296 'Journal of Cotnparattve Neurology and Psychology. 



ing power and location of the nucleus. A comparison with Fig. 

 3 from the mature rat, shows that in the one day form the cells 

 are narrower and deeper. The long diameter is perpendicular 

 to the base of the cell, while in the adult it is parallel with it. 

 Many measurements give the following averages. Cells from the 

 one day old plexus are 13 micra in height, and 8 micra in width. 

 Cells from the adult are 9 micra in height, and 1 1.6 micra in width. 

 This would indicate that the young cells were a trifle the smaller. 

 An interesting question here is : How does growth take place ? 

 The extent of the plexus is greater at maturity than it is at birth, 



and it is therefore necessary to account 

 for the extension of the epithelial layer 

 to cover the larger area of the adult 

 plexus. A partial answer is contained 

 in the fact shown above, that the adult 

 epithelial cells are about one-third wider 

 than those of the foetal type and so cover 

 about twice the area. This is possibly 

 sufficient to account for growth from the 

 foetus to the adult. Growth m.ight, how- 

 ever, take place in another way by a 

 regular mitotic division of the epithelial 

 cells. So far as known this has not been 

 observed in epithelial cells of the plexus 

 in late stages of growth, but it has been 

 reported as occurring in the ependyma. 



The location of the nucleus in the one 

 day form is apical, while in the adult it 

 is central or basal. In the former the 

 nucleus is removed from the base of the 

 cell by three-fourths the vertical diameter of the cell; in the 

 latter, it is removed by one-half the vertical diameter. 



The contents of the cell in the one day plexus are well nigh 

 unstainable, at least by ordinary methods. The basal part of the 

 cell shows a few radiating lines of microsomes, but it is for the 

 most part clear. The nucleus stains deeply and so diff'usely, that 

 in only a few cells is its structure visible. Under favorable con- 

 ditions of staining, reticulations are visible. Between the nucleus 

 and the apex of the cell, the cytoplasm stains somewhat diff^usely, 

 but a reticulation with granules in the meshes can be made out. 



Fig. 7. Loop of a plexus from 

 a one day old rat. The cells are 

 high and narrow, and the nucleus 

 is peripheral, and most of the pro- 

 toplasm fails to stain. Magnifica- 

 tion X 1500. 



