14 GEORGE T. HARGITT 



and criticism of this position, points out its weakness and con- 

 siders the recourse to invisible units as a hindrance and not an 

 assistance to an understanding of the facts. In any effort to test 

 the theory by observed results in hydroids one is met by the 

 distinct statement that when germ cells arise from body cells the 

 latter contain invisible and unrecognizable materials. If the 

 germ plasm be really invisible and unrecognizable, the theory 

 need not be discussed, since it cannot be proved or disproved. 

 In the following pages evidence bearing upon the theory is pre- 

 sented from various lines of investigation, but the point of view 

 is taken that there must be recognizable differences of some sort, 

 or else an unbroken line must be traceable from germ cell to germ 

 cell in the life cycle. 



2. Evidence from Hydrozoa 



a. Germ cells. The earlier section of this paper upon the origin 

 of the germ cells is pertinent here, and should be considered in 

 its entirety as a part of the evidence. It may be repeated that 

 the facts show an absence of differentiation of germ cells in early 

 ontogeny; an absence of a definite migration and germ-track; and 

 the formation jof germ cells at the time of sexual maturity from 

 different layers and cells of the body. It has been possible to 

 trace the germ cells back to tissue cells and observe the method 

 by which they are produced; Weismann's own observations 

 confirm this perfectly. It is even possible to prove that there 

 cannot be present in the body cells which form germ cells any 

 invisible germ-cell determinants. Goette ('07) and the author 

 ('13, '16) find cases where di^dsion of a tissue cell results in the 

 formation of two cells, one of which becomes a germ cell while the 

 other persists as an epithelial body cell. If invisible germ plasm 

 be present in the chromatin, as Weismann distinctly states, how 

 is it possible for one of the two cells to become a germ cell and 

 the other a tissue cell when the chromatin is equally divided 

 and none of it lost? This is crucial evidence, and it gives the 

 facts demanded by Weismann himself to prove his contention 

 incorrect, as Goette and the author have already pointed out. 



