Page:Popular Science Monthly Volume 75.djvu/148

144, and that in regeneration the nervous net forms earlier than the muscles. By taking jellyfishes at the appropriate stage in regeneration, it was found that a stimulus applied to one side of a regenerated area was followed by a muscular response on the other side of this area without any observable movement in the area itself. Hence transmission through the regenerated region must have been by nervous means, doubtless by the nervous net.

In jellyfishes the nervous net will transmit apparently in any direction and in this respect it is in strong contrast with the central nervous organs of the higher metazoans, where, especially in the vertebrates, a polarized condition generally prevails. Thus in the spinal nerves of vertebrates, it is easy to send impulses through from a dorsal root to a ventral one, but impossible to send them in the reverse direction. Apparently the cord contains some structure on its path of conduction that is valve-like and allows impulses to pass in one direction only. Such a condition does not exist in the nervous net of the jellyfishes.

The neuromuscular organs of the coelenterates have been considered by so many investigators as the most primitive in the animal, kingdom that it is not inappropriate to consider at this place the relations of some of the older views on this subject to those expressed in these articles.

The discovery by Kleinenberg (1872) of the so-called neuromuscular cells (Fig. 5) in Hydra led this investigator to the belief that these cells represented a complete neuromuscular apparatus in that each cell-body could be regarded as a receptor and its fibrous portion as an effector. By growth and cell division, according to Kleinenberg, separate receptors and effectors would be differentiated simultaneously from such single cells.

The simultaneous differentiation of nervous and muscular elements (Fig. 6) was also accepted by the brothers Hertwig (1878), but in their opinion the two types of tissue did not arise from a common cell as claimed by Kleinenberg, but from separate cells which became simultaneously differentiated, some to form nerve-cells (sense-and ganglion cells) and others to form muscle-cells. This view has come to be commonly accepted by the majority of investigators.

The independent origin of the nervous system and its secondary