Meek, Choroid Plexus. 295 



osmic acid, but he does not intimate that they are anything Hke 

 the large droplets found in the rabbit. As before noted, these 

 clear spaces can be seen in the fresh tissues even before the blood 

 has ceased circulating. For this reason, it is not believed that they 

 can be due to any error in technique or to post-mortem processes. 

 Vacuoles have been mentioned in a general way as occurring in 

 the typical cells of the plexuses, but to our knowledge, nothing 

 similar to these clear areas has been described. It is not believed 

 that they represent the chief secretion of the cells, since they have 

 not been found in the other forms examined, and it is therefore 

 best to consider them as of secondary importance. 



A second feature in the rabbit's plexus is the development of 

 the modified structures at the apex of the epithelial cells. In the 

 rat, the marginal zone is at best but a double contoured line. In 

 the rabbit, however, it is wider, and composed of filaments placed 

 perpendicularly to the surface of the 

 cell, and embedded in some kind of a 

 matrix. This gives the cells the ap- 

 pearance of ciliation, but this cannot 

 be confirmed by ciliary movements 

 in the fresh tissue. The structure is ^ F'«- ^- "^'^^ ""^ ^^"^ ^ '^^g'^ 



1 ^ -17 , , /'^„\ J "U ^1 plexus. The animal was killed with 



what ViGNON (01) describes as the ^ . . , , „ 



jj, , , , ^ ' ,, ,, , illuminating gas, and the cells are in a 



bordure de brOSSe or filamentous .^sting state. Magnification X 1800. 



plateau. At the base of the filaments 



are cytoplasmic microsomes, which take up the stain and simu- 

 late basal bodies. Terminal bars may be seen in cross section 

 at the corner of the cells. 



Fig. 6 shows the epithelium of a dog's plexus. The stain is 

 rather diffuse, and the reticulations show poorly, but it differs 

 from the rabbit's in no particular way, except by the absence of 

 the clear spaces or droplets. The epithelial cells of the guinea 

 pig's plexus are somewhat peculiar in having a great many nucle- 

 oli. There are usually two or three, and often four or five. In 

 other respects, the plexus is similar to that of the dog, as shown in 



The preceding description has referred entirely to the adult 

 plexuses of the rat, sheep, rabbit and dog. If a late fcetal or a new- 

 born specimen be examined, striking differences will be noticed. 

 Fig. 7 is a section from a one day old rat. The epithelial cells 

 differ from those of the adult in three particulars: Shape, stair.- 



