Papers from the Marine Biological Laboratory at Tortugas. 33 



their power of contractibility, and under these conditions the muscles 

 respond only when directly stimulated. If the subumbrella tissue be 

 heated, however, the nerves continue to function for several degrees beyond 

 the point whereat the muscles cease to respond by contractions. 



The subject of the neurogenic nature of the pulsation-stimulus in 

 Cassiopea is discussed by Mayer (1906, p. 19) ; and, as is well known, Carlson 

 has presented convincing proof that the pulsation-stimulus in the heart of 

 Limulus is neurogenic. 



The neurogenic nature of pulsation in these invertebrates is the more 

 remarkable because recent work has decidedly strengthened the myogenic 

 theory for the beating of the vertebrate heart. Thus Paton (1907) shows 

 that both the heart and the axial muscular system of Pristiurus functions 

 spontaneously before the development of definite nerve-fibrils from the 

 central nervous system, although protoplasmic connectives are seen ex- 

 tending from cell to cell, and these may transmit impulses. 



Also, Hooker (191 1) finds, that the heart differentiates and functions in 

 a normal manner in frog embryos which have developed without a central 

 nervous system; and a crucial experiment has been made by Burrows 

 (1912), who observed that single, isolated muscle-cells of chick hearts, if 

 placed in proper media, pulsate as does the heart itself. 



As the experiments of which we are about to speak involve alterations 

 in the composition of the sea-water, it is important that we should under- 

 stand the composition of the Tortugas sea-water in respect to the cations 

 of sodium, magnesium, calcium, and potassium. Accordingly the follow- 

 ing analysis of such of its components as may affect the experiments is 

 presented. 



COMPOSITION OF TORTUGAS SEA-WATER. 



As a deduction from George Steiger's analysis,^ made under the direc- 

 tion of Professor F. W. Clarke, it appears that in so far as its ingredients 

 (sodium, magnesium, calcium, and potassium) are concerned, we may make 

 up Tortugas sea-water by mixing the solutions shown under A, each being 

 practically isotonic with the others, and with the sea-water as a whole: 



A. B. 



81. 1 volumes of 0.6006 molecular NaCl. 0.487 molecular NaCl. 



14.36 " " 0.398 " MgCla. 0.0571 " MgCl,. 



2.84 " " 0.389 " CaClj. o.oiio " CaCla. 



1.7 " " 0.597 " KCl. o.oioi " KCl. 



100.00 



Thus any given volume of Tortugas sea-water may be considered to con- 

 sist of an atmosphere composed as in B, all occupying one and the same space. 



At 25° C, however, this being near the usual temperature of the 

 tropical sea-water, the NaCl is dissociated to about 75 per cent; and this 

 is also true of approximately 80 per cent of the MgClj, 88 per cent of the 

 CaCU, and 95 per cent of the KCl. 



^ Carn. Inst., Year Book, No. 9, 1910, p. 122. 



