280 H. E. JORDAN 



chromatic nucleoli (fig. 2) and a delicate wide-meshed granular 

 reticulum. Many of the cells are undergoing mitosis. The 

 cell wall appears distinct. But there is no indication of terminal 

 bars nor brush borders, such as have been described for the ento- 

 dermal cells of the human yolk-sac by Branca (2). In Giemsa- 

 stained preparations the cytoplasm is colored dark blue, the 

 nuclei light blue, and the nucleoli bluish orange or lilac. 



The most striking feature of these cells is the presence of a 

 generous amount of delicate filaments (basal filaments; ergas- 

 toplasmic filaments) scattered throughout the finely granular 

 basophilic cytoplasm. They are oriented in general parallel 

 to the long axis of the cell. They may be coarser or finer, in 

 length equal to that of the entire cell or much shorter; and they 

 may be apparently homogeneous or segmented (fig. 31). The 

 latter condition would seem to indicate the possibility that they 

 may fragment into secretion granules, but the evidence for this 

 conclusion is not wholly satisfactory. Their probable signifi- 

 cance and nature will be discussed in a later section. It may 

 suffice here to state that the cells of the liver (fig. 32) and those 

 of the mesonephric tubules contain apparently identical cyto- 

 plasmic threads; and that in no case do they bear any direct 

 relationship to mitochondria, which must have been dissolved 

 by the fixing fluids used. 



b. The mesotheliimi 



The outer surface of the yolk-sac wall, like the homologous 

 layer of the splanchnopleure generally, is characterized by a 

 layer of greatly flattened cells each bulging more or less at the 

 point where the nucleus is located. The cytoplasm is delicately 

 reticular like that of the underlying mesenchyma, with which the 

 mesothelial cells are apparently in syncytial continuity (figs. 2 

 and 4). The nuclei are generally relatively large, oval, vesicular 

 structures, with one or several small irregular net-knots, and a 

 delicate wide-meshed nuclear reticulum (figs. 9 and 20). In 

 their general form, structure and light staining capacity they 

 are practically identical with the nuclei of the mesenchyma and 



