TRANSMISSION AND CONDUCTION 215 



ease. The four tissue masses are generally regarded as 

 nervous in character and the very definite differential 

 staining of these and of the strands arising from them 

 make it highly probable that the strands are actually 

 nerves. Each strand terminates in an expanded end in 

 contact with the base of the first plate of each row. 

 Methylene blue has been very widely used as a more or 

 less specific stain intra mtam for nervous structures, and 

 the very definite and sharply bounded differential 

 staining of these structures in ctenophores leaves little 

 doubt as regards their nervous nature. Moreover, in 

 forms like Mnemiopsis, in which four strands arise from 

 the apical masses and each strand divides into two and 

 passes to two plate rows, the two plate rows thus 

 " innervated'' always show the same rhythm of beats, 

 while in Pleurobrachia and other forms, in which two 

 strands arise separately from each mass and pass with- 

 out division to the two plate rows of a quadrant, these 

 two rows often show an independent rhythm when 

 the animal is more or less quiescent, but beat synchron- 

 ously in stimulated animals. The cells of the plate row 

 also stain with methylene blue earlier and more deeply 

 than the general body surface, but no structures which 

 can be identified as distinctly nervous appear in them. 

 In the light of all these facts I am inclined to believe 

 that actual nervous connection exists between the plate 

 row and the apical nervous tissue, and that the plate 

 row, as regards its relation to this tissue, represents a 

 motor end organ or effector, but one capable of initiating 

 and transmitting impulses. The paths from the apical 

 nervous tissue are, however, not the only paths of trans- 

 mission, for, as various authors have noted, the plate 



