Meek, Choroid Plexus. 



293 



The writer's own results are decidedly in favor of there being 

 but a single layer of the epithelial cells. In about 100 plexuses 

 examined, there has never been the slightest suggestion of strati- 

 fication. This refers, of course, to normal tissue. Pathological 

 proliferation would be both likely and possible. No such cases, 

 however, were noted. It may be that observers have been misled 

 by oblique sections of some of the villi. It is not possible to have 

 all of the tissue when it is covered with villosities, at right angles 

 to the plane of section. In such a case, an oblique section might 

 lead one to believe that the tissue had several layers of cells. 



Intercellular spaces have been found in ependymal tissue by 

 Obersteiner ('01), Renaut ('99) and Studnicka ('00). Re- 

 NAUi's idea is that the spaces are filled with a delicate cement sub- 



FiG. 4. Cross section of a portion of an adult rabbit's plexus. Magnification X 1500- ^. Micro- 

 somes simulating basal \)odies; b, capillary with blood corpuscles; c, clear space (fat droplet); a, 

 marginal zone. 



stance, while Studnicka holds that they are true lymph spaces. 

 Studnicka has found similar spaces between the epithelial cells 

 of the choroid plexuses in the case of the shark, Notidanus cin- 

 ereus. Milian ('04) and Pettit and Girard ('02-03), on the 

 contrary, believe that normally the cells are closely appressed. 

 They have noticed that such spaces increase in number as the 

 result of poor fixation and post-mortem changes. They therefore 

 regard them as due to changes in the cytoplasm, occasioned by 

 faulty fixation, or post-mortem alterations. We have observed 

 intercellular spaces in but a single specimen, that of a sheep s 

 plexus, which had been fixed with the brain in formalin, the ven- 

 tricles not having been opened. In freshly fixed tissue, there was 

 not even a suggestion of spaces between the cells. There is no 

 reason for doubting that the observations on the ependymal cells 



