Meek, Choroid Plexus. 287 



were opened to allow an easy access of the fixative. Often the 

 plexus was left untouched on the floor of the ventricle while the 

 overlying nervous matter was trimmed away. These devices all 

 avoid mechanical injury, and they also preserve the position of the 

 plexus in the ventricle. Only in the largest brains were the plex- 

 uses removed and fixed separately. 



The period of immersion in the fixative is important. When an 

 entire brain is fixed the average time required by the fixative may 

 be allowed, but if the plexuses are exposed the length of time 

 should be reduced to about one-third. In the case of Carnoy's 

 solution, about 20 minutes gave by far the best results. 



The usual methods of embedding in paraffin and sectioning 

 were employed. 



Sections were cut 3 and 4 micra thick. Many diff^erent stains 

 were used in the hope of diff^erentiating various structures. The 

 most satisfactory results were obtained with iron haematoxylin 

 followed by acid fuchsin. Other stains, copper-chrome haema- 

 toxylin,^ toluidin blue, erythrosin, van Gibson's acid fuchsin, 

 and Ehrlich's triacid stain were used as controls, and for the 

 sake of comparison. 



On account of their accessibility and their size, the plexuses of 

 the lateral ventricles were used exclusively. So far as known, the 

 plexuses of the other ventricles are precisely the same in function 

 and structure. Mammalian material was studied from the fol- 

 lowing forms: albino rat, rabbit, guinea pig, cat, dog, sheep and 

 man. 



GENERAL MORPHOLOGY. 



The choroid plexuses of the lateral ventricles are due to an 

 ingrowth of the pia mater pushing the mesial wall of the hemi- 

 spheres into the ventricles. The arachnoid is not supposed to be 

 present, although the plexus is but a fringe of the velum interposi- 

 tum, into the structure of which the arachnoid does enter. The 

 neural wall is of course preserved, but consists only of a simple 

 epithelium. The plexuses then are thin laminae covered with an 

 epithelium, beneath which is a connective tissue stroma containing 

 an extraordinarily rich network of blood vessels. 



In many animals, the laminae are smooth, but in others, they 

 are covered with projecting villi. Between these two extremes 



' The formula for this stain was kindly given by Dr. Bensley of the Department of Anatomy, Univer- 

 sity of Chicago. 



