Book - Text-Book of the Embryology of Man and Mammals 4
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Hertwig O. Text-book of the embryology of man and mammals. (1892)
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General Discussion of the Principles of Development
A SIMPLE principle has exclusively controlled the embryonic processes hitherto considered. By means of the cleavage of the eggsubstance, or cell- division, alone the originally simple elementary organism has been converted into a cell-colony. This presents the simplest conceivable form, inasmuch as it is a hollow sphere, the wall of which is composed of one or several layers of epithelial cells. But the principle of cell-division is not adequate for the production, out of this simple organism, of more complicated forms with dissimilar organs, such as the adult animals are ; further progress in development can be brought about from this time forward only by the supervention of two other principles, which are likewise simple ; namely, the principle of unequal growth in a cell-membrane, and the principle of the division of labour, together with the histological differentiation connected with it.
Let us consider first the principle of unequal growth. When in a cell -membrane the individual elements continue to divide uniformly, the result will be either a thickening or an increase in the surface of the membrane. The former takes place when the plane of division has the same direction as the surface of the membrane, the latter when it is perpendicular to the surface. With the increase in the extent of surface the cells which were at first present are uniformly and gradually crowded apart by the introduction of the new daughtercells, inasmuch as they are soft and plastic, and are joined together only by means of a soft cementing substance. Were we to assume that only such a growth took place in the case of the blastula during its further development, nothing else could come of it except an ever larger and thicker-walled hollow sphere of cells,
The operation of an unequal growth of the surface produces quite another result. When in the middle of a membrane the cells of a single group within a short time repeatedly undergo " division " by vertical planes, they will be suddenly compelled to claim for themselves much greater surface, and they will consequently exert a vigorous pressure, due to growth, upon the cells in their vicinity, and will tend to push them apart. But in this case a separation of contiguous cells, such as takes place with gradual and uniformly distributed interstitial growth, will be impossible ; for the surrounding cells, remaining in a passive condition, will constitute, as it were, a rigid frame, as His has expressed it, around the extending part, which, in consequence of accelerated growth, demands an increased area. It must therefore secure room for itself in another manner, and increase its surface by abandoning the level of the passive part through the formation of a fold in either one direction or the other. The fold will be still further increased, and forced farther from the original level, if the increased activity of the process of cell-division in it continues. Thus by means of unequal growth there has now arisen out of the originally uniform membrane a new recognisable part, or a special organ.
When the folding membrane encloses a cavity, as is the case with the blastula, there are two cases conceivable in the formation of folds. In the first place, the membrane may be folded into the interior of the body, a process which in embryology is called invagination or involution. Secondly, there may arise by evagination a fold, which projects free beyond the surface of the body.
In the first case numerous variations in the details are possible, so that the most various organs, as, e.g., the glands of the animal body, parts of the sensory organs, the central nervous system, etc., are formed.
In the origin of glands a small circumscribed circular part of a cellular membrane is infolded as a hollow cylinder (fig. 39 1 and 4 ), towards the interior of the body, into the underlying tissue, and by continuous growth may attain considerable length. The invagination develops into either the tubular or the alveolar form of gland (FLEMMING). If the glandular sac possesses from its mouth to its blind end nearly uniform dimensions, we have the simple tubular gland (fig. 39 l ), the sweat glands of the skin, LIEBERKUHN'S glands of the intestine. The alveolar form of gland differs from this in that the invaginated sac does not simply increase in length, but expands somewhat at its end (fig. 39 5 , db\ while the other part remains
Fig. 39. Diagram of the formation of glands.
1, Simple tubular gland ; 2, branched tubular gland ; 3, branched tubular gland with anastomosing branches ; 4 and 5, simple alveolar glands ; , duct ; db, vesicular enlai'gement ; 0, branching alveolar gland. +++++++++++++++++++++++++++++++++++++++++
narrow and tube-like and serves as its duct (a}. More complicated forms of glands arise, when the same processes to which the simple glandular sac owes its origin are repeated on the wall of the sac 12 34 (5 when on a small tract of it a more growth takes place, and a part begins to grow out from the main tube as a lateral branch (fig. 39 2 and 6 ). By numerous repetitions of such evaginations, the originally simple tubular gland may acquire the form of a much - branched tree, upon which we distinguish the part formed first as trunk, and the parts which have arisen by outgrowths from it as chief branches and branchlets of first, second, third, and fourth order, according to their ages and correlated sizes. According as the lateral outgrowths remain tubular or become enlarged at their tips, there arise either the compound tubular glands (fig. 39 2 ) (kidney, testis, liver), or the compound alveolar glands (fig. 39 G ) (sebaceous glands of the skin, lungs, etc.).
