The Reactions of the Vertebrate Embryo to Stimulation etc. 541 



to pass from the lateral position to that maintained by the adult 

 fish are probably the result of the shape of the body combined with 

 the propulsion through the water and ave not to be attributed to 

 the direct influence exerted by the nervous system. Even embryos 

 of 20 mm. or more in length rest upon their side when not forging 

 ahead. It is impossible to determine the time when the preservation 

 of a stable equilibrium may be said to become a function of the 

 nervous system. In every instance the change that takes place 

 between movements, obviously the result of purely chemical and 

 physical processes in the organism, and those reflecting the modifying 

 and coordinating action produced by the interposition of a nervous 

 system is exceedingly graduai and no distinguishing characteristics 

 mark the advent of this domination. 



Gradually as the embryo increases in length the movements 

 become more and more complicated. Already in the case of PristiurKs, 

 Scyllium and Torpedo at 15 mm. a rotatory motion appears and at 

 20 mm. this has become a true screw movement which adds con- 

 siderably to the rapidity of the progression. At 25 mm. the embryo 

 is almost constantly engaged in twisting and untwisting the long 

 narrow body and tail. 



Trout embryos of 15 mm. are capable of swimming once or twice 

 around a dish of 5 cm. in diameter, but in ali the fish examined 

 definite progression is possible long before this stage is reached; 

 thus for example at 9 or 10 mm. the embryo is quite capable, when 

 detached, of forging slowly ahead. 



When we come to consider the reactions that follow an incideut 

 Stimulus, such as the touch or prick of a needle, the difficulties of 

 accurately observing and interpreting the phenomena are very great. 

 The results themselves are often apparently conflicting and con- 

 sequently any attempt to Interpret even those that seem to be the 

 most favorable and positive should be made with the greatest caution. 



In Scyllium embryos of 7 — 8 mm. it is frequeutly observed, that 

 when the tail, head or body is gently touched with a needle whose 

 point is guarded by a piece of blotting paper, that a slight but 

 positive increase occurs in the number as well as in the extent of 

 the movements. If the head is prevented from swinging, by gently 

 pressing the needle stili covered with paper against it, the lashing 

 of the tail becomes more pronounced. On the other band at this 

 stage it is stili very difficult to determine with accuracy whether 

 when the surface is pricked with an extremely fine and sharp needle 



