v.] FOUNDATION OF A THEORY OF HEREDITY. 209 



value, because they serve to connect the extreme cases. It is 

 quite impossible to maintain that the germ-cells of Hydroids 

 or of the higher plants exist from the time of embryonic 

 development, as indifferent cells, which cannot be distinguished 

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

 Such a view is contradicted by the simplest mathematical con- 

 sideration ; for it is obvious that none of the relatively few 

 cells of the embryo can be excluded from the enormous 

 mcrease by division, which must take place in order to produce 

 the large number of daughter individuals which form a colony 

 of polypes. It is therefore clear that all the cells of the embryo 

 must for a long time act as somatic cells, and none of them can 

 be reserved as germ-cells and nothing else : this conclusion is 

 moreover confirmed by direct observation. The sexual bud of a 

 Coryme arises at a part of the Polype which does not in any 

 way differ from surrounding areas, the body wall being uni- 

 formly made up of two single layers of cells, the one forming 

 the ectoderm and the other the endoderm. Rapid growth then 

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

 produced are transformed into germ-cells, which did not pre- 

 viously exist as separate cells. 



Strictly speaking I have therefore fallen into an inaccuracy 

 in maintaining (in former works) that the germ-cells are them- 

 selves immortal ; they only contain the undying part of the 

 organism — the germ-plasm ; and although this substance is, as 

 far as we know, invariably surrounded by a cell-body, it does 

 not always control the latter, and thus confer upon it the 

 character of a germ-cell. But this admission does not materially 

 change our view of the whole subject. We may still contrast 

 the germ-cells, as the undying part of the Metazoan body, with 

 the perishable somatic cells. If the nature and the character 

 of a cell is determined by the substance^f the nucleus and not 

 by the cell-body, then the immortality of the germ-cells is 

 preserved, although only the nuclear substance passes unin- 

 terruptedly from one generation to another. 



G. Jager ^ was the first to state that the body in the higher 



^ Gustav Jagcr, ' Lehrbuch der Allgcmeinen Zoologie,' Leipzig, 1878 ; 

 II. Abtheilung. Probably on account of the extravagant and superficial 

 speculations of the author, the valuable ideas contained in his book 

 have been generally overlooked. It is only lately that I have become 



