grey matter become more and more developed, and the fissures between the white columns deepen, while the connective tissue or pia-matral septa run more completely inwards through the white substance.

Angular cells with radiating processes make their appearance in the grey matter, and the nerve-fibres both of the grey and white matter become more distinct.

The cylindrical cells lining the central canal retain their distinctness, and they are more completely separated from the grey matter by the delicate tissue of the ependyma. Throughout the greater part of the spinal marrow the dorsal part of the primary medullary hollow is obliterated to form the fissure, but in the sacral region of birds it opens out in the rhomboidal sinus, and in the filum tcrminale of the human spinal marrow the whole primary medullary cavity remains.

The SPINAL COED has been found by Kolliker already in the form of a cylinder in the cervical region of an embiyo four weeks old. Un-united borders have been seen by Tiedemann in the ninth week towards the lower end of the cord, the perfect closing of the furrow being delayed in that part, which is slightly

750

DEVELOrMENT OF THE ^'EPLVO^S SYSTEM.

enlarged, and presents a longitudinal median slit, analogous to the rliomboidal sinus in bii-ds.

The anterior fismirc of the cord is developed very earlj', and contains even at fii-st a process of the pia mater.

Fin

<-/

icw'^'

Fig. 550. — Transverse Section OF Half of the Spinal Cord of the Chick OF Seven Days (from Foster and Balfour). Magnified.

pciv, posterior, lew, lateral, and acw, anterior •white columns ; pc, jjosterior cornu of grey matter with small cells ; ac, anterior grey cornu with large cells ; ep, epithelium of the canal ; c, the upper part now open and filled with tissue in the posterior fissure ; S2')c, the lower division of the primitive medullary cavity, which remains as the permanent canal ; «/, anterior fissure left between the ]3rojecting anterior columns ; ajc, anterior grey commissure.

aye

The cervical and lumhar cnlarricmcnts, opposite the attachments of the brachial and crural nei-ves, appear at the end of the third month : in these situations the central canal, at that time not filled up, is some'ndiat larger than elsewhere (see figs. 556 and 5.58).

At first the cord occupies the whole length of the vertebral canal, so that there is no Cauda equina. In the fourth month the veitebraB begin to grow more rapidly than the cord, so that the latter seems as it were to have been retracted within the canal, and the elongation of the roots of the nerves which gives rise to the Cauda equina is commenced. At the ninth month, the lower end of the cord is opposite the third lumbar vertebra. (Kolliker, Ent'wickelungsgeschichte ; Lockhart Clark in the Phil. Trans. 1862; Bidder und Kupfer, Untersuch. iib d. Eiickenmark, Leipz., 1857. Foster and Balfom-, Elements of Embryology.)

Till lately it was believed that the roots and ganglia of the spinal nerves are at first distinct from the medullary substance of the cord, ;and that they originate by differentiation of cells in the mesoblastic f5ubstance of the protovertebral plate. But recent observations, to bo more particularly referred to hereafter, have shown that they arise in 2)art at least in close connection with the spinal cord itself.

THE BRAIN OR ENCEPHALON.

1. — General phenomena of development as ascertained in birds and mammals. — A reference has previously been made to the simple form in which the brain at first presents itself in the anterior dilated portion of the primitive medullary tube, and its partial division into the three primary cerebral vesicles. This is placed within simple cranial walls formed by the cephalic inflection of the blastoderm, without face or any other parts ; so that the head of the embryo consists at first of no more than the wider part of the medullary tube and the simple enclosing wall.

Fig. 551. — Four Views of the Brain of an Embrto-kitten in the Stage of First Division into the Five Cerebral Eudiments, magnified Three Diameters (from Reichert).

A, from above ; B, from the side ; C, vertical section showing the interior ; D, from below.

1, Cerebral hemisphere, prosencephalon ; 2, thalamencephalon ; 3, mesencephalon, still single ; 4, cerebellum, epencephalon ; 5, mj-elencephalon, medulla oblongata ; o, optic nerves ; V, fifth ^pair ; "VIII, eighth pair or glossopharyngeal and pneumogastric ; i, infundibulum ; v, v', general ventricular cavity, opening at v, into the lateral ventricle by the foramen of Monro.

In the base of this wall, it will be remembered that the notochord extends forward beneath the posterior and middle of the vesicles, and occupies, therefore, the part of the cranium corresponding to the occipito-sphenoidal basis, while the trabecute cranii, developed forwards

Fig. 552. — Vertical Sections of Embryonic Brains in two Stages of Transition from the Rudimentary Condition, Magnified THREE Diameters (from Eeichert).

A, Brain of the embryo pig in commencing state of transition. 1, Right cerebral hemisphere ; 2, thalamencephalon and position of the pineal gland ; 3, midbrain, with a large cavity ; /, foramen of Monro ; i, infundibulum ; 4, cereliellum ; 5, medulla oblongata.

B, Brain of the embryo of the cat more advanced, c, Cerebral hemisphere passing backwards so as to cover the other parts in succession ; I, olfactory bulb ; II, optic ner^'e ; th, thalamus opticus ; /, foramen of Monro ; cc, cor25US callosum ; ^3, pineal gland ; i, infundibulum ; cq, corpora quadrigemina, not yet divided ; 3, third ventricle ; cr, crura cerebri, the aqueduct of Sylvius, now reduced in width ; c', cerebellum ; 4, fourth ventricle ; jpv, Pons Varolii ; mi, medulla oblongata.

from below the anterior vesicle, are prolonged in the anterior or sphenoethmoidal part. The latter cerebral rudiment, therefore, which corresponds to the thalami optici and third ventricle, and which may with Huxley be conveniently called thaJamcncepiialon, is at first the foremost part of the brain, and the region of the ijituitary fossa lying below it is the foremost part of the cranial basis. The manner in which the development of the trabecule and other elements of the face modifies the form of this re,e,'ion of the head has already been adverted to, aud need not be repeated here.

As regards the earliest phenomena of development in the brain itself, there are three changes which mainly tend to modify its form in the most marked degree, viz., 1st, the development from the anterior vesicle on each side of the primitive ocnlar vesicle ; 2nd, the expansion from another part, somewliat later, of the vesicles of the cerebral hemispheres; and 3rd, the formation in the forepart of the posterior encephalic vesicle of a new cerebral rudiment corresponding to the cerebellun).

Fig. 553. — Sketches op the Primitive Parts OF THE Human Brain (from Kolliker).

1, 2, and 3 are from the human embryo of about seven weeks. 1, view of the wliole embryo from behind, the brain and spinal cord exposed ; 2, the posterior, and 3, the lateral view of the brain removed from the body ; h, the cerebral hemisphere (prosencephalon) ; i, the thalamenceiihalou ; i', the infundibulum at the lower part of the same ; m, the middle ^jrimary vesicle (mesencephalon) ; c, the cerebellum (epencephalou) ; m 0, the medulla oblongata. Figure 3 shows also the several curves which take place in the development of the parts from the primitive medullary tube. In 4, a lateral view is given of the brain of a human embrjo of three months : the enlargement of the cerebral hemisphere has covered in the optic thalami, leaving the tubercula quadrigemina apparent.

The formation of the primitive ocnlar vesicles, by an evolution of the lateral wall of the primitive medullary tube, gives to the first vesicle and the adjacent part of the head a much greater lateral width ; but the cranial wall, though pushed out by the enlarging oculai: vesicles, does not follow closely the inflection of their surfaces. As the subsequent contraction of the stalk of the ocular vesicles progresses, these vesicles are thrown more backwards and downwards by the change next to be described.

Tills is the evolution or expansion of the wall of the anterior encephalic vesicle into the two cerebral hemispheres, which takes place in front and at each side, so that the vesicles of the right and left hemispheres are from the first separate and distinct. As these vesicles become dilated, the cranial wall undergoes a corresponding expansion in the forepart of the head, and the vesicle of the thalamencephalon, which was at first the foremost part of the embryo-head, is thrown downwards and backwards into a deeper position.

The middle encephalic vesicle, increasing greatly in size, takes the most prominent part of the head superiorly, both from its own greatei relative magnitude, and from the sudden bend which the head now takes below this vesicle in the great cranial curvature.

The formation of the cerebellum begins by a thickening in the upper and lateral walls of the part of the posterior primitive vesicle which is next to the midbrain, and is accompanied by a deep inflection of the medullary tube between it and the remaining part of the vesicle which forms the medulla oblongata.

FIVE TRIMARY DIVISIONS OF THE BEAIN.

753

There are thus distinguished the rudiments of five fundamental constituents of the brain, under which it will be found convenient to bring the notice of the development of the several parts forming the full grown organ, and which m^y in this association be shortly enumerated as follows, viz. : —

1. The cerebral hemispheres, with their ventricular hollows or lateral ventricles, the corpora striata, and the olfactory lobes, — a set of parts to which, as a whole, the name of proceredrum or j)rosejice2}haIo?i may be Q-iveu.

Fig. 554. — Sketches of the early form OF THE PARTS OF THE CeREBRO-SPINAl Axis in the Human Embryo (after Tiedemann).

A, at the seventh week, lateral view ;

1, spinal cord ; 2, medulla oblongata ; 3, cerebellum ; 4, mesencej^halon ; 5, 6, 7, cerebrum. B, at the ninth week, posterior view; 1, medulla oblongata; 2, cerebellum ; 3, mesencephalon ; 4, 5, tlialami optici and cerebral hemispheres. C and D, lateral and posterior views of the brain of the human embryo at twelve weeks, a, cerebrum ; b, corpora quadrigemina ; c, cerebellum ; d, medulla oblongata ; the thalami are now covered by the enlarged hemispheres. E, posterior view of the same brain dissected to show the deeper parts. 1, medulla oblongata ; 2, cereliellum ; 3, corjDora quadrigemina ; 4, thalami optici ; 5, the hemisphere tuined aside ; 6, the corpus striatum embedded in the hemisphere ; 7, the commencement of the aorpus callosum. F, the inner side of the right half of the same brain separated by a vertical median section, showing the central or ventricular cavity. 1, 2, the spinal cord and medulla oblongata, still hollow ; 3, bend at which the pons Varolii is formed ; 4, cerebellum ; 5, lamina (supierior cerebellar peduncles) passing up to the corpora quadrigemina ; 6, crura cerebri ; 7, corpora quadrigemina, still hollow ; 8, third ventricle ; 9, infundibu lum ; 10, thalamus, now solid ; 11, optic nerve ; 12, aperture leading into the 1 ventricle ; 13, commencing corpus callosum.

2. The thalamencephalon with its cavity or third ventricle, the primary ocular pedicles, and the infundibulum.

3. The mcsenceplialon, which is the same with the original middle vesicle, and comprises the corpora quadrigemina and crura cerebri with its contracted internal hollow, the iter a tertio ad quartuni ventriculum of human anatomy.

4. The next part in succession is the cerehellimi, along with which is included the pons Varolii and the fourth ventricle.

5. The hinder part, which passes into the spinal marrow, is the medulla oblongata, with the continuation of the medullary cavity in the fourth ventricle and into the central spinal canal.

In these five fundamental parts or rudiments of the brain, arising out of very simple modifications of the primitive medullary tube, it is mainly by an increased thickening of the medullary wall in some of the parts, and the relative thinning, or even the removal of the substance in others, that the changes accompanying the formation of the cerebral masses are effected, while as a consequence of these and other modifications of form, the several parts of the internal cavity or ventricles of the brain acquire the different degrees of expansion and contraction, or the comparatively closed or open condition which they exhibit in after life. Thus the cerebral hemispheres, and corpora striata are the main masses formed by the lateral thickening and expansion of the medullary walls of the procerebrum, while the corpus callosum and fornix are formed later by a deeper median development in connection with these parts : the thalami optici are the most solid parts of the lower and lateral region of the second rudiment, while in the upper wall the pineal gland, and in the lower the infundibulum with the hypophysis cerebri, are added : the corpora quadrigemina are thickenings of the upper wall of the third rudiment, while the crura cerebri arise by increased deposit in its lower part ; the cerebellum is a large deposit in the upper wall of the fourth radiment, while the pons Varolii is a thickening of its lower wall ; and the parts composing the medulla oblongata are principally formed by increased deposit in the lower and lateral wall of the fifth rudiment.

Thus, also, the lateral ventricles are two lateral expansions of the forepart of the original ventricular cavity which follow the dilatation of the vesicles of the right and left cerebral hemispheres, and communicate with the central or third ventricle by the common foramen of Monro. The central or third ventricle, originally the foremost part of the medullary hollow, is narrowed on the sides by the increased development of the thalami optici, while inferiorly it is prolonged and projects downwards as infundibulum into the pituitary fossa ; and above the wall of this ventricle comes to be opened up by the thinning away of its medullary substance, excepting at the place where it is crossed by the pineal gland. The continuation backwards of the original ventricular hollow, greatly narrowed by the ultimate thickening of the substance of the corpora quadrigemina and crura cerebri, forms the aqueduct of Sylvius, or the iter a tertio ad quartura ventriculmn, and is succeeded by the more expanded cavity of the fourth ventricle, lying between the cerebellum and the lower wall. The upper wall of the latter cavity undergoes great thinning, like that of the third ventricle, so as to be reduced m the part before the cerebellum to the valve of Vieussens, and in the part behind it to be covered only by membrane, and to present an opening from the cavity into the posterior sub-arachnoid space.

Fig. 555. — Vertical Section of thr Brain of a Human Embryo of fourteen weeks, Magnified THREE Diameters (from Reicliert).

c, cerebral bemispliere ; cc, carpus callosum beginning to pass back , /, foramen of Monro ; p, membrane over tbe third ventricle and the pineal gland ; tk, thalamus opticus ; 3, third ventricle ; I, olfactory bulb ; cq, corpora quadrigemina, mesencephalon : cr, crura cerebri, and above them the aqueduct of Sylvius still wide ; c', cerebellum, and below it the fourth ventricle ; _/)c, Pons Varolii ; m, medulla oblongata.

From what has before been said of the relation of the fundamental parts of the brain to the basis of the skull, it will be seen that the cerebral development is intimately connected also with the great cranial flexure which occurs at the pituitary fossa ; for while the infundibular prolongation of the thalaraencephalon projects down into this fossa, and the lamina terminalis rises in front in the position of the original foremost part of the encephalon, certain parts of the brain may be considered as situated posterior to this point, viz., the mesencephalon with crura cerebri, cerebellum with pons Varolii, and medulla oblongata, while the cerebral hemispheres, with the corpora striata, corpus callosum, and fornix, notwithstanding their enormous proportional development, may be considered as formed by forward expansion, and as situated in front of this turning point. But though the connections of the cerebral hemispheres with the rest of the brain may thus be considered as anterior to the cranial centre, and while in their earlier stages, and still of small size, they are actually placed as in the lowest Vertebrates, entirely in front of it, yet by the later great proportional development in the higher animals, and especially in man, the cerebral hemispheres come to progress backwards, and successively to cover superiorly the thalami, corpora quadrigemina, the cerebellum, and the medulla oblongata.

The connection of the several parts of the brain, with the five fundamental parts respectively, may be stated in the following tabular form :

I" Cerebral Hemispheres, Corpora Striata,

{1. Prosencephalon.* s Corpus Callosum, Fornix, Lateral Ven(. tricles, Olfactory bulb (Rhiuencephalou). 2. Thalamencephalon. f Thalami Optici,Pinealglaud,Pituitai-y body, (Diencephalon.) ( Third Ventricle, Optic nerve (iirimarily).

ir Middle urimarv Vesicle •? Mesenceiilnlon J Corpora Quadrigemina, Crura Cerebri, Aque11. -MKime primal y vesicle, o. Jiebencepiiaion. j duct of Sylvius, Optic nerve (secondarily).

III. Posterior primary Vesicle,

< Cerebellum, Pons Varolii, anterior part of f- 4. Eiicncephalon. \ the Fom-th Ventricle.

"j ( Medulla Oblongata, Fourth Ventricle, Au 1.5. Meteneephalon. ( ditory nerve.

The changes which take place in the growth of the brain were first elaborately described by Tiedemann ; they have been investigated by Von Baer, Bischoff, Remak, Reichert, Kolliker, and others. (Tiedemann. " Anatomic und Bildiingsgeschichte des Gehu-ns," Nilremberg, 1816 ; Reichert, Bau des Menschlichen Gehirns," Leipzig, 1859, 1861 ; F. Schmidt, '• Beitragez. Entwick. des Gehims," in " Zeitschr. f. Wissen. Zool.," 1862 ; Kolliker, " Entwicklmig,sgeschichte," 1861,)

FARTHER DEVELOPMENT OF THE BRAIN IN MAN AND MAMMALS.

The medulla oblongata is not completely closed in above bj^ nervous matter. The open part of the medullary tube constitutes the floor of the fourth rent ride, which communicate.^ below with the canal of the spinal cord at the place where the calamus scriptorius is eventually formed, and opens posteriorly into the subarachnoid space.

• TliLs and the four following terms are adopted as applicable to the principal secondary divisions of the primordial medullary tube, and a.s corresponding to the commonly received names of the German embryologists, viz., Vorderhirn, Zwischenliirn, Mittelhirn, Hinterhirn, and Nachhirn ; or their less used English translations, viz., forebrain, interbraiu ('tweenbrain), midbrain, hiudbrain, and afterbrain.

The three constituent parts of the medulla oblongata beg-in to ]je distrnguisheJ about the thii-d month ; first the rr.stiform bodies, which are connected with the commencing cerebellum, and afterwards the anterior pjTamids and olives. The anterior puminidx become prominent on the surface and distinctly defined in the fifth month ; and by this time also their decussation is evident. The oUranj fasciculi are early distinguishable, but the proper olirary hochj. or tubercle, doe's not appear till about the sixth month. The fam'wlai c'merea; of the fourth ventricle can be seen at the fomth or fifth month, bub the nvhltc strue not until after birth. •

Cerebellum

In the human embryo the cerebellum exists at the end of the second month, as a delicate medullary lamina, forming an arch behind the corpora quadrigemina across the widely open primitive medullary tube.

According to Bischoff, the cerebellum does not commence, as was previously supposed, by two lateral plates which grow up and meet each other in the middle line ; but a continuous deposit of nervous substance takes place across this part of the medullary tube, and closes it in at once. This layer of nervous matter, which is soon connected with the coi-pora restiformia, or inferior peduncles, increases gradually up to the fourth month, at which time there may be seen on its under sui'face the commencing corpus th-ntatvm. In the fifth month a division into five lobes has taken place ; at the sixth, these lobes send out folia, which are at first simple, but aftei-wards become subdivided. Moreover, the hem i. '<])?! eres of the cerebellum are now relatively larger than its median portion, or worm. In the seventh month the organ is more complete, and thejfocoihis and 2Mt<terior rein in, with the other parts of the mferior vennifoiin process, are now distinguishable, except the amyrjdalo', which are later in theii' appearance. Of th.ej}eil-incles of the cerebellum, the inferior pair (corpora restiformia) are the first seen — viz., about the thii-d month ; the middle peduncles are perceptible in the foui-th month ; and at the fifth, the superior pedmicles and the Vieussenian valve. The pons VfiroHi is formed, as it were, by the fibres from the hemispheres of the cerebellvun embracing the pyramidal and olivary fasciculi of the meduUa oblongata. According to Baer, the bend which takes place at this part of the encephalon thrusts down a mass of nervous substance before any fibres can be seen ; and m this substance transverse fibres, continuous with those of the cerebellum, are afterwards developed. From its relation to the cerebellar hemispheres the pons keeps pace with them in its gi'owth ; and. in conformity with this relation, its transverse fibres are few, or entirely wanting, in those animals in which there is a con-esponding deficiency or absence of the lateral parts of the cerebellum.

Fig. 556. — Brain and Spinal Cord exposed feom behind A FcETus OF THREE MONTHS (from Kolliker).

h, the hemispbei-es ; in, the mesencephalic vesicle or corpora qnadrigemina, c, the cerebellum ; below this are the medulhi oblongata, mo, and fourth ventricle, with remains of the menibrana obturatoria. The spinal cord, s, extends to the lower end of the sacral canal and presents the brachial and crural enlargements.

Parts connected with, the Mesencephalon. — The eorjioni qvndricfemina are formed in the upper part of the middle cephalic vesicle ; the hollow in the interior of A^diich communicates with those of the first and thhd vesicles. the coi-pora qnadrigemina, in the early condition of the human embryo, are of great proportionate volume, in harmony with what is seen in the lower vertebrata ; but subsequently they do not grow so fast as the anterior parts of the encephalon, and are therefore soon overlaid by the cerebral hemispheres, which at the sixth month cover them in completely. Moreover, they become gradually solid by the deposition of matter within them : and as, in the meantime, the ccrchval jwdunclcs are increasing rapidly in size in the floor of this

Fig. 557. — Brain of the Human EmBRYO OF THREE MONTHS. NATURAL SIZE (from Kolliker).

In 1 the view is from above, the Upper fiart of the cerebral hemispheres and mesencephalon having been removed. /, fore-part of the divided wall of the hemisphere ; /', hind part of the same turned in which becomes the hippocampus ; est, corpus striatum ; tha, thalamus opticus.

In 2 the lower surface is represented \ to, ti'actus opticus ; and in front of this the olfactory bulbs and tracts ; c m, single mass of the corpora mammillaria not yet divided ; p, pons Varolii. The cerebellum and medulla oblongata are seen behind and to the sides in both figures.

middle cephalic vesicle, the cavity in its interior is quickly filled up, with the exception of the narrow passage named the SijJrian aqueduct. The fillet is distinguishalble in the fourth month. The corpora quadrigemma of the two sides are not marked off from each other by a vertical median groove until about the sixth month ; and the transverse depression separating the anterior and posterior pau-s is first seen about the seventh month of intra-uterine life.

Fig.. 558. — Brain and Spinal Cord op a Foetus of four MONTHS, seen from BEHIND (from Kolliker).

h, hemispheres of the cerebrum ; m, corpora quadrigemina or mesencephalon ; f, cerebellum ; m o, medulla oblongata, the fourth ventricle being overlapped by the cerebellum ; s s, the spinal cord with its brachial and crural enlargements.

Thalamencephalon

From the sides of this vesicle, as has already been described, the optic vesicles are fonned. and from its forepart on the two sides the vesicles of the cerebral hemispheres are developed. Reichert first pointed out that there is left between the hemu2)?ier(Tcslclcs of opposite sides a wedge-shaped interval, which forms the thu-d ventricle. He points out that the tenninal extremity (lamina terminalis) of the cerebrospinal tube is at the tip of this wedge, and is placed immediately in front of the optic commissure, at the lamina cinerea ; and that therefore the infundibulum is not that extremity, as had been previously supposed by Baer, but is an expansion of the vesicle downwards. The formation of the pituitary body has akeady been described. The infundibulum of the thalamencephalon

becomes connected with it superiorly, and seems for a time even to form a part of it.

The pineal gland, according to Baer, is developed from the back part of the thalami, where those bodies continue joined together ; but it is suggested by Bischoff that its development may be rather connected with the pia mater. It was not seen by Tiedemann until the fourth month in the human foetus ; subsequently its growth is very slow : and it at first contains no gritty deposit : this, however, was found by Sommeiiing at birth.

The two optic thalamic fonned from the posterior and outer part of the anterior vesicle, consist at first of a single hollow sac of nervous matter, the cavity of which communicates on each side in front with the anterior of the commencing their interior behind, below, and at the sides, the thalami become solid, and at the same time a cleft or fis&ure appears between them above, and jjenetrates down to the internal cavity, which continues open at the back part opposite the entrance of the Sylvian aqueduct. This cleft or fissure is the tJiird rentriclc. Behind, the two thalami continue united by the jyoxfrrinr C(i/ii»iix.\ure, -which, is distinguishable about the end of the third month, and also by the ped7i?icle.i of the 2>i»ii glnnrl. The Koft commissure probably exists from an early period, although it could not be detected by Tiedemann until the ninth month.