Again, the invagmating part of an originally flat membrane assumes other forms in the production of sense organs and the central nervous system. For example, the part of the organ of hearing which bears the nerve terminationsthe membranous labyrinth is developed out of a small tract of the surface of the body, which becomes depressed into a small pit (fig. 40) in consequence of its acquiring an extraordinary vigor in growth. The edges of the auditory pit then grow toward one another, so that this is gradually converted into a little sac, which still opens out at the surface of the body by means of a narrow orifice only (fig. 40 a). Finally, the
Fig. 40. Diagram of the formation of the audi
lit ; b, auditory vesicle, which has arisen by a process of constriction, and still remains connected with the outer germ-layer by means of a solid stalk of epithelium. +++++++++++++++++++++++++++++++++++++++++
narrow orifice closes. Out of the auditory pit there has arisen a closed auditory sac (5), which then detaches itself completely from its parent tissue, the epithelium of the surface of the body. Afterwards, simply by means of the unequal growth of its different regions, by means of constrictions and various evaginations, it acquires such an extraordinarily complicated form, that it has justly received the name of membranous labyrinth, as will be shown in detail in another chapter.
The development of the central nervous system may serve as the last example of invagination. Spinal cord and brain take their origin at an early epoch from the layer of epithelial cells which limits the outer surface of the body of the embryo. A narrow band of this epithelium lying along the axis of the back becomes thickened, and is distinguished from the thinner part of the epithelium, which produces the epidermis, as the medullary plate (fig. 41 A mp\ Inasmuch as the plate grows more rapidly than its surroundings, it becomes infolded into a gutter which is at first shallow, the medullary groove. This becomes deeper as a result of further increase of substance. At the same time the edges (fig. 41 B vif), which form the transition from the curved medullary plate to the thinner part of the cellular membrane, become slightly elevated above the surrounding parts, and constitute the so-called medullary folds. Subsequently these grow toward each other, and become so apposed that the furrow becomes a tube, which still remains temporarily open to the outside by means of a narrow longitudinal fissure. Finally, this fissure also disappears (fig. 4 1 (7) ; the edges of the folds grow together ; the closed medullary tube (ft), like the auditory vesicle, then detaches itself completely along the line of fusion (suture) of the cell-membranes of which it was originally a component part and becomes an entirely independent organ (ti).
Let us now examine somewhat more closely the mechanism of the fusion and detachment of the neural tube.
The two medullary folds are each composed of two layers, which are continuous with each other at the edge of the fold, the thicker medullary plate (wyo), which lines the furrow or tube, and the thinner epidermis (ep), which has either a more lateral or a more superficial position. When, now, the folds conie into contact, they fuse, not only along a narrow edge, but over so extensive a tract that epidermis is joined to epidermis, and that the edges of the medullary plate are joined to each other. The medullary tube thus formed, and the continuous sheet of epidermis that stretches across it, are by means of an intermediary cell-mass still in continuity along the suture produced by the concrescence. But a separation soon takes place
+++++++++++++++++++++++++++++++++++++++++ Fig. 4l Cross sections through the dorsal halves of three Triton larvae.
A, Cross section through an egg in which the medullary folds (mf) begin to appear.
B, Cross section through an egg whose medullary furrow is nearly closed.
C, Cross section through an egg with closed neural tube and well-developed primitive segments. mf, Medullary folds ; mp, medullary plate ; n, neural tube (spinal cord) ; ch, chorda ; ep, epidermis, or corneal layer ; -ink, middle germ-layer ; mk\ parietal, ink", visceral subdivision of the middle germ-layer ; ik, inner germ-layer ; ush, cavity of primitive segment. +++++++++++++++++++++++++++++++++++++++++ along this line, inasmuch as the intermediary band of substance becomes narrower and narrower, and one part of it unites wit.li the epidermis, while the other part is annexed to the medullary tube. Thus in the formation of the suture processes of fusion and of separation occur almost simultaneously, a condition which often recurs in the case of other imaginations, as in the constricting off of the auditory vesicle, the vesicle of the lens, etc.
The neural tube having once become independent is subsequently segmented in manifold ways by the formation of foldings, in consequence of inequalities in the rate of surface growth, especially in its anterior enlarged portion, which becomes the brain. There are formed out of this by means of four constrictions five brain-vesicles, which lie in succession one after another ; and of these the most anterior, which becomes the cerebrum with its complicated furrows and convolutions of first, second, and third order, serves as a classical example when one desires to show how a highly differentiated organ with complicated morphological conditions may originate by the simple process of folding.
In addition to invagination the second method in the formation of folds, which depends upon a process of evayination, plays a no less important part in the determination of the form of animal bodies, giving rise to protuberances of the surface of the body, which may likewise assume various forms (fig. 42). As a result of exuberant growths of small circular territories of a cell-membrane there arise rodlike elevations, resembling the papillae on the mucous membrane of the tongue (c), or the fine villi (a) in the small intestine (villi intestinales), which are so closely set that they give a velvety appearance to the surface of the mucous membrane of the intestine. Just as the tubular glands may be abundantly branched, so tufted villi are here and there developed out of simple villi, since local accelerations of growth cause the budding-out of lateral branches of a second, third, and fourth order (fig. 42 b). . We recall the external tufted gills of various larvae of Fishes and Amphibia, which project out from the neck-region free into the water, or the villi of the chorion in Mammals, which are characterised by still more numerous
+++++++++++++++++++++++++++++++++++++++++ Fig. 42. Diagram of the formation of papillae and villi. , Simple papilla ; b, branched papilla or tufted villus ; c, simple papilla, the connective-tissue core of which runs out into three points. +++++++++++++++++++++++++++++++++++++++++
branchings. The formation of the limbs is also referable to such a process of external budding.
