IX 



MID- AND INTEK-BKAIN 



495 



Steiner (1885) divided Amphioxus into two transverse halves, 

 cephalic and caudal ; both parts fall immediately to the bottom 

 of the vessel, and lie motionless, but if after a few minutes the 

 two parts are stimulated mechanically, each begins to move with 

 perfect regularity, maintaining its equilibrium and . always 

 advancing head -end forward. If the animal is divided into 

 three or four segments, each of these, after a suitable interval, 

 responds by locomotor movements to external stimuli. Steiner 

 concluded from these observations that Amphioxus consists of a 

 number of metameres which in no way differ physiologically, 

 and that it has no true brain or controlling centre for general 

 movements. 



Danilewsky obtained somewhat different results from his later 



FIG. 250. Diagram of decussation of optic nerve-fibres in chiasma. (Vialet.) 1, optic nerve on 

 left ; 1', on right side ; 2, 2', optic tract on left and right side ; a, direct ; ft, crossed bundle ; 

 c, macular bundle, partly crossed, partly direct ; d, Gudden's commissure. 



experiments. After bisecting the animal, he saw that the anterior 

 half was capable of executing spontaneous rhythmical extension 

 and flexion movements, but not true locomotion; the posterior 

 half, on the contrary, remained motionless for a long time. Arti- 

 ficial stimulation elicited motor reactions more readily from the 

 head than from the tail end. 



When the head is cut off, spontaneous movements cease ; the 

 animal remains one to two days motionless unless artificially 

 stimulated. The reflex movements are normal but weak, and 

 excitability seems more depressed than in the anterior part of the 

 divided animal. 



From these and similar observations, Danilewsky concluded 

 that the so-called "brain" of Amphioxus contains the centres 

 for voluntary movement, that is, controlling centres for all the 

 other segments of the neuraxon. 



In fishes in general the brain is but little developed. In 





