404 E. V. COWDRY 



neuroblast from the hind-brain on the right side opposite the otic 

 invagination of an embryo of 15 somites, of 5,8 mm. in length 

 and fort}'^ hours incubation prepared by Cajal's silver nitrate 

 method. The neurofibrils occur throughout the cytoplasm which 

 has assumed a rather dark yellowish tint. Figure 8 shows the 

 neurofibrils in a cell of the same type in a more advanced embryo. 

 Here the neurofibrils seem to be of finer diameter and more con- 

 tinuous, while the ground substance of the cytoplasm is of a 

 distinctly lighter and more brilliant yellow tint. 



Neurofibrils also occur in cells which seem to lie within the 

 lumen of the neural tube, and in the processes of cells which 

 extend along the ventricular border of the central canal. Fig- 

 ure 9 has been drawn from the left side of the hind-brain opposite 

 the otic invagination of an embryo of 24 somites, length 7.5 mm. 

 and sixty-five hours incubation treated by Cajal's silver nitrate 

 method. The cell (a) is apparently within the cavity of the 

 neural tube, and the process (b) of the cell (c) extends toward the 

 membrana limitans interna, .all three of which contain well dif- 

 ferentiated neurofibrils. Figure 6, which represents a cell from 

 the same region of the same embryo, shows a similar condition. 

 Appearances such as these are distinctly rare, seem to be 

 restricted to the region of the hind-brain and do not obtain in 

 embryos during the early stages of neurofibrillar formation. 



3. Region of the cytoplasm in ivhich neurofibrils originate 



Figures 10, 12 and 14 are from an embryo of 15 somites, length 

 5.8 mm. and incubation forty hours at 39°C., which represents 

 the stage of development at which neurofibrils may first be 

 distinguished. All of these three figures show that the neuro- 

 fibrils are first formed in the immediate neighborhood of the 

 nuclei. The fact that these primitive fibrils are in the form of a 

 network is best illustrated at a figure 14. In the cells containing 

 neurofibrils the distal region of the cytoplasm (i.e., that near the 

 membrana limitans externa) stains a darker yellowish brown 

 color than does the more proximal part, on the opposite side 

 of the nuclei, whereas in cells without neurofibrils the cytoplasm 



