. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. 272 DEVELOPMENT OF THE LYMPHATIC SYSTEM is formed by the fusion of spaces appearing in the mesenchyme immediately external to the intima of degenerating veins; hence the spaces are termed extraintimal spaces. These at first have no endothelial lining and they are never in connection with the lumina of the veins. They are perfectly independent structures and any connections they may«nake with the venous system are entirely secondary. This mode of origin from extraintimal spaces is not confined
. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. 272 DEVELOPMENT OF THE LYMPHATIC SYSTEM is formed by the fusion of spaces appearing in the mesenchyme immediately external to the intima of degenerating veins; hence the spaces are termed extraintimal spaces. These at first have no endothelial lining and they are never in connection with the lumina of the veins. They are perfectly independent structures and any connections they may«nake with the venous system are entirely secondary. This mode of origin from extraintimal spaces is not confined to the thoracic duct, according to the authors mentioned, but is the method of development of all parts of the lymphatic system, with the exception of the jugular sacs. According to the supporters of the direct venous origin the peripheral lymphatic stems develop, like blood-vessels, as outgrowths from the stems already present. Lymph nodes nave not been observed in human embryos until toward the end of the third month of development, but ' !.<l'-V''\LY. they appear in pig embryos of 3 cm. X^Hi^. Their unit of structure is a blood-vessel, breaking up at its termination into a leash of capillaries, around which a con- densation of lymphocytes occurs in the mesenchyme. A structure of this kind forms what is termed a lymphoid follicle and may exist, even in this simple condition, in the adult. More frequently, however, there are associated with the follicle lymphatic vessels, or rather the follicle develops in a network of lymphatic vessels, which, become an investment of the follicle and form with it a simple lymph node (Fig. 166). This condition is, however, in many cases but transitory, the artery branching and collections of lym- phoid tissue forming around each of the branches, so that a series of follicles are formed, which, together with the surrounding lym- phatic vessels, become enclosed by a connective-tissue capsule to. Fig. 166.—Diagram of a Primary Lymph Node o
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