614 GEORGE T. HARGITT 



ORIGIN OF GERM CELLS IN THE SECONDARY MEDUSAE 



The germ cells of the primary medusae are not our chief 

 concern, except for their form and their position relative to the 

 place of formation of secondary medusae. According to the 

 theory of the continuity of the germ plasm all germ cells are 

 genetically related through a direct line of germ cells. There 

 can be no new formation of germ cells, no transformation of body 

 cells into germ cells, but only by a division of a germ cell may 

 another be produced. In Coelenterates in which the sexual 

 individuals may secondarily produce other sexual individuals 

 by asexual means, there should be, on the theory, a migration 

 of some cells from the gonad of the primary individual to the 

 budding secondary individuals. 



Hybocodon is admirably adapted to test such a thesis. Figure 

 39, while a somewhat diagrammatic representation of a longi- 

 tudinal section of a medusa, is made by carefully combining two 

 or three sections, each of which was drawn with a camera lucida. 

 The outlines of the bell, wrinkled in the sections, are made smooth 

 but no other modification has been made ; the form, the size, the 

 relation of parts are absolutely as observed in the sections. 

 The gonad is on the wall of the stomach, and the swollen base 

 of the tentacle, the only point from which medusae bud, is at 

 the lower corner of the medusa at the right of the figure. Germ 

 cells to pass from the gonad to the tentacle base must proceed 

 aborally to the point where the stomach meets the bell, and 

 must then migrate the entire distance through the bell to the 

 tentacle. 



Evidence on the migration of germ cells. In the bell migrating 

 cells might go through the jelly, or along the ectoderm of the 

 sub-umbrella, or in some way get into the radial canal and pro- 

 ceed to the tentacle, there to pass into the developing bud. The 

 jelly of the bell is non-cellular and any cell migrating through it 

 would at once be observed; since no cells were ever observed in 

 this jelly the latter is plainly not a path of migration. The 

 ectoderm of the sub-umbrella is a delicate sheet of cells in a 

 single layer so thin as to be difficult of demonstration except with 



