Meek, Choroid Plexus. 303 



The choroid plexuses are due to the invagination of the neural 

 wall by the pia mater. They are thin laminae with an epithelial 

 covering, derived from the neural wall, a connective tissue stroma, 

 and a rich supply of blood vessels. Villi are absent in the rat,^ 

 guinea pig, and mouse, but they are present in man, horse and 

 ox. Villi are poorly developed in most birds, but they are especi- 

 ally well developed in the selachians and the crocodile. 



In area, the plexuses have a greater free surface than would be 

 supposed. This free surface is quite large enough to account for 

 the secretion of the cerebro-spinal fluid. 



In the case of the albino rat, the plexuses increase in size grad- 

 ually, from their first appearance, until they reach their maximum 

 growth, and at no time do they seem to be relatively enlarged, or to 

 fill all of the ventricular space. 



Microscopic examination of the fresh plexuses show that they 

 are semi-transparent membranes, covered with ill defined cubical 

 cells. The cytoplasm is finely granular. Cell walls and nuclei 

 are dimly visible. 



Stained material shows that the capillaries have an endothelial 

 intima, which is strengthened by connective tissue. The epithe- 

 lial cells in the albino rat are 8-10 micra high, and 10-12 micra 

 wide. The nuclei are oval or circular and after birth located basally 

 or centrally. The cytoplasm is reticular with granules in the meshes. 

 The free or apical edge of the cell is convex, and consists of a cuticle 

 which is little developed in the rat, but is more highly differentiated 

 in the rabbit and dog, where it appears as a filamentous plateau. 

 Iti no case have cilia been found on these cells after birth. 



The epithelial cells of the plexuses have lost all basal projections 

 which are characteristic of the ependymal cells from which they 

 were derived. 



The epithelium consists of a single layer, and the cells are closely 

 ap pressed without intervening intercellular spaces. In the case of 

 the rabbit, many of the epithelial cells contain droplets of fat. These 

 increase in size as they approach the apex of the cells, and appear to 

 he extruded without causing a destruction of the cell from which 

 they come. Vasomotor nerves to the blood vessels have been 

 reported by Benedikt ('73) and Findlay ('99). 



The epithelial cells of embryonic and young forms, differ from 



' Italics are used to indicate the results which are new, or to which the author believes he may have 

 made some contribution. 



