. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. 224 THE FORMATION OF THE BLOOD true capillaries, on the other hand, are more definitely tubular in form, are usually imbedded in mesenchymatous connective tissue and are developed in the same manner as the primary capillaries of the area vasculosa, by the aggregation of vasifactive cells to form cords, and the subsequent hollowing out of these. Whether these vasifactive cells are new differentiations of the embryonic mesen- chyme or are budded off from the walls of existing capillaries which
. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. 224 THE FORMATION OF THE BLOOD true capillaries, on the other hand, are more definitely tubular in form, are usually imbedded in mesenchymatous connective tissue and are developed in the same manner as the primary capillaries of the area vasculosa, by the aggregation of vasifactive cells to form cords, and the subsequent hollowing out of these. Whether these vasifactive cells are new differentiations of the embryonic mesen- chyme or are budded off from the walls of existing capillaries which have grown in from extra-embryonic regions, is at present undecided. The Formation of the Blood.—The mesamceboids, which are gl i i. A ,\ Fig. 131.—The Vascular Areas of Rabbit Embryos. In B the Veins are Represented by Black and the Network is Omitted.—(van Beneden and Julin.) the first formed blood-corpuscles are all nucleated and destitute or nearly so of haemoglobin. They have been held by some observers to be the only source of the various forms of corpuscles that are found in the adult vessels, while others maintain that they give rise only to the red corpuscles, the leukocytes arising in tissues external to the blood-vessels and only secondarily making their way into them. According to this latter view the red and white corpuscles have a different origin and remain distinct throughout Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original McMurrich, J. Playfair (James Playfair), 1859-1939. Philadelphia : P. Blakiston's son & co.
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