Book - Text-Book of the Embryology of Man and Mammals 17
Embryology - 27 Apr 2024    Expand to Translate
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Hertwig O. Text-book of the embryology of man and mammals. (1892) Translated 1901 by Mark EL. from 3rd German Edition. S. Sonnenschein, London.
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The Organs of the Intermediate Layer or Mesenchyme
- The Organs of the Intermediate Layer or Mesenchyme
- The Development of the Blood-vessel System
- the first developmental conditions of the vascular system
- of the heart
- vitelline circulation, allantoic and placental circulation
- the further development of the vascular system up to the mature condition
- the metamorphosis of the tubular heart into a heart with chambers
- the development of the pericardia! sac and the diaphragm
- metamorphoses of the arterial system
- venous
- the first developmental conditions of the vascular system
- The Development of the Skeleton
- the development of the axial skeleton
- the development of the vertebral column
- head -skeleton i. bones of the cranial capsule ii. ., visceral skeleton
- concerning the relation of the head-skeleton to the trunk-skeleton
- the development of the skeleton of the extremities
- pectoral and pelvic girdles
- skeleton of the free extremity
- development of the joints
- the development of the axial skeleton
- The Development of the Blood-vessel System
THE grounds which made it appear necessary to distinguish in addition to the four epithelial germ-layers a special intermediate layer or mesenchyme have already been given in the first part of this text-book. This distinction is also warranted by the further progress of development. For all the various tissues and organs which are derived in many ways from the intermediate layer allow, even subsequently, a recognition of their close relationship. Histologically the various kinds of connective substance have been for a long time considered as constituting a single family of tissues.
It will be my endeavor to emphasise the relationship of the organs of the intermediate layer, and whatever is characteristic of them from a morphological point of view, more than has been hithei'to customary in text-books, and to do the same in a formal way by embracing these organs in a chapter by themselves and discussing them apart from the organs of the inner, middle, and outer germ-layers.
It is the original province of the intermediate layer to form a packing and sustentative substance between the epithelial layers, a fact which stands out with the greatest distinctness particularly in the lower groups, as for example in the Ccelenterates. It is therefore closely dependent upon the epithelial layers in the matter of its distribution. When the germ-layers are raised up into folds, it penetrates between the layers of the fold as a sustentative lamella ; when the germ-layers are folded inwards, it receives the parts that are being differentiated as for example in the Vertebrates, the neural tube, the masses of the transversely striped muscles, the secretory parenchyma of glands, the optic cups, and the auditory vesicles and provides them with a special envelopment that adjusts itself to them (the membranes of the brain, the perimysium, and the connective-tissue substance of the glands). In consequence of this the intermediate layer, in the same proportion as the germ-layers become more fully organised, becomes itself converted into an extraordinarily complicated framework, and resolved into the most divergent organs, by the formation of evaginations and invaginations and the constricting off of parts.
The form of the intermediate layer thus produced is of a secondary nature, for it is dependent upon the metamorphosis of the germlayers, with which it is most intimately connected. But in addition, the intermediate layer, owing to its own great power of metamorphosis, acquires in all higher organisms, particularly in the Vertebrates, an intricate structure, especially in the way of liistological differentiation or metcvnwrphosis. In this way it gives rise to a long series of various organs the cartilaginous and bony skeletal parts, the fasciae, aponeuroses, and tendons, the blood-vessels and lymphatic glands, etc.
It is therefore fitting to enter here some\vhat more particularry upon a discussion of the principle of histoloyical differentiation, and especially to inquire in what manner it is concerned in the origin of organs differentiated in the mesenchyme.
The most primitive and simplest form of mesenchyme is gelatinous tissue. Not only does it predominate in the lower groups of animals, but it is also the first to be developed in all Vertebrates, out of the embryonic cells of the intermediate layer, and is here the forerunner and the foundation of all the remaining forms of sustentative substance.
In a homogeneous, soft, quite transparent matrix, which chemically considered contains mucous substance or mucin, and therefore does not swell in warm water or acetic acid, there lie at short and regular intervals from one another numerous cells, which send out in all directions abundantly branched protoplasmic processes and by means of these are joined to each other in a network.
