56 THE ELEMENTARY NERVOUS SYSTEM 



vertebrates must be admitted to have a purely myogenic 

 beat and at this stage to be an absolutely clear instance of 

 an independent effector such as is shown in the muscles 

 of sponges. 



If further grounds were needed to support the opinion 

 that the embryonic vertebrate heart has a myogenic beat, 

 they can be found in the work of Burrows (1912), who cul- 

 tivated the heart muscle from chick embryos in vitro. 

 Pieces of the beating heart of young embryos were trans- 

 ferred to culture fluids where they became centers of out- 

 growths of cells. In time many of these cells began 

 rhythmic beating at a rate of 50 to 120 beats per minute, 

 a rhythm typical for beating pieces of the ventricular 

 muscle. As some of the beating cells were absolutely dis- 

 connected from the rest of the tissue, they afforded most 

 conclusive evidence of myogenic activity. These unicellu- 

 lar mechanisms are models, so to speak, of the embryonic 

 vertebrate heart, whose nature as an independent effector 

 is thereby exemplified. The independent rhythmic con- 

 traction of skeletal muscle has also been demonstrated 

 through this method by Lewis (1915). 



The modes of action of the heart muscles of other ani- 

 mals than the vertebrates present considerable diversity. 

 Thus Carlson (1904) has shown that in the King crab, 

 Limulus, the heart-beat is purely neurogenic and is as 

 dependent on extraneous nerve centers as are most other 

 arthropod muscles. On the other hand, the heart of the 

 silkworm is said by Pigorini (1917) to continue to beat 

 even after it has been separated from the central nerv- 

 ous system and cut into isolated segments, a condition 

 that is also claimed for the heart in tunicates. 



The tunicate heart, in consequence of the remarkable 

 periodic reversal in the direction of its beat, has excited 



