154 Papers from the Department of Marine Biology. 



Vacuoles are formed in cells when changes in surface tension occur 

 such that the pressure inside the cell-wall is greater than that of the 

 surrounding medium. The electroblasts are growing at an abnormal 

 rate in the longitudinal direction and not at all in the other direction, 

 so that the pressure inside the cell must be tremendous. If this ten- 

 sion reaches a certain point, the vacuoles may increase in size until they 

 burst through the cell-wall. That this is happening in these electro- 

 blasts is shown by the long, narrow shape of the vacuoles at the ven- 

 tral end, where many have already broken through to form the papillae. 

 This explains whj^ the striations at this end appear to be broken across 

 instead of being separate striations formed for each papilla. It also 

 explains why in the adult electroplax many of the papillae have con- 

 necting bridges of the same material as themselves (Dahlgren 36), for 

 in such cases the vacuoles have merely persisted instead of breaking 

 through. The tension at the ventral end far exceeds that of the dorsal 

 end, for the small, round vacuoles finally disappear without breaking 

 through the wall and the dorsal surface becomes straight and smooth, 

 the dorsal cytoplasm homogeneous and unvacuolated. 



The electric organ develops in four parts on each side of the eye, each 

 of which is called an electric organ for convenience during its develop- 

 ment. The rectus externus is the largest of the six eye-muscles and 

 has a proportionately large electric organ. It is the first of the four 

 to begin development. The central part of the tissue forms a broad 

 band lying close to the muscle and curving from its dorsal to its ven- 

 tral surface in a spiral. The ends of the tissue separate from the mus- 

 cle ventrally, just caudad to the origin of the muscle, running caudad 

 and ventrad, and dorsally, near the point of insertion, running craniad. 

 This separation of the organs from the muscle with the turning and 

 bending of the organs is accomplished by the elongation of the electro- 

 blasts at the point of separation. The growth of the cells, there- 

 fore, begins at either end of the organ and later progresses toward the 

 center. Also, the electric organ of the rectus externus muscle, which 

 was the first to begin development, is the first to begin to separate 

 from the muscle-tissue (plates ii and iii, R. ex.). 



The long, narrow electric organ of the rectus internus Ues close to 

 its surface and curves spirally from a point cranial and medial around 

 the dorsal to the ventral side of the muscle. Dorsally the organ re- 

 mains closely attached to the muscle, but ventrally it separates to run 

 caudad a short distance. It is the second of the four electric organs to 

 begin development and the second to begin to separate from the muscle 

 (plates II and iii, R. int.). 



The electric organ of the rectus superior muscle Ues close to the 

 median side and does not curve around it nor separate from it. The 

 obUquus superior is the last of the four muscles to develop an electric 

 organ, which is correspondingly small, forming a short, broad band, 