Fig. 559. — Remimagrammatic Views of the Inner Surface op the Right Cerebraii Hemisphere of the Fcetal Brain at various stages of development (from Schmidt).

1, 2, and -3, are from foetuses of the respective ages of eight, ten, and sixteen M'eeks ; 4, from a foetus of six months, a, lamina terminalis or imrt of the first primary vesicle which adheres to the sella turcica ; b, section of the cerebral i)eduncle as it passes into the thalamus and corpus striatum ; the arched line which surrounds this bounds the gi-eat cerebral fissure ; c, anterior part of the fornix and the sej)tum lucidum ; d, inner part of the arch of the cerebrum, afterwards the hippocampus major and posterior part of the fornix ; (, corpus callosum very short in 3, elongated backwards in 4 ; in 4, /, the superior marginal convolution ; /', fronto-parietal fissure ; g, gyrus fornicatus ; p', the internal vertical fissure descending to meet the fissure of the hippocampus ; I, olfactory bulb ; F, P, 0, T, frontal, parietal, occipital and temporal lobes.

cerebral hemispheres, and behind with that of the middle cephalic vesicle (corpora quadrigemina). Soon, however, by increased deposit taking jilace in

Fig. 560. B

560. — The Surface of the Fcetal Brain at Six Months (from R. Wagner).

This figure is intended to show the commencement of the formation of the principal fissures and convolutions. A, from above ; B, from the left side. F, frontal lobe ; P, parietal ; O, occipital ; T, temporal ; a, q, a, slight appearance of the several frontal convolutions ; s, the Sylvian fissure ; s', its anterior division ; within it, C, the central lobe or convolutions of the island ; r, fissure of Rolando ; p, the vertical fissure (external part).

At an early period the ojriie tracts may be recognised as hollow i^rolongations from the outer part of the wall of the thalami while they are still vesicular. At the fourth month these tracts are distinctly fonned. They subsequently are prolonged backwards into connection with the coi-pora quadiigemina.

Prosencephalon

Each hemispherevesicle becomes divisible into two parts : one of these is the part which from the interior appears as the corpus striatum, and from the exterior as the island of Eeil, or central lobe ; the other fonns the expanded or covering portion of the hemisphere, and is designated by Reichert the mantle. The aperture existing at the constricted neck of the hemispherevesicle, Schmidt and Reichert have recognised as the foramen of Monro.

The corpora striata, it will be observed, have a different origin from the optic thalami ; for, while the latter are formed by thickening of the cu-cumferential wall of a part of the first cerebral vesicle, and thus correspond in their origin with all the parts of the encephalon behind them, which are likewise derived from portions of the cerebro-spinal tube, the corpora striata appear as thickenings of the floor of the hemisphere-vesicles, which are lateral ofl:'-shoots from the original cerebro-spinal tube. On this account, Reichert considers the brain primarily divisible into the stem, which comprises the whole encephalon forwards to the taenia semicircularis, and the hemisphere-vesicles, which include the corpora striata and hemispheres.

Fig. 561. — View of the Inner Surface of the Right Half OF THE Foetal BrAIN OF ABOUT SIX MONTHS (from Reichert).

F, frontal lohe ; P, parietal ; 0, occipital ; T, temporal ; I, olfactory bulb ; II, right optic nerve ; f p, fronto-parietal fissure ; p, vertical fissure ; ;/, internal vertical fissui'e ; /(, hippocampal fissure ; ff, gyrus formicatus ; c, c, corpxis callosum ; s, septum lucidum ; /, placed between the middle commissure and the foramen of Monro ; V, in the upper part of the third ventricle immediately below the velum interpositum ami fornix ; v', in the back part of the third ventricle below the pineal gland, and pointing l)y a line to the aqueduct of Sylvius ; v" , in the lower part of the third ventricle above the infundibuhim ; )•, recessus pinealis passing backwards from the tela choroidea ; p r,pons Varolii ; C c, cerebellum.

The cerebral hemispheres enlarging, and having their walls increased in thickness, fonn, dming the fourth month (Tiedemann), two smooth shell-like lamellfe. which include the cavities afterwards named the lateral ventricles, and the parts contained within them. Following out the subsequent changes affecting the exterior of the cerebral hemispheres, it is found that about the foiuth. month the first traces of some of the conrolvtions appear, the intermediate svlci commencing- only as very slight depressions on the hitherto smooth sm-face. Though the hemispheres continue to grow quickly upwards and backwards, the convolutions at first become distinct by comparatively slow degrees ; but towards the seventh and eighth months they are developed with gi-eat rapidity, and at the beginning of the last month of intra-uterine life, all the principal ones are marked out.

The Sf/!r!fi)i _p\\:finv, which afterwards separates the anterior from the middle lobe of each hemisphere, begins as a depression or cleft between them about the fourth month, and, after the great longitudmal, is the first of the fissures to make its ajipearance. It is followed by the fissure of Rolando, and the vertical or parieto-occipital fissure, and somewhat later by the internal fronto-parietal fissui-e. After this, the various subordinate fissures dividing the convolutions gradually make their appearance. By the end of the third month the hemispheres have extended so far backwards as to cover the thalami ; at the fourth they reach the corpora quadrigemina ; at the sixth they cover those bodies and great part of the cerebellum, beyond which they project still fiuther backwards by the end of the seventh month.

During the growth of the hemisphere the aperture of the foramen of Monro is extended backwards ; the arched margin of this opening is curved downwards at its extremities, and forms anteriorly the fornix, and posteriorly the corpus fimbriatum and hippocampus major ; above the margin a part of the wall of each hemisphere comes into contact with its fellow, and in the lower part forais the septum lucidum, while above this the hemispheres are united by the development of the great commissure, the coipus callosum.

The corpus callo.sum is described by Tiedemann as being first seen about the end of the third month, as a narrow vertical band, extending across between the forepart of the two hemispheres, and subsequently growing backwards. With this view the observations of Schmidt coincide. Reichert, however, maintains that the commissural structure seen at the forepart of the hemispheres is the anterior white commissure, and that the coiiaus callosum appears in its whole extent at once.

The corpora alhtcantui at fixst form a single mass : so also do the anterior pillars of the fornix, which are distinguished before the posterior pillars. The posterior pillars are not seen until the fourth or fifth month. At that period the hippocampus minor is also discernible.

DEVELOPMENT OF THE NERVES.

Spinal Nerves

Very little is yet known as to the mode of origin of the nerves. In their peripheral extension the great majority of them seem to arise more immediately from mesoblastic formative elements, and the manner in which this takes place has been adverted to in the General Anatomy at p. 161. The ganglia and roots of the spinal nerves are first seen to make their appearance in some very close association with the protovertebral segments. In this the ganglion comes to be distinguishable as a mass by itself, and the anterior and posterior roots follow, with their jimctiou in the part forming the nerve-trunk beyond the ganglion. But according to recent observations by Balfour in ElasmoBranch fishes (Scyllium and Torpedo), it would appear that both the anterior and posterior roots may arise iu these animals in a closer connection with the nervous centre than was previously believed, and as independent outgrowths from the involuted epiblast of the neural canal. The posterior roots are the first to appear, and commence by an outgrowth at the summit (dorsal median groove) of the neural canal, and gradually pass outwards from thence to reach their permanent place of origin in a posterior lateral furrow. A subsequent division of the nerve rudiment takes place into root, ganglion, and a part of the nerve beyond.

The anterior roots spring by an outgrowth from the antero-lateral angles of the cord, one for each muscular plate nearly in the place which they permanently occupy, and after they have attained some size they iniite with the posterior roots beyond the ganglion.

But although the roots of the nerves may thus be traced in their commencement to elements of epiblastic nature, it is probable that their sheaths and blood-vessels arise from mesoblastic tissue introduced later into them. The ganglia are at first of proportionally very large size, causing even a considerable part of the projections on the surface of the body, as in the human embryo of from four to seven weeks, which are usually attributed to the vertebrate segmentation (Kolliker). The union of the roots with the grey matter soon becomes apparent, being most obvious in the anterior roots.

The nerves, like the other elementary parts, are at first composed entirely of cells, but fibres are soon formed by transformation of the cells. On the sixth day in the chick, Foster and Balfour found the fibres developed, but were unable to trace them into connection with the ganglionic cells, but at a later period the connection was observed by Lockhart Clarke to be formed.

The very early development of the trunks of the nerves of the limbs, and their progress outwards into the first part of the commencing limb, were observed and figured by Remak.

Cranial Nerves

The optic nerve and the retina, arise from epiblast by an extension of the primary medullary wall of the brain, as already described, and may therefore, in some sort, be regarded as an extension of the brain itself. The olfactory tract and bulb are still more to be looked upon as constituent parts of the cerebrum ; but the manner in which the peripheral olfactory nerves which pass through the cribriform plate into the nose are formed has not been yet observed.

Among the other cranial nerves there are four important ones of which the rudiments are seen to be formed at an early period ; taking their origin as has been supposed in the mesoblastic wall of the cranial cavity, and extending thence into the facial or visceral plates. These are the fifth pair or trifacial, the facial or portio dura of the seventh pair, the glossopharyngeal and the pneumogastric nerves. The two first of these are situated in front, and the two last behind the otic vesicle and tympanoeustachian passage ; and according to Parker each of these nerves forks or divides into two above one of the visceral clefts. Thus the fifth pair gives its naso-maxillary division in fi-ont, and its infero-maxillary division behind the oral cleft ; the facial gives its vidian or superior petrosal before and its descending part behind the tympano-eustachian passage ; the glosso-pharyngeal has its lingual and pharyngeal branches divided by the first branchial cleft, and in animals the pneumo-gastric is similarly divided at the second and succeeding clefts.

The third, fourth and sixth pairs of nerves are of subordinate importance, and may be considered as related, the two first to the fifth pair, and the last to the facial nerve. Their peripheral parts are developed in connection with the muscles of the eyeball, but the mode of the formation of their roots in connection with the nervous centres has not been ascertained.

The hypoglossal nerve, although it passes through the exoccipital bone in man, may be compared to a spinal nerve, and probably takes its origin much in the same manner.

The Sympathetic Nerves. — Remak observed the development of the great sympathetic nerves from the lateral plates in loops or arches connecting them with the spinal nerres ; wlnle the great sympathetic itself consisted at first of a chain of rounded masses representing; the ganglia, connected together, but so closely set that scarcely any intervening nervous cord was at first perceptible. He also observed the separate formation in the mesentery of birds of the large visceral nerve which he discovered in these animals. The gangliated cord of the sympathetic has been described and figured by Kiilliker in the human foetus of eight or ten lines long. The peripheral sympathetic nerves are also formed at a very early period, and are perceptible in a foetus of three months. In the hinder part of the abdomen their origin appears to be intimately connected in some way with the formation of the suprarenal bodies.

DEVELOPMENT OP THE EYE

The embryonic structures forming the eyeball and its contents may be considered as proceeding from three sources, viz., 1st, by evolution or expansion from the medullary wall of the first encephalic vesicle (thalamencephalon), giving rise to the retina, in its nervous and pigmental structure and optic nerve ; 2nd, by involution or depression and development of a part of the cuticular epiblast, forming the foundation of the lens and the epithelium of the conjunctiva ; and 3rd, by the intrusion of mesoblastic elements between and around the other parts, so as to furnish the materials out of which are formed the general coverings of the eyeball, cornea and sclerotic, the fibrous and vascular choroid, the ciliary apparatus and iris, the capsule of the lens and the capsulo-pupillary membrane, the vitreous humour, and all the fibrous and vascular parts of the organ.

The very early formation of the primary optic vesicles by the expansion of the lower and anterior parts of the wall of the anterior primary encephalic vesicle has already been described, and the manner in which each of these vesicles forms a hollow pediculated chamber communicating by its stalk with the general ventricular cavity of the primitive brain. The first important change which the primary optic vesicles undergo is connected with the depression of the rudimentary lens from without, and consists in a doubling back or inwards of the medullary wall of each vesicle, so as to form a depression or cup at its lower and anterior part, into which the commencing lens in part sinks. This depression has been called the secondary optic vesicle, or the opfic cup (Foster and Balfour). From a very early period the outer fold of this cup undergoes a much greater thickening by the rapid development of its cellular constituents than the inner or that towards the brain, and from this, as well as the increase of the inward folding, the original cavity of the primary optic vesicle becomes in a great measure obliterated or narrowed, and the outer and inner folds are closely approximated, while the stalk or pedicle becomes proportionally much diminished. The continued increase of cellular development in the outer fold of this cup leads to the formation of the various elements composing the retina ; while in the thinner inner fold only pigment cells are formed. The transition at the bend from the thick nervous part tc

Fig. 562. — Section of the Head through the Primitive Optic Capsule uf one side in an Embryo-Calf of 9 mm. long, MAGNIFIED (from Julius Arnold).

To the right is seen the optic capsule with its contracted pedicle and its outer wall depressed by the thickening of the corneous layer which forms the commencement of the formation of the lens. The optic stalk is in communication with the thalamencephalon. Mesoblast is seen between the optic capsule and the lens rudiment.

Fig. 563. — Section through the Eye op an Embryo-Calf OF TWELVE 3IM., OR HALF AN INCH, LONG, MAGNIFIED (froui Julius Amold).

The lens follicle Ls now closed in and detached fi'ora the corneous laj'er, and its cavity contains loose cells which are the remains of the superficial corneous cells. The oj^tic vesicle or cajisule is now completely doubled back, so as to present towards the lens side the secondary ocular capsule or cup ; its outer wall now much thickened by the commencement of the development of retina. Mesoblastic tissue is seen to have passed in from the periphery between the optic capsule and the lens, as well as in front of the lens. The commencement of a vascular circle is shown round the exterior of the ocular capsule, and extending also between the lens follicle and the cuticle.

the thin pigmental part is quite sudden, and as soon as pigment cells begin to be developed, a very marked distinction is perceptible between it and the nervous structure of the retina. These cells were formerly regarded as a part of the choroid membrane, but they are now looked upon as belonging rather to the retina, — a view which is supported by the mode of development now described.

Fig. 564. — Diagrammatic Sketch op a Vertical Longitudinal Section THROUGH THE EyEBALL OP A Human Foetus OF FOUR WEEKS (after Kolliker).

The section is a little to the side so as to avoid passing through the ocular cleft, c, the cuticle, where it becomes later the cornea ; I, the lens ; o ij, optic nerve formed by the pedicle of the primary oj)tic vesicle ; v 2'>, primary medullary cavity of the optic vesicle ; p, the pigment-layer of the outer wall ; r, the inner wall form- . ing the retina ; v s, secondary optic vesicle containing the rudiment of the vitreous humour.

The fold which produces the optic cup proceeds from above downwards, and when the lens is formed it seems as if it enclosed the lens but left for a time an aperture or depression below. This is the choroidal fold or fissure, which may easily be distinguished in the embryo-head after pigment has been deposited, from the circumstance that the pigment is absent from the cleft, which thus appears for a time as a broad white line, particularly obvious in the embryo bird, running from the circumference in upon the lens.

The lens is developed in the part of the cuticle opposite to the -most projecting part of the primary optic vesicle, or at the place where this vesicle comes in contact with the surftice of the head. In this situation there is seen from a very early period a thickening of the epiblast, which seems to reside chiefly in its deeper layer of cells, and in birds and mammals it would appear that an actual involution of the cuticle takes place, so that first an open follicle and next an enclosed ball of cuticle is formed. Although, however, both the corneous and the deeper layer (sensory of Strieker) of the cuticle are enclosed, it is only the cells of the deeper layer which undergo development into the fibres of the lens. The ball of the lens separating from the external cuticle, which passes freely over the surface, a cavity filled with loose cells, the remains of cells of the corneous layer, exists for a time within the lens ball. Then the cells of the hinder or inner wall are seen to rise from the bottom by their elongation, and thus a rapid growth of fibres from that side of the ball takes place, while the anterior or outer wall undergoes no similar change, but retains its simply cellular structure. Figures 565, and 566, show sufficiently clearly the manner in which the fibres thus developed from cells rise fi'om the bottom of the lens ball and come to constitute the solid part of the lens.

The optic cup receives the enlarging lens in its anterior and lower opening, and the reflected margins of the cup closely embrace the margin of the lens ; but there is a considerable space intervening between the lens and the hollow of the optic cup (or secondary ocular vesicle), which comes to be occupied by the vitreous humour. Into this space connective tissue and blood-vessels developed from mesoblastic

Fig. 565. — Sectiojt of the Eye in an Embryo-C^lf OF 18 MM. LONG, MAGNIFIED (from JuUus Amold).

The posterior or inner v.-all of the lens follicle is now much thickened by the elongation of its fibres, each of which presents a nucleus, and the whole causing a bulging of the posterior wall. The outer la^'er of the lens capsule consists of columnar cells. The cavity of the lens follicle is still visiljle, but is now widened and flattened. Two layers of mesoblastic tissue are now visible between the lens and the cuticle, viz., a deeper vascular layer and a superficial non-vascular one containing nuclei. The secondary optic capsule is now occupied bj' connective tissue nuclei and numerous blood-vessels. The retinal section of the primitive ocular vesicle is now thicker. Pigment begins to ajapear in the choroidal section, and numerous bloodvessels surrovmd the whole exterior.

elements are projected from below, so as ..'^,|y| to furnish the materials for the formation

^ '"^r of the vitreous humour and the blood-vessels

-W which pass through it to the lens, and also to surround the lens with vascular and fibrous elements, out of which are produced the capsulo-pupillary membrane, and probably also the capsule of the lens. It results from the observations of Lieberkiihn that in mammals the fold which produces the ocular cup or secondary vesicle runs back into the stalk so as to fold in the optic nerve for a considerable space, and by the simultaneous intrusion of mesoblastic tissue, thus to lead to the introduction of the central blood-vessels of the retina within the nerve. But in birds,, according to the same observer, no such infolding of the stalk occurs, so that in them the vessels are excluded from thenerve. The malformation termed coloboma iridis is to be attributed to a persistence of the choroidal cleft or fold, and the pecten of birds, close to the optic nerve, with the vascular fold farther forwards, and the falciform fold of the eyes of fishes are to be regarded as fibro-vascular structures formed by original projection through the same fold.

The further development of the parts of the eye may be briefly stated as follows : —

The expansion of the ocular cup continuing to proceed, the chamber for the vitreous humour enlarges, and that structure gradually comes to occupy its space between the retina and the lens.

The marked distinction between the nervous and the pigmental portions of the primitive ocular vesicle goes on increasing by the continued deposit of pigment in the latter, and its proportional thinning, and by the great addition to the thickness and the textural differentiation of the substance of the former. Thus the cells in the retinal or nervous portion, by their rapid multiplication, soon l^ecome several layers thick ; certain parts of these cells assume the spindle shape, and exhibit elongation into fibres, while others retain the nuclear form, and thus there is foreshadowed the division into the fibrous, ganglionic, and nuclear layers of the retina. On the exterior a limiting membrane

Fig. 566. — Section of the Eye of AN EjIERYO-CaLP of 30 MM. LONG, MAGNIFIED (froiu Julius Arnold).

the cavity of the lens is much reduced in size from the increased development of fibres from behind. The intersection space begins to be formed posteriorly, and the zone of nuclei is thrown forward. The cornea is now formed, covered externally by the cuticular epithelium, and with the separation of the aqueous chamber internally. Close to the lens is the caiDsulo-pupillary membrane, which is continuous with the vascular structure occupying the secondary ocular capsule or cavity for the vitreous humour, and with the choroid membrane round the margin of the ocular capsule, where

iris and ciliary processes will afterwards be formed. The layer of pigment is now more developed, and the tissue of the sclerotic is begun to be formed. The eyelids are beginning to project as folds of the skin.

makes its appearance, and in connexion with it the rudiments of the cells composing the layer of rods and cones. The space between the retinal and pigmental layers rapidly contracts, and finally the rods and cones are closely united with the layer of pigment cells.

The optic nerve, as already described, is at first connected by its origin with the vesicle of the third ventricle or thalamencephalon, and for a time it retains its original hollow form. But as the cerebral hemispheres are developed forwards, the eye and the optic nerve are thrown backwards and downwards, and a new connection is established between the optic nerve (or tract) and the vesicle of the midbrain (mesencephalon) : the rudiment of the optic commissure is at the same time formed by the median approximation of the stalks and the growth of one over the other. Each stalk then becomes more and more solid by the development of nerve fibres along with the intruded connective tissue which forms the sheath substance of the nerve.

Lens

The development of fibres from the hinder wall of the primitive lens-follicle continuing to take place, the cavity of the follicle is first greatly narrowed and then completely filled up by the lengthening fibres, and the lens takes more and more of its fall spherical shape. The new fibres continue to be formed towards the margin of the lens ; each fibre retaining its nucleus, so as to produce the "nuclear zone which runs through the whole lens. This zone is at first situated far back in the lens while the fibres are still short, but as they elongate, its place is advanced, so that it comes to be situated considerably in front of the equatorial plane of the lens. It is most distinct towards the margin where the fibres are newly formed. The anterior wall of the lens-follicle remains as a simple cellular layer. The greater number of the fibres now follow the general curve of the surface of the lens, presenting therefore their concavity towards its centre, but the curvature gradually diminishing in those nearest the middle, where they are straight or nearly so. Only the external short and recently formed fibres present a concavity towards the exterior. The intersecting stars of the anterior and posterior poles of the lens now make their appearance by the collection of cells in the peculiarly shaped triradiate space in these two situations, and the ends of the fibres are now traceable to the edges of these spaces, so that the fibres gradually take the arrangement round the poles of the lens which belongs to the adult.

Fig. 567.— Transverse Vertical Section of the Eyeball op a Human Embryo of four weeks (from Kolliker). '""

The anterior half of the section is represented. pr, the remains of the cavity of the primary optic vesicle ; ii, the inflected part of the outer layer, forming the retinal pigment ; r, the thickened inner part giving rise to the columnar and other structures of the retina ; r, the commencing vitreous humour within the secondary optic vesicle ; v', the ocular cleft through which the loop of the central blood-vessel, a, projects from below ; 1, the lens with a central cavity.

The capsule of the lens, according to Lieberkiihn's and Julius Arnold's most recent observations, owes its origin to the thin innermost pellicle of mesoblast which is introduced atan early period of development between the lens and the secondary ocular vesicle.

Cornea

There is at first no aqueous chamber in the eye, and even after the solution of continuity which gives rise to this space lias occurred, the cavity is not dilated with fluid, till near the time of ])irth. Even then it is very shallow and the lens is placed very near to the cornea. The formation of the cornea is due to a differentiation of the tissue in the layer of mesoblast whichis introduced from the neighbouring wall of the head, between the primitive lens-follicle and the corneous epiblast, the cavity of the aqueous humour arising by the separation of the corneous part from a layer of the mesoblastic tissue lying within it. The latter gives rise to the anterior part of the vascular capsulo-pupillary membrane, while a still deeper lamina closely embracing the lens, remaining non-vascular, is converted into the lens capsule. Along with the latter is also formed the suspensory ligament of the lens.

Vitreous humour

The enlargement of the space for the vitreous humour progressing, the cells of the mesoblast which form its foundation become stellated and very sparse from the effusion of a large quantity of fluid, and the hyaloid membrane surrounding this structure takes its origin from the same mesoblastic elements.

Choroid and other membranes

The mesoblastic substance which surrounds the ocular vesicle externally is the source of a number of important parts. Among these may be mentioned first the choroid membrane, the cellular (membrana fusca), fibrous, and vascular layers of which are developed out of the deeper division of the mesoblastic substance, and to the same source may be traced in a later stage of development the ciliary processes, ciliary muscle and iris ; while the zonula ciliaris may be regarded as a part of the deeper mesoblastic tissue connected with the formation of the hyaloid membrane and membrana capsulo-pupillaris. The folds of the ciliary processes gradually increasing, encroach upon the space outside the margin of the lens and indent the zonula ciliaris and canal of Petit.

