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and the anterior medullary velum constitute parts of the isthmus rhombencephali (Figs. 45, 63).
The ventral portion of the primitive mid-brain is converted into the two peduncles of the cerebrum of the adult brain, and the dorsal portion is transformed into four rounded elevations, the colliculi or corpora quadrigemina.
The transformations which take place in the region of the primitive fore-brain or prosencephalon are numerous and complicated; therefore its ventral, lateral, and dorsal walls require separate consideration.
By the expansion of its cephalic (anterior) extremity is formed the secondary fore-brain, which becomes divided, as already explained, into the two secondary vesicles which are the rudiments of the cerebral hemispheres of the completed brain. After the formation of the rudiments of the cerebral hemispheres, which
constitute the telencephalon of the adult, the primitive fore-brain and the undivided stalk of the secondary fore-brain diverticulum become the diencephalon.
The cephalic or anterior end of the diencephalon is closed by the lamina terminalis (see p. 33), in association with which are subsequently developed two columns which run dorsoA. The different subdivisions of the brain are marked off from each other by dotted ventrally, the lines, and the dotted line running in the long axis of the neural tube indicates the columns of the separation of the alar from the basal lamina of the lateral wall.
FIG. 45.-DIAGRAMS TO ILLUSTRATE THE ALAR AND
cases the embryonic brain is represented in mesial section (His).
fornix (O.T. anB. Medial section through the brain of a human embryo at the end of the first month. Dotted lines mark off the different regions and also the alar and basal terior pillars), and two transverse
laminæ from each other.
H, Buccal part of hypophysis cerebri; RL, Olfactory lobe; C.STR, Corpus striatum; commissures, one A, Entrance to optic stalk; 0, Optic recess; I, Infundibular recess; T, Tuber of which connects cinereum; M, Mamillary eminence. together the two cerebral hemispheres and is called the anterior commissure, whilst the other is the optic chiasma in which the medial fibres of the optic nerves decussate.
From the cephalic or anterior end of the ventral wall of the diencephalon a diverticulum is projected ventrally towards the dorsal wall of the primitive mouth. The ventral end of this diverticulum becomes the posterior lobe of the hypophysis (O.T. pituitary body) of the adult, the dorsal end becomes the tuber cinereum, and the intermediate part is the infundibulum which connects the tuber cinereum of the adult brain with the posterior lobe of the hypophysis.
Caudal to the hypophyseal diverticulum a single elevation appears in the ventral wall of the diencephalon. It is the corpus mamillare, which afterwards separates into the paired corpora mamillaria of the adult brain.
Still more caudally the ventral wall of the diencephalon takes part in the formation of the substantia perforata posterior, which lies between the two peduncles of the cerebrum and is partly developed from the cephalic or anterior end of the ventral wall of the primitive mid-brain.
The greater part of the dorsal wall of the diencephalon is ultimately reduced to a single layer of epithelial cells, but near its caudal end a diverticulum is projected dorsally. This is the epiphysis or pineal body, which remains quite
rudimentary in man as contrasted with many other animals. At a later period two transverse bands of fibres appear in the dorsal wall of the diencephalon, one in front of and the other immediately behind the root of the epiphyseal recess. The anterior band is the dorsal or habenular commissure, and the posterior is the posterior commissure of the adult brain.
These structures, collectively, together with a small diverticulum of the epithelial roof, which appears anterior to the dorsal commissure, and is called the supra-pineal recess, constitute the so-called epithalamus.
Each lateral wall of the diencephalon is differentiated into a dorsal and a ventral part. The dorsal part forms a large gray mass called the thalamus, and on the posterior end of the thalamus are developed two rounded eleva
tions, the medial and the A. Diagram of a transverse section of a spinal medulla which has not differentiated into groups of cells. B. Diagram of a transverse section of a spinal medulla showing positions of germinal cells.
lateral geniculate bodies, which constitute the metathalamus of the adult brain.
The ventral or basal portion of the lateral wall of the diencephalon, together with the adjacent part of the ventral wall, forms the hypothalamus of the fully developed brain.
Spongioblast Ependyma cells
The Fate of the Spinal Portion of the Primitive Neural Tube.-The spinal portion of the neural tube, during the first three months of intra-uterine life, develops equally in its whole extent, but after that period a longer cephalic or anterior (superior in the erect posture) and a shorter caudal portion are recognisable. The cephalic portion undergoes still further development and is converted into
Columnar cells of roof-plate
the spinal medulla of the adult, but in the smaller caudal or posterior portion retrogressive changes occur, and it is transformed into the non-functional filum terminale of the completed medulla spinalis.
