ORGANIZATION AND GROWTH 219 



tht> t i|> of one of the tubes is touched, this only may contract. 

 It is generally believed that when this occurs the external 

 stimulus which brings about a change in the sensory nerve 

 endings is conducted to the central organ, here to be trans- 

 formed into a motor impulse which causes the muscles to 

 contract. These ideas harmonize with the facts. But it is 

 also generally believed that without the central nervous sys- 

 tem the reflexes are no longer possible. This idea, as the 

 following experiments will show, is not true in Ciona intes- 

 tiiialis. When the ganglion has been removed from a Ciona, 

 it at first remains fully contracted. After some time, how- 

 ever, in favorable cases, as early as the next day the animal 

 again relaxes. If a drop of water is allowed to fall into the 

 basin of water, the entire Ciona contracts rapidly, just as an 

 animal whose central nervous system is intact. 



If, therefore, a normal Ciona and one without the central 

 nervous system are kept in the same vessel, both have the 

 same reflex irritability qualitatively. It is nevertheless pos- 

 sible to differentiate clearly between the reaction of the nor- 

 mal and the brainless Ascidian. In ike latter the thresh- 

 old of stimulation is much hiylier than in flic former. To 

 determine this I utilized a procedure which I have employed 

 in a series of comparative experiments on the irritability of 

 the lower animals, and which mi^ht be used with advantage 



' O O 



in human beings. I allow a drop of water to fall from a 

 pipette upon the organ to be stimulated, which lies a certain 

 distance (varying according to circumstances) beneath the 

 surface of the water. If the same pipette is always used, 

 the weight of the drop is nearly always the same, so that the 

 height which the drop must fall just to bring about a reac- 

 tion is a convenient measure of the threshold of stimulation. 

 In what follows I shall give the height of the fall of a drop 

 of water which just sufficed to bring about a contraction of 

 the entire Ciona. The normal and the brainless r.nimals 