Fig. 568. — Blood-vessels of the CapSULO-PUPILLARY MEMBRANE OF A New-born Kitten, magnified (from Kolliker).

The drawing is taken from a preparation injected by Tiersch, and shows in the central part the convergence of the net-work of vessels in the pupillary membrane.

The sclerotic coat is due to a process of differentiation occurring in the outer layer of the enveloping mesoblastic tissue, which occurs considerably later than those which bring the choroid and the cornea into existence, but there is from the first continuity between the corneal tissue and that of the sclerotic coat.

The capsulo-pupillary membrane, already referred to, may be looked upon as at first a complete fibro-vascular investment of the lens, which owes its origin to the deepest pcart of the intruded mesoblast. The vessels of this membrane are svipplied by a branch of the central artery of the retina, which passes foi"n-ards in the axis of the globe, and breaks up at the back of the lens into a brush of rapidly subdividing twigs. The forepart of this tunic, adherent to the pupillary margin of the iris, forms the jnipUhir// mcmhrane by which the aperture of the pupil is closed in the middle periods of foetal life. In the human subject, the whole tunic, to<^ether with the artery which supplies it, becomes atrophied, and is lost sight of before birth, but in some animals it remains apparent for a few days after birth. According to Kolliker, the anterior chamber expands only a short time before birth by the intervention of the aqueous humour between the iris and cornea.

The eyelids make their appearance as folds of integument, subsequently to the foi-mation of the globe. "When they have met together in front of the eye, their edges become closely glued together ; and they again open before birth.

The lachrymal canal may be regarded as a persistently open part of the fissure between the lateral frontal process and maxillai-y lobe of the embryo.

The iii'st discovery of the mode of development of the eye as it is now generally understood was made by Husclike in 1832, and was published in Meckel's Archiv for that year. In addition to the vaiious systematic works on Development previously quoted, the reader is refen-ed to the following, viz., Lieberkiihn, Uber das Auge des Wirbelthier-enibryo, 1872 ; and Julius Arnold, Beitrage zur Entwick. de^ Auges, Heidelberg, 1874.

DEVELOPMENT OF THE EAR

The first origin of the organ of hearing as an involuted follicle from the superficial epiblast of the head, constituting the primary auditory

Fig. 569. — Outlines showing the Formation of the External Ear in the Fietus.

A, head and upper part of the body of a human foetus of about four weeks (from nature). \' Four branchial plates (the first, forming the lower jaw, is marked 1), and four clefts are shown ; the auditory vesicle (a), though closed, is visible from the transparency of the parts, and is placed beliind the second branchial plate.

B, the same parts in a human fcetus of about six weeks (from Ecker). \ The third r.nd fourth plates have nearly disappeared, and the third and fourth clefts are closed ; the second is nearly closed ; but the first (1') is somewhat widened posteriorly in connection with the formation of the meatus externus.

C, human fcetus of about nine weeks (from nature). \ The first branchial cleft is more dilated, and has altered its form along with the integument behind it in connection witli the formation of the meatus externus and the auricle.

or otic vesicles, has already been shortly described. From numerous cbservations there is now no doubt that both in birds and mammals

Fig. 570. — Transverse and slightly Oblique Section of the Head op a Fietal iSlikep, in the Eegion op the Hind Erain (from Foster and Balfour after Boettclier).

HD, inner surface of the thickened walls of the hind brain ; RL, recessus vestilnili ; VB, commencing vertical semicircular canal ; CC, canalis cochlew, Avith the cavity of the primitive otic vesicle. On the left side parts only of these structures are seen ; GC, cochlear ganglion of the right side ; on the left side, Gr', the ganglion, and N, the auJitoiy nerve connected with the hind brain.

the otic vesicle forms at first for a time a follicle open to the surface, ami that it has therefore no original connection with the nervous centre. Its position is at the side of the medulla oblongata, and in a l)lace opposite to the interval between the first and second postoral visceral arches. The outer opening of the follicle very soon contracts and becomes entirely closed. The follicle sinks down towards the basis of the cranium, and becomes imbedded in the formative mesoblastic tissue lying between the basioccipital and alisphenoid matrices, undergoing chondrification and ossification at a very early period, as has been already described under the development of the head. There are therefore to be distinguished from an early period a part corresponding to the internal membranous labyrinth proceeding from the epiblast, and an outer cartilaginous or bony and fibrous wall, together with other adventitious structures arisingin the mesoblast. Labyrinth. — In the development of the primary otic vesicle after it becomes completely closed, a series of very remarkable changes by extension of its cavity in different directions gives rise to the formation of the different parts of the labyrinth. The first complication which the vesicle exhibits is by the extension of a process upwards and backwards, which remains permanent in the lower vertebrata, but in mammals is obliterated, its vestiges remaining in the aqueduct of the vestibule. The semicircular canals next appear as elongated elevations of the surface of the primary vesicle : the middle portion of each elevation becomes separated from the rest of the vesicle by bending in of its walls under it, and thus the elevation is converted into a tube open at each end, vyhich subsequently becomes elongated and acquires an ampullar dilatation. The cartilage which forms the osseous labyrinth is continuous with that of the rest of the primordial cranium. The cartilaginous walls of the cavity are united by connective tissue to the vesicle ; this connective tissue, according to Kolliker, becomes divided into three layers, of which the outer forms the lining periosteum, the inner forms the external walls of the membranous labyrinth, while the intervening; layer swells up into gelatinous tissue, the meshes of which become wider and wider, till at last the space is left which ultimately contains the perilymph.

Fig. 571. — Labyrinth of tee Human Fcetus of FOUR aveeks, magnified (from Kolliker).

A, from behind ; B, from before ; v, the vestibule ; rv, recessus vestibuli, giving rise later to the aqueduct ; r.s', commencement of the semicii'cular canals ; a, upper dilatation, belonging perhaps to another semicircular canal : c, cochlea.

Fig. 572. — Transverse Section of the Head op a Fcetal Sheep of four-fifths of AN INCH IN LENGTH (from Foster and Balfour after Boettclier).

RV, recessus vestibuli ; VB, vertical semicircular canal ; CC, cochlear canal ; G, cochlear ganglion ; HB, horizontal canal.

Fig. 573. — Tr.ANsvERSE Section of THE Cochlea in a Fcetal Calf, magnified (from Kolliker).

C, the wall of the cochlea, still cartilaginous ; c c, canalis cochlear ; I s, placed in the tissue occupying the place of the scala vestibuli, indicates the lamina spiralis ; n, the central cochlear nerve ; (/, the jjlace of the spiral ganglion ; S, the body of the sphenoid • ch, remains of chorda dorsalis.

Fig. 574. — Views of the Cartilage of Meckel and parts connected WITH THE First and Second Branchial Plates.

A (after Kolliker), bead of a fcetus of about eighteen weeks, showing the cartilage of Meckel in connection with the malleus and the surrounding parts.

M, placed upon the lower jaw indicates the cartilage of IMeckel of the right side.

B (from nature). An enlarged sketch explanatory of the above view ; z, the zygomatic arch ; ma, the mastoid process ; ml, jDortions of the lower jaw of which the parts near the angle and the symphysis have been removed ; M, the cartilage of Meckel of the right side ; M', a .sratdl part of that of the left side, joining the left cartilage at s, the symphysis ; T, the tympanic ring ; m, the malleus ; i, the incus ; s, the stapes ; sta, the stapedius muscle ; st, the styloid process ;p,A,<7, the stylo- j)haryngeus, stjdohyoid, and stylo-glossus muscles ; stl, stylo-hyoid ligament attached to the lesser cornu of the hyoid bone ; hij, the hyoid bone ; th, thyroid cartilage.

The cochlea appears at first as a prolongation downwards from the auditory vesicle, but afterwards becomes tilted forwards. This prolongation of the auditory vesicle is the rudimentary canalis membranacea. Close to it is placed the cochlear nerve, with a gangliform extremity. The canal becomes elongated in a spiral direction, and the ganglion, which is elongated with it, becomes the ganglion spirale. Between the canal and the cartilaginous wall which afterwards surrounds it a large amount of connective tissue intervenes, and in this tissue the cavities of the scala vestibuli and scala tympani gradually appear at a later period, precisely as does the space for the perilympli, in the vestibule. The modiolus and spiral lamina, according to Kulliker, are ossified without intervention of cartilage. Within the canalis membranacea Kulliker finds in the embryo a continuous epithelial lining, thin on the membrane of Reissner and on the outer wall, but forming a thick elevation in the position of the rods of Corti, and a large'r elevation more internally, filling up the sulcus spiralis. On the surface of this latter elevation he has observed a transparent body, the membrane of Corti.

The auditory nerve is said to be developed, separately from both the brain and the labyrinth, in the intermediate mesoblastic wall of the head ; the canal termed meatus auditorius internus being left in the bones round it and the facial nerve. The auditory nerve becomes secondarily connected with the medulla obloug-ata in a manner not yet ascertained, and its fibres are extended into relation with the delicate terminal structiu-es formed in the membranous labyrinth.

Middle and External Cavities of the Ear

It has been already explained that the external meatus, the tympanic cavity, and the Eustachian tube, are formed in the posterior or upper part of the first postoral visceral cleft, which remains permanently open as the tympano-eustachian passage, except at the place where it is interrupted by the formation of the membrana tjmipani ; and also that the malleus is formed in the first visceral plate from the proximal part of Meckel's cartilage, and the incus, stapes, and stapedius muscle and the styloid process probably in the second plate. It is pointed out by Kolliker that during the whole period of foetal life the tympanic cavity is occupied by connective tissue, in which the ossicles are imbedded ; and that only after respiration has been established this tissue recedes before an expansion of the mucous membrane.

The pinna is gradually developed on the posterior margin of the first visceral cleft. It is deserving of notice that congenital malformation of the external ear, with occlusion of the meatus and greater or less imperfection of the tympanic apparatus, are observed in connection with abnormal development of the deeper parts of the first and second visceral plates and the intermediate cleft ; while cases have been observed of the persistence in the neck of the adult of one or more of the branchial clefts situated behind the first. (Allen Thomson, Proceed. Eoy. Soc. of Edin. 1844, and Edin. Journ. of Med. Sc. 1847.)

DEVELOPMENT OF THE NOSE

The organ of smelling, as was first pointed out by V. Baer, owes its origin, like the primary auditory vesicle and the crystalline lens of the eye, to a depression of the integument, or what may be more precisely designated as epiblast. This depression, the primary olfactory groove, is at first encircled by a uniform wall, and is unconnected with the mouth. This staple has been observed by Kolliker in the human embryo of four weeks. The olfactory groove is enclosed in the anterior extremity of the nasal cartilages prolonged forward from the trabeculas cranii (Parker). Soon, however, by the unequal growth of the surrounding parts, the groove so formed, descending from above, passes into the mouth. Thus the middle frontal process is isolated between the grooves of opposite sides, while the lateral frontal process separates the nostril from the eye. The maxillary lobes, growing forwards from behind the eyes, complete the boundaries of the nostrils, which then open into the fore part of the mouth. Kolliker observed this stage in the latter half of the second month. The palate subsequently grows inwards to the middle line, as has been elsewhere stated, and separates the nasal from the buccal cavity ; leaving only the small communication of the incisor foramen. Meanwhile, with the growth of the face, the nasal fossae deepen, and the turbinated bones make their appearance as processes from their walls. The ethmo-turbinal cartilages are at first simple, but rapidly extend themselves to take the more or less complex shape which they present in different animals or in man.

Fig. 575. - Views op the Head op Human Embryoes, illustrating the Development OF the Nose.

A, Head of an embryo of three weeks (from Ecker). 1, anterior cerebral vesicle ; 2, middle vesicle ; 3, nasal or middle frontal process ; 4, superior maxillary process ; 5, eye ; 6, inferior maxillary process or first visceral plate, and below it the first cleft ; 7, 8, and 9, second, third, and fourth plates and clefts.

B, Head of an embryo of about five weeks (from Ecker). 1, 2, 3, and 5, the same parts as in A ; 4, the external nasal or lateral frontal process, inside which is the nasal groove ; 6, the superior maxillary process ; 7, the inferior maxilla ; x , the tongue seen within the mouth ; 8, the first visceral cleft which becomes the outer part of the meatus auditorius externus and tynipano-eustachian passage.

C, View of the head of an embryo of eight weeks seen from below, the lower jaw having been removed (from Kolliker). *

n, the external nasal apertures ; ?', premaxillary or incisor process, and to the outer side of this the internal nasal aperture ; m, one of the palatal processes of the upper jaw, which, advancing inwards from the sides, form the partition between the mouth and nose ; p, common cavity of the nose, mouth, and pharynx.

Observations are still wanting to determine whether the olfactory nerves are developed from the bulbs, and have thus a cerebral origin, or are separately formed from peripheral blastema like all other nerves, with the exception of the optic.

DEVELOPMENT OF THE ALIMENTARY CANAL AND ORGANS ARISING FROM THE HYPOBLAST

The whole alimentary canal, from the fauces to the anus, together with the rudiments of certain organs associated with it in their commencement, viz., the thyroid gland, lungs, trachea and larynx, the liver, and pancreas, as well as the allantois, owe their origin more immediately to inflections of the hypoblast layer of the germinal membrane, which supplies the epithelial lining of their principal cavities ; but in all these organs parts of their structure are supplemented, and some other organs, such as the mesentery and spleen, are wholly formed from the mesoblast, whence proceed the vascular, fibrous, and parenchymatous elements, and also the serous coverings of the organs, where these exist.

ALIMENTARY CANAL,

The primary digestive cavity of birds and mammals, as it extends from one end of the embryo to the other below the vertebral axis, presents at first a manifest division into three parts. One of these^ occupying the part of the embryo which is enclosed by the cephalic fold, and which may be named the foregut, comprises the rudiments of the pharynx and gullet, the stomach and duodenum. The posterior division, which is comparatively short, occupies the caudal fold of the embryo, and corresponds mainly to the lower part of the colon and rectum. Both of these parts have from the first a tubular form, and are closed respectively by the Inflection of the whole blastodermic layers at the anterior and posterior extremities of the body. The middle division has primarily the form of a long and wide groove, lying close below the corresponding part of the vertei)ral bodies, leading at its opposite ends into the cephalic and caudal portions of the gut, and is freely open throughout on its ventral aspect into the cavity of the yolk-sac, with the blastodermic walls of which, as formerly described, the constituents of the intestinal walls are directly continuous (see fig. 576).

The mouth, as elsewnere explained, is no part of the primitive alimentary canal, but is formed by involution of parts of the face, and receives, therefore, its lining membrane from epiblast. It is separated for a time from the pharynx, which is the foremost part of the primitive alimentary canal, by the reflection of the layers of the blastoderm, and the communication which is later established between the mouth and pharynx at the posterior arch of the fauces, is due to a solution of continuity in these layers, which occurs in the chick on the fourth day of incubation, and has been traced at a corresponding period of development in several mammals. The aperture has at first the form of a vertical slit, which widens later as it becomes the opening from the pharynx into the common cavity of the nose and mouth. The diverticulum of the pituitary gland, it will be remembered, occupyingthe place which becomes the top of the permanent pharynx, is formed in connection with the epiblastic or buccal, and not the hypoblastic or pharyngeal division of the alimentary passage (see fig. 535, A and B, py).

The hypoblastic layer of the germinal membrane, from which is derived the epithelial lining of the whole alimentary canal and passages communicating witli it, is at first extremely thin and simple, and is composed of flat cells ; but as soon as this layer comes to form a part of the. inflected alimentary tube, its character is completely altered, its cells become cylindrical, and it attains a great proportional thickness, which it preserves for a considerable time.

Fig. 576. — Early form OF THE Alimentary' Canal (from Kolliker after Bischotf).

In A a front view, and in B an antero-posterior section are represented.

a, four pliaryngeal or visceral plates ; h, the phai-ynx ; c, c, the commencing lungs ; d, the stomach ; /, /, the diverticula connected with the formation of the liver ; g, the yolk-sac into which the middle intestinal gi'oove opens ; h, the posterior part of the intestine.

The outer surface of the inflected hypoblast of the alimentary tube is more or less in contact with the splanchno-pleure division of the mesoblast. In the head no marked separation of the splanchnopleure and somatopleure divisions of the mesoblast takes place, but the elements of the former are no doubt combined with the hypoblast in the walls of the pharynx, and the formation of the tympano-eustachian and following pharyngeal clefts is therefore due to the perforation of both epiblastic and hypoblastic layers with intervening mesoblastic tissue, just as occurs in the formation of the opening of the fauces. But in the thorax and abdomen, the primitive alimentary canal is brought into relation with the pleuro-peritoneal cavity, and receives in various parts a serous investment from the lining membrane which becomes developed in that space. In the thorax the right and left cavities remain distinct as the two pleura, while a central portion is separated for the formation of the pericardium, and thus the gullet, as well as the lungs, is brought into relation with the pleura, and receives partial covering from them. The formation of the diaphragm, which does not at first exist, and which grows down from the vertebral column as a partition between the thorax and abdomen, leads to the ultimate separation of the peritoneum from the pleurte. Some examples of diaphragmatic hernia may be considered as arising from the persistence of the original connection between the two cavities. In the abdomen, also, the right and left peritoneal cavities are at first distinct, but when the intestine assumes a tubular form, the right and left cavities are thrown into one across the middle plane of the body.

As the development of the alimentary canal proceeds, the middle open part shortens, more and more of it being converted into the tubular intestiue, and at last, as before explained, there remains only the narrow opening by which the gradually lengthening ductus vitellointestinalis leads into the umbilical vesicle. The middle part of the intestinal canal has, when first produced, more or less the form of a straight tube lying close to the vertebral column ; but as it increases in length, it is thrown into the shape of a loop bent downwards to the umbilicus, — a change which is accompanied by the formation of the mesentery. The latter structure is undoubtedly entirely due to splanchnopleure mesoblastic elements, which, extending themselves between the proto-vertebral masses and the elongating gut, become developed into the vascular and other parts of the mesentery, as was long ago shown by Yon Baer. But the mesoblast, also, by its splanchnopleure division, furnishes the contractile fibrous, vascular, and connective tissue elements of the intestinal walls. The extent to which the glandular elements of the alimentary canal are supplied by the hypoblast, to which their origin was entirely attributed by Remak, or furnished rather by mesoblast from the proto vertebral mass, as held by Schenk, is not yet determined.

Fig. 577. — Human Embryo of TniRxr-FiVE DAYS SEEN FROM BEFORE (from Kcilliker after Coste).

3, left external nasal process ; 4, superior maxillary process ; 5, lower maxillary jirocess ; z, tongue ; b, aortic bulb ; b', first perrmmeut aortic arch, which becomes ascending aorta ; b" , second aortic arch ; b"' , third aortic arch or ductus Botalli ; ?/, the developing pulmonary arteries ; c, the superior cava and right azygos vein ; c', the common venous sinus of the heart ; c", the common stem of the left vena cava and left azygos ; o', left auricle of the heart ; v, right v', left ventricle ; a e, lungs ; c, stomach ; j, left omphalo-mesenteric vein ; s, continuation of the same behind the pylorus, which becomes afterwards the vena portaj ; x, vitello-intestinal duct ; a, right omphalo-mesenteric artei-y ; m, Wolffian body ; ?', rectum ; n, umbilical artery ; Vi umbilical vein ; 8, tail ; 9, anterior, U', posterior limb. The liver has been removed.

As development proceeds in the forepart of the alimentary canal, a change in its form manifests itself, by which one part, becoming dilated, forms the commencement of the stomach, while the others remain of smaller diameter as gullet and duodenum ; and in connection with different parts of these 'the rudiments begin to appear of the lungs, liver, and pancreas.

When the tubular parts of the gut have attained to some length, a change of position gradually accompanies their further development. While the oesophageal part remains comparatively straight, the dilated portion of the tube which forms the stomach turns over on its right side, so that the border, which is connected to the vertebral cohminby the membranous fold (or true mesogastrium) comes to be turned to the left, — the position of the tube being still vertical, like the stomach of some animals. By degrees it becomes more dilated, chiefly on what is now the left border but subsequently becomes the great curvature, and assumes first an oblique and finally a transverse position, carrying with

Fig. 578. — Outline of the Form ,and Position of the Alimentary Canal in Successive Stages of its Development.

A, alimentary canal, &c., in an emln-yo of five weeks ; B, at eight weeks ; C, at ten ■weeks ; D, at twelve weeks ; I, tbe primitive lungs connected with the pharynx ; s, the .stomach ; d, duodenum ; /, the small intestine ; i', the large ; c, the ciecum and vermiform appendage ; r, the rectum ; c /, in A, the cloaca ; a, in B, the anus distinct from s /, the sinus uro-genitalis ; r, the yolk-sac ; v i, the vitello-intestiual duct ; u, the urinary bladder and urachus leading to the allantois ; g, the genital ducts. In B, and 0, the thickness of the colon is erroneously represented as greater than that of the ileum.

it the mesogastrium, from which the great omentum is afterwards produced. A slight indication of the pylorus is seen at the third month. Upon the surface of the part of the canal which immediately succeeds the stomach, and which forms the duodenum, the rudiments of the liver, pancreas, and spleen are simultaneously deposited, in the manner to be stated in the description of the development of these organs.

The place of transition from the small to the large intestine, which is soon indicated by the protrusion of the cfECum, is at a point just below the apex or middle of the simple loop already mentioned, as accompanying the first elongation of the tubular gut. As the small intestine grows, the part below the duodenum forms a coil which at first lies in the commencing umbilical cord, but retires again into the abdomen about the twelfth week ; aftei'wards it continues to elongate, and its convolutions become more and more numerous.

The large intestine is at first less in calibre than the small. In the early embryo there is at first no csecum. This part of the bowel gradually grows out from the rest, and in the first instance forms a tube of uniform calibre, without any appearance of the vermiform appendix: subsequently the lower part of the tube ceases to grow in the same proportion, and becomes the appendix, whilst the upper portion continues to be developed with the rest of the intestine. The caecum now appears as a protrusion a little below the apex of the bend in the primitive intestinal tube, and, together with the commencing colon, and the coil of small intestine, is at first lodged in the wide part of the umbilical cord W'hich is next the body of the embryo. The ileo-cascal valve appears at the commencement of the third month. When the coils of intestine and cecum have retired from the umbilicus into the abdomen, the colon is at first entirely to the left of the convolutions of the small intestines, but subsequently the first part of the large intestine, together with the meso-colon, crosses over the upper part of the small intestine, at the junction of the duodenum and jejunum. The ca3cum and transverse colon are then found just below the liver ; finally, the ca3cum descends to the right iliac fossa, and at the fourth or fifth month the parts are in the same position as in the adult. At first, villous processes or folds of various lengths are formed throughout the whole canal. After a time these disappear in the stomach and large intestine, but remain persistent in the intermediate portions of the tube. According to Meckel, the villous processes are formed from larger folds, which become sen-ated at the edge, and divided into separate villi.

The formation of the hinder part of the gut is complicated with the development of the allantois, which arises as a projection or outgrowth of the hypoblast and mesoblast from the lower wall of its terminal portion. This part rapidly buds out in the pleuro-peritoneal space, having from a very early period a rich network of the branches of the umbilical vessels in its outer layer. The anal or cloacal portion remains behind the allantoid pedicle, and by the fifth day in the chick the whole of the tissues which close the terminal fold thin rapidly away, and become perforated so as to form the primitive anal, or rather the cloacal opening. The separation of the permanent anus fi'om the urogenital orifice is the result of a later process of development.

