214 THE ORIGIN OF THE NERVOUS SYSTEM 



Before considering the behavior of the plate row, 

 the question of the nature of the transmission requires 

 some attention. Verworn (1890) believed it to be 

 mechanical and others have supposed that a true nervous 

 transmission is concerned, but Parker (1905) has con- 

 cluded on the basis of experimental evidence that 

 transmission in the plate row cannot be mechanical 

 and that, since definite nervous structures have not 

 been found, it must be neuroid. I agreed with this 

 conclusion, but more recent observations of my own on 

 several species raise the question whether true nervous 

 transmission is not more intimately concerned in at 

 least some portions of the path of the impulse than has 

 been previously believed. About the apical sense 

 organ are four tissue masses corresponding to the four 

 quadrants of the body. In some ctenophores, e.g., 

 Mnemiopsis, Bolina, a single ciliated groove passes 

 orally from each of these four masses and at some dis- 

 tance from the apical end each groove divides into two, 

 and each of the eight grooves thus formed connects with 

 the apical end of a plate row. In other forms, e.g., 

 Pleurobrachia, two ciliated grooves arise separately 

 from each of the four masses and connect with the plate 

 rows without division. When the animals are vitally 

 stained with methylene blue a very definite differential 

 staining occurs. The four tissue masses about the 

 apical sense organ stain rapidly and deeply and appear 

 as aggregations of cells. Each ciliated groove shows a 

 perfectly definite, deeply stained strand, and the origin 

 of this strand from the cell mass, its division into two in 

 those forms in which it does divide and its connection 

 with the plate row can be followed with the greatest 



