No. 520] ELECTRICITY TISSUES IX FISHES 



197 



layer both on the papilla- and on the posterior surface 

 itself. 



The whole cylinder, both elcctroplaxes and electric con 

 nective tissue, is separated from the surrounding mus- 

 cle tissue by a connective tissue sheath in which a few 

 isolated pigment cells are to be seen. The elcctroplaxes 

 do not exactly correspond in the adult either to the myo- 

 tomes or to the vertebrae. 



The significant development of the electric tissue takes 

 place between the ninth day of embryonic life, at which 

 time the embryo contains unchanged muscle tissue in its 

 tail, and the 40-day embryo which possesses the organs in 

 practically its adult condition. The critical changes take 

 place within even closer limits; from the 11th day to the 

 15th would include them. The stages used in this exami- 

 nation were but four in number, the ninth, eleventh, 

 twelfth and fortieth, four out of the seven valuable em- 

 bryos given the writer by Dr. Arthur Shipley, Dr. Richard 

 Assheton and Dr. J. Graham Kerr, to whom many thanks 

 are due. This material was collected by Mr. John Sam- 

 uel Budgett in Africa, he most unfortunately losing his 

 life from fever shortly afterwards. 



The ninth-dav embrvo shows no trace of any electric 

 tissue. The myotomes, as shown in a longitudinal section 



of many perfect muscle fibers with the myofibrils devel- 

 oped in several bundles in the peripheral cytoplasm. 

 These muscle fibers are all parallel with one another and 

 with the long axis of the body. Connective tissue is but 

 little developed although it is found sparingly between 

 the muscle fibers. 



In the embryo of eleven days the electric tissue has be- 

 gun to form. Eight regions in each muscle segment, 

 each region composed of a few (10-40) of the young 

 muscle fibers, have become prominent through the slightly 

 greater density of their cytoplasm and the beginning de- 

 generation of the muscle cells which immediately sur- 

 round them. These eight regions in each myotome lie 

 directly in front of and behind eight corresponding 