The mode of development of the alimentary canal explains, in some measure, the complicated folds of the peritoneum. The stomacii being originally more nearly in the line of the alimentary canal and mesial in position, the small omentum aad gastro-phrenic ligament are originally parts of a mesial fold with the free edge directed forwards, which afterwards forms the anterior boundary of the foramen of Winslow. Thus the anterior wall of the sac of the omentum, as far as the gi'eat curvature of the stomach, may be considered as formed by the right side of a mesial fold, while the peritoneum in front of the stomach belongs to the left side of the same, and a sac of the omentum is a natural consequence of the version and disproportionate growth of the tube between the duodenum and the cardiac orifice of the stomach. It is obvious that the view of the omental sac, according to which its posterior layers are held to return to the duodenum and posterior wall of the body before proceeding to form the transverse meso-colon (p. 484) is more consistent with the phenomena of development now described.

FOLDING OF THE PERITONEUM.

than that which would make them directly enclose the colon. On the other hand, the farther elongation of the omental sac and the whole disposition of the peritoneum, with respect to the colon, must be regarded as having taken place after the assumption by the great intestine of its permanent position.

Fig. 579. — Sketch op the Human Embryo of the Tenth Week, showing THE Coil of Intestine in the Umbilical Cord. (A. T.)

the amnion and villous chorion have been ojiened and the embryo drawn aside from them ; r, the umbilical vesicle or yolk-sac placed between the amnion and chorion, and connected with the coil of intestine, i', by a small or almost linear tube ; the figure at the side represents the first part of the umbilical cord magnified ; i, coil of intestine ; v i, vitello-intestinal duct, alongside of which are seen omphalo-mesenteric blood-vessels.

The occuiTence of umbilical hernia in its various degrees may be referred to the persistence of one or other of the foetal conditions in which a greater or less portion of the intestinal canal is contained in the umbilical cord ; and it has been shown that the most common form of abnormal diverticula from the small intestine is connected with the original opening of the ductus vitello-intestinalis into the ileum.

DEVELOPMENT OP THE LIVER, PANCREAS, AND SPLEEN

The Liver

The liver is one of the earliest formed abdominal organs. It consists at first, according to most observers, of two solid masses in connection with the lower surface of the duodenal portion of the alimentary canal. Schenk, however (Lehrbuch, p. 93), states that the blastemic mass of the liver is single. A hollow cavity soon appears within the mass, which is the commencement of the main excretory duct (ductus choledochus communis). This cavity is lined by hypoblastic epithelium ; and, according to the commonly received view, is produced as a diverticulum of the hypoblast of the intestine. Through the mass, but at first unconnected Avith its substance, there passes the main stem of the veins from the umbilical vesicle and allantois (umbilical vein or meatus venosus).

In the rudimentary mass composing the liver there are soon observed a number of solid cylinders of blastemic cells which branch out from the hypoblast into the mesoblast, and as these come to unite together by their ends, they at last form a network of solid cords with which the hypoblastic diverticula are connected. In the meantime bloodvessels are developed in the mesoblast lying between the cylinders, which vessels become united as branches with the umbilical vein passing through the liver. Hollow processes also extend themselves from the hypoblastic diverticula and stretch into the solid cylinders of the hepatic parenchyma ; but the greater part of this remains solid for a time, consisting of reticulated strings of cells between which there is nothing but blood-vessels.

According to some embryologists, as Schenk, the hypoblast forms no more than the lining epithelium of the bile-ducts and gall-ljladder, and the hepatic or glandnlar cells are entirely derived from mesoblast ; but, according to Foster and Balfour, following Reiuak and the earlier observers, the cellular elements of the gland are stated to derive their origin from the h3'poblast, and the mesoblast is mainly converted into blood-vessels and the fibrous tissue of the ducts.

Fig. 580. — E.VRLT Condition OF THE Liver IN THE Chick on thk Third Bxy of Incubation (from J. ]\IiilLr.) \"

] , the lieai-t as a .simple curved tube ; 2, 2, the intestinal tube : 3, conical protrusion of the coat of the commencing intestine, on which the blastema of the liver (4) is formed ; 4, portion of the layers of the germinal membrane, passing into the yolk-sac.

The gall bladder is formed by extension from the wall of the main duct.

The blood-vessels formed in the liver become branches of the main vein, which passes tlumigh the cellular mass. These are distinguishable as an anterior and posterior set, the arrangement of which is such that the blood flows from stem to branches in the anterior, and from branches to stem in the posterior. Thus the distinction is established between portal and hepatic veins (sec the Development of the Yeins).

The solid cylinders of the blastema represent the hepatic lobular tissue, the hollow processes the hepatic ducts ; but the origin of the finest ducts is not known. Perhaps each cellular cylinder may be looked upon as a collection of hepatic cells, in the centre of which is the minute duct, according to the view now taken of the structure of the adult liver (Foster and Balfour).

The gall-bladder is at first tubular, and then has a rounded form. The aU'eoli in its interior appear about the sixth month. At the seventh month it first contains bile. In the fcetus its direction is more horizontal than in the adult.

The following are the principal peculiarities in the liver of the foetus : —

Size. — In the human fcetus, at the fifth or sixth week, the liver is so largo that it is said to constitute one -half of the weight of the -n-hole body. This proportion gradually decreases as development advances, until at the full pei'iod the relative weight of the foetal liver to that of the body is as 1 to IS.

In early foetal life, the right and left lobes of the liver are of equal, or nearly equal, size. Later, the right preponderates, but not to such an extent as after birth. Immediately before birth the relative weight of the left lobe to the riglit is nearly as 1 to I'G.

Position. — In consequence of the nearly equal size of the two lobes, the position of the foetal liver in the abdomen is more symmetrical than in the adult.

In the very yoimg foetus it occupies nearly the whole of the abdominal cavity ; at the full period it still descends an inch and a half below the margin of the thorax, overlaps the spleen on the left side, and reaches nearly down to the crest of the ilium on the right.

Form, Colour, — The foetal liver is considerably thicker in proportion from above downwards than that of the adult. It is generally of a darker hue. Its consistence and specific gravity are both less than in the adult.

During foetal life, the umbilical vein runs from the umbilicus along the free margin of the suspensory ligament towards the anterior border and under surface of the liver, beneath which it is lodged in the umbilical fissm-e. and proceeds as far as the transverse fissure. Here it divides into tn-o branches ; one of these, the smaller of the two, continues onward in the same du'ection, and joins the vena cava ; this is the ductus rciumix. which occupies the posterior part of the longitudinal fissure, and gives to it the name of the fossa of the ductus venosus. The other and larger branch (the tnink of the umbilical vein) turns to the right along the transverse or jjortal fissure, and ends in the vena portaj, which. })roceeding from the veins of the digestive organs, is in the foetus comparatively of small dimensions. The umbilical vein, as it lies in the umbilical fissure, and before it joins the vena porta3. gives off large lateral branches, which pass dii-ectly into the right and left lobes of the liver. It also sends a few smaller branches to the square lobe and to the lobe of Spigelius.

Fig. 581. — Under Surface of the FcETAL Liver, with its gre.\t Blood-vessels, at the full Period.

a, the iimhilical A-ein, lying in the umbilical fissure, and turning to the ri^dit side, at the transverse fissure (o), to join the vena portte (p) : the branch marked d, named the ductus venosus, continues straight on to join the vena cava inferior (c) : some branches of the umliilical vein pass from a into the substance of the liver ; >j. the gall-bladder.

Fig. r.8i.

The blood which leaves the liver by the hepatic veins, and is carried into the heart along with that of the vena cava inferior, consists of the following parts, viz. ; 1. That of the mnbilical vein, which passes on directly by the ductus venosus ; 2, that portion of the blood which is distributed to the liver by branches proceeding immediately from the trunk of the umbilical vein ; and 3, the blood from the digestive organs of the foetus arriving by the vena porta3.

After birth the umbilical vein becomes obliterated from the umbilicus up to the point of its giving off branches to the liver. The ductus venosus is also obliterated, but the veins which were given as l^ranches from the umbilical vein to the liver remain in communication with and appear as branches of the left division of the portal vein.

The Pancreas

This organ takes its origin in a mass of mesoblastic tissue, which thickens the wall of the duodenum close to the place where the rudiment of the liver is first seen, but placed more to the left side. This mass may be seen on the third day in the chick. There is, however, also a diverticulum from the primary wall of the intestine or hypoblast. The same doubt prevails, as in regard to the liver, with respect to the exact share of the hypoblastic and mesoblastic elements in the formation of the glandular cells. The main duct and its branches undoubtedly owe their origin to diverticula proceeding from the intestinal hypoblast, and the epithelial lining of the ducts is doubtless derived from that source. By those who consider that the glandular cells also arise from the hypoblast, solid processes of that layer are described as stretching into the mass of mesoblast. Into these the diverticular cavities subsequently extend in more than one main division. The blood-vessels and the connective tissue of the gland are undoubtedly due to the mesoblastic elements, and these are very soon combined with the imrts proceeding from hypoblast.

The Spleen

This organ appears soon after the pancreas as a thickening of the mesogastrium, and is therefore entirely mesoblastic in its origin. (Peremeschko, Vienna Acad., 18G7, and W. Mliller in Strieker's Histol.) The gradual formation of the trabecular structure from one set of cells and of the blood-vessels and cellular elements of the organ from the blastemic substance has been traced. The pulp is formed in connection with the veins, and the arteries are formed along with the Malpighian corpuscles. The spleen is closely related to the pancreas in its origin, but it is later of being formed and contains no hypoblastic elements. In the human foetus of about twelve weeks its shape can be recognised, but the Malpighian bodies are not visible till the middle of foetal life.

Lymphatic Glands

The development of the mesenteric lymphatic glands has been observed by Sertoli in mammals. (Vienna Acad., 18G6.) The blastema from which they are produced is imbedded in the mesentery, and is therefore entirely mesoblastic. The gradual differentiation of the blastema gives rise in succession to the lymph spaces, the trabecuke and the lymph cells, and the distinction follows between inferent and efferent lymphatic vessels. The development of lym])hatic vessels has been described in the General Anatomy, p. 191.

The Thymus and Thyroid Glands

The development of these bodies has been described in an earlier part of this volume, pp. 297 and 299. The thymus gland proceeds entirely from mesoblastic tissue ; but, according to the researches of W. Miiller (Jenaisch. Zeitsch., 1871), it would appear that the thyroid body arises at first as a diverticulum from the pharynx, and it therefore contains some hypoblastic elements.

DEVELOPMENT OF THE LUNGS AND TRACHEA

The lungs first appear as two small protrusions upon the front of the oesophageal jiortion of the alimentary canal, completely hid by the rudimentary heart and liver. These primitive protrusions or tubercles are visible in the chick on the third day of incubation, and in the embryos of mammalia and of man at a corresponding stage of advancement. Their internal cavities communicate with the oesophagus, and are lined by a prolongation of the hypoblast. At a later period they are connected with the oesophagus by means of a long pedicle, which ultimately forms the trachea, whilst the bronchia and air-cells are developed by the progressive ramification of the internal cavity in the form of cffical tubes, after the manner of the ducts of glands.

The diverticulum of hypoblast is surrounded by a mass of mesoblastic cells, so that the pulmonary parenchyma, like that of the glands, owes its origin to both hyiioblastic and mesoljlastic elements. The substance of the mesoblast, thickening round the primary diverticula, becomes penetrated by secondary diverticula formed from the hypoblast processes ; these are succeeded by tertiary branches which develop the bronchia, and ultimately have the air-cells formed as their terminations. The formative process consists essentially in the budding of hypoblastic into mesoblastic substance ; the hypoblast furnishing the lining epithelium of the tubes, and the mesoblast the other tissues, such as muscular fibres, cartilage, blood-vessels, elastic tissue, c^-c.

In the formation of the trachea and bronchi the wall of the primitive oesophagus is projected downwards (or forwards), and by the gradual folding in of the sides a second median tube is separated from the primitive alimentary canal. This new tube grows out at its hinder end so as to bulge at the two sides, and thus the commencement of a right and left bronchus is formed. The subsequent division of the diverticular hollow goes on by budding of the hypoblast from within into the masses of pulmonary blastema. The division into larger lobes externally, three in the right and two in the left lung, may be seen at a very early period in the human foetus. As the bronchial subdivision extends within the lungs, a tubercular or coarsely granular appearance is seen over the outer surface, as observed by Kiilliker in the human foetus in the latter half of the second montli. This is produced by the primitive air-cells placed at the extremities of ramified tubes, which occupy the whole of the interior of the organ : the ramification of the bronchial twigs and multiplication of air-cells goes on increasing, and this to such an extent that the air-cells in the fifth month are only half the size of those which are found in the fourth month.

Fig. 582. — Sketch iLLUSTRATiNa the DeveLOPMENT Oi THE RESPIRATORY ORGANS (from Fatlike). A. B O

A, oesophagus of a cliick, on the fourth day of incubation, with the rudimentary lung of the left side, seen laterally ; 1, the front, and 2, the hack of the oesophagus ;

3, rudimentary lung protruding from that tube ; 4, stomach. B, the same seen in front, so as to show both lungs. C, tongue and resjjiratory organs of embryo of the horse; 1, tongue; 2, larjmx ; 3, trachea j

4, lungs seen from behind.

In birds the principal air-sacs, three in number, are formed in direct connection with the lung in the course of its early development, and the riidiments of these sacs may be seen at an early period, as bulging constituent parts of the rudimentary lungs.

Pleurae

Each lung receives a covering externally from the lining' membrane of the common pleuro-peritoneal cavity of its own side. This is at first only on the outer side; but, as the lungs enlarge, a fissure separates their solid substance from the outer wall of the oesophagus, and the pleura is carried round the lung-mass so as to encircle the gradually narrowing root of each lung. The two pleurse remain separated by the mediastinum and heart.

Pulmonary Vessels

The blood-vessels of the lungs which arise in the mesoblastic tissue seem to be of comparatively late formation.

penetrating into the mesoblast only on the twelfth day in the chick. The pulmonary arteries are developed in mammals in connection Avith the fifth branchial arch of the left side, but the manner in which they become connected with the vessels formed in the lung-substance, and the manner in which a union is established between the pulmonary veins and the left auricle have not yet been ascertained.

DEVELOPMENT OF THE HEART AND BLOOD-VESSELS.

In the account of the general phenomena of development the establishment of the first circulation of blood, by the simultaneous formation of the simple heart and of the first blood-vessels and blood in the body of the embryo and in the vascular area of the blastoderm, has already been described, and in the General Anatomy (p. 180) an account has

Fig. 583. — Outlines of the anterior HALF OF THE EjIBRYO ChICK A'IEWED FROM BELOW, SHOWING THE HeART IN ITS EARLIEST STAGES OF FORMATION (after Kemak).

A, Embryo of abouut 20 to 30 lioxirs ; B, of about 36 to 40 hours ; a, anterior cerebi'al vesicle ; h, j)roto-vertebral segments ; c, amniotic fold ; 1, 1, primitive omphalo-mesenteric veins entering the Iieart posteriorly ; 2, their union in the auricle of the heart ; 3, the miLldle part of the tube corresponding to the ventricle ; 4 (in B) the arterial bulb.

been given of the histological changes occurring in the first development of the blood-vessels and blood.

DEVELOPMENT OF THE HEART

Origin of the Heart

Simple Tubular Foi'in. — The heart takes its origin in the form of an elongated sac or dilated tube in the substance of a thickening of the splanehuo-pleure layer of the mesoblast, in the ventral aspect of the cephalic portion of the primitive alimentary canal, immediately in front of the fovea cardiaca. Doubts have existed as to the exact mode of production of the cavity of the organ, but the observations of Aflanasieff and Klein, and especially those of Foster and Balfour, appear to show that the substance in which the first rudiments of the heart arise is produced by a thickening of the lower wall of the mesoblastic layer of the primitive intestine, and that the cavity is formed by a solution of continuity or liquefaction of tliis substance in such a manner that, while the outer cells constitute the foundation of the commencing fibrous walls,, a deep set of cells very soon or from the first arrange themselves in the form of an endo-vascular lining of the cavity. The oi-gan has at first the form of an elongated sac or dilated tube of symmetrical shape, widening out behind into two lateral orifices, each of which is connected with an omphalo-mesenteric vein of its own side bringing the nascent blood back from the vascular area, while the anterior part of the rudimental heart leads into two arterial vessels, one of which arches over each side of the primitive pharynx and turns backwards below the proto-vertebrge to form one of the two primitive aortic tubes. From each of these last the omphalo-mesenteric arteries pass off into the vascular area.

According to recent observations by Kolliker and by Hensen (loc. cit.) a still earlier condition of the heart has been perceived in the embryo of mammals, in which there are two separate tubes hollowed out of the lateral parts of the cephalic fold. Each of these tubes is connected with a vein or entering vessel posteriorly, and an artery or out-going vessel anteriorly : these slowly come together and unite by fusion in the middle, in a limited space at first, and then more and more till the single tubular heart results. Each tube is in relation with the isleui-operitoneal cavity of its own side, and when the median fusion takes place the union of these two becomes the pericardium.

Fig. 584. — Diagrammatic longitcdixal section through the Axis op an Embrto.

The section is supposed to be made at a time when the head-fold has commenced, but the tail-fold has not yet appeared. A, epiblast ; B, mesoblast ; C, hypoblast ; FSo, fold of the somatopleure ; Sp, and FiSp, fold of the splanclmopleure ; Avi, commencing (head) fold of the amnion ; NO, neural canal, closed in front, but still open behind ; C/i, notochord, in front uncleft mesoblast in the ba«e of the cranium ; D, the commencing foregut, or alimentary canal ; Ht, heai-t ; jU^, pleuro-i>eritoneal cavity.

The rudimental heart in the form now described, exists in the chick at the thirty-sixth hour of incubation, and already, while still consisting of formative cells not differing greatly from those composing the other parts of the mesoblast, begins to exhibit motions of alternating systole and diastole, by slow contractions which begin behind and pass forward to the anterior extremity of the tube ; and a small quantity of imperfectly formed blood is propelled through the cavity.

The elongation which the tubular heart now undergoes causes it to lose the symmetrical form ; and its middle part now becomes detached from the lower side of the alimentary canal, and projects downwards (or forwards in the body) with an inclination to the right side of the embryo.

The heart is now found to be surrounded on the ventral aspect by a median cavity, which is a part of the pleuro-peritoneal space intervening between the wall of the heart as splanchno-pleure, and the somato-pleure forming the thoracic wall. This cavity becomes the pericardial sac.

As the development of the tubular heart progresses, the bend increases, and the venous is doubled back upon the arterial end. The tube also becomes divided by two slight constrictions into three

Fig. 585. — Human Embryos AT DIFFERENT EARLY STAGES OF DEVELOPMENT, SHOWING THE Heart in its tubular CONDITION.

A, upper half of the body of a human embryo of three weeks, viewed from the abdominal side (from Coste) ; «, frontal plate ; h, protovertebrse, on which, the primitive aortiB are lying ; 3, the middle of the tube of the heart, below it the place of entrance of the great veins, above it the aortic bulb.

B, lateral view of a human embryo more advanced than that last referred to, and somewhat imperfectly developed (from A. Thomson) ; a, the frontal part of the head ; h, the vertebral column ; v, the wide communication of the umbilical vesicle or yolk-sac with the intestine ; u, communication with the allantois or urachus ; 2, auricular part of the heart connected with the veins posteriorly ; 8, ventricular part of the bent tube ; 4, the aortic bulb ; near the extremities of the tube the divided pericardium is seen portions, of which that originally posterior and receiving the veins is the widest, and constitutes the primitive auricle ; the middle one, next

Fig. 586.— Diagrammatic Outlines of the Heart and First Arterial Vessels OP THE Embryo, as seen from the Abdominal Surface.

A, at a period corresponding to the 36th or SStli hour of incubation in the chick • B, and C, at the 48th hour of incubation ; 1, 1, primitive veins ; 2, auricular part of the heart ; 3, ventricular part ; 4, aortic bulb ; 5, 5, the primitive aortic arches and their continuation as descending aorta ; these vessels are still separate in their whole extent in A, but at a later period, as shown more fully in C, have coalesced into one tube in a part ot the dorsal region ; in B, below the upper 5, the second aortic arch is formed, and farther clown the dotted lines indicate the position of the succeeding arches to the number of rive in all ; 5', 5', the continuation of the main vessels in the body of the embryo ; 6, 6, the omphalo-mesenteric arteries imssing out of the body of the embryo into the vascular area of the germinal membrane.

in width and most strongly bent upon itself, becomes the ventricular portion ; and the third, situated anteriorly and retaining most the simple tubular form, is the arterial or aortic bulb. This tubular stage of the rudimental heart has been observed in the human embryo by Coste and Allen Thomson (see fig. 585, A and B).

Division into Single Auricle, Ventricle, and Arterial Bulb. — By a continued increase of the inflection of the heart-tube, a change in the relative position of the several parts is effected, so that the auricular cavity comes to be placed above or behind (dorsally) and to the left of the ventricular part, the veins being carried forwards along with it, while the arterial bulb is attached by its extremity in front to the neck of the embryo immediately behind the lower visceral plates. There is as yet only a single passage through the heart, but the distinction of the auricular and ventricular cavities becomes more apparent, both by an increase in the diameter of each, and by the constriction which separates them, and by the much greater thickness acquired ]:»y the walls of the ventricular and bulbous parts as compared with the auricular portion.

The three parts of the heart have now the appearance of being very closely twisted together. The ventricular part becomes considerably wider transversely, and the auricular part shows two projecting pouches, one on each side of the arterial bulb, which are the first indications of the future auricular appendages. At the same time the constriction between the auricular and ventricular parts increases consideral:)ly, and the constricted part elongating produces what has been called the canalis auricular is.

Division of the Cavities. Ventricles

The next series of changes in the developing heart consists in the division of each original single cavity of the ventricle, auricle, and arterial bulb into two compartments, so as to form the right and left ventricles and auricles, and the stems of the -pulmonary artery and aorta. The first of these changes occurs in the ventricular portion, and is to be seen in progress on the fourth day in the chick, and the sixth and seventh week in the human embryo. The ventricular chamber of the heart, increasing considerably in breadth, that part of it which ultimately becomes the apex of the heart is thrown towards the left side, and in most mammals, and especially in the human embryo, a blmit cleft or depression appears betAveen this and the right part of the ventricle, which causes an external division into two portions corresponding to the future right and left ventricles ; and if the interior of the ventricular cavity be examined at this time, there is perceived a crescentic partition rising from the anterior or lower border of the right wall and projecting into the cavity, at first narrow and placed opposite the external notch, but gradually growing more and more towards the auriculo-ventricular aperture. As development progresses the external division becomes more or less effaced, when the apex of the heart formed by the left ventricle becomes more pointed, and the whole heart takes more of the conical form which belongs to its more advanced condition ; but the depression is still perceptible as the interventricular groove of the adult heart, which, as is well known, varies considerably in depth in different cases. In some animals, as the rabbit, the temporary external division of the ventricles is greater than in the human embryo, while in others, as in ruminants, there is very little of the external notching, and in them, as in birds, the heart very early assumes the conical form. The dugong presents a remarkable example of the persistence of the complete external separation of the ventricles, and there appears to be a tendency to the occasional occurrence of the same in the seal.

Fig. 587. — Head op the Embryo of the Dog with the Heart seen from below (from Kolliker, after Bischoff). Magnified.

a, cerebral hemispheres : h, eyes ; c, midbrain ; d, inferior maxillary plates ; e, superior maxillary processes ; /, /', /", second, third, and fourth branchial or visceral plates ; g, right, h, left auricle of the heart ; h, right, i, left ventricle ; 1, aortic or arterial bulb, with three paii's of aortic or vascular arches protruding from it.

The internal septum of the ventricles continuing to rise between the right and left divisions of the cavity, reaches at last the base where it is placed in relation with both the auriculo-ventricular orifice and the root of the arterial bulb ; but at this place there remains for a time a communication over the still free border of the septum between the right and left ventricles, wdiich is interesting, as this is the seat of the abnormal communication between the right and left ventricles in almost all cases of malformation of the heart presenting that condition.