Histological Differentiation of the Walls of the Neural Tube. -In the earliest stages of its development the walls of the neural tube consist of a mass of nucleated protoplasm, more or less distinctly differentiated into cell areas, of columnar form, which extend between and are connected with an internal limiting membrane, bounding the neural canal, and an exsurrounds the whole tube. At ternal limiting membrane, which
Columnar cells of floor-plate
Fig. 47.—SHOWING ELEMENTS OF CENTRAL NERVOUS SYSTEM. this time the outline of a transverse section of the primitive neural tube is somewhat ovoid. The cavity of the tube is compressed laterally into a dorsoventral cleft, which is bounded by dorsal, ventral, and lateral walls. In the dorsal and ventral walls, called respectively the roof- and floor-plates, the columnar character of the primitive epithelial elements of the medulla spinalis is retained. throughout the whole of life, but the peripheral parts of some of the cells are converted into fibrils.
In the lateral walls of the embryonic medulla spinalis some of the cells soon
assume a spherical form. These spherical cells have large deeply staining nuclei, and they are termed germinal cells.
For many years it was believed that the germinal cells were the predecessors of the primitive nerve elements or neuroblasts, and that the remaining cells, called spongioblasts, became transformed into the reticular sustentacular tissue of the central nervous system. It appears, however, from the results of more recent researches, that some of the descendants of the germinal cells become spongioblasts whilst others become neuroblasts or primitive nerve-cells. Moreover, there appear to be two groups of germinal cells; the descendants of one group are directly transformed into the ependymal or lining cells of the central canal, whilst those of the other group form in the first instance indifferent cells, some of whose descendants become neuroblasts and others spongioblasts. The fate of the cells present before the germinal cells appear, and which do not become germinal cells, is uncertain, but they probably take part in the formation of the spongioblastic tissue.
It is believed, therefore, that all the nerve-cells are the descendants of the germinal cells, and that the spongioblasts which become developed into the cells of the neuroglia or sustentacular reticulum are derived partly from the nongerminal cells of the primitive neural tube and, partly, they are descendants of the germinal cells.
As differentiation proceeds three layers and two membranes are gradually defined in the walls of the neural tube: (1) a central layer of columnar ependyma cells immediately surrounding the central canal; (2) an intermediate or mantle layer consisting of neuroblasts and their processes, the nerve-fibres, intermingled with spongioblasts; (3) a peripheral reticular layer consisting, at first, of processes of the bodies of the spongioblasts. The membranes are an external limiting membrane, surrounding the exterior of the tube, formed by the fused outer ends of the spongioblastic cells, and an internal limiting membrane bounding the central canal and continuous with the inner ends of the ependyma cells. Throughout the whole of the spinal medulla and the brain, the ependyma cells become transformed into the columnar ciliated cells which line the cavities of the adult brain and spinal medulla. The mantle layer becomes converted into the gray matter of the adult central nervous system.
The peripheral reticular layer, in the spinal region, becomes permeated by nerve-fibres, which are merely processes of the nerve-cells, and it is thus converted into the white matter of the adult spinal medulla. In the brain region it is either transformed in the same way into white matter, or it remains in a more rudimentary condition as a thin peripheral layer of neuroglia on the surface of the gray matter. On the other hand, in the brain region white matter is formed internal to the gray matter by the growth of nerve-fibres which insinuate themselves between the mantle layer externally and the bodies of the ependyma cells internally.
As the histological differentiation of the walls of the neural tube is proceeding each lateral wall is divided into a dorsal part, the alar lamina, and a ventral part, the basal lamina, by a sulcus-like dilatation of the central canal called the sulcus limitans. After the limiting sulci are formed the parts of the walls of the neural tube are a roof-plate, a floor-plate, and two lateral walls, each of which consists of an alar lamina, essentially sensory in function, and a basal lamina, essentially motor in function (Fig. 44).
The Fate of the Cavities of the Primitive Brain. The cavity of the spinal portion of the primitive neural tube becomes the central canal of the spinal medulla of the adult. The cavities of the primitive brain vesicles are transformed into the ventricles, foramina, and aqueduct of the adult brain. The cavities of the telencephalic divisions of the secondary fore-brain become the right and left lateral ventricles of the adult brain. The cavity of the undivided portion of the secondary fore-brain vesicle, together with the cavity of the primary fore-brain, become the third ventricle or cavity of the diencephalon, and the apertures of communication between the third ventricle and the cerebral hemispheres are the interventricular foramina (O.T. foramina of Monro).
The cavity of the hind-brain vesicle becomes the fourth ventricle, and the
cavity of the primitive mid-brain is converted into the aqueductus cerebri, which connects the third with the fourth ventricle.
After the anterior and posterior neuropores (p. 31) are closed, the cavity of the neural tube is, for a time, a completely enclosed space. Subsequently the mesoderm, which in the meantime has surrounded the tube, becomes differentiated, in its immediate neighbourhood, into three membranes. The innermost of the three is closely connected with the walls of the neural tube and is called the pia mater. The outermost, known as the dura mater, is dense and resistant, and the intermediate membrane is a thin lamella called the arachnoid.
