- GEKM CELLS OF COELENTERATES 13 



fore, the present discussion will be limited to a consideration of 

 the theory in relation to observed facts in the Hydrozoa. In 

 order to have clearly in mind the essential features of the theory 

 and its method of application to the Hydrozoa attention is 

 directed to the statements of the author of the theory. 



In every ontogenj^, a part of the specific germ-plasm contained in 

 the parent egg cell is not used up in the construction of the body of the 

 offspring, but is reserved unchanged for the formation of the germ cells 

 of the following generation (Weismann, '91, vol. 1, p. 170). 



This splitting up of the substance of the ovum into a somatic half, 

 which directs the development of the individual, and a propagative 

 half, which reaches the germ cells and there remains inactive, and later 

 gives rise to the succeeding generation, constitutes the theory of the 

 continuity of the germ plasm, which I first stated in the year 1885 

 (Weismann, '04, vol. 1, p. 411). 



. . . . In hydroids the germ cells do not appear in the 'person' 

 which is developed from the ovum at all, and only arise in a much 

 later generation, which is produced from the first by continued budding. 

 . . . . In all the last mentioned cases the germ cells are not present 

 in the first person arising by embryogeny as special cells, but are only 

 formed in much later cell generations from the offspring of certain cells 

 of which this first person was composed. These ancestors of the germ- 

 cells cannot be recognized as such: they are somatic cells — that is to 

 say, they, like the numerous other somatic cells, take part in the con- 

 struction of the body, and may be histologically differentiated in different 

 degrees (Weismann, '93, p. 185). , 



Invisible, or at any rate unrecognizable, masses of unalterable germ- 

 plasm must have been contained in the body cells in all cases in which 

 such a transformation has apparently occurred (Weismann, '93, p. 19). 



In the hydroids, then, Weismann notes the germ cells as unrec- 

 ognizable till the period of maturity; their origin at that time is 

 from body cells which are morphologically differentiated and 

 physiologically specialized to perform certain functions of the 

 animal. This is a statement of fact which is confirmed by the 

 work of the authors referred to in section II of this paper. These 

 facts do not fall into line sufficiently with the theory as stated 

 in the first two quotations, and Weismann thereupon assumes 

 the presence of invisible and unalterable determinants which lie 

 latent in the body cells till activated in some way not specified. 

 This point of view is one to which the greatest objection has 

 been raised. Lloyd Morgan ('91), in a very searching analysis 