Division of the Auricles

Although the auricular cavity presents externally some appearance of being divided into two at a period antecedent to the partition of the ventricles, in consequence of the formation of the right and left auricular appendages before mentioned, the internal division of the cavity does not take place till some time later, as on the fifth and sixth days in the chick, and in the eighth week in the human embryo. The auricular septum commences as an internal fold proceeding from the anterior wall of the common cavity, and starting from the septum of the ventricles, it grows backwards towards the entrance of the common vein or sinus, but stops short of it some distance. For a time, therefore, the veins enter the back part of the auricular cavity in common. It is proper to explain, however, that, by the time at which the auricular septum is forming, the venous sinus has been modified so as to produce three veins entering the auricle at its .back part. Of these, two correspond with the right superior cava and the inferior cava veins, and the third to the left superior cava and connected with what afterwards becomes the coronary sinus. For a time, all the three vessels open so as to communicate freely with the whole auricular cavity. But changes now occur which cause the left superior cava and the inferior cava to be directed towards the left side, while the right superior cava is placed more immediately in connection with the right part of the auricular cavity.

The auricular septTim,in extending itself backwards, is not completed, but leaves an oval deficiency in its lower and middle part, as the foramen ovale, and the inferior cava opens immediately behind this. Some time later in the human embryo, or in the course of the tenth or eleventh weeks, two new folds make their appearance in the auricles posteriorly. One of these constituting the Eustachian valve, of a crescentic form, is placed to the right of the entrance of the inferior vena cava, and in the angle between it and the orifice of the left superior cava (or great coronary sinus), and besides separating these two veins, and thus throwing the opening of the left superior cava into communication with the right auricle, this fold, as it runs forward into the annulus ovalis or border of the anterior auricular septum, has the effect of deepening the entrance of the inferior cava into a groove which lies close to the foramen ovale, and directs the blood entering by that vessel through the passage into the left auricle.

Fig. 588. — Shows the position and FORM OP THE HeART IN THE HuMAN Embryo from the Fourth to the Sixth WEEK.

A, upper half of the body of a human embryo of Dearly four weeks old (from Kolliker after Coste) ; B and C, anterior and posterior views of the heart of a human embryo of six weeks (from Kolliker after Ecker) ; a, frontal lappet ; h, mouth ; c, below the lower jaw and in front of the second and third branchial arches ; d, upijer limb ; e, liver ; /, intestine cut short ; 1, superior vena cava ; 1', left superior cava or brachio-cephalic connected with the coronary vein ; 1", opening of the inferior vena cava ; 2, 2', right and left auricles ; 3, 3', right and left ventricles ; 4, aortic bulb.

The other fold referred to advances from the posterior wall of the common auricle to meet the anterior auricular septum, but yet to the left of the border of the foramen ovale. To this border, however, it adheres as it grows forwards, and thus gradually fills up the floor of the fossa ovalis. Up to the middle of foetal life, this posterior septum being incomplete, there is a direct passage from right to left through the foramen ; but, after that period, the fold in question, having advanced beyond the anterior border of the annulus ovalis and lying to the left, it does not adhere to this or the fore part of the annulus, but leaves a passage between, and appears as a crescentic fold in the left auricle, which, as it passes beyond the annulus, constitutes a very perfect valve against the return of blood from the left into the right auricle.

Division of the Arterial Bulb

The third important change occurring in the heart belongs to the arterial bulb, by which there are developed from this tube the first parts or main stems of the pulmonary artery and the ao'-ta. Within the thick walls of this arterial tube there is at first only a single cylindrical cavity, continued from the originally single ventricle ; but, a short time after the partition of the ventricular cavity has commenced, or in the seventh week of the human embryo, a division of the bulb by an independent process begins to take place. This consists in the projection inwards of a lateral fold of the wall on the two sides, affecting, however, only the inner and middle coats, and not perceptible externallj^ ; so as to divide the cavity of the bulb into two channels, which may be described as respectively anterior and posterior, but which from the spiral direction taken by the folds are somcAvhat twisted on each other, so that the channel which at the ventricular end is placed anteriorly becomes connected with the right ventricle and forms the pulmonary stem, and that which is placed posteriorly becomes connected with the left ventricle and forms the commencement of the aorta. In the distant portion of the bulb, however, the pulmonary channel is situated to the left and posteriorly, and the aortic channel is to the right and most forwards, and at this end these channels are respectively connected with different aortic arches, giving rise to the permanent pulmonic and systemic vessels in the manner afterwards described.

It is further to be noted that the partition of the bulb begins at the remote extremity, and progresses towards the ventricles. There is a time, therefore, during which the ventricular septum, and the septum of the bulb, advancing towards each other, are incomplete and disunited ; and from the difference in their direction it is obvious that there must be a peculiar twist of one or both, in order that they may finally unite so as to become continuous.

Fig. 589. — View op the Front and Right Side of the Fcetal Heart, at four MONTHS, THE RIGHT AuRICLE BEING LAID OPEN (from Kilian).

a, the right auriculo-ventricular opening ; h, a probe passed iip the vena cava inferior and throiigh the fossa ovalis and foramen ovale into the left auricle ; r, vena cava inferior ; c, Eustachian valve ; v, valve of the foramen ovale ; s', vena cava superior.

Fig. 590. — View of the Posterior and left surface of the Heart of a Foetus of four months, the left Auricle being opened (from Kilian).

o, left auriculo-ventricxilar orifice ; c, inferior vena cava, through which a probe b, is passed from below, and thence by the foramen ovale into the left auricle ; e, left auricular appendage laid open ; o, valve of the foramen ovale seen to be attached to the left side of the annulus ovalis of the septum.

The completion of the partition of the aortic and pulmonary stems is afterwards effected by the progress of the division from within outwards through the external walls of the tubes ; but the two vessels remain united externally by a common envelope of pericardium.

The remarkable cases sometimes observed of abnormal transposition of the two great arterial stems from their natural connection with their respective ventricles may be explained by reference to the history of the development of the parts of the heart before given.

Formation of the Valves

The formation of the auriculo-ventricular and semilunar valves begins during the time of the changes previously described by the projection of thick folds from the inner wall of the heart. In the case of the semilunar valves the trifid division is early perceived, but the cavities or sinuses within the valves are late of being developed. In the auriculo-ventricular valves there is at first an entire or annular projecting fold of the inner substance round the orifice, and this becomes gradually divided into segments, and the chordse tendinae are gradually produced by perforation of the valve plate. (See Tonge in Proceed. Eoy. Soc, 1868.)

The manner in which the pulmonary veins, which are formed separately in the lungs, come to be connected with the left auricle has not yet been ascertained.

No further important changes occur in the internal structure of the heart, but there are some w^hich affect the external form and thickness of its walls. In early foetal Hfe the size of the heart bears a considerably greater proportion to that of the body than at a later period. At birth it is still proportionally large. For some time the auricular portion remains more voluminous than the ventricular, but in the latter half of foetal life the permanent proportion is more nearly established. The walls of both ventricles are also thicker than in after life, and it is especially deserving of notice that the wall of the right is up to near the time of birth quite as thick as that of the left, — a peculiarity which may be connected with the orifice of the right ventricle to propel the blood of the foetus through the extended course of the ductus arteriosus, the descending aorta and the placental circulation.

DEVELOPMENT OF THE BLOOD-VESSELS

The Principal Arteries. The Aorta

The most interesting part of this history is that relating to the development of the aorta and the larger vessels arising from it. The double condition of the main trunk of "the aorta has already been referred to as existing in the chick up to near the end of the second day. About the fortieth hour the inedian fusion or coalescence of the two vessels begins to take place in the dorsal region, by their external union, at first in a very limited space, and very soon afterwards by the formation of a perforation through their united walls. The union of the two vessels which begins in the dorsal region extends itself backwards towards the lumbar vertebrae, and when it reaches the place where the omphalo-mesenteric arteries pass out on each side, these vessels, each of which was originally the continuation merely of one of the aortas, appear now as branches of a single and median aorta. The iliac vessels are the next large vessels formed from the hinder part of the aorta. The first vessels belonging to these trunks are not, however, those of the lower limbs, for these are not yet formed ; but the umbilical or hypogastric arteries, developed at a very early period in connection with the allantois, and subsequently attaining to a large size along with the growth of the placenta. As the limbs are formed, arteries are developed in them, and these are branches of the main aorta ; but they are for a long time comparatively small, w'hile the umbilical arteries speedily attain a large size, so that, even up to the conclusion of foetal life, they appear to form the principal part of the two large vessels into which the aorta divides at its lower extremity. The middle sacral artery may be looked upon as the continuation of the median stem of the aorta, and probably originates from a double vessel in the same manner as the aorta itself.

Fig. 591. — Transverse Section through the Dorsal Region op an Embryo-Chick OP THE Second Day (from Foster and Balfour, after His). M, medullary canal ; Pv, proto -vertebral column ; w, rudiment of Wolffian duct in the intermediate mass; Ch, notocliord ; Ao, one of the two aortas ; A, epiblast ; C, hypoblast ; BC, splanchnopleure ; Pp, pleuroperitoneal space.

Fig. 592.- — Transverse section through the Dorsal region op an Embryo Chick, END op Third Day (from Foster and Balfour).

Am, amnion ; m p, muscle plate ; CV, cardinal vein; Ao, dorsal aorta at the point where its two roots begin to join ; Ch, notochord (the line does not quite reach it) ; Wd, Wolffian duct; Wb, commencement of formation of Wolffian body ; ep, epiblast ; so, somatopleure ; Sp, splanchnopleure ; hy, hypoblast. The section passes through the place where the alimentary canal {hy) communicates with the yolk-sac.

The double state of the main aorta when first formed in the foetus was discovered by Serres, and described bj him in his 4tli Memoir on Transcendental Anatomy (Annal. des Scien. Nat., 18130), but was doubted by Von Baer, as Serres's observations did not show the relation of the primitive trunks of the aorta to the pharyngeal vascular arches. The fact of the original double condition was, however, placed beyond doubt by Allen Thomson (Edin. New Philos. Journal, ISoO) by the method of tranverse sections, then fii-st employed as a means of embryological investigation, and the process of median union was traced in full detail. The relation of this process to the occurrence of a permanent double canal in the aorta as a malformation, as described by Vrolik. Schroder van der Kolk and Cruveilhier. and obseiwed in at least one case by Allen Thomson, has already been referred to in vol. i., p. 350.

According to Serres, the vertebral arteries within the cranium are originally separate, and the basilar artery results from their mesial union or fusion in the same manner as occurs in the aorta, and the median union of the anterior cerebral arteries in the forepart of the Circle of Willis is another example of the same process. It seems probable that the internal cross band observed by John Davy in the interior of the basilar artery (Researches Physiol, and Anatom., lSo9, p. 301) may be a remains of the septum or united walls of the two vertebral arteries.

Aortic OX' Branchial Arches

The two primitive arterial arches which lead into the dorsal aorta from the arterial bulb of the rudimentary heart, at the time of the establishment of the first circulation, are the most anterior of a series of five pairs of vascular arches which are developed in succession round this part of the pharynx ; and which, since their discovery by Eatlike in 1S25 (Oken's Jsis, 1825) have been regarded with much interest, as corresponding with those vessels which are the seat of development of the subdivided blood-vessels of the gills in fishes and amphibia. These vascular arches thus exhibit in the amniota, along with the branchial or pharyngeal clefts and visceral plates, a typical resemblance to the structure of gills, although no full development of these respiratory organs occurs in such animals, but they furnish by their various transformations the basis of formation of the permanent pulmonary and aortic stems and the main vessels to which they give rise.

The form and position of the primitive aortic arches, up to the time of their transformation into permanent vessels, is nearly the same in reptiles, birds and mammals ; and the main differences in the seat and distribution of the large permanent vessels are to be traced to changes in the openness and extent of growth of the several arches. The five pairs Of arches do not all co-exist at the same time, for they are developed in succession from before backwards, and by the third day of incubation, or by the corresponding period of the fourth week in the human embryo, when the posterior arches have been formed, already a part of the anterior arches, beginning with the first one, has become obliterated. Each of the first four branchial arches occupies a place in the substance of the pharyngeal or visceral plates, and in front of one of the pharyngeal clefts. The first or anterior is therefore situated in the inferior maxillary plate, and in front of the tympano-Eustachian, or first pharyngeal cleft ; and the fifth arterial arch is placed behind the fourth pharyngeal cleft, and in the substance of the neck, in which there is no distinct bar or plate in the higher animals, but which is the seat of a developed branchial bar in some aquatic animals.

The vessels forming the arterial arches are given off on each side in succession from two short canals, into which the primitive arterial bulb divides immediately in front of the place where it joins the neck. These may be named the lower (ventral) or anterior aortic roots ; and similarly, when they have passed round the wall of the pharynx, the branchial arches unite in succession into a vessel on each side, thus forming the upper (dorsal) or posterior aortic roots.

On the third and fourth days in the chick, and from the fourth to the sixth week in the human embryo, there are still three complete pairs of arterial arches passing round the pharynx, and connected both before and behind with the anterior and posterior aortic roots previously mentioned. The transformations of these arches were in part traced by Von Baer and various other observers, but the fuller knowledge of their changes is due to the later researches of Eatlike (Mem. of

Fig. 593. — Diagram of the Aortic or Branchial Vascular Arches of the Mammal, with their transformations giving rise to the permanent arterial Vessels (accordiug to Ilatlike, slightly altered).

A, Primitive arterial stem or aortic Lnlb, now divided into A, the ascending part of the aortic arch, and P, the pulmonary ; a, the right ; a', the left aortic root ; A', the descending aorta. On the right side, 1, 2, 3, 4, 5, indicate the five Ijranchial primitive arterial arches ; on the left side, I, II, III, IV, the four branchial clefts, which, for the sake of clearness, have been omitted on the right side. It will be observed, that while the fourth and fifth pairs of arches rise from the part of the aortic bulb or stem, which is at first undivided, the first, second, and third pairs are branches above c, of a secondary stem on each side. The permanent systemic vessels are represented in deep shade, the pulmonary arteries lighter ; the parts of the primitive arches, which have only a temporary existence, are drawn in outline only, c, placed between the permanent common carotid arteries ; ce, the external carotid arteries ; ci, c'l, the right and left internal carotid arteries ; s, the right subclavian rising from the right aortic root beyond the fifth arch ; r, the right vertebral from the same opposite the fourth arch ; v', s', the left vertebral and subclavian arteries rising together from the left or permanent aortic root opposite the fourth arch ; P, the pulmonary arteries rising together from the left' fifth arch ; d, the outer or back part of the left fifth arch, forming the ductus arteriosus ; j)n, 2^n , the right and left pneumogastric nerves, descending in front of the aortic arches, with their recurrent branches represented diagrammatically as passing behind, with a view to illustrate the relations of these nerves respectively to the right subclavian arteiy (4) and the arch of the aorta and ductus arteriosus {d).

Vienna Acad., 1857), and although some 'points are still left in doubt, their history may now be given from these observations, and the supplemental illustration derived from the investigation of the various examples of congenital malformation, the greater number of which are manifestly related to variations in the natural mode of transformation. This will be explained by reference to the diagram in fig. 593.

From these researches it appears that the permanent vessels owe their formation to the persiscence of certain of the foetal arches or parts of them, while other arches or portions of them become obliterated and disappear. Thus it is ascertained that in mammals the main aortic arch, which in the adult passes to the left of the trachea and gullet, is formed by the persistence of the fourth embryonic arterial arch of the left side, which not only remains patent, and becomes connected with the aortic stem of the arterial bulb, but keeps pace by its increased width and the development of its walls with the rate of growth in the other parts of the body, so that it soon surpasses all the rest of the arches in its width of calibre and thickness of its walls. In birds, however, the permanent aortic arch passes to the right of the trachea and gullet, and it is formed by the persistence of the fourth embryonic arch of the right side ; while, in all reptiles, as there are two permanent aortic arches, it is by the persistence of both the right and left foetal arches that the two aortas are produced, the right being that which is most directly connected with the systemic or left ventricle.

The pulmonary arteries of mammals would appear by Ratlike's observations to be developed in connection with only one foetal arterial arch, viz., the fifth of the left side, from the middle part of which they appear as branches, and the whole fifth arch of the right side undergoes rapid atrophy and ultimate obliteration. The first part of the left fifth arch, becoming the common pulmonary artery, is connected with that division of the arterial bulb which is separated as the pulmonary stem ; but the remote part of this arch also remains fully patent, and undergoing equally with the rest of it full development, continues to lead into the left root of the aorta as ductus arteriosus Botalli, which serves to convey the blood from the right ventricle of the foetal heart into the descending aorta, but becomes obliterated at the time of birth.

This duct is therefore in mammals due to a persistent condition of the fifth left branchial arch. But, in birds and reptiles, it appears that the process of transformation is somewhat different, for in them the right and left pulmonary arteries (excepting in those serpents in which there is .only one lung developed) are formed in connection with the respective right and left fifth branchial arches, and there are thus two ductus arteriosi during foetal life, the short one of the right side corresponding to that which is left in mammals, and the longer one of the left side passing round the pharynx into the left aortic root. Both of these arches are obliterated at the time of the exclusion of the bird from the eg^ ; but in some reptiles the ductus arteriosi remain permanently open during life.

The subclavian and vertebral arteries were shown by Ratlike to spring from the posterior aortic roots at a place between the junction of the fourth and fifth arches. In mammals, the vessels on the left side are from the first in direct connection Avith the aortic root at the place which they permanently occupy ; but upon the right side, as the fourth arch and the aortic root are obliterated posteriorly, the passage for blood from the aortic stem into the subclavian trunk is formed by the persistence of the forepart of the fourth right arch as far as the place where it meets the origin of the subclavian and vertebral arteries.

The common carotid trunks, occupying the region which afterwards becomes the neck, but which is at first absent or extremely short, are formed by the anterior divisions of the aortic roots ; while the external carotid artery is due to the persistence of a channel in the continuation of each anterior aortic root, and the internal carotid artery arises from the persistence of the crossing third arch and the upper part of the posterior aortic root.

Thus it falls out that, in man and a certain number of mammals, an innominate artery is formed on the right side by the union of the first part of the fourth right aortic arch leading into the right subclavian, and the right anterior aortic root which forms the common carotid ; while, on the left side, the carotid and subclavian vessels rise separately from the permanent aortic arch in consequence of the distance lying between them in the original foetal condition.

It does not come within the scope of this chapter to describe the further steps of development of these vessels, nor to enter into an explanation of the manner in which abnormal position of the arch of the aorta and its branches, or of the pulmonary arteries, may be supposed to arise. For further information on this subject the reader is referred to the short account of the varieties given in the description of the blood-vessels in the first volume of this work, as well as to the third volume of Henle's Handbuch, and to the special works of Tiedemann and Eichard Quain on the Arteries.

DEVELOPMENT OF THE GREAT VEINS

In the early embryo, before the development of the allantois, a right and a left omphalo-mesenteric vem bring- back the blood from the walls of the nmbilical vesicle, and unite to form a short trunk, the meatus venosus, which is continued into the auricular extremity of the rudimentary heart.

In the first commencement of the placental circulation, or in the fourth week of foetal life, two umbilical veins are seen coming from the placenta, and uniting to form a short trunk, which opens into the common omphalo-mesenteric vein. Very soon the right omphalo-mesenteric vein and right umbilical vein disappear. In connection with the common trunk of the umbilical and omphalo-mesenteric veins, two sets of vessels make their appearance in the yomig liver. Those furthest from the heart, named vena; Iwpatlca advehentcs, become the right and left divisions of the portal vein ; the others are the hepatic veins, vencv kcpaticce. Tcvclwnifs. The portion of vessel intervening between those two sets of veins forms the ductnn rcnoKiix, and the part above the hepatic vein, being subsequently joined by the ascending vena cava, forms the upper extremity of that vein. Into the remaining or left omphalo-mesenteric vein, open the mesenteric and splenic veins. The part above the latter forms the trank of the portal vein ; and the portion of vessel between the union of this with the umbilical vein and the origin of the venaj hepaticaj advehentes is so altered that the portal tiimk opens into the commencement of the right vena advehens.

At the time of the commencement of the placental cu-culation, two short transverse venous trunks, the ductii of Ci/ricr, open, one on each side, into the auricle of the heart. Each is formed by the union of a superior and an inferior vein, named the itrimitive jugular and the cardinal.

The primitive jugular vein receives the blood from the cranial cavity by channels in front of the ear, which are subsequently obliterated : in the greater part of its extent it becomes the external jugiilar vein : and near its lower end it receives small brandies, which, grow to be the external jugular and subclavian veins. The cardinal veins are the primitive vessels which return the blood from the Wolffian bodies, the vertebral column, and the parietes of the trunk. The inferior vena cava is a vessel of later development, whichopens into the trunk of the umbilical and omphalo-mesenteric veins, above the vense heijaticas revehentes. The iliac veins, which unite to form the inferior vena cava, communicate with the cardinal veins. The inferior extremities of the cardinal veins are persistent as the internal iliac veins. Above the iliac veins the cardinal veins are obliterated in a considerable part of their course ; their upper portions then become continuous with two new vessels, the j;osfe?-ipr vertchral veins of Ratlike, which receive the lumbar and intercostal twigi.

Fig. 594. — Diagrams illustrating HE DEVELOPMENT OF THE GrEAT Veins (after KiJlliker).

A, plan of the principal veins of the fretus of about four weeks, or soon after the first formation of the vessels of the hver and the vena cava inferior.

B, veins of the liver at a somewhat earlier period.

C, principal veins of the foetus at the time of the first estabhshment of the placental circulation.

D, veins of the liver at the same period.

dc, the right and left ducts of Cuvier ; ca, the right and left cardinal veins ; j. j,ihe jugular veins ; s, the subclavian veins ; az, the azygos vein ; u, the umbilical or left umbilical vein ; «', in B, the temporary right umbilical vein ; o, the omphalo-meseuteric vein ; o', the right omphalo-mesenteric vein ; m, the mesenteric veins ; p, the jjortal vein ; p', p', the vente advehentes ; I, the ductus venosus ; V, I', the hepatic veins ; c'l, vena cava inferior ; il, the division of the vena cava inferior into common iliac veins ; cr, the external iliac or crural veins ; h, the hypogastric or internal iliac veins, in the line of continuation of the jirimitive cardinal veins.

In C, li, in dotted lines, the transverse branch of communication between the jugular veins which forms the left innominate vein ; ri, the right innominate vein ; ca, the remains of the left cardinal vein by whish the superior intercostal veins fall into the left innominate vein ; above lo, the obliquely crossing vein by which the hemiazygos joins the azygos vein.

As development proceeds, the direction of the ducts of Cuvier is altered by the descent of the heart from the cervical into the thoracic region, and becomes continuous with that of the primitive jugular veins. A communicating branch makes its appearance, directed transversely from the junction of the Jeft subclavian and jugular veins, downwards, and across the middle line to the rig-ht jugular ; and further down in the dorsal region between the posterior verteljral veras a communicating branch passes obliquely across the middle line from right to left. The communicating branch between the primitive jugular veins is converted into the left innominate vein. The portion of vessel between the right subclavian vein and the termination of the communicating branch becomes the right innominate vein. The portion of the primitive jugular vein below the communicating vein, together with the right duct of Cuvier. forms the vena cava superior, while the cardinal vein opening into it is the extremity of the great vena azygos. On the left side, the portion of the primitive jugular vein placed below the communicating branch, and the cardinal and i^osterior vertebral veins, together wdth the cross branch between the two posterior vertebral veins, are converted into the left superior intercostal and left superior and inferior azygos veins. The variability in the adult arrangement of these vessels depends on the various extent to which the originally continuous vessels are developed or atrophied at one point or another. The left duct of Cuvier is obliterated, except at its lower end, which always remains pervious as the coronal sinus. Even the adirlt, traces of the existence of this vessel can always be recognised in the form of a fibrous band, or sometimes even a narrow vein, which descends obliquely on the left auricle ; and in front of the root of the left lung there remains a small fold of the serous membrane of the pericardium, the re.sfii/ial fold of the pericardium, so named by Marshall, to whom is due the first full elucidation of the nature and relations of the left primitive vena cava.

