PLATE 2 



EXPLANATION OF FIGTJUES 



All the preparations illustrated on this plate have been drawn from speci- 

 mens prepared by Cajal's silver nitrate method (formula I). Figures 6 to 9 show 

 the different types of cells in which neurofibrils are formed; figures 10, 11, 12 and 

 14 the appearance of the earliest neurofibrils to become differentiated (i.e., in an 

 embryo of 15 somites) and figure 13 the neurofibrils passing out from the neural 

 tube among the cells of the myoblast in a later stage. Unfortunately the 

 preparations could not be illustrated in their original colors on account of the 

 cost of reproduction. 



6 From the right side of the hind-brain, opposite the otic invagination, of an 

 embryo of 24 somites, length 7.5 mm. and sixty-five hours incubation. This 

 figure shows a cell sending out a process, containing neurofibrils, which pierces 

 the membrana limitans interna {m.l.int.) and extends for some distance within the 

 cavity of the neural tube (p. 404). 



7 Bipolar cell from the same region of the same embryo. The neurofibrils 

 are black. The membrana limitans interna {m.l.int.) and externa (m. I. e.vt.) are 

 shown for orientation (p. 403). 



8 Marginal neuroblast from the same region of the same embryo. The neu- 

 rofibrils seem to form a more or less continuous network (p. 403). 



9 From the same region of the same embryo but on the left side. The cell 

 (a) is apparently within the cavity of the neural tube and the process (b) of the 

 cell (c) extends toward the neural tube. All three contain neurofibrils (p. 404). 



10 Cells from the right side of the forebrain of an embryo of 15 somites, 

 length 5.8 mm., incubation forty hours at 39°C, being slightly further advanced 

 than Duval's embryo of thirty-three hours, (fig. 268, p. 56). The darkly staining 

 portion of the middle cell affixed to the distal pole of the nucleus contains 

 within it a blackened network which probably represents the first stage in the 

 histogenesis of the neurofil)rils (p. 402). 



11 More completely formed neurofibrils occurring in a marginal neuroblast 

 in the hind-brain on the right side, opposite the otic invagination of the same 

 embryo (p. 402). 



12 Cells from the right side of the mid-brain of the same embryo showing a 

 stage in the formation of neurofibrils similar to that represented in figure 10 (p. 

 402). 



13 Xeural tube with the adjacent myoblast in the region of the sixth somite 

 on the right side of an embryo of 29 somites, length 8.1 mm. and incubation sixty- 

 three hours. The neurofibrils are seen winding in and out among the mesoblastic 

 cells (p. 411). 



14 Cells from the same region of the same embryo as figure 10 showing, rather 

 more satisfactorily than figures 10 and 12, the darkly staining distal part of the 

 cytoplasm in which the neurofibrillar network is formed (p. 404). 



422 



