Heredity and the Origin of Variations. 139 



cases. " It is quite impossible," lie says,* " to maintain 

 that the germ- cells of hydroids, or of the higher plants, 

 exist from the time of embryonic development, as undif- 

 ferentiated cells, which cannot be distinguished from 

 others, and which are only differentiated at a later period." 

 The number of daughter-cells in a colony of hydroid 

 zoophytes is so great that "all the cells of the embryo 

 must for a long time act as body-cells, and nothing else." 

 Moreover, actual observation (e.g. in Coryne) convinces Dr. 

 Weismann that ordinary body-cells are converted into 

 reproductive cells. After describing the parts of the body- 

 wall in which a sexual bud arises as in no way different 

 from surrounding areas, he says, "Bapid growth, then, 

 takes place at a single spot, and some of the young cells 

 thus produced are transformed into germ-cells which did not 

 previously exist as separate cells. "f 



This transformation of body-cells or their daughter- 

 cells into germ-cells seems therefore, if it be admitted, to 

 negative the continuity of germ-cells as such. But this 

 fact, says Weismann, does not prevent us from adopting a 

 theory of the continuity of germ-plasm. "As a result of 

 my investigations on hydroids," he says,J "I concluded 

 that the germ-plasm is present in a very finely divided 

 and therefore invisible state in certain body-cells, from 

 the very beginning of embryonic development, and that it 

 is then transmitted, through innumerable cell-generations, 

 to those remote individuals of the colony in which the 

 sexual products are formed." 



This germ-plasm resides in the nucleus of the cell ; 

 and it would seem that by a little skilful manipulation it 



* Weismann, " Essays on Heredity," p. 205. 



t A few pages earlier (p. 200) in the same essay, Professor Weismann 

 says, "A sudden transformation of the nucjeo-plasm of a somatic cell into 

 that of a germ-cell would be almost as incredible as the transformation of a 

 mammal into an amoeba." This at first sight does not seem quite consistent 

 with the subsequent sentence which I have quoted in the text; for here, at 

 any rate, the daughters of " mammals " are said to be converted into " amoebae." 

 But this is no doubt because the amoebae (germ-plasms) are contained in the 

 mammals (body-cells). (See the quotations that follow in the text.) 



Weismann, " Essays on Heredity," p. 207. 