Fig. 595. — A and B. — Diagrammatic Outlines op the Vestige of the Left Superigr Cava and of a Case of its Persistence (sketched after Marshall), h,

A, brachio-cephalic veins with the superior intercostal, azygos, and principal cardiac veins.

B, the same in a case of persistence of the left superior cava, showing its communication with the sinus of the coronary vein. The views are supposed to be from before, the parts of the heart being removed or seen through

1, 1', the internal jugular veins ; 2, 2', subclavian veins ; 3, right innominate ; 3', right or reguhir sujjerior cava ; 4, in A, the left innominate ; in B, the transverse or communicating vein between the right and left superior vemg cavte ; 5, in A, the oj)ening of the superior intercostal vein into the innominate ; 5', vestige of the left superior cava or duct of Cuvier ; 5, 5', in B, the left vena cava superior abnormally persistent, along with a contracted condition of 4, the communicating vein : 6, the sinus of the coronaiy vein ; 6', branches of the coronary veins ; 7, the superior intercostal trunk of the left side, or left cardinal vein ; 8, the principal azygos or right cardinal vein ; 7', 8', some of the upper intercostal veins ; 9, the opening of the inferior vena cava, with the Eustachian valve.

The left duct of Cuvier has been observed persistent as a small vessel in the adult. Less frequently a right and left innominate vein open separately into the right am'icle. an arrangement which is also met witli in birds and in certain mammals, and which results from the vessels of the left side being developed similarly to those of the right, while the cross branch remains small or absent. (Quain on the Arteries, plate 58, figs. 9 and 10.)

Fig. 596. — View op the Foetal Heart AND Great Vessels, from the left SIDE, TO SHOW THE VeSTIGE OF THE Left Superior Cava VEI^f in situ. (This figure is j)lanned after one of Marshall's, and slightly altered according to an original dissection.)

a, right auricle ; b, left auricle and pulmonary veins ; c, the conns arteriosus of the right ventricle ; d, the left ventricle ; c, descending aorta ; + , vestigial fold of the loericardium ; /, arch of the aorta, with a part of the pericardium remaining superiorly ; r/, main pulmonary artery and ductus arteriosus ; [/', left pulmonary artery ; 1,1', right and left internal jugular veins ; 2, 2', subclavian veins ; 3, 3', right innominate and superior vena cava ; 4, left innominate or communicating vein ; 5, 5', remains of the left superior cava and duct of Cuvier, passing at + in the vestigial fold of the pericardium, joining the coronary sinus, 6, below, and receiving above the superior intercostal vein, 7 ; 7', 7', the upper and lower intercostal vein, joining into one.

A case is recorded by Gruber, in which the left vena azygos opened into the coronary sinus, and was met by a small vein descending from the union of the subclavian and jugular. (Reichert and Dubois. RejTnond's Archiv, 18(54, p. 729.) In this case, the jugular veins had been developed in the usual manner, while the left vena azygos continued to pour its blood into the duct of Cuvier.

(Consult Kolliker, Entwickelungsgeschichte, p. 41-1, et seq. ; J. Marshall on the Development of the great Anterior Veins in Man and Mammalia, in Phil. Trans., part i., 1850 ; and Wenzel Gruber, Uber die Sinus Communis und die Valvulaj der Vense Cardiaca3, &c., in Mem. de I'Acad. imper. des Scien. de St. Petersbourg, 1SG4 ; and in Virchow's Archiv, Jan. 1SG6.)

PECULIARITIES OP THE FOETAL ORGANS OP CIRCULATION

It may be useful here to recapitulate shortly the peculiarities of structure existing in the advanced stage of the formation of the foetal organs of circulation with reference to their influence in determining the course of the blood during intra-uterine life, and the changes which occur in them in consequence of the establishment of pulmonary respiration at birth.

The so-called foramen ovale retains the form of a free oval opening in the septum auricularum up to the fourth month, but in the course o"f that month and the next there takes place the growth from below and on the left side of a flat plate or curtain, which advancing upwards fills up the floor of the fossa ovalis, adheres to its left borders as far as its anterior part, and then becoming free and passing beyond the anterior border of the fossa, converts the aperture into an oblique passage or slit over the valvular margin of the fold, so that in the last three and a half months the arrangement is completed, by which blood may readily pass from the right into the left auricle, but not in a contrary direction.

The Eustachian Valve constitutes a crescentic fold of the lining structure of the heart, which is situated to the right of the opening of the inferior vena cava and fossa ovalis, deepens that fossa, and directs the blood entering it from the inferior cava towards the opening of the foramen ovale ; while it throws the opening of the great coronary vein into connection with the right auricle, into which the superior vena cava also opens.

The formation at an early period of foetal life of the transverse vein of the neck uniting the left with the right brachio-cephalic veins, carries the whole of the blood returning from the head and neck, together with that from the main azygos, into the stream entermg the heart by the superior cava.

The ductus arteriosus passes from the main pulmonary artery into the aorta, at the hollow part of the arch, a short distance beyond the place of origin of the left subclavian artery. It is nearly of the same width with the pulmonary stem, while the right and left pulmonary arteries are of comparatively small size, so long as the lungs have not been expanded by air in rfespiration.

Umbilical Vessels

Besides the usual branches of the descending aorta intended to supply the abdominal viscera and the lower limbs, two large vessels, named hypogastric or iimhilical arteries, are prolonged from the common iliacs, and passing out of the abdomen, proceed along the umbilical cord, coiling round the umbilical vein to reach the placenta. The commencement of each of these vessels afterwards forms the tmnk of the corresponding- internal iliac artery, but, from their size, they might be regarded in the fojtus as the continuations of the common iliac arteries into which the aorta divides. From the placenta the blood is returned by the umbilical vem. which, after entering the abdomen, communicates by one branch with the portal vein of the liver, and is continued by another, named ductus rcnosus, into one of the hepatic veins, through which it joins the main stem of the vena cava inferior.

Fig. 597. — Semi-diageammatic view of the Organs of Circulation in the Fcetus FROM BEFORE (from Luschka, modified, and from Nature). |

a, front of the thyroid cartilage ; 6, right side of the thyroid body ; c, trachea ; d, surface of the right lung turned outwards from the heart ; e, diai^hragm below the apex of the heart ; /, right lobe of the liver, dissected to show ramifications of the portal and hepatic veins ; /', the middle part and left lobe of the liver in the same manner, .showing branches of the umbilical veins and ductus venosus ; g, right, fj', left kidney ; ij" , suin-arenal bodies ; h, right, li, left ureter ; i, portion of the small intestine tunied towards the side, to show the veins from it going to the portal vein ; Ic, urinary bladder ; I, is placed below the umbilicus, which is turned towards the left of the fcetus, and points by a line to the urachus ; m, rectum, divided and tied at its upjier part.

A, A. right auricle of the heart opened to show the foramen ovale : a probe, introduced through the large divided right hepatic vein and vena cava inferior, is seen passing through the fossa ovalis into the left auricle : at the lower part of the fossa ovalis is seen the Eustachian valve, to the right and inferioi'ly the auriculo-ventricular orifice ; B, the left auricular appendix ; C, the surface of the right ventricle ; D, placed on the inner surface of the left lung, i)oints to the left ventricle.

1, ascending part of the arch of the aorta ; 1', back part beyond the ductus arteriosus ; 1", abdominal aorta; 2, stem of the pulmonary artery ; 2', the place of division into right and left pulmonary arteries and root of the ductus arteriosus : the left pueumo-gastric nerve is seen descending over the arch of the aorta ; 3, superior vena cava ; 3', right, 3", left innominate vein ; 4, stem of the inferior vena cava, between the junction of the hepatic vein and the right auricle ; 4', continuation of the vena cava inferior below ; 5, umbilical vein within the body of the fojtus ; 5 x , without the bo'ly, in the umbilical cord ; 5', 5', ductus venosus ; between 5 and 5', the direct branches of the umbilical vein to the liver ; 5", 5", hepatic veins, tlu-ough one of which a probe is passed into the fossa ovalis and through the foramen ovale ; 6, vena portD3 ; 6', its left branch joining the umbilical vein; (i", its right branch; 7, placed on the right iliac vein, points to the right common iliac ai-tery; 7', left common iliac artery; 8, right, 8', left umbilical arteries coming from the internal iliac arteries ; 8 x , umbilical arteries without the body, in the umbilical cord ; 9, 9', external iliac arteries ; 10, placed below the right renal vessels ; 11, inferior mesenteric artery, above the root of which .ore seen the two spermatic arteries.

Fig. 597,

Course of the Blood in the Foetus

The right auricle of the foetal heart receives its blood from the two venre cavfe and the coronary vein. The blood brought by the superior cava is simply the venous blood returned from the head and upper half of the body ; whilst the mferior cava, which is considerably larger than the superior, conveys not only the blood from the lower half of the body, but also that which is returned from the placenta through the umbilical vein. This latter stream of blood reaches the vena cava inferior, partly by a direct passage — the ductus venosus, and partly by the hepatic veins, which bring to the vena cava inferior all the blood circulating through the liver, whether derived from the supply of placental blood entering by the umbilical vein, or proceeding from the vena portfe or hepatic artery.

The blood of the superior vena cava, descending in front and to the right of the Eustachian valve, and mixed with a small portion of that from the inferior cava, passes on into the right ventricle, and is thence propelled into the trunk of the pulmonary artery. A small part of it is then distributed through the branches of that vessel to the lungs, and retuins by the pulmonary veins to the left auricle ; but, as these vessels remain comparatively undilated up to the time of birth, by far the larger part passes through the ductus arteriosus into the dorsal aorta, entering that vessel beyond the place of origin of the arteries of the head and upper limbs, and, mixed perhaps with a small quantity of the blood flowing into the aorta from the left ventricle, is distributed in part to the lower half of the body and the viscera, and in part is conveyed along the umbilical arteries to the placenta. From these several organs it is returned by the vena cava inferior, the venaj portsB, and the umbilical vein : and, as already noticed, reaches the right auricle through the trank of the inferior cava.

Of the blood entering the heart by the inferior vena cava, only a small part is mingled with that of the superior cava, so as to pass into the right ventricle ; by far the larger portion, directed by the Eustachian valve through the foramen ovale, flows from the right into the left auricle, and thence, together with the small quantity of blood returned from the lungs by the pulmonary veins, passes into the left ventricle, from whence it is sent into the arch of the aorta, to be distributed almost entu-ely to the head and upper limbs. A small portion of it, may, however, flow on into the descending aorta, and join the fuller stream of blood from the ductus arteriosus. From the upper half of the body the blood is returned by the branches of the superior cava to the right auricle, from which its course into the right ventricle and pulmonary trunk has been already traced.

There is probably a considerable difference in the early and more advanced stages of foetal life, m the distribution of the stream of blood entering the heart by the vena cava inferior. In the early stages, a lai-ge part of the cuiTcnt being directed into the left, but in the three last months, and as the foetus approaches maturity, more and more of the blood of the inferior cava joins the stream from the superior cava ; and, indeed, the coui-se of the blood, and the relative position of the veins, as well as other original peculiarities of the foetal heart, become gradually altered, in preparation, as it were, for the important changes which take place at birth. It seems also probable that very little of the blood propelled from the left ventricle passes into the descending aoi-ta beyond the ductus arteriosus diu-ing those months of foetal life in which the peculiarities of the circulation are most complete.

From the preceding account of the course of the blood in the foetus, it will be seen that, whilst the modified blood from the placenta is principally conveyed to the upper or cephalic half of the foetus, the lower half of the body is chiefly supplied with the blood which has already circulated through the head and upper limbs. The larger portion of the blood, however, which passes into the descending aorta, is sent out of the body to the placenta. This duty is principally performed by the right ventricle, which after birth is charged with an office somewhat analogous, in having to propel the blood through the lungs. But the passage of the blood through the vessels of the umbilical cord and placenta is longer and subject to greater resistance than that of the pulmonary chculation, and the right ventricle of the foetus, although probably aided by the left in the placental circulation, also takes a large shave in the systemic through the lower half of the body ; and this, ijerhaps, may be the reason wliy the wall of the right equals in thickness that of the left ventricle in the fa^tus.

Sabatier was the first to call attention particularly to the action of the Eustachian valve in separating the currents of blood entering the right auricle by the superior and inferior venaj cavse. (Traite d'Anat., vol. ii., p. 22i.) This separation, as well as that occm-ring between the currents passing through the aortic arch and the ductus arteriosus into the descending aorta, were illustrated experimentally by John Reid. (See art. '• Heart," in Cyclop, of Anat. and Phj'siol., and Edin. Med. and Sui-g. Journal, ISo.j.) A striking confirmation of the extent to which the last mentioned division of the two currents of the foetal blood may take place, without distm'bance of the chculation up to the time of bii-th, is afforded by the examples of malformation in which a complete obliteration has existed in the aortic trunk immediately before the place of the union of the ductus arteriosus with the posterior part of the aortic arch.

CHANGES IN THE CIRCULATION AT BIRTH

The changes which occur in the organs of circulation and respiration at birth, and lead to the establishment of their permanent condition, are more immediately determined by the inflation of the, lungs with air in the first respiration, the consequent rapid dilatation of the pulmonary blood-vessels with a greater quantity of blood, and the interruption to the passage of blood through the placental circulation. These changes are speedily accompanied by shrinking and obliteration of the ductus arteriosus, in the space between the division of the right and left pulmonary arteries and its junction with the aorta, and of the umbilical arteries from the hypogastric trunk to the place of their issue from the body by the umbilical cord ; — by the cessation of the passage of blood through the foramen ovale, and somewhat later by the closure of that foramen, and by the obliteration of the umbilical vein as far as its entrauce into the liver, and of the ductus venosus within that organ.

The process of obliteration of the arteries appears to depend at first mainly on the contraction of the coats, but this is very soon followed by a considerable thickening of their substance, reducing ra^mlly their internal passage to a minute tube, and leading in a short time to final closure, even although the vessel may not present externally any considerable diminution of its diameter. It commences at once, and is perceptible after: a few respirations have occurred. It makes rapid progress upon the first and second days, and by the third or fourth days the passage through the umbilical arteries is usually completely interrupted. The ductus arteriosus is rarely found open after the eighth or tenth day, and by three weeks it has in almost all instances become completely impervious.

The process of closure in the veins is slower, there not being the same thickening or contraction of their coats ; but they remain empty of blood and collapsed, and by the sixth or seventh day, are generally closed.

Although blood ceases at once to pass through the foramen ovale from the moment of birth, or as soon as the left auricle becomes filled with the blood returning from the lungs, and the pressure ^Yithin the two auricles is equalised, yet the actual closure of the foramen is more tardy than any of the other changes now referred to. It is gradually effected by the union of the forepart of the valvular fold forming the floor of the fossa ovalis with the margin of the annulus on the left side ; but the crescentic margin is generally perceptible in the left auricle as a free border beyond the place of union and not unfrequently the union remains incomplete, so that a probe may be passed through the reduced aperture. In many cases a wider aperture remains for more or less of the first year of infancy, and in certain instances there is such a failure of the union of the valve as to allow of the continued passage of venous blood, especially when the circulation is disturbed by over-exertion, from the right to the left auricle, as occurs in the malformation attending the morbus coeruleus.

DEVELOPMENT OE THE GENITAL AND URINARY ORGANS

The development of the permanent genital and urinary organs in birds and mammals, is preceded by the formation of a temporary glandular organ with which the principal parts of both these sets of organs are in their origin connected. These bodies are named the Wolffian bodies, after their discoverer, C. F. Wolff". From this close association of these organs, it becomes necessary to describe their development together.

Fig. 598. — Enlarged View from before op the Left Wolffian Body before the Establishment op the DISTINCTION op Sex (from Farre after Kobelt).

a, a, h, d, tubular structure of the WoliBan bodj ; e, Wolffian duct ; /, its upper extremity ; <y, its termination in .r, the iiro-genital sinus ; h, the duct of Miiller ; i', its upper still closed extremity ; I:, its lower eud terminating in the uro-genital sinus ; I, the mass of blastema for the reproductive organ, ovary or testicle.

PRIMARY FORMATION OP THE URO-GENITAL SYSTEM.

Wolffian bodies

The Wolffian bodies occupy a considerable space in the abdominal cavity of birds and mammals from an early period of fcetal life, extending at first from the fifth or sixth protovertebral segments to near the caudal extremity, in the form of two reddish prominent ridges, one on each side of the primitive intestine, and below the protovertebral columns and primitive aortaa. They are thickest in the middle of their length, and taper somewhat at their upper and lower extremities. They consist, when fully formed, of short slightly convoluted tubes running transversely, connected on the inner side with vascular glomeruli, very similar to the Malpighian corpuscles of the permanent kidneys, and leading along the outer border into a tube named the Wolffian duct, which terminates on each side in the cloaca. The Maipighian glomeruli were first discovered by Katlike, who pointed out their vascular structure, and their vessels derived from neighbouring branches of the aorta. The ducts of the Wolffian bodies are found to contain a whitish fluid, and the bodies are believed to perform the glandular office of kidneys during a part of foetal life in the higher vertebrata, and they have accordingly received the name of irrimonlial Iciihieys, a designation which is quite appropriate, as it appears that in fishes and amphibia, they remain as the whole in some, and a part in others, of the permanent kidneys.

Fig. 599. — Human Embryo of from 25 to 28 days, viewed from before the thorax and abdomen opened (from Kolliker after Coste).

0, the eye ; in, the maxillary plate ; mn, the inferior maxillary plate ; h, the second postoral plate ; h, the heart ; w, Wolttiaii bodies and ducts on their outer borders ; I, the liver ; c', the upper and t", the lower limbs ; a, the allantoid pedicle, and on each side of it the umbilical arteries ; i, i, tue upper and lower parts of the intestine of which the middle parts with the vitello-intestinal duct have been removed, leaving the mesentery sti'etched between.

In the human foetus they begin to be formed along with the allantois, at a very early period, probably before the third week, as they are already very apparent in the fourth. They have attained their full size by the sixth week, and in the seventh and eighth are rapidly diminishing in size, in connection with the changes which accompany the development of the genital organs and the permanent kidneys.

Fig. 600. — Transverse Section through: the Embryo of the Chick and Blastoderm ON THE Second Day (from Kolliker). d d, hypoblast ; ch, chorda dorsalis ; u w, primordial vertebrae ; m r, medullary plates ; h, corneous layer or epihlast ; u lo h, cavity of the primordial vertebral mass ; in p, mesoblast dividing at s p into h p I, somatopleure, and d f, splanchnopleui'e ; un g, Wolffian duct, beginning in the intermediate cell-mass.

As development advances the "Wolffian bodies rapidly become proportionally shorter and thicker : they shrink towards the lower part of the abdominal cavity, and soon become almost entirely wasted. By the middle of the third month only traces of them are visible in the human embryo.

First origin of the Wolffian bodies. — Difference of opinion has for some time existed among embryologists as to the exact source of the rudiments of the uro-genital system, but it now appears to be fully ascertained by the concurrence of a variety of observers, more especially of Waldeyer, Scheuk, and Balfour, that the Wolffian duct, which is the iirst part formed, and the formative substance of the Wolffian tubes

fig. 601. — Tkaxsverse Section THRotJcn the Abdominal Region of the Chick ok THE Third Day (from Kolliker).

The explanation of the letters is the same as that in the previous figure.

Fig. 602. — Transverse Section of the Embryo-Chick of the Third Day (from Kolliker).

mr, medullary canal and medulla of the spinal cord ; en, notochord ; mch, primordial •vertebral mass ; 7n, muscle-plate ; dr and (//, groove of the primitive intestine as formed by the hyijoblast and splanchnopleure ; cio, one of the two aoi-tte : mw, Wolffian body ; unff, Wolffian duct ; re, vena cardinalis ; h, epiblast ; hp, somatopleure and its reflection into the amnion ; ^^j the p] euro-peritoneal space.

and glomeruli, proceed from the mesoblast, and as these form the foundation of the principal urinary and genital organs, it follows that this system as a wliole has its Inundation in the mesoblastic layer. In birds and mammals the duct, which is first formed, appears in its commencement as a sohd cord in the upper part of a group of cells, projecting below the epiblast, in the interval between the protovertebral mass and the united somatopleure and splanchnopleure of the mesoblast, and thence called the mtermcdiate cell mass (fig. GOO, vnfi). This cord becomes hollow, and gradually changes its place by sinking downwards in the cellular mass in which it is imbedded, towards the pleuro-peritoneal cavity, while the tubular and glomerular structures of the Wolffian body are developed as diverticula from the duct in connection with the neighbouring cellular blastema.

Fig. 603. — Kidneys, Wolffian Bodies, Wolffian and MtJLLERIAN ducts of a foetal bird. Magnified (after J. Miiller).

a, kidney ; 5, tuliular part of Wolfflan body : c, the ovary ; d, suprarenal body \ e, ureter ; /, Wolffian duct ; g, duct of Miiller.

The intermediate cell-mass now forms a considerable projection to the outside of the mesentery, which occupies a median position (figs. 602 and C04), and the epithelium on its surface exhibits a considerable thickening in two places, first, along the inner side, where it becomes columnar, and forms an opaque whitish ridge, the //«'we^;//7itflium, the seat of after formation of the primitive ovigerms ; and second, along the outer side in a line inside the seat of the Wolffian duct, where, by a process of grooved involution, there is gradually formed the duct named Miiller ian, after its discoverer, Johannes Miiller. It is now fully ascertained that both the Wolffian and Miillerian ducts are constantly present in all embryoes of birds and mammals, whatever the sex they may be destined afterwards to assume ; but the respective ducts have a difterent sexual destination, for the duct of ]\lUller becomes converted into the oviduct of the female, while in the male the Wolffian duct forms the vas deferens, or main seminal duct of the testicle ; and while vestiges of the duct of Miiller are perceptible in the developed male, remains of the Wolffian duct are almost always present in the female in a manner afterwards to be described.

The permanent kichicijs of birds and mammals take their origin in connection with the Wolffian duct and formative substance deposited near the Wolffian bodies. Their first rudiments consist in a diverticulum from the upper or dorsal aspect of the Wolffian duct near its posterior extremity, which constitutes the commencement of the ureter ; and from this the tubular and glandular parts of the kidney are formed by extension into the neighbouring mass of blastema at a period somewhat later than that of the development of the Wolffian body itself.

The researches of Waldeyer and others have shown that the procluciive glands of the generative organs in the two sexes, ovary and testis, arise from nearly the same part of the intermediate cell mass, but in a manner somewhat different. Both are mainly produced in the substance which lies along the inner border of the blastemic mass alreadyreferred to, and which may therefore be named the common reproductive blastema ; but with this important difference between them, that in the female the primitive ova originate more immediately from the cells of the surface in the germ ejntheUum, and become afterwards imbedded as Graafian follicles in the deeper substance of the mass which forms a stroma round the ova ; while the glandular substance of the testicle is apparently developed within the cell mass, without any direct concurrence of the superficial or germ epithelium, — which, though at first existing in male as well as in female cmbryoes, and even exhibiting some tendency to the enlargement of some cells as ovigerms (Waldeyer), soon becomes atrophied and reduced in thickness in the male as the structure of the testicle becomes developed.

Fig. 604. — Transverse Section op the Wolffian Body and Rudiment of the Ovary AND THE Duct op Muller in an Embryo Chick at the end of the fourth DAY (from Waldeyer).

WK, Wolffian body ; y, section of the Wolffian duct ; a, germ epithelium with, o, o, cells enlarging into ovigerms ; a', epithelium near the place of involution of MviLler's duct, z ; E, stroma of the ovai-y ; m, mesentery ; L, lateral wall of the abdomen.

