48 H. V. NEAL 



(e.g., fig. 21), as the neuraxon process reaches the apex of the 

 myotome, some of the pseudopodial processes extend median 

 and some lateral to the myotome. It appears to be undeter- 

 mined whether, in its fm-ther extension, the neuraxon ma}^ grow 

 between the myotome and the neural tube or between the myo- 

 tome and the ectoderm. The significant fact presented b}- such 

 a section and by all the sections which show the cells of Rohon- 

 Beard with their greatly elongated processes, is that there is 

 not the slightest suggestion of a direct primary connection be- 

 tween the cell and its later connection with the ectoderm in the 

 extra-embrj'onic blastoderm. 



Such relations as are shown in figure 20, afford still further 

 evidence of the genetic relations of ganglion cell and neuraxon. 

 The section shows a Rohon-Beard cell lying at the apex of a 

 sensor}' ganglion, and which has reached its present position by 

 migration from the dorsal wall of the neural tube. The con- 

 tinuity of the cell bodj^ and of the neuraxon and evidence of 

 their genetic relation to each other is more clearly shown than 

 in the case of sections where the Rohon-Beard cell lies within 

 the wall of the neural tube. The objection to the inference that 

 somatic neuraxons are formed as elongated. processes of medul- 

 lary neuroblasts, on the ground that it is impossible to trace 

 the protoplasm of the neuroblast through the wall of the neural 

 tube into the growing nerve anlage is met by the evidence pre- 

 sented in this figure. 



Doubt remains only as to the inference that the process is 

 formed b}' a migration of protoplasm from the neuroblast into 

 the neuraxon. ^^^ly not infer that the neuraxon is formed from 

 intercellular bridges in situ? In answer to this question, it is 

 sufficient to state that in the earlier stages when the neuraxons 

 of the Rohon-Beard cells first appear, there are no protoplasmic 

 bridges extending in the direction taken later by the growing 

 neuraxons, but simpl}" a non-staining plasma. Furthermore, it 

 is as logical to conclude that the bod}'- of the Rohon-Beard cell 

 shown in figure 20 was differentiated in situ as that its neuraxon 

 was formed in situ. On the contrary, if there be good reason 

 for inferring that the Rohon-Beard c.ell has reached its present 



