Meek, Choroid Plexus. 297 



The free margin of the cell shows double contoured lines. As the 

 rat grows older, the cells gradually assume the adult type. Plex- 

 uses four days old show the nuclei removed by two-thirds the 

 diameter of the cell from the base, and by the seventh day the 

 position of the nucleus is similar to that in the adult. A change 

 in the staining reactions of the cells occurs at the same time. 



The primitive columnar condition of the epithelial cells is 

 therefore retained until extra-uterine life is well begun. Whether 

 this means that the cells do not function until this time cannot be 

 said. It would certainly indicate that their work could not be 

 the same as in the adult. This rapid change in the epithelium 

 after birth seems to occur in the plexuses of other forms than the 

 white rat. We have observed it in the rabbit and cat, and judging 

 from Studnicka ('00, Plates XXXII, XXXIII), it occurs in man 

 also. 



SECRETORY PHENOMENA. 



Thus far we have studied the plexus in what might be termed 

 its resting stage. Careful examination shows that in any adult 

 plexus there are always some cells that have the appearance of 

 secretory activity. It is to this phenomenon and its relation to 

 the production of the cerebro-spinal fluid that we would now 

 direct attention. 



For many years there has been a suspicion that the choroid 

 plexuses secreted, or at least aided in the secretion of the cerebro- 

 spinal fluid, but the idea, until recently, remained without much 

 supporting evidence. Even as late as 1901, Charpy ('01) stated 

 that the origin of the cerebro-spinal fluid was unknown. 



Although the possibility had been mentioned the century before, 

 it was E. Faivre ('54) who first definitely stated that the plexuses 

 were probably concerned in the production of the fluid. Luschka 

 ('55) published an important monograph in which he produced 

 evidence to show that the cerebro-spinal fluid could not be a transu- 

 date, and that it must be considered as a secretion produced by 

 the membranes of the brain. 



For many years no further evidence appeared. In 1897 Paul 

 Claisse and Charles Levi ('97) reported a case of internal 

 hydrocephalus, in which there was hypertrophy of the plexuses. 

 There was a great amount of granulation, and the veins were 

 dilated and gorged with blood. Kingsbury ('97), while working 



