PLATE 3 



EXPLANATION OF FIGURES 



All the figures of this plate were drawn in the same way as those on plates 

 1 and 2. Magnification reduced one-third in reproduction. The series of figures 

 on this and the following plate illustrate stages in the extension of the neu- 

 raxon processes of the Rohon-Beard cells. The phenomena appear analogous 

 with those shown in the formation of the anlagen of ventral motor nerves (plates 

 I and II). 



13 A portion of a cross-section (IJ 4-1-22) of a 6 mm. embryo in the middle 

 trunk region. A portion of the ectoderm and of the dorsal wall of the neural 

 tube are shown. The amoeboid process (pi.) of a medullary cell is strikingly 

 similar to those which form the ventral nerve anlagen. Finer branches of the 

 protoplasmic processes show definite relations to the vacuolated intercellular 

 plasma, but no evidence that this relation is a genetic one. The phenomena are 

 entirely in harmony with the supposition that the amoeboid protoplasmic 

 process has a genetic relation to the medullary cell. Earlier stages show no 

 evidence of protoplasmic continuity between neural tube and ectoderm. 



14 A portion of a cross-section (IK 2-6-37) of a 6 mm. embryo in the middle 

 trunk region, showing a portion of the dorso-lateral wall of the neural tube and 

 a Rohon-Beard cell with deeply stained neuraxon process extending beyond the 

 limiting membrane of the neural tube. Similar outflows of adjacent cells may 

 also be seen, and the phenomena strikingly resemble those presented in the for- 

 mation of the somatic motor nerves (plates 1 and 2). 



15 A portion of a cross-section (sections IK 2-3-22 and IK 2-3-23 combined) 

 of a 6 mm. embryo in the middle trunk region, showing a part of the dorso-lateral 

 wall of the neural tube and the greatly elongated neuraxon process of a Rohon- 

 Beard cell which may be traced through the wall of the neural tube into the 

 plasma-filled space between the neural tube and the ectoderm. The pseudo- 

 podia-like extensions of the distal extremity of the neuraxon show relations to 

 the vacuolated plasma, but such evidence does not prove any genetic relation- 

 ship between the two, nor a primary protoplasmic relationship between the 

 neuroblast cell and its peripheral distribution. 



16 A portion of a cross-section (sections II 5-1-31, II 5-1-32 and II 5-1-33 

 combined) of a 6 mm. embryo in the middle trunk region, showing the dorso- 

 lateral wall of the neural tube and the neuraxon processes of two adjacent Rohon- 

 Beard cells, one of which extends into the space between myotome and ectoderm 

 the other between myotome and neural tube, the neuroblast cell of the latter 

 not shown. The neuraxon processes appear distinctly fibrillar only in the prox- 

 imal portion. Since the distribution of these neuraxon processes in later stages 

 is essentially the same, viz., the extra-embryonic ectoderm, the different rela- 

 tions to the dorsal part of the myotome appears to favor the view that the path 

 by which a neuraxon finds its way to its terminal organ is a matter of chance 

 rather than a predetermined one along an intercellular bridge. Moreover, the 

 amoeboid processes do not extend in one direction, as would seem demanded by 

 the latter assumption, but irregularly in different directions. 



17 to 20 (See p. 170). 



168 



