Meek, Choroid Plexus. 



291 



*cj 



out. Very small refractive globules may be seen within it. No 

 pigment has been noted in the rat's plexuses. The most careful 

 examination has failed to show the presence of cilia in the adult. 

 These are present in younger forms but it seems reasonably certain 

 that cilia are not present in the adult. At the same time, cilia 

 are often noticed in the adult on the ependyma, which has been 

 "torn from the ventricular walls. The ependyma, in certain por- 

 tions, is ciliated at all ages. 



The microscopic appearance of the plexuses of other forms is 

 similar to that of the albino rat. In most forms, particularly the 

 guinea pig, dog and rabbit, the marginal or apical zone of the 

 epithelial cells seems more specialized. The rabbit's plexus is 

 remarkable in being studded with 

 clear drop-like spaces within the 

 cells. These will be discussed more 

 at length in the next section. 



II. Examination of Stained Ma- 

 terial. Under the influence of fix- 

 atives and stains, many other details 

 of the cells are brought out. At 

 best, there must be some distortion, 

 but judging from the appearance 

 of the fresh tissues it is believed 

 that the most successful prepara- 

 tions are a fair representation of the ^ ^ . , . , 



' . iiG. 3. Cross section or a portion ol an 



Structures m the fresh condition. ^^^1^ ,^,,, p,^^,^. Magnification x 1500. 



Fig. 3 shows the cross section a, capillary; i, connective tissue. 



of a small loop of an adult rat's 



plexus. The two capillaries represented, show about the two 

 extremes in size, the smaller being about 12 micra in diameter. 

 The capillaries consist of a delicate endothelial intima, with elon- 

 gated nuclei. This intima is strengthened by connective tissue cells 

 and fibrils. Findlay ('99) considers this as an adventitial coat. 

 Between this adventitial and the epithelium are more connective 

 tissue cells and their processes. In young animals, lymphocytes 

 are occasionally noticed. 



The epithelium covering the plexus of the rat is composed of 

 cubical cells, which in cross section average about 10-12 micra 

 in width, and 8-10 micra in height. The basal wall is rather 

 poorly defined. Often it is difficult to seperate the cell wall from 



