294 Journal of Comparative Neurology. 



two types of the neuroglia elements just mentioned. From the 

 previous descriptions, the origin of the type "a" from the ecto- 

 blast will be easily explained, since it exhibits a close similarity 

 to th^ nuclei of the nerve cells. An explanation of the origin 

 of the cells of the type "b," however, is difficult, for they 

 have none of the structural characters of the ectoblast cells. 

 For the solution of this problem, attention was directed to the 

 structure of the nuclei of the capillary wall. 



The capillary wall is composed of a single layer of the 

 endothelial cells (Figs, i, 2, 3, 4, 5, 6, 7). An outline of an 

 individual cell, however, can not be made out by the technique 

 employed for the present investigation. The nuclei, however, 

 as can be seen in most cases, are placed very close together, 

 while in some cases, a long space appears between the two 

 nuclei (Fig. 6). In other cases, the nuclei are placed so closely 

 that the one partly overlaps the other (Fig. 4). The external 

 form of these nuclei shows considerable variation, as will be 

 seen from the figure 8, as well as from the other figures (Figs. 

 I, 2, 3, 4, 5, 6, 7). This is due in part to the plane in which 

 the nucleus has been sectioned. Curiously enough, the shape 

 and size of the nuclei of the capillary wall coincide very closely 

 with that of the nuclei or the neuroglia nuclei which belong to the 

 type "b;" that is, it appears oblong, somewhat spindle shaped as 

 well as flattened from side to side. The only form of the type 

 "b" not found in the capillary wall is that which appears like 

 an amoeba in motion. An examination of the internal struc- 

 ture of the nuclei in the capillary wall reveals still other points 

 of similarity as shown by the numerical relations, as well as the 

 peripheral aggregation of the acidophile particles, the absence 

 of the nucleoli and linin filaments, and a strong affinity for the 

 acid dyes. In these particulars it coincides exactly with the 

 nuclei of the type "b" of the neuroglia. The writer was un- 

 able to see any structural differences between these two forms 

 just mentioned ; this suggests a close connection between them. 



On examining the capillary wall, the writer noticed, very 

 frequently, a nucleus projecting outwardly (Figs, i, 2, 5, 6), 

 and in some cases, these nuclei were isolated from the capillary 



