PLATE 3: EXPLANATION OF FIGURES (CONTINUED) 



17 A portion of a cross-section (sections IK 2-4^15 and IK 2-4-16 combined) 

 of a 6 mm. embryo in the middle trunk region, showing the processes of two 

 Rohon-Beard cells, one of which lies out of the plane of the section. The con- 

 ditions resemble those shown in figure 16, except that the processes appear con- 

 fluent instead of divergent. The neurofibrillae are well differentiated in the 

 proximal portion of the neuraxon. 



18 and 19 Portions of cross-sections (IK 2-5-6 and IK 2-5-37) of a 6 mm. 

 embryo in the middle trunk region, showing the distal termination of the grow- 

 ing neuraxon process of a Rohon-Beard cell as it extends into the space between 

 the myotome and the ectoderm. In the case of the process in figure 18 the grow- 

 ing end lies nearer the myotome, while in figure 19 the growing end lies nearer 

 the ectoderm. Comparison with earlier stages shows that in this region there 

 are no protoplasmic bridges in the region shown in the drawing — the intercellular 

 spaces are filled with an unstained vacuolated plasma. The brush-like termi- 

 nation is a characteristic feature of the neuraxon process. 



20 A portion of a cross-section of a 7 mm. embryo in the middle trunk region, 

 showing a Rohon-Beard cell on the top of a spinal ganglion. Its deeply staining 

 neuraxon process contrasts strongly with the surrounding mesenchyma, and the 

 evidence of the genetic relation of ganglion cell and neuraxon process seems 

 more convincing than in the case of the cells which are imbedded in the wall 

 of the neural tube. The process may be traced to a point lateral to the myo- 

 tome. The section figured however does not show the peripheral termination 

 of the process, which bends out of the plane of the section. 



PLATE 4 



EXPLANATION OF FIGURES 



21 and 22 Portions of cross-sections of 6 mm. embryos in the middle trunk 

 region, showing Rohon-Beard cells in somewhat advanced stages of the exten- 

 sion of the neuraxon processes. Figure 21 is a combination drawing from three 

 sections (II 4-4-26, 27 28). Figure 22 is a combination of two sections (IK 

 2-6-29, 30). The presence of neurofibrillae in the protoplasmic processes evinces 

 their nervous character. The distal extremity of these neuroblast processes, 

 however, is granular, vacuolated and e.xtended into pseudopodial processes. 

 Figure 21 does not appear to harmonize with the assumption of predetermined 

 paths for the extension of the nervous outgrowths, but rather with the view that 

 neuraxons are pseudopodia-like outgrowths of neuroblasts. The variation in 

 the form of the amoeboid termination of the neuraxon processes shown in the 

 two figures is striking and significant. 



170 