The ducts of Muller open at their anterior extremities into the pleuro-peritoneal cavity by the orifice which ultimately becomes the infundibulum and fimbriated ostium abdominale ; and, as their lining membrane has originally been formed by an involution of the epithelium (germ-epithelium) of that cavity, it follows that the lining membranes of the female passages (Fallopian tubes and uterus) which in their later development assume the characters of mucous membrane, and are described as such, have in reality the same origin as the lining membrane of the pleuro-peritoneal cavity.

Fig. 605. — DiAGRAiniATic Outline OF THE Wolffian Bodies in THEIR RELATIONS TO THE EUDIJIENTS OF THE RErRODUcTivE Organs (A. T.).

ot, Seat of origin of the ovaries or testes ; W, Wolffian bodies ; 11', w, Wolffian chicts ; m, m, jVIiillerian ducts ; r/c, genital cord ; iig, sinus iirogeuitalis ; /, intestine ; cl, cloaca.

These ducts at first unite with the Wolffian ducts on each side separately, but later they become separated from them and conjoined at their lower or posterior extremity, and in the development of the female type the uterus results from the further growth of this median or united part, while in the male sex the prostatic vesicle and gland may be looked upon as its nearest representative, and other partial vestiges of the female passages are to be found in the human species and in various degrees in different mammals.

The Wolffian duct, as has already been stated, becomes the vas deferens of the testicle, while the secreting part of the gland, comprising the tubuli seminiferi and the rete testis, are developed in the reproductive blastema of the intermediate cell mass. The union of these two parts of the male organs through the coni vasculosi and the epididymis is brought about by the development of the eflFerent vessels in the upper part, or what may appropriately be termed the sexual part of the Wolffian body, as this structure has been shown by Banks and others to differ from the lower and larger part of the organ by the absence of the vascular tufts or glomerular arrangement in connection ^vith its tubes. The convoluted tubes forming the efferent vessels, which fi-om the time of their first production are in communication with the upper part of the Wolffian duct, become subsequently connected with the vessels of the rete testis, and thus the original Wolffian duct becomes in its upper part the tube of the epididymis, and in its lower the main excretory duct or vas deferens of the testis.

Homologies of the Wolffian body

An interesting view of the correspondence of the urino-genital organs in different animals is presented by the recent observations of embryologists on the formation of the Wolffian bodies. It was ascertained by His, Bornhaupt, Rosenberg and Goette, that in the lower vertebrates a second body similar to the Wolffian was formed later in connection with its main duct ; and the researches of Balfour and Semper have shown that in the selachians the permanent kidneys, which had long been believed to be the same with the Wolffian bodies, consist in reality of two sets of tubular organs, of which one corresponds to the Wolffian bodies of the embryoes of the amniota, while the other tubnlar body, already referred to as being of later formation and as connected with the main Wolffian duct, corresponds to the permanent kidneys of the higher animals. Balfour has also ascertained (Jour, of Anat. and Physiol., yoI. x., 1875) that in the selachians both the ducts are found which exist in the amniota, viz., both the Wolffian and the Miillerian ducts, but that they arise in a somewhat different manner from that by which they are produced in birds and mammals, inasmuch as in the selachians the duct of Miiller arises by the formation of a septal partition which divides the original duct through a considerable part of its length into two canals : one of these, the Miillerian duct, is in communication with the pleuro-peritoneal cavity in front, and opens into the cloaca behind as a separate tube ; the other corresponding with the Wolffian, besides being the excretory duct of the primordial kidneys, becomes the vas deferens of the testicle. In the selachians, therefore, the permanent kidneys consist of two parts, of which one, the anterior, is homologous with the temporary kidneys or Wolffian bodies, while the other, or posterior part, corresponds with the permanent kidneys of birds and mammals.

Fig. 606. — Diagram op toe Primitive Uro-genital Organs IN THE Embryo previous TO Sexual Distinction.

The parts are shown chiefly in profile, but the MuUerian and Wolffian ducts are seen from the front. 3, ureter ; 4, urinary bladder ; 5, urachus ; ot, the mass of blastema from which ovary or testicle is afterwards formed ; W, left Wolffian body ; X , part at the apex from which the coni vasculosi are afterwards developed ; w, w, right and left Wolffian ducts ; m, on, right and left Miillerian ducts uniting together and with the Wolffian ducts in g c, the genital cord ; ug, sinus urogenitalis ; ?', lower part of the intestine ; cl, common opening of the intestine and urogenital sinus ; cp, elevation which becomes clitoris or penis ; Is, ridge from which the labia majora or scrotum are formed.

Balfour and Semper have made further the interesting discovery that the transverse tubes of the two parts of the primordial kidney of the lower animals correspond in number and position with the vertebral segments of the region of the embryo in which they are situated, — a fact of great interest in vertebrate morphology, and, according to the authors, leading also to important views of the morphological correspondence of the organs in question with similar organs in the annelida. The tubes of the kidneys in the lower vertebrata are therefore named segmental tiilios, and their common duct (Wolffian), the segmental duct. lu the amniota, however, the same correspondence between vertebrate segments and Wolffian body tubes no longer exists. The External Organs. — The existence in the embryo at first of a single outlet or cloaca, for the urogenital passages and the alimentary canal in common, has already been referred to. This condition of the parts connected with the surface continues even beyond the time when the sexual distinction has begun to become manifest in the deeper organs, as up to the seventh day in the chick and the end of the eighth week in the human foetus. Previous to this time the cloaca presents itself in the form of a Avide cavity, into the middle of which the intestine descends on the dorsal aspect. The pedicle of the allantois opens by a deep groove or recess anteriorly or on the ventral aspect, and on each side there is a widening, into wliich, in succession from the ventral to the dorsal aspect, open the Mullerian and Wolffian ducts and the ureters. The external opening has the form of a vertical slit wider above and below, and is situated in a raised portion of the common integument, from which all the other parts retire more and more within the cavity of the pelvis as it gradually deepens.

Fig. 607. — Development of the External Sexual Organs in the Male AND Female from the Inbifferext Type (from Ecker).

A, the external sexual organs in an embryo of about nine weelis, in which external sexual distinction is not yet established, and the cloaca still exists ; B, the same in an embryo somewhat more advanced, and in which, without marked sexual distinction, the anus is now separated from the urogenital aperture ; C, the same in an embryo of aljoutten weeks, showing the female tj^se ; D, the same in a male embryo somewhat more advanced. Throughout the figures the following indications are employed ; }■)€, common blastema of penis or clitoris ; to the right of these letters in A, the umbilical cord ; p, penis ; c, clitoris ; cl, cloaca ; \({f, urogenital opening ; «, anus ; I s, cutaneous elevation which becomes labium or scrotum ; I, labium ; caudal or coccygeal elevation.

The first change which takes place in the rudiments of the external organs, and which is common to all embryoes, and therefore to botli sexes, consists in the advance from the sides and behind of the partition which separates the intestinal portion from the rest, thus throwing the urogenital ducts into connection with a wide ventral part ot the lower aperture, urogenital sinus, while the intestine is left in communication with the narrower dorsal section. The anus, strictly so called, now appears as the opening of the alimentary canal, and in front of it the urogenital aperture forms a narrow vertical slit wider behind than before, and leading into the urogenital sinus.

In front of the last-named aperture there now rises a well-marked prominence of the integument, the rudiment of the still indifferent organ rei:)resenting the clitoris or penis. Into this jirominence the nrogenital groove runs forward, and surrounding the prominence in front, and continued downwards on each side of the urogenital opening, there is a raised ridge of integument, which is the foundation of the future lal)ia majora in the female, and of the two halves of the scrotum in the male.

The description of the later changes which occur in these parts in the development of fuller sexual differences will be given hereafter. Here it will be sufficient to state their general nature. In embryoes which are assuming the male type, the common eminence becomes gradually longer, more cylindrical and deeply grooved along its lower surface. The lateral ridges of the urogenital opening become united from behind forwards along the middle line, and this union is gradually continued into the ridges of the groove below the penis, so as to enclose a canal which becomes the urethra with its tegumental and spongy vascular coverings, and to form below this the scrotum, in which the raphe is the remains of the median union of the integmnent.

In female embryos, on the other hand, the cylindrical eminence remains comparatively small, and the groove along its lower surface ■widens into two folds, forming the laljicc minorcc or nymphae ; while the larger lateral integumental folds, retaining their prominence and remaining separate, constitute the labia majora. The groove is not closed, but widened and shortened so as to become the vulva, while more deeply the sinus urogenitalis shortens itself considerably so as to form the limited atrium vagiiuc, into which open the urethra from the urinary bladder and the now united lower portion of Midler's ducts forming the yagina.

From the previous statement, it appears that both the urinary and the reproductive organs take their origin in symmetrical pairs from the intermediate cell-masses of the mesoblast, which are situated to the outside at first, and subsequently below, the protovertebral columns. The earliest formed of these organs are the Wolffian bodies, by which the others are all intimately connected together in their development, so as, to form one great system. It further appears that, while the urinary organs are developed in an entirely similar manner in all embryoes, there are in the sexual organs certain departures from the common type b}^ which the peculiarities of the male and female are established. The general plan of development of these organs having been previously described, the history of the process will now be completed by an account of the further changes which they undergo.

FURTHER HISTORY OF THE3 DEVELOPMENT OF THE UROaENITAL

ORGANS.

The Kidneys and their Ducts

These organs are developed together from a mass of formative cells situated posteriorly on the dorsal aspect of the Wolffian bodies, their first hollows being formed as diverticula from the Wolffian duct.

The formative blastema of the kidney, as observed 133- Eatlike in the foetal calf, soon contams a series of club-shaped bodies v^'hich have their larger ends free and turned outwards, and their smaller ends or pedicles directed inwards towards the futiire hilus, where they are blended together. As the organ grows these bodies increase in number, and finally, becoming hollow, form the uriniferous tuhes. At fii-st, short, wide, and dilated at their extremities, the tubuli soon become elongated, narrow, and flexuous, occupying the whole mass of the kidney, which then appears to consist of cortical substance only. At a subsequent period, the tubuli nearest the hilus become straighter. and thus fomi the medullary substance. The tubuli, as sho-wTi by Valentin, are absolutely, as 'u-ell as relatively, wider in the early stages of formation of the kidney. The Malpighian corpuscles have been seen by Ratlike in a sheep's emin-yo, the kidneys of which measured only two and a half lines in length. Koliiker observed the kidneys already foi-med in the human embryo of between six and seven weeks, the ureter being hollow, and communicating with dilated cavities within the rest of the blastema. At eight weeks they had assumed their characteristic reniform shape, and about the tenth week they are distinctly lobulated. The separate lobules, generally about fifteen in number, gi-adiially coalesce in the manner already described ; but at birth, indications of the original lobulated condition of the kidney are still visible on the surface, and the entire organ is more globular in its general figui-e than in the adult. The kidneys are then also situated lower do-mi than in after-life.

In the advanced fcetus and in the new-born infant, the kidneys are relatively larger than in the adult, the weight of both glands, compared with that of the body, being, according to Meckel, about one to eighty at bu-th.

The Suprarenal Bodies

These organs have their origin in a mass of blastema, placed in front of and between the kidneys and the Wolffian bodies. They appear to originate in a single mass, and afterwards to become divided. Koliiker has also observed them in close connection with the substance in which the large sympathetic plexus of the abdomen is produced, but it is not ascertained that they have a common origin.

In the human emlnyo the suprarenal bodies are at the seventh or eighth week larger than the kidneys, and quite conceal them, but after that time their relative size diminishes, so that at about the tenth or twelfth week they are smaller than the kidneys. At six months, according to Meckel, the proportion of the suprarenal bodies to the kidneys is as 2 to 5 ; at birth the proportion between them is 1 to 3, whilst in the adult it is about 1 to 22. They diminish much in aged persons, and are sometimes scarcely to be recognised.

The Urinary Bladder and Urachus

It has elsewhere been stated that in the human eniljiTO the vesicular part of the allantois extending beyond the umbilicus is closed at a very early period. Its pedicle, however, remains in communication with the urogenital sinus, and receives the ureters from the developing kidneys. The lower part of the pe'dicle undergoes a gradual dilatation to form the urinary hladder, while at the connection of this part with the urogenital sinus a constriction occurs in the part which gives rise to the urethra. Tliis in the female opens at once into the original urogenital sinus, but in the male the passage is continued onwards through the penis by the median union of the parts below that organ.

The part of the allantois situated above or in front of the bladder within the abdomen remains very much narrowed as the urachus, a tapering process of the upper extremity of the bladder into which at first the internal cavity is prolonged, but which later consists only of the muscular and fibrous coats. This process may for a time be traced for a short distance within the umbilical cord, but at an early period all vestiges of its farther prolongation disappear.

Genital Cord; — In both sexes, as was first fully shown by Tiersch and Leuckart in 1852, the two Wolffian ducts become united by surrounding substances into one cord behind the lower part of the urinary bladder ; but retaining internally their separate passages until they reach the sinus urogenitalis. With this cord the Miillerian ducts are incorporated posteriorly, so that at one time there are four passages through the whole of the genital cord. The Miillerian ducts next coalesce into one at some little distance from their lower ends, and this fusion, progressing upwards and downwards for a considerable space, a single median cavity is produced which lies between the still separate canals of the Wolffian ducts. A large accumulation of tissue in its walls gives to the genital cord great thickness as compared with the neighbouring parts of the ducts where they emerge from its enclosure. The lower ]3art of the united Miillerian ducts thus comes afterwards to form the foundation of the vagina and lower part of the uterus in the female, and the corresponding prostatic vesicle with its occasional accompaniments, or the uterus masculinus of the male.

Pig. 608. — Transverse Sections of the Genital Cord in a Female Calf Embryo. Magnified 14 diameters (from Kolliker).

1, near the upper end ; 2 and 3, near the middle ; 4, at the lower end ; a, anterior, p, posterior aspect ; m, Miillerian ducts, united or separate ; W, Wolffian ducts.

REPRODUCTIVE ORGANS.

In the farther history of the development of the genital organs it will be expedient to consider them in the two sexes in succession under

Fig. 609. — Internal Genital Organs op a Male Human Embryo of S^ inches long (from Waldeyer).

t, body of the testicle with seminal canals formed ; c, epididymis, or upper part of Wolffian body ; w, Wolffian body, lower part, 1>ecoming paradidymis or organ of Giraldes ; w', Wolffian duct, becoming vas deferens ; ff, gubernaculum.

the three heads of 1st, the productive organs ; 2nd, the conducting passages ; and 3rd, the external organs.

Reproductive Glands

It has already been explained that although the male and female productive organs lake their origin fi'om a mass of blastema which is on the whole identical in the two sexes, yet there are such differences in the development ol' the essential parts of the respective structures of the ovary and testicle as almost to warrant the conclusion that these organs are from the firsfc in some measure distinct.

The distinction of sex begins to be perceptible in the internal organs of the human eminyo in the seventh week, and becomes more apparent in the eighth. The reproductive gland is from the first connected with the Wolffian body, of which its blastema seems to be actually a part ; and it remains attached to it, or after its disappearance to the structure whichoccupies its place, by a fold of the peritoneal membrane, constituting the mesorchiura or mesovarium. Upper and lower bands fix the Wolffian body ; the upper passing to the diaphragm may be named the diaphragmatic ; the lower running down towards the groin from the Wolffian duct, contains muscular fibres and constitutes the future gubemaculum testis and round ligament of the uterus.

The Testicle

In male embryoes at the tenth week already seminal canals are visible, being at first, according to Kolliker, entirely composed of cells, but by the eleventh and twelfth weeks the tubes have become somewhat smaller, longer, and are now branched and possess a membrana propria. There is also by the end of the third month a commencement of lobular division, and the body of the testis is now covered with a condensed laj-er of fibrous tissue which forms the tunica albuginea.

In connection with the development of the spermatic filaments or spennatozoa, the essential part of the male reproductive element, previously referred to at p. 448 of this volume, it may here fui'ther be stated that renewed researches by Neumann (Ai'chiv filr Microsc. Anat., vol. xi., p. 292), appear to show that the doubts thrown by Sertoli and Merckel on the statements of V. Ebner are not well founded, that there really exist within the seminal ducts protoplasmic columns stretching from within the wall of the tube into its cavity, and that the spennatic filaments are produced in connection with the inner ends of the columns as branched lobes, amounting- in general to ten or twelve in number, in which the heads lie outwards imbedded in the protoplasmic stalk, and the filaments or tails are directed inwards towards the central lumen of the tube. Each stalk, or sjjcrmatohlant, as Neumann proposes to name it, possesses a large clear nucleus with nucleolus, and previous to the formation of the heads there are nuclei corresponding in number to them, which do not, however, appear to arise directly from division of the main nucleus of the stalk, but rather to be formed as free nuclei in the protoplasm. Each spermatozoon consists of three parts, which are most easily disting-uished in those which have not reached their stage of full development. These parts are, 1st, the head, or, as it may from its form in some animals be called, the hook ; 2nd, the body or middle pai-t, forming a slight thickening, and frequently of a vesicular appearance ; and 3rd, the filament or tail. The fii-st of these proceeds more immediately from a nucleus, the second is the remains of the protoplasmic covering of a spennatoblastic lobe, the feird is a ciliated production from the last. The bases of the spermatoblasts ai-e attached to the inner sm-face of the fibrous coat of the seminal canals, to which they furnish a complete lining, being set closely upon it like a layer of hexagonal plates. The stalks rise as tapering processes from these plates, and in the intervals between the stalks, necessarily largest towards the periphery, there is a number of opaque gi-anular spherical cells, the exact nature of which is not ascertained, but which it is conjectured may be the source of new spermatoblasts.

An interesting view is presented by Neumann of the analogy of these spennatoblasts of the seminal tubes with the much elongated ciliated cells whichare found in the canals of the coni vasculosi and tube of the epididymis, in accordance with which it may be held that the spermatic filaments are a peculiar forai of ciliary structm-e, developed from protoplasmic elements of a cellular nature, but which undergo a peculiar modification in connection with the special destination. cf the spermatozoa.

The Ovary

Considered as a glandular organ the ovary differs from other glands by the absence from it of excretory ducts, and by the

Fig. 610. — Internal Okgans of a Female human foetus of 3.| INCHES LONG. MAGNIFIED (from Waldeyer).

o, the ovai7 full of primordial ova ; e, tubes of the upper part of the WoMan body forming the epoophoron (parovarium of Kobelt) ; \V, the lower part of the Wolffian body forming the paroophoron of His and Waldeyer ; W', the Wolffian /'- .C"f ' 1 duct; M, the Miillerian duct; m, its upper fimbriated opening separation of its conducting passages from the glandular or productive part of its structure. Like the testicle it begins to manifest its peculiar characteristics by the seventh or eighth week, when the germ-epithelium has attained

considerable thickness, and forms a decided prominence on the mesial side of the Wolffian body. The farther development of the glandular part of the organ consists mainly in the formation of ovigerms and ova, and the implantation of these in Graafian follicles by a peculiar combination or intermixture of the superficial germinal cells with the deeper blastema which forms the stroma of the organ.

In a former part of this volume, imder Ovary, p. 478, the development of the primordial ova from a certain number cf the cells of the germ-epithelium and their enclosure in Graafian follicles by the growing stroma of the ovary have been described according to the most recent obsen'ations of "Waldeyer, Kolliker and J. Foulis. The publication of the very careful researches of the last observer enables us to add some important details to the previous description.

Figure 611, copied from some of Foulis's plates (Trans. Roy. Soc, Edin., 1875) will best show what from these observations appears to be the most j^robable view of the mode of development lof ova in the human ovary. At e, fig. 611. B, is seen a portion of the germ-epithelium, and at c', one of the cells undergoing enlargement and conversion into an ovigerm or primordial ovum. Of this the outer jirotoplasm becomes the yolk, and the nucleus the germinal vesicle with its nucleolus or macula. At o, a single o'S'igerm, and at o\ clusters of ovigerms in various stages of development have sunk into the ovarian stroma, and are beingsurrounded collectively and individually by the growth of the connective tissue of the ovarian stroma advancmg from below. Some of the o^^germs in the clusters are more advanced than the rest, and in these, as also in the isolated ovigerm represented in C, a covering of altered connective tissue coi-puscles is seen to be forming round the yolk protoplasm. This is the oi-igin of the cells of the tunica granulosa, which Foulis has shown are not produced, as W^aldeyer believed, from germ-epithelial cells, but from the interstitial connective tissue of the deeper ovarian stroma. In A, o, o, the cell fibres of the stroma («, «,) are seen suiTOunding several individual ova, so as to furnish the first elements of the wall of the Graafian follicles enveloping the ova, and covering immediately the granular cells. In D, representing an ovum somewhat farther advanced, the enlarged yolk-protoplasm and the geiininal vesicle are shown entire, with a fragment of the granular cell covering and fibro-cellular wall of the Graafian follicle ; but the zona pellucida is not yet perceptible.

The further steps in the formation of the ovum, as ascertained by the observations of Foulis, consist mainly in the enlargement of the mass of yolk protoplasm, the formation of a certain quantity of albuminous and fatty granules in combination with it (deutoplasm of Edw. van Beneden) ; and the formation externally of the zona pellucida or yolk-membrane by a consolidation cf the outer layer of the yolk substance. And here it may be remarked that the recent observations of Oellacher and Balfour on the radiated structure of the yolk protoplasm may explain in some degree, or be connected with the linear radiated marking of the zona pellucida.

Fig. 611. — Views of the Formation op Ova and Graafian Follicles in the Ovart

(from Foulis). A, small portion of the ovary of a human fcetus of SJ montbs, showing primordial ova imbedded in the stroma ; o, larger primordial ova ; o cluster of earlier ova ; n, fusiform corpuscles of the stroma. B, portion of the ovary near the surface in a human fcetus of 7 A months, showing the manner of inclusion of the germ epithelium corpuscles in groups in the ovarian stroma ; e, germ epithelium ; e', one of the cells enlarging into a primordial ovum before sinking into the stroma ; o, a larger cell imbedded, becoming an ovum ; o', groups of ovigerms or germ cells which have been surrounded by the stroma. C, young ovum from the same ovaiy, isolated ; p, yolk protoplasm. D, ovum more advanced, enclosed in condensed stroma, whichbegins to form a Graafian follicle ; p, yolk protoplasm ; V, germinal vesicle with macula ; g, the fusiform corpuscles now converted into the granular cells ; Gf, condensed stroma forming the wall of the Graafian follicle.

Such is the number of ova formed in the manner now described, that in the human foetus of six to seven months the whole substanc-e appears to consist of them and their newly formed Graafian follicles, by which each primordial ovum is closely embraced. A uniform layer of such ova of nearly equal size is especially to be found tovs^ards the surface ; but in the two later months of foetal life some of the ova and follicles advance to a farther stage of development, and increase in size, and this advance is invariably accompanied by a change of position of these ova to a deeper stratum of the ovary. The most advanced of the ova, therefore, are situated deepest in this the earlier stages of the ovarian development. It is different, however, when some "years after Ijirth, and still more towards the age of puberty, a few of the Graafian follicles expand to a great extent, and ultimately when mature reach the diameter of about a quarter of an inch, for then the expanding Graafian follicle gradually approaches the surface of the ovary, or perhaps rather, during the rapid expansion of the follicle, the ovarian stroma gives way by absorption between the follicle and the surface.

