294 Papers from the Department of Marine Biology. 



MATERIAL AND METHODS. 



The material includes three embryos, with yolk-sac attached, of 5, 

 6, and 7 mm. length. The embryos were collected in March 1912 at 

 Montego Bay, Jamaica, British West Indies, while with the scientific 

 expedition of the Department of Marine Biology of the Carnegie 

 Institution of Washington, under the leadership of the Director, Dr. 

 Alfred G. Mayer. The embryos were fixed in Kelly's fluid, stained 

 in toto with Delafield's hematoxylin, lightly counterstained with eosin, 

 and sectioned in paraffin at 10 microns. From the viewpoint of hemo- 

 poietic phenomena the three embryos are practically identical. Since 

 the 5 mm. embryo seems to possess a slight advantage in respect of 

 abundance of crucial stages and of differential staining, the following 

 description will pertain almost exclusively to this embryo. The tissue 

 is perfectly normal, as is indicated by the abundant mitoses in prac- 

 tically every tissue. The cell-clusters of the aorta show a progressive 

 increase in size and differentiation from the 5 to the 7 mm. embryo. 



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 m). 1 



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. 



x The photomicrographs were made by Mr. William S. Dunn, Cornell University Medical 

 School, New York. 



