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Hemopoiesis in Yolk-Sac

cells, nucleus, cytoplasm, fig and mesothelium

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HEMOPOIESIS IN YOLK-SAC.

The yolk-sac wall consists of three layers: (1) the thin superficial mesothelial layer; (2) the wide middle mesenchymal layer, filled with endothelium-lined blood-channels; (3) the inner entodermal lining, consisting of a single layer of large cuboidal cells more or less flattened (fig. 2, plate I; and fig. 23, plate iii).' The entodermal cells are characterized by the typical cytology de scribed for those of the 10-mm. pig embryo—large vesicular nucleus and a granulo-alveolar cytoplasm frequently containing long, delicate basal filaments. The cytologic evidence indicates a secretory function. A number of the cells are in mitosis.

The mesothelium consists of greatly flattened cells with long oval vesicular nuclei. The mesothelium is in syncytial continuity with the middle mesenchymal layer. Where mesothelium and endothelium abut the two tissues become continuous, and no differential marks, either nuclear or cytoplasmic, appear to identify the two. Cells of either tissue may have larger or smaller nuclei, more chromatic or less chromatic, with a delicately or coarsely granular reticulum, and a more deeply or less deeply staining cytoplasm, depending probably upon the particular phase of modification or function. Exactly the same description will hold also for the mesenchyma. Except for a frequently stellate shape of the mesenchymal cells, the three tissues mesothelium, endothelium and mesenchyma—are structurally prac tically identical at this stage. Mesothelium and endothelium are deriv atives of the mesenchyma, apparently under the operation principally of the mechanical factor of pressure.

The study of the origin of the blood-cells is best approached by a classification of the different types of cells found free in the blood vessels, both yolk-sac and intraembryonic. The preponderating type of cell is the erythroblast with pale homogeneous cytoplasm and a spherical vesicular nucleus. The nucleus generally contains one or several nucleoli and a delicate, more or less granular chromatic retie ulum (fig. 2). Many of these cells are in mitosis; an occasional cell

shows a nuclear condition suggestive of amitosis (fig. 2). The homo geneous cytoplasm has a slightly acidophilic staining reaction; these cells correspond to those of the pig embryo, which, in Giemsa-stained material, have a grayish-pink color. An occasional cell has a slightly larger size than the average of the eythroblasts and is characterized by a finely granular cytoplasm, slightly acidophilic (fig. 12b). This is a still younger erythroblast, and corresponds to Maximow's megaloblast type. The granules are more probably the initial hemoglobin content. These cells are transition forms between basophilic hemoblasts and acidophilic erythroblasts. A few cells occur which are characterized by a denser, deeper-staining nucleus and a more highly acidophilic cytoplasm; these represent more differentiated erythroblasts and may be called normoblasts; no non-nucleated erythroplastids are yet present.

Hemoblasts also appear in considerable numbers, more abundantly in the yolk-sac vessels (fig. 1, a and b). These are characterized by a relatively large granular and vesicular nucleus, usually with several large chromatic nucleoli and a relatively thin shell of basophilic, ap parently homogeneous, cytoplasm. In this material not a single mitotically dividing hemoblast was seen. In the pig also a mitotically dividing hemoblast was an extremely rare occurrence. In both instances, however, nuclear amitotic phenomena are abundant. The initial stage is characterized by a kidney-shaped nucleus. The hemoblast may take very irregular shapes (see figure lb) indicating amceboid activity. Figure 1, c and d, shows two late differentiation stages of a hemoblast; the nucleus stains less intensely, and the cyto plasm has become less basophilic. Figure ld is binucleated, probably the result of a direct division of the nucleus. Tfinucleated hemoblasts also are of frequent occurrence.

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