542 Stewart Paton 



tbe character of tlie movements made by the embryo is materially 

 altered. 



An interesting phenomenou, and one of the earliest reactions 

 following an impiuging Stimulus may be observed in Trout, if the 

 sac of the embryo after removal from the Shell is gently pricked 

 or squeezed with the forceps. Instantaneously all the movements 

 of the youug fish become greatly exaggerated and persist for several 

 seconds to be renewed immediately when the Stimulus is again 

 applied. I first noticed tbis reactiou several years ago but have 

 never discovered a satisfactory explanation for its occurrence. Pos- 

 sibly it may be due to transmission of an impulse by means of tbe 

 fluid, which fills and distends the sac, to the skeletal muscles. No 

 neurofibrils are to be found in the wall of the sac at tbis period. 

 As far as I know tbis reaction, or rather series of reactions, follow- 

 ing Stimulation of the sac has not been described. 



In very young embryos gentle pressure over the cardiac area 

 causes a slight change in the rhythm and rapidity of the bodily 

 movements, a reaction that may be set down to tìie interference 

 with the circulation, and not to any sensory anomaly. 



The first premonition conveying to the observer the distinct 

 Impression that tbe movements following direct Stimulation may be 

 tbe result of the dominating and coordinating influence of the nervous 

 system occurs at the time when the neurofibrils begin to^ develope 

 in tbe lateral line organs. Tbis period is represented in Selachians 

 of from 9 — 10 mm. in length and has been also noted at early stages 

 in trout embryos. 



One of tbe earliest responses to Stimulation to appear, almost 

 coincidently with that associated with tbe development of the lateral 

 line organs, is obtained by pricking the belly of the fish. The im- 

 mediate change in position following Stimulation, or the altera- 

 tion in the rhythm of the swing of the head and tail indicate the 

 presence of genuine reflex activities. In studying these reac- 

 tions it has been considered that a definite change in position 

 of the embryo, or an alteratiou in the number and character of the 

 movements, if following immediately and constantly upon an incident 

 Stimulus were evidence of the beginning functional activity of so- 

 called reflex arcs and centres. The difficulties connected with any 

 attempt to localize tbe areas on the body, tbe Stimulation of which 

 are immediately followed by reflex movements, are so great as to 

 make an accurate investigatiou impossible. Tbe difference however 



