EXPLANATION OF PLATE. 



The following drawings were made with the aid of a camera lurida, a 1.5 mm. apoehromatic Zeiss objective, and 

 No. 6 compensating Zeiss ocular. All drawings are at a magnification of 1,650 diameters. 



The Nissl substance appears as a background of bluish-green masses, between which are the unstained canals 

 of the caiialicular system. Against this background the mitochondria appear as bright-red dots. All cells were drawn 

 without changing the focus, so that sometimes the mitochondria are clumped within the Niss.1 substance and some- 

 times within the canals, although this latter appearance is due to their presence in the thin layer of cytoplasm sur- 

 rounding the canals. If the focus were changed sufficiently mitochondria would occupy the spaces now free. The 

 counting, however, was done by focussing through the whole depth of the section. Some of the mitochondria seem to 

 be smaller and less definite than others ; this, in all the drawings, is due to the fact that they were slightly out of focus, 

 yet sufficiently clear to be included in the drawing. Where they occur as chains of granules they are probably broken 

 filaments. The granules, when brought clearly into focus, are of approximately the same size. 



PLATE 1. 



1. A typical cell of the motor nucleus of the VII cranial nerve. Note its similarity to the cell of the XII nucleus 



(fig. 5), both with respect to the appearance of the Nissl substance and the number of mitochondria, the 

 number per unit volume of cytoplasm for both being 15. 



2. A cell from the ventral cochlear nucleus of the VIII nerve. The Nissl substance here is a diffuse violet, and the 



mitochondria stain more intensely than those in the other nuclei. 



3. A motor cell from the nucleus of the VI nerve. 



4. A motor cell from the nucleus of the IV nerve. It will be noted that there appear to be more mitochondria in 



the cytoplasm of the cell of the mesencephalic nucleus (fig. 7) than in this, although the counts show that the 

 number per field is practically the same. This is due to the fact that the cytoplasm of the mesencephalic cell 

 is much more transparent, lacking the great clumps of relatively opaque Nissl substance which characterizes 

 the IV nucleus cell; and so in a single optical plane, mitochondria may be seen to a greater depth in the 

 mesencephalic cell. Since focussing was done throughout the depth of the section, the Nissl substance did 

 not interfere with the accuracy of the counts. 



5. A motor cell from the nucleus of the XII nerve. 



6. A motor cell from the nucleus of the III nerve. 



7. A sensory cell from the mesencephalic nucleus of the V nerve. The mitochondria appear closely crowded, even 



on one plane. 



8. A cell from the dorsal motor nucleus of the X nerve. 



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