292 journal of Comparative Neurology and Psychology. 



the connective tissue beneath. The lateral walls of the cells are 

 also obscure, but they may usually be identified by a hazy line. 

 Under a low power, the cytoplasm appears finely granular and homo- 

 geneous, but with higher magnification, it is easily seen that it is 

 finely reticular. In the meshes are deeply staining granules, 

 which are often collected into small irregular masses. For the 

 most part the reticulation is more pronounced near the periphery. 

 Neither vacuoles, clear spaces of any kind, nor pigment granules 

 have been found in the plexuses of the rat. The nuclei of the 

 epithelial cells are oval or circular in outline, and centrally located 

 in the adult. They average about 6 micra in diameter. Nucleoli 

 are frequently present. The free edge of the cell is slightly con- 

 vex. It consists of a thin apical plate or cuticle, which is not very 

 apparent. 



Embryologically the epithelial cells of the plexuses are derived 

 from the inner layer of the neural tube which also produces the 

 ependymal cells, and which is called the ependymal zone. The 

 ependymal cells and the epithelial cells of the plexuses are there- 

 fore parts of the same layer. This may be plainly seen by tracing 

 the plexus back to where its epithelial covering joins the ependyma 

 lining the ventricle. Here the two types of cells pass into each 

 other by an easy gradation. Fig. 2 illustrates this for the embryo, 

 and the condition is not essentially different in the adult. It is 

 generally conceded that the epithelium of the plexus has lost all 

 vestiges of the neuroglia, which normally underlies the ependyma. 

 Catola ('02), however, reports that he has found glia fibers by 

 the Weigert method. So far as known, his observations have 

 not been confirmed. The epithelial cells have also lost all pro- 

 jections from the base, a characteristic of so many ependymal cells. 



There has been some discussion as to whether or not the epithe- 

 lium of the plexuses consists of a single layer. Haeckel ('60), 

 Heldt ('74), Dejerine ('95) and Kolliker ('96) describe only 

 a single layer. Luschka ('55), however, recognizes several layers 

 with transitional forms. Haeckel admits that in pathological 

 cases the cells may greatly increase in number. Findlay ('99) 

 reports that there may be from one to four, or even more layers, 

 and that the cells change in character as they pass from the basal 

 layers toward the surface. He finds this condition in the sheep, 

 calf, ox and man, and believes it occurs too frequently to be 

 explained as pathological. 



