492 PATON— CORRELATION OF STRUCTURE [April i8, 



doses of these drugs until the peripheral nervous system is developed. 

 We find the action of thyroid extract as well as strychnine is modi- 

 fied to some extent by the development of the sympathetic nervous 

 system ; an occurrence taking place about the fourth day. We are 

 not yet prepared to state exactly what the character of this mechanism 

 is, although for the present we may consider it highly probable that 

 the increased activity of the heart brought about by moderate doses 

 of the two substances mentioned is the result of the functional activ- 

 ity of the sympathetic system. The symptoms of irregularity in the 

 heart's activities which develop after a certain period deserve con- 

 sideration and show a remarkable degree of similarity for both 

 strychnia, thyroid extract and magnesium chloride. As will be 

 noticed in chick no. 3 the rapidity of the heart decreased after the 

 embryo was placed in a solution containing thyroid extract. Sud- 

 denly, and this seemed to be the characteristic effect of all the 

 substances used — the organ stops pulsating, remaining motionless 

 for a period varying, as a rule, from ten to thirty seconds, or even 

 two minutes. Then it suddenly begins to pulsate again, the rhythm 

 gradually increasing in strength and rapidity until a point of maxi- 

 mum intensity is reached and then after one-half or one minute the 

 cycle ends again. The abrupt manner in which the pulsations cease 

 and the subsequent incidence of the beats, often after prolonged 

 intervals of rest, are strikingly similar to the phenomena taking place 

 when an embryo has been poisoned by an excess of magnesium 

 chloride. In these early stages of development it is extremely inter- 

 esting to compare the action upon the heart of three substances, 

 possessing chemical qualities as dififerent as thyroid extract, mag- 

 nesium chloride and strychnine sulphate. The characteristic primary 

 toxic effects as shown in the adult by the rapid rhythm of the heart 

 do not appear until the period when the nervous system has attained 

 a relatively high degree of differentiation. 



In addition to the substances already mentioned, solutions of 

 NaOH (1:500) and CH3OOH (1:500-1:1,000) were employed. 

 No positive results, except a gradual slowing of the heart, were noted 

 in connection with the former, but the latter seemed to exert a 

 marked inhibitory action upon the heart; the stronger solutions 