When the growth of the membrane takes place along a line, the free edges form ridges or folds directed outward, such as the valves of KERKRING folds of the small intestine or the gill-plates on the gill-arches of Fishes.
From the examples cited it is clearly to be seen how the greatest variety of forms may be attained by the simple means of invagination and evagmation alone. At the same time, the forms may be modified by two processes of subordinate importance, by separations and by fusions which affect the cell-layers. Vesicular and sac-like cavities acquire openings by the thinning out of the wall at a place where the vesicle or sac lies near the surface of the body, until there is a breaking through of the separating partition. Thus in the originally closed intestinal tube of Vertebrates there are formed the mouth-opening and the anal opening, as well as the gill-clefts in the neck-region.
The opposite process fusion is still more frequently to be observed. It allows of a greater number of variations. We have already seen how the edges of an invagination may come in contact and fuse, as in the development of the auditory vesicle, the intestinal canal, and the neural tube. But concrescence may also take place over a greater extent of surface, when the facing surfaces of an invaginated membrane come more or less completely into contact, and so unite with each other as to form a single cell-membrane. Such a result ensues, for example, in the closure of the embryonic gill-clefts, in the formation of the three semicircular canals of the membranous labyrinth of the ear, or, as a pathological process, in the concrescence of the surfaces of contact of serous cavities. Moreover fusions may take place between sacs which come in contact with their blind ends, as very often occurs in the compound tubular glands (fig. 39 3 ). Of the numerous lateral branches which sprout out from the tubule of a gland, some come in contact at their ends with neighboring branches, fuse with them, and establish an open communication with them by the giving way of the cells at the place of contact. It is by this means that branched forms of tubular glands pass into the net-like forms to which the testis and the liver of Man belong.
In addition to the formation of folds in epithelial layers, which under a great variety of modifications determine in general the organisation of the animal body, there were mentioned, as a second developmental principle, of fundamental significance, division of labor and the histological differentiation associated with it. In order to understand fully the significance of this principle in development, we must proceed from the thesis that the life of all organic bodies expresses itself in a series of various duties or functions. Organisms take to themselves substances from without ; they incorporate in their bodies that which is serviceable, and eliminate that which is not (function of nutrition and metastasis) ; they can alter the form of their bodies by contraction and extension (function of motion) ; they are capable of reacting upon external stimuli (function of sensibility) ; they possess the ability to bring forth new organisms of their own kind (function of reproduction). In the lowest multicellular organisms each of the individual parts discharges in the same manner as the others the enumerated functions necessary for organic life ; but the more highly an organism is developed, the more do we see that its individual cells differentiate themselves for the duties of life, that some assume the function of nutrition, others that of motion, others that of sensibility, and still others that of reproduction, and that with this division of labor is likewise joined a greater degree of completeness in the execution of the individual functions. The development of a specialised duty likewise leads invariably to an altered appearance of the cell : until the physiological division of labor there always goes hand-in-hand a 'morphological or histological differ en tiation.
Elementary parts which are especially concerned in the duties of nutrition are distinguished as gland-cells ; again others, which have developed the power of contractility to a greater extent, have become muscle-cells, others nerve-cells, others sexual cells, etc. The cells which are concerned in one and the same duty are for the most part associated in groups, and constitute a special tissue.
Thus the study of the embryology of an organism embraces chiefly two elements : one is the study of the development of form, the second the study of histological differentiation. We may at the same time add that in the case of the higher organisms the morphological changes are accomplished principally in the earlier stages of development, and that the histological differentiation takes place in the final stages.
A knowledge of these leading principles will materially facilitate the comprehension of the further processes of development.
Text-Book of the Embryology of Man and Mammals: Description of the Sexual Products | The Phenomena of the Maturation of the Egg and the Process of Fertilisation | The Process of Cleavage | General Discussion of the Principles of Development | The Development of the Two Primary Germ-Layers | The Development of the Two Middle Germ-Layers | History of the Germ-Layer Theory | Development of the Primitive Segments | Development of Connective Substance and Blood | Establishment of the External Form of the Body | The Foetal Membranes of Reptiles and Birds | The Foetal Membranes of Mammals | The Foetal Membranes of Man | The Organs of the Inner Germ-Layer - The Alimentary Tube with its Appended Organs | The Organs of the Outer Germ-Layer | The Development of the Nervous System | The Development of the Sensory Organs | The Development of the Skin and its Accessory Organs | The Organs of the Intermediate Layer or Mesenchyme | The Development of the Blood-vessel System | The Development of the Skeleton
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