In the lower Vertebrates the gelatinous tissue persists at many places, even when the animals are fully grown ; in Man and other Mammals it early disappears, being converted into two higher forms of connective substance, either intojlbrillar connective tissue or into cartilaginous tissue. The first-named arises in the gelatinous matrix by the differentiation of connective-tissue fibres on the part of the cells, which are sometimes close together, sometimes widely scattered. These fibres consist of collagen and upon boiling produce glue. At first sparsely represented, these glue-producing fibres continually increase in volume in older animals. Thus transitional forms, which are designated as foetal or immature connective tissue, lead from gelatinous tissue to mature connective tissue, which consists almost exclusively of fibres and the cells which have produced them. This is capable of a great variety of uses in the organism, according as its fibres cross one another in various directions without order, or are arranged parallel to one another and grouped into special cords and strands. Thus, in connection with other parts derived from the germlayers, it gives rise to a great variety of organs. In some places it forms the foundation for epithelial layers of great superficial extent ; together with them it produces the integument, composed of epidermis, corium, and subcutaneous connective tissue, and the various mucous and serous membranes ; in others it unites with masses of transversely striped muscle, and arranges itself under the influence of their traction into parallel bundles of tense fibres, furnishing tendons and aponeuroses. Again at other places it takes the form of firm sheets of connective tissue, which serve for the separation or enveloping of masses of muscle, the intermuscular ligaments and the fasciae of muscles.
The second metamorphic product of the primary mesenchyme, cartilage, is developed in the following manner : At certain places the embryonic gelatinous tissue acquires as a result of proliferation a greater number of cells, and the cells secrete between them a cartilaginous matrix, chondrin. The parts which have resulted from the process of chondrification exceed in rigidity to a considerable extent the remaining kinds of sustentative substance, the gelatinous and the glue-producing intermediate tissue ; they are sharply differentiated from their softer surroundings, and become adapted, in consequence of their peculiar physical properties, to the assumption of special functions. Cartilage serves in part to keep canals open (cartilage of the larynx and bronchial tree), in part for the protection of vital organs, around which they form a firm envelope (cartilaginous cranium, capsule of the labyrinth, vertebral canal, etc.), and in part for the support of structures projecting from the surface of the body (cartilage of the limbs, branchial rays, etc.). At the same time they afford firm points of attachment for the masses of muscle imbedded in the mesenchyine, neighboring parts of the muscles entering into firm union with them. In this manner there has arisen through histological metamorphosis a differentiated skeletal apparatus, which increases in complication in the same proportion as it acquires more manifold relations with the musculature.
Cartilaginous and connective tissues, finally, are capable of a further histological metamorphosis, since the last form of sustentative substance, osseous tissue, is developed from them in connection with the secretion of salts of lime. There are therefore bones that have arisen from a cartilaginous matrix ami others from one of connective tissue. With the appearance of bone, the skeletal apparatus of Vertebrates has reached its highest perfection.
Even if the rnesenchyme has by these processes experienced an extraordinarily high degree of differentiation and a great diversity of form, the histological processes of differentiation which take place in it are nevertheless not yet exhausted. In the gelatinous or connective-tissue matrix canals and spaces arise in which blood and lymph move in accomplishing their function of intermediating in the metastasis of the organism, not only conveying the nutritive fluids to the individual organs, but also conducting away both the substances which owing to the chemical processes in the tissues -have become useless and the superfluous fluids. Out of these first beginnings arises a very complicated organic apparatus. The larger cavities constitute arteries and veins, and acquire peculiarly constructed thick walls, provided with non-stria te muscle-cells and elastic fibres, in which three different layers can be distinguished as tunica intinia, media, and adventitia. A small part of the blood-passages, especially distinguished by an abundance of muscle-cells, is converted into a propulsive apparatus for the fluid -the heart. The elementary corpuscles that circulate in the currents of the fluid, the blood-cells and lymph-cells, demand renewal the more frequently the more complex the metastasis becomes. This leads to the formation of special breeding places, as it were, for the lymph-corpuscles. In the course of the lymphatic vessels and spaces there takes place at certain points in the connective tissue an especially active proliferation of cells. The substance of the connective-tissue framework assumes here the special modification of reticular or adenoid tissue. The surplus of cells produced enters into the lymphatic current as it sweeps past. According as these lymphoid organs exhibit a simple or a complicated structure, they are distinguished as solitary or aggregated follicles, as lymphatic ganglia and spleen.
Finally there are formed at very many places in the intermediate layer, as especially in the whole course of the intestinal canal, organic [non-striate] muscles.
After this brief survey of the processes of differentiation in the intermediate layer, which are primarily of an histological nature, I turn to the special history of the development of the organs which arise from it, the blood-vessel and skeletal systems.
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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Cite this page: Hill, M.A. (2024, April 27) Embryology Book - Text-Book of the Embryology of Man and Mammals 17. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Text-Book_of_the_Embryology_of_Man_and_Mammals_17
- © Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G
