544 Stewart Paion 



The following table gives a general idea of the rapiditv of the 

 heart beats at ditferent periods in the lite of the embryo. 



Length of embiyo Number of beats per minute after 



removal from egg-case and embiyo 

 placed in S% salt solution 

 Pristiurus 5 mm 16 — 20 



10 > 27—29 



14 > 36—38 



17 » 43—45 



This table with only very slight moditications is applicable to all 

 The different species of Selaehians I have stndied. 



In Trout and Salmon on the other band the rate of cardiac pul- 

 sation is eonsiderably greater; for example at 10 mm. it is 25 or 28 

 and at 15 mm. it has inereased to 75 or 80 per minute. 



I bave freqnently notieed that when the Selachian embryo is 

 first brought into the dish containing salt solution that the rapidity 

 of the heart's action is less than it is after an interval of half a 

 minute. This change may be the result of shock. 



Unfortunately I bave not studied with any degree of accuracy 

 the eflFect of changes of temperature uf the salt solution upon the 

 heart, but even to the casual observer there cannot be any doubt 

 that comparatively slight changes in the temperature of the fluid 

 medium in which the embryo swims modify very materially the 

 rate and rh\ thm of the cardiac cvcle. 



Section 2. • 

 a; General Description of Nervous System. 



In Order to avoid repetition the general appearances of the 

 nerrous system of an embryo of Torpedo ocellata of 7 mm. in length 

 will be described and the discussion of the coudition noted in 

 the earlier sfages postponed until considering the questions relating 

 to the finer histological structure Section 3 . By following this plan 

 it is hoped that the reader will get as concise and clear an idea, 

 as the actual facts permit, of the general relation existing between 

 the jìhysiological reactions that have already been described and 

 the correlative changes occurring in the central and peripheral ner- 

 vous system. 