As the membranes are formed, spaces are differentiated between them. The space between the dura mater and the arachnoid is the subdural space, and that between the arachnoid and the pia mater is the subarachnoid space.
After a time a median perforation, the median aperture of the fourth ventricle (O.T. foramen of Magendie), and two lateral perforations pierce the dorsal wall of the fourth ventricle and the pia mater which covers it, and thus the fourth ventricle becomes connected with the subarachnoid space. It is stated also that a perforation passes through the medial wall and the covering pia mater of a portion of each lateral ventricle which is called its inferior horn, throwing those portions of the lateral ventricles also into communication with the subarachnoid space, but it is doubtful if the statement is correct.
THE FORMATION OF THE EMBRYO.
The transformation of the relatively flat embryonic area into the form of the embryo is due, in the first instance, to the rapid extension of the median part of the area, as contrasted with the slower growth of its margins, and the later modelling of the various parts of the embryo is due to different rates Cephalic end of
of growth in different parts of
mesoderm Mesoderm of
By the rapid proliferation entoderm vesicle
Ectoderm of amnion
Mesoderm of amnion,
of cells from the nodal grow-
geal and the pericardial areas become reversed in position, and a cephalic or head fold is formed. This fold is bounded, dorsally, by what is now the cephalic portion of the embryo, ventrally, by the reversed pericardial region, and its cephalic end is formed by the extremity of the head region and the bucco-pharyngeal
FIG. 48.-SCHEMA OF SAGITTAL SECTION OF EMBRYONIC AREA AND
Ectoderm of amnion
Allantoic diverticulum from entoderm vesicle
posterior neuropore Cloacal
FIG. 49. SCHEMA OF SAGITTAL SECTION OF EMBRYONIC AREA SHORTLY AFTER THE FOLDING HAS COMMENCED. The pericardial mesoderm is carried into the ventral wall of the fore-gut and the cœlom has extended through it. The cephalic end of the neural tube and the caudal part of the primitive streak are bent ventrally, and the latter now forms the cloacal membrane.
The growth at the nodal point not only produces a head fold, but at the same time it forces the cephalic end of the primitive streak caudally over the caudal end of the embryonic area, thus forming a tail fold.
As the head and tail folds of the embryo are produced by the longitudinal increase of the embryonic area, transverse growth of the area results in the formation of right and left lateral folds (Figs. 37, 39), and as the various folds are formed the embryo rises, like a mushroom, into the interior of the amnion cavity.
The portion of the entodermal sac which is enclosed within the hollow embryo, formed by the folding of the embryonic area, is the primitive entodermal alimentary canal. The part which remains outside the embryo is the yolk sac, and the passage of communication between the two is the vitello-intestinal duct.
That portion of the primitive entodermal alimentary canal which lies in the head fold is termed the fore-gut, the part in the tail fold is the hind-gut, and the intermediate portion which is in free communication with the yolk-sac is the mid-gut.
As the extension of the embryonic area and its folding proceed the margin of the area, which remains relatively stationary, becomes the margin of an orifice, on
FIG. 50.-SCHEMA OF SAGITTAL SECTION OF EMBRYO AFTER THE FOLDING HAS DEFINED BOTH THE FORE-GUT AND HIND-GUT AREAS.
the ventral aspect of the embryo, through which the primitive alimentary canal of the embryo and the intra-embryonic part of the coelom communicate, respectively, with the yolk sac and the extra-embryonic portion of the cœlom. This orifice is the primitive umbilical orifice.
Not only does the primitive alimentary canal communicate with the yolk sac, and the intra-embryonic with the extra-embryonic cœlom, at the margin of the umbilical orifice, but also the body walls of the embryo, formed by the somatopleure, becomes continuous, at the same margin, with the wall of the amnion.
The young embryo is connected also with the inner surface of the chorion by a band of tissue which is part of the median portion of the caudal part of the wall of the amnion sac. The mesoderm in this region is thickened, and contains in its interior a diverticulum, allantoic diverticulum, which is primarily derived from the entodermal sac, but is afterwards connected with the hind-gut. This strand consists of ectoderm and mesoderm, and it contains not only the allantoic diverticulum but also the blood-vessels passing between the embryo and the chorion. It was called, by His, the body stalk, but the term is not fortunate, for it takes no part in the formation in the body of the embryo. On the other hand, its mesodermal and entodermal constituents represent a diverticulum from the wall of the hind-gut, present in many mammals and known as the allantois; it might with advantage, therefore, be termed the allantoic stalk.
At first the umbilical orifice is relatively large as contrasted with the total size