Pearl, Reactions of Litmdus. 147 



topples over and rolls down till it lies in the usual position in 

 the bottom of the "vicarious chorion" with the haemal side 

 up. I have seen this series of actions repeated time after time 

 by the same embryo. It usually takes several minutes of stren- 

 uous labor for the embryo to get righted in the "vicarious 

 chorion," and then within a few seconds of the time it attains 

 that position, it suddenly loses its balance and falls back to the 

 point from which it started. Then the same series of events 

 begins again. There is nothing in the behavior which would 

 in the least suggest any process of "learning by experience," 

 or of perfecting a reaction by practice. 



The stimulus which induces these almost continuous leg 

 movements in the embryo is probably of the same sort as that 

 which causes the righting reaction in the adult Limulus. If adult 

 Limuli are placed in water so that the haemal side is in contact 

 with the bottom, they immediately give a characteristic reaction 

 which brings them into the normal position. From a long series 

 of observations and experiments on the adult animals it appears 

 that this apparent • 'equilibrium sense" is primarily due to a strong 

 positive thigmotaxis of the neural surface of the body, together 

 with a negative thiymotaxis of the haemal surface A similar con- 

 dition of affairs has been shown to be the cause of the righting 

 reaction in other organisms (cf. Peakl, :03 for Planaria). It 

 seems to me probable that the leg movements of the embryos 

 when they are in an inverted position are thigmotactic responses. 

 The only essential difference between embryo and adult in re- 

 spect to this thigmota.xis would then be that the definite, 

 purposeful reaction with which the adult meets and solves the 

 difficulty has not yet developed in the embryo. Instead the 

 embryonic thigmotactic reaction is simply a generalized response 

 to a general stimulus. Tlie re.xtion becomes specialized and 

 better adapted to the accomplishment of its end as development 

 proceeds. 



The movements of the thoracic appendages which have 

 been described are the only ones which I have observed before 

 the time of hatching. None of the complex, coordinated re- 



