. Anatomy, descriptive and applied. Anatomy. DEVELOPMENT OF THE BLOOD-VASCVLAR SYSTEM 755 DEVELOPMENT OF THE BLOOD-VASCULAR SYSTEM. There are three distinct stages in the develo])im'iit of the C'irculatory system, each in accord- ance with the manner in which nourishment is i)r()\idcd for ;it different periods of the existence of the individual. In the first stage there is the eitcUinc circulation, during which nutriment is extracted from the vitellus or contents of the yolli sac. In the second stage there is the placental circulation, during which nutriment is obtained by means of the placent
. Anatomy, descriptive and applied. Anatomy. DEVELOPMENT OF THE BLOOD-VASCVLAR SYSTEM 755 DEVELOPMENT OF THE BLOOD-VASCULAR SYSTEM. There are three distinct stages in the develo])im'iit of the C'irculatory system, each in accord- ance with the manner in which nourishment is i)r()\idcd for ;it different periods of the existence of the individual. In the first stage there is the eitcUinc circulation, during which nutriment is extracted from the vitellus or contents of the yolli sac. In the second stage there is the placental circulation, during which nutriment is obtained by means of the placenta from the blood of the mother. In the third stage, commencing after birth, there is the complete circu- lation of the adult, during which nutrition is provided for by the organs of the individual.'. I Mesoderm. Entoderm. Blood-island. Fig. 529.—Section through vascular area to show commencing development of bloodvessel. (Semidiagrammatic.) Bloodvessels first make their appearance in the mesodermal wall of the yolk sac, i. e., outside the body of the embryo. Here the cells become arranged into solid strands or cords which join to form a close-meshed network. The peripheral cells of these strands become flattened and joined to each other by their edges to form the walls of the primitive bloodvessels. Fluid col- lects within the strands and converts them into tubes, and the more centrally situated cells of the cell cords are thus pushed to the sides of the vessels and appear as masses of loosely arranged cells which project toward the lumen of the tube. These masses are termed hlood islands (Fig. 529); their cells acquire coloring matter (hemoglobin), and are then detached to form the blood- corpuscles or erythroblasts (Fig. 530).^ Later, red cells are formed in organs where the circulation. 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 perfect
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectanatomy, bookyear1913
