^'" ORIGIN OF NER VE- FIB RES 



353 



and also in spermatozoa," the "great importance possessed 

 by these elements as the instruments of an enormous develop- 

 ment," by the suggestion that the nuclei originate tliis 

 development, " a supposition which is considerably supported 

 by the most recent observations" (1878). 



Originally even the multicellular animals had no special 

 paths of nervous conduction. Possibly in the ectoderm-cells 

 of Hydra a network of ordinary protoplasmic threads still 

 constitutes the communication between cell and cell, nucleus 

 and nucleus. 



, Oeigin of ISTerve-Fibres and their Vicarious Action. 



But a more distinct morphological effect of the exercise of 

 nervous action is seen in ectoderm-cells, in which the course 

 of nervous impulses is through the cells lengthwise to the 

 nerves or muscles connected with them. For instance, in the 

 ectoderm-cells over the nerve-ring of Medusae, which are con- 

 tinued into nerve-fibrils, and also in the sensory portion of 

 neuro-muscular cells, the cell-plasm is, as I have already 

 mentioned, longitudinally striated in consequence of its 

 transformation into extremely fine fibrill?e. In the former 

 instance these fibrillse pass directly into nerve-fibrils ; in the 

 latter, they pass into the muscular portion of the cell, after 

 having in both cases come into connection witli the 

 nucleus. These relations are quite similar to those described 

 in ciliated cells — that is, the fibrillse are nerve-strands which 

 have been produced by functional activity from the proto- 

 plasm of ordinary epithelial-cells.^ 



It follows from the preceding that the nerves of Zoophytes 

 originally consisted of chains of cells derived from the ecto- 

 derm. This is also the most probable origin of the nerves of 

 the higher animals. In the gelatinous tissue of Scyphomedusa?, 

 in Aurelia aurita for example, we find that the amoeboid cells 



1 Cf. Die Medusen, Taf. xii. figs. 8, 12 ; Taf. xi. fig. 6, and others. 



2 A 