As the Graafian follicle expands with the slightly enlarging ovum, the thickness of the layers of condensed connective tissue or stroma round the ovum increases, and thus there are gradually formed the layers which liave been described as the follicular walls, while blood-vessels penetrate into them so as to form the vascular network of the covering. Within the follicle the granular cells multiply so as to form several layers lining the •whole follicle and closely covering the ovum. As yet there is no space between the ovum and wall of the follicle except that which is occupied by the granular cells, and for a long time the follicle is not larger than to enable it to enclose the ovum ; but in the more advanced stages a proportionally great enlargement of the follicle takes place, in consequence of the separation of two layers of the granular cells, so as to form a space in which fluid accumulates, and thus one or more layers •of cells are left lining the expanded follicle and constituting its tunica granulosa, while those covering the ovum, which is now thrown to one side of the follicle, form the investment known as the discus proligerus, -which appears as a reflected portion of the tunica granulosa (see figs. 335 and 336, p'3. 473 and 475).

As connected with the difference in the seat and mode of development of the ■essential parts of the male and female productive organs, the important question presents itself of the possibility or reality of the simultaneous coexistence in any cases of malformation of ovaries and testes on one or both sides of the body in the same individual. From what has been stated above, the possibility of such coexistence may perhaps be theoretically admitted. On this subject the reader may consult an interesting account by Dr. C. L. Heppner of St. Petersburg (Reichert's and Dubois Reymond's Ax-chiv for 1870, p. 679), of a hermaphroditic child which lived two months after biiiih, in which, along with a considerable amount of the better kno-mi conditions of approximation or mingling of the sexual charapters, it appeared that two organs coexisted, in one of which, agreeing in all respects with the ovary, primordial ova in Graafian folUcles were observed, and in another of a distinctly rounded iorm and compact structm'e, and so far corresponding to the testicle and unlike any of the other known vestigial organs, branched and coiled tubes, filled with cells in a manner exactly the same as those of the seminal canals, were ascertained by microscopic observation to exist. The jiarovarium (epididymis or coni vasculosi) also existed.

The genital passages

The existence of two sets of tubes between the iuteinal productive organs and the external parts has already been adverted to as a feature common to both sexes. The female organs contrast Avith the male in the large development of one of these tubes, viz.. the Miallerian ducts into their passages, and in the abortive disappearance of the greater part of the Wolffian ducts ; while in the male the ducts of Miiller suffer in a great measure the abortive retrogradation, and the seminal conducting tubes are produced out of canals formed within special parts of the Wolffian body and the whole of the Wolffian duct. But as in all embryoes of whatever sex both sets of tubes are originally present, while a different one of the original tubes becomes developed into the respective permanent conducting passages, vestiges of the other original tubes are invariably present in various degrees in both sexes.

The Female passages. — In the female, the vagina, uterus, and Fallopian tubes are formed out of the Miillerian ducts. That portion of the ducts in which they become fused together is developed into the vagina, the cervix, and part of the body of the uterus ; and the pecu

Fig. 612. GKAJI OF THE FEMALE Type op Sexual Organs.

This and figure 615 represent diagrammatically a state of the parts not actually visible at cue time ; but they are intended to illustrate the general type in the two sexes, and more particularly the relation of the two conducting tubes to the develop inent of one as the natural passage in either sex, and to the natural occurrence of vestiges of the other tube, as well as to the persistence of the whole or parts of both tubes in occasional instances of hermaphroditic nature.

1, the left kidney ; 2, suprarenal body ; 3, ureter, of which a i)art is removed to show the

parts passing within it ; 4, urinary bladder ; 5, urachus ; o, the left ovary nearly in the place of its original formation ; p o, parovariiun, ejioophoron of Waldeyer ; W, scattered remains of Wolffian tubes near it, parooi^horon of Waldeyer ; d G, remains of the left Wolffian duct, such as give rise to the duct of Gaertner, represented by dotted lines ; that of the right side cut short is marked w ; /, the abdominal opening of the left Fallopian tube ; u, the upper part of the body of the uterus, presenting a slight appearance of division into cornua ; the Fallopian tube of the right side cut short is marked in ; g, round ligament, corresponding to gubernaculum ; i, lower part of the intestine ; V a, vagina ; 7i, situation of the hymen ; C, gland of Bartholin (Cowper's gland), and immediately above it the urethra ; c c, corpus cavernosum clitoridis ; s c, vascular bulb or corpus spongiosum ; n, nympha ; I, labium ; v, vulva.

liaritv of the mode of fusion accounts for the occurrence, as a rare anomaly, not only of double uterus, but of duplicity of the vagina, coincident with communication between two lateral halves of the uterus. The next following part of the Miillerian duct, constitutes in animals with horned uteri, the cornu of the uterus ; but in the human subject it remains comparatively short, entering into the formation of the upper part of the organ. The remaining upper portion of the Miillerian duct constitutes the Fallopian tube — becoming at first open and subsequently frino-ed at a short distance from its upper extremity.

The pediculated hydatid of the fimbriated extremity (Hydatid of Morgagni) appears to be the remains of the original upper end of the Miillerian tube. The additional or accessory fimbrise and openings referred to at p. 471, and by Henle in his Handbuch, vol. ii., p. 470, may admit of explanation on the supposition of the duct of Miiller having remained open at these places.

In the human embryo of the third month the uterus is two-homed, and it is by a subsequent median fusion and consolidation that the triangular body of the entire organ is produced. The comua uteri, therefore, of the human uterus correspond with the separate comua of the divided uterus in animals, and this explains the occasional malformation consisting in the gi-eater or less division of the uterine cavity and vagina into two passages. There is no distinction in the human foetus in the third and fourth month between the vagina and uterus. In the fifth and sixth months the os uteri begins to be formed, and the neck is subsequently gradually distinguished. Thickening succeeds in the walls of the uterine portion ; but this takes place first in the cervix, which up to the time of birth is much larger and thicker than the body of the uterus (KoLLiker).

Fig. 613.— Female Genital Okgans of the Embryo with the Remains of the Wolffian Bodies (after J. Miiller).

A, fi'om a foetal sheep ; n, the kidneys ; h, the ureters ; c, the ovaries ; d, remains of ■Wolffian bodies ; c, Fallopian tubes ; /, their abdominal openings ; g, their union in the body of the uterus. B, more advanced from a foetal deer ; a, body of the uterus ; b, comua ; c, tubes ; d, ovaries ; e, remains of Wolffian bodies. C, still more advanced from the hviman fcetus of three months ; a, the body of the uterus ; b, the round ligament ; c, the Fallopian tubes ; d, the ovaries ; e, remains of the Wolffian bodies.

In the meantime the Wolfiian bodies undergo a partial atrophy, and their ducts become more or less obliterated and abortive in different parts. The most constant vestige of the "Wolffian bodies in the female is the now well-known body of Rosenmiiller or Parovarium of Kobelt (Eosenmiiller, Quoidam de Ovariis Embry. Human., Lipsiaj. 1802 ; Kobelt. der Nebeneierstock des Weibes, Heidelberg-, 1847), which has already been described at p. 480 of this volume, the ciHwj)horon of Waldeyer, and which, being produced out of the same elements as the epididymis of the male, presents a remarkable resemblance to that body. The canal uniting the radiating tubes (coni vasculosi) of this organ is also usually persistent, but ceases at a short distance below. In the sow and several ruminants, however, the subdivided upper tubular part or epoophoron has disappeared, and the main tube (middle part of the Wolffian duct) remains in the (hu-t of Gaertner, a strong, slightly undulated tube, which is traceable, first free in the broad ligament of the uterus, and lower down becoming incorporated with the wall of the uterus and vagina, upon which last it is lost.

Fig. 614. — Adult OvARr, Parovarium anu FalluI'Iaii Tube (from Farre, after Kobelt).

a, a, Epoophoron (parovarium) formed from the xipper part of the Wolffian body ; b, remains of the uppermost tubes sometimes forming hydatids ; c, middle set of tubes ; d, some lower ati-ophied tubes ; c, atrophied remains of the Wolffian duct ; /, the terminal bulb or hydatid ; h, the Fallopian tube, originally the duct of Midler ; i, hydatid attached to the extremity ; I, the ovary.

The Male Passages

The conversion of the Wolffian duct into the vas deferens of the testicle was first demonstrated in animals by Ratlike, in correction of the views of J. Miiller (Meckel's Archiv, 1833), and was further proved and illustrated by H. Meckel and Bidder (H. Meckel, Zur Morphol. der Ham uud Geschlechts-Organe der Wirbelthiere, Halle, 1848 ; Bidder, Male Organs in the Amphibia, Dorpat, 184G). KuUiker showed that a similar process occurs in the human embryo, and that a communication established between the seminal tubes of the testicle (rete testis) and some of the upper tubes of the Wolffian body gave rise to the epididymis.

The observations of Cleland and Banks first pointed out clearly the difference between the structure of the upper nonglomerular, or simple tubular part of the Wolffian body, and that of the lov.er and glomerular, or primordial-kidney part.

In the male, the Miillerian ducts are destined to undergo little development and are of no physiological importance, while the ducts of the Wolffian Iwdies, and probably also some part of their glandular substance, form the principal part of the excretory apparatus of the testicle. The rmited portion of the Miillerian ducts remauis as the vesicula prostatica, which accordingly not only corresponds with the uterus, as was shown by Weber, but likewise, as pointed out by Leuckai-t. contains as much of the vagina as is represented in the male. In some animals the vesicula prostatica is prolonged into comua and tubes ; but in the human subject the whole of the ununited parts of the Miillerian ducts disappear, excepting, as suggested by Kobelt. their upper extremities, which seem to be the source of the hydatids of Morgagni. The excretory duct of the Wolffian body, from the base of that body to its orifice, is converted into vas deferens and ejaculatory duct, the vesicula seminalis being formed as a diverticulum from its lower part (Waldeyer).

With respect to the fomiation of the epididymis, it appears certain that the larger convoluted seminal tube, which foi-ms the body and globus minor of the epididymis, arises by a change or adaptation of that part of the Wolffian duct which runs along the outer side of the organ. The vas aben-ans or vasa aberrantia of Haller appear to be the remains also, in a more highly convoluted foi-m, of one or more of the tubes of the Wolffian body still adhering to the excretory duct of the organ, and their communication with the mam tube of the epididymis receives an explanation from that circumstance. As to the coni vasculosi in the upper part of the epididymis, it has been customary to regard them as produced by a transformation of the tubes and duct in the upper part of the Wolffian body, according to the views most fully given by Kobelt ; but, according to the more recent observations of Banks, the origin of the coni vasculosi is most jn'obably due to a process of development occm-ring in a new stnictiure or mass of blastema which had been jireviously observed by Cleland, and which is foiTued in connection with the upper end of the "Wolffian body, and close to the Miillerian duct. "Within this blastema Cleland showed that the tubes of the efferent seminal vessels and the coni vasculosi, together with the tube which connects them, are formed anew, while the tubes of the lower primordial -kidney part of the "Wolffian body are undergoing an atroi^hic degeneration. This has been confirmed by the detailed observations of Banks, who has further shown the continuity of their uniting tube with the "\Yolffian excretory duct.

Fig. 615. — Diagram of THE Male Type of Sexual Organs.

1, 2, 3, 4, and 5, as in iigure 612 ; t, testicle in the place of its original formation ; e, caput eiiididymis ; v d, vas deferens ; W, scattered remains of the Wolffian body, constituting the organ of Girakles, or the paradidymis of Waldeyer ;

V h, vas aberraus ; on, Miillerian duct, the upper part of -which remains as the hydatid of Morgagni, the lower part, represented by a dotted line descending to the prostatic vesicle, constitutes the cornu and tube of the iiterus masculinus ; g, the gubernaciilum ;

V s, the vesicula seminalis ; p r, the prostate gland ; C, CowiJer's gland of one side ; c f, corpora cavernosa penis cut sliort ; s p, corpus spongiosum urethraj ; s, scrotum ; t' , together with the dotted lines above, indicates the - direction in Avhich the testicle and epididymis change place in their descent from the abdomen into the scrotum.

According to this view, the caput epididymis must be regarded, not simply as a conversion of the ujiper part of the "Wolffian body, but rather as a new formation, or superinduced development of tubes in blastema connected with it.

The coni vasculosi, so fonned, become connected with the body of the testicle by means of a short straight cord, which is afterwards subdivided into the vasa efferentia. The peritoneal elevation descending from the testis towards the lower extremity of the "Wolffian body, is the upper part of the plica gubernatrix, and becomes shortened as the testicle descends to meet the lower end of the epididjTiiis ; the peritoneal elevation which passes do'OTi into the scrotum, and is continuous with the other, is the more important part of the plica gubernatrix, connected with the gnbernactilum testis. The spennatic arteiy is originally a branch of one of those which go to the "Wolffian body, and ascend from the surface of the Wolffian body to the upper part of the testis, along the ligaments connecting them ; but, as the testis descends, the artery lies entnely above it, and the secreting substance of the Wolffian body remains adherent to it ; and hence it is that the organ of Gii-aldes, which consists of persistent "\\'olffian tubules, is found in a position superior to the ei^ididymis. (For a fuller account of this subject the reader is referred to Banks "-On the Wolffian Bodies," Edin 18G-i.)

Fig. 616. YiEW FROM BEFORE OF THE Adttlt Testis and EpididtMis (from Farre, after Kobelt).

a, a, convoluted tubes in ■ the head of the epididymis developed from the upper part of the Wolffian body ; b and /, hydatids in the head of the epididymis ; c, coni vasctdosi ; (/, va.sa aberrantia ; /(, remains of the duct of ]\Udler ■with i, the hydatid of IMorgagni at its upper end ; , body of the testis.

The Descent of the Testicles

The testicles, which are originally sitixated in the abdominal cavity, pass down into the scrotum before birth. The testicle enters the internal inguinal ring in the seventh month of foetal life : by the end of the eighth month it has usually descended into the scrotum, and, a little time before birth, the naiTow neck of the peritoneal pouch, by which it previously communicated with the general pei-itoneal cavity, becomes closed, and the process of peritoneum, now entu-ely shut off from the abdominal cavity, remains as an independent serous sac. The peritoneal pouch, or processus vaginaliB, which, passes do-mi into the scrotum, precedes the testis by some time in its descent, and into its posterior part there projects a considerable columnar elevation already alluded to, which is filled with soft tissue, and is termed 2>lica (juhcrnatrix . There is likewise a fibrous structui'e attached inferiorly to the lower part of the scrotum, and surrounding the peritoneal pouch above, which may be distinguished as the (jul}t'rnacular cord, both this and the plica gubernatrix being included in the general tei-m g uhernaciiluvi testis (J. Hunter). The gubernacular cord consists of fibres which pass downwards from the sub-peritoneal fascia, others which pass upwards from the superficial fascia and integmnent, and others again which pass both upwards and downwards from the internal oblique muscle and the aponeurosis of the external oblique ; it exhibits, therefore, a fusion of the layers of the abdominal wall. Superiorly, it surrounds the processus vaginalis, without penetrating the plica gubernatrix ; and the processus vaginalis, as it grows, pushes its way down through the gubernacular cord and disperses its fibres. By the time that the testis enters the internal abdominal ring, the processus vaginalis has reached a considerable way into the scrotum ; and. as the testis follows, the plica gubernatrix becomes shoiler, till it at last disappears ; but it cannot be said that the shortening of the plica is the cause of the descent of the testicle, and much less that (as has been held by some) the muscular fibres of the gubernacular cord are the agents which effect this change of position. The arched fibres of the cremaster muscle make their appearance on the surface of the processus vaginalis as it descends, while its other fibres are those which descend in the gubernacular cord. (See, for a further account of this process, and the various views which have been held with regard to the descent of the testicles, Cleland, " Mechanism of the Gubemaculum Testis." Edinburgh, IS.JG.)

The External Organs

In the human embryo, as in that of animals, the external organs are up to a certain time entirely of the same form in both sexes ; and the several organs which aftenvards distinguish the male and female externally take their origin respectively from common masses of blastema of precisely similar structure and connections. The common cloaca exists till after the fifth week, and the genital emmence from which the clitoris or penis is formed makes its appearance in the course of the fifth and sixth weeks in front of and within the common orifice. In the course of the seventh and eighth weeks the common orifice is seen to become divided into two parts, viz., the longer slit of the genito-urinary apeiture anteriorly, and the naiTower and more rounded anal aperture posteriorly : but the exact manner in which the separation of these two apertures takes place has not yet been acciu'ately traced. It is intimately connected with the formation of the urogenital cord as an independent stmcture, and is probably mainly effected by the advance from the sides and posteriorly of septal bands which separate the lower pai"t of the intestine. Somewhat later, or in the ninth and tenth weeks, a transverse integumental band completes the division between the anal and the urogenital orifices, which band forms the whole of the so-called perineum of the female, and the part of the perineal integument in the male which is situated behind the scrotum ; the raphe being most obvious in the male sex.

The cutaneous folds, or cii-cular genital ridge, which are afterwards converted into mons Veneris, labia majora, and scrotum, as well as the lips of the urogenital furrow, which are converted into the nymphte of the female and unite as integument below the penis in the male, are both of early formation and at first precisely the same in all embryoes. In this condition, which continues until the eleventh or twelfth week, the parts appear alike in both sexes, and resemble very much the more advanced female organs. The rudiments of JJartholui's or Ceirpcrs glands are, it is said, seen at an early period, near the root of the rudimentary clitoris or penis, on each side of the genito-uininary passage.

In the female, the tvro lateral cutaneous folds enlarge, so as to cover the clitoris and form the labia majora. The clitoris itself remains relatively smaller, and the gToove on its under surface less and less marked, owing to the opening out, and subsequent extension backwards, of its margins to form the ntjinplKP. The vascular bulbs remain distinct and separate, except at one point where they run together in the glans clitoridis. The hijmcn begins to appear about the fifth month as a fold of the lining membrane at the opening of the genital passage into the urogenital sinus. Within the vestibule, which is the shortened but ■widened remains of the urogenital sinus, the urethral orifice is seen, the lu'ethra itself undergoing considerable elongation.

In the male, on the contrary, the j;c'«w continues to enlarge, and the margins of the groove along its under surface gradually unite from the primitive urethral orifice behind, as far forwards as the glans, so as to comiilete the long canal of the male itrcthra, which is therefore a prolongation of the urogenital sinus. This is accomplished about the fifteenth week. "When this union remains incomplete, the abnormal condition named iLUpo.^padlas is produced. In the meantime the prcjniec is formed, and, moreover, the lateral cutaneous folds also unite from behind forwards, along the middle line or raphe, and thus complete the scrotum, into which the testicles descend in the course of the eighth month of fojtal life.

The corpora cavernosa, which are at first separate, become united in their distal portions in botli sexes ; but the corpus spongiosum lu'ethraj which is also originally divided in all embryoes, and in the female remains so in the greater part of its extent, becomes enlarged in the male in the glans penis, and its two parts united mesially both above and below the urethra, so as to enclose the whole of that tube from the bulb forwards to the glans.

TYPE OF DEVELOPMENT AND ABNORMAL FORMS OF THE GENITAL ORGANS

The type of development of the genital organs may be stated to differ in the several parts of the system in the two sexes as follows, viz. : —

1st. It is single and homological in the external organs.

2nd. It is double and heterological in the middle organs or passages.

3rd. It is partially double and heterological in the productive organs.

Accordingly the congenital malformations of the reproductive organs admit of being distributed under the following divisions : —

1st. Abnormal forms attributable to deficient, redundant, or abnormal modes of development of one or more of the external organs in either sex, producing an approach to the fonn of the other sex.

2nd. Forms referrible to deficient, redundant, or abnonnal modes of development of one or other of the two sets of sexual passages, viz., of the Wolffian or Miillerian ducts, so as to lead to the greater or less predominance of sexual characters in a part or the whole of these passages inconsistent with tliose prevailing in other parts of the system, or to the coexistence of both sets of passages in whole or in i:)art.

8rd. Extremely rare forms referrible to the possible coexistence of the productive parts of testicles and ovaries in the same individual, usually combined with more or less of the foregomg kinds of malformation.

Upon the subject of these malformations the reader may consult the learned and able article Hermaphroditism by Sir James Y. Simpson in the Cyclop, of An at. and Physiol.

Upon the subject of malformations in general the following works are recommended, viz. : —

Isid. Geoff. St. Hilaire, Hist. Gen. et Partic. des Anomalies de I'Organisation, &c., 3 torn. Paris, 1832 — 6 ; Cruveilhier, Anat. Pathol., &c., Paris, 1S;>() — 42. Otto, Sexcentorum Monstrorum desc Anat. Vratisl., 1841 ; Th. L. W. Bischoff. Uber Missbildungen, &c., in R. Wagner's Handworterbuch der Physiol., 1843 ; Wm. Vrolik, Tab. ad illustr. Embryol. Hom. et Mammal, tam Natiu-. quam Abnormem, Amstel., 1849, and the article " Teratology"' by the same author in Todd's Cyclop, of Anat. and Physiol. ; Aug. Forster, Die Missbildungen des Menschen. &c., Jena, 1861 ; as also the systematic works of Rokitanski and others on Pathological Anatomy.

The following tabular scheme of the Coreesponding Parts of the ffenito-urinary organs in the two sexes, and of their relation to the Formative Rudiments of the common embryonic type, may be useful in fixing attention on the more important points of the foregoing description, and indicating more clearly the homologies of the parts : —

Fejule Perjiaxext. Common Embryonal. Male Permanent.

I. -COMMON BLASTEMA OF REPRODUCTIVE GLANDS.

Ovary. Body of Testicle.

Fnniislies the ovigerms and re- 1. Germ-epithelium covering . . Uisuppear.s, and is replaced by mains on the .surface. serous covering of tunica

vaginalis.

Forms stroma of the ovary and 2. Deeper blastema Forms glandular seminal tubes

the Graalian follicles. of the testis.

II.- WOLFFIAN BODIES.

Transverse tubes of epoophoron 1. Upper tubular non-glonieru- Vasa efferentia and coni vas or organ of Rosennuilkr lar part. culosi of the epididymis. (Parovarium).

Paroophoron (Wald.) 2. Lower glomerular part (pri- Paradidymis (Wald.), organ of

mordial kidneys). Giraldes, and ^'asa aberrantia.

Koimd ligament of the uterus . . 3. Ligament of the "Wolffian Gubemaculum testis.

body.

Ill- WOLFFIAN DUCTS.

Tube of the Epoophoron 1. Upper and middle parts .... Convoluted tube of the epididymis.

Ducts of Gaertner, in cow and pig 2. Lower part Vas deferens and vesiculse

seminales.

IV— MULLERIAN DUCTS.

Pimbriated abdominal opening 1. Upiwr extremity Hydatid of Morgagni.

and terminal and oecasiunal hydatids

Paliopian tubes 2. Middle part Occasional tubular pi'olonga ticns (it uterus masculinus.

Vagina and uterus 3. Lower single or meiiian part Uterus masculinus (vesicuU

prostatica).

v.— GENITAL CORD AND SINUS UROGENITALIS.

Tissue uniting female urethra and 1. Substance surrounding geni- Prostate gland. Muscular and vagina. tal cord. glandular tissue.

Female urethra 2. Upper part of cavity or Upper part of prostatic portion

urinai-j' pedicle. of the urethra.

Ostium vaginae . Hymen 3. Confluence of urinary and Verumontanum.

genital ]iarts. Vestibule 4. Lower part Lower ]iart of prostatic portion and membranous part of urethra. Glands of Bartholin 5. Common blastema Cowper's Glands.

VL— EXTERNAL ORGANS.

1. — Vascular parts.

Crura and corpus clitoridis a. Corpora cavern<jsa Crura and corpus penis.

Glans clitoridis and vascular 0. Corpora spongiosa Glans penis and spongy body

bulbs separate) of urethra (united). 2. Integumental parts.

Preputium clitoridis «. On genital eminence Preputium penis.

Integumental folds of nymphee b. Lijjs of genital furrow Integument and raphe below

(separate). . ]ieiiis.

Labia majora (se]iarate) c. Genital ridges (lateraP Scrotum and raphe (united).

Perineum of female, with raphe . U. Transverse interauogcnital Perineum of male beliind

baud. scrotum, with raphe.