290 Journal of Comparative Neurology and Psychology. 



ning of the plexus in the albino rat. The exact age of this embryo 

 unfortunately is not known, but the neural walls are thick and 

 the ventricles large. In a two centimeter embryo rat, which has 

 about the same brain development as a three months human 

 foetus, the plexus has elongated somewhat more than in Fig. 2, 

 but it is not essentially different. From this time to birth, there 

 seems to be a steady growth of the plexus, until the structure 

 assumes the appearance of Fig. i. At no time is there any evi- 

 dence that the organ is enlarged or distended, or that it fills any- 

 thing like all of the ventricular space. 



MICROSCOPIC HISTOLOGY. 



I. Examination of Living Tissue. — From a freshly killed adult 

 albino rat, the plexuses may be removed and examined in cerebro- 

 spinal fluid, or in normal salt solution. Often under these con- 

 ditions the blood will continue to flow through the capillaries for 

 several minutes. This circulation is due to a gradual shrinkage 

 of the tissues and the weight of the cover slip, both of which aid 

 in forcing the blood out of the ruptured capillaries. In case the 

 arterioles are not clogged, this circulation may be in a reverse 

 direction, since resistance would be less toward the open arterioles. 

 Not a great amount of detail can be obtained in this way, still it 

 serves as a control for the sections subjected to reagents. 



Under low powder, the plexuses of the rat appear as a thin, semi- 

 transparent membrane, which is threaded by an immense number 

 of capillaries. The edges are somewhat sinuous, and these curves 

 are closely followed by the blood vessels, so that the appearance 

 is not unlike the scalloped edge possessed by many leaves. Under 

 a high power, it is readily seen that the lamina consists of an upper 

 and a lower layer of epithelial cells, w^th the blood vessels embed- 

 ded between. Details are plainest at the edges. The cells are 

 cuboidal, and can best be located by their nuclei, which appear as 

 shadowy discs. The cell boundaries can be made out only with 

 much difficulty. Occasionally on a flat surface, the ordinary 

 mosaic appearance of the cell walls may be brought into focus. 

 The apical plate or marginal zone of the cell is visible as a thin 

 refractive line. The apical end of the cell is curved with the con- 

 vexity outward. At times, the lateral cell walls may be detected, 

 but not usually. The cytoplasm appears finely granular through- 